Alternative Powertrains and Extensions to the Conventional Powertrain [2 ed.] 9783662655696, 9783662655702

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Commercial Vehicle Technology

Michael Hilgers

Alternative Powertrains and Extensions to the Conventional Powertrain 2nd Edition

Commercial Vehicle Technology Series Editor Michael Hilgers, Weinstadt, Baden-Württemberg, Germany

Michael Hilgers

Alternative Powertrains and Extensions to the Conventional Powertrain 2nd Edition

Michael Hilgers Daimler Truck Stuttgart, Germany

ISSN 2747-4046 ISSN 2747-4054  (electronic) Commercial Vehicle Technology ISBN 978-3-662-65569-6 ISBN 978-3-662-65570-2  (eBook) https://doi.org/10.1007/978-3-662-65570-2 © Springer-Verlag GmbH Germany, part of Springer Nature 2021, 2023 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Responsible Editor: Markus Braun This Springer Vieweg imprint is published by the registered company Springer-Verlag GmbH, DE, part of Springer Nature. The registered company address is: Heidelberger Platz 3, 14197 Berlin, Germany

Preface to the Second Edition

For my children Paul, David and Julia, who derive just as much pleasure from trucks as I do, and for my wife, Simone Hilgers-Bach, who has shown so much understanding for us.

I have been working in the commercial vehicle industry for many years. Time and again I am asked, “So you work on the development of trucks?” Or words to that effect. “That’s every boy’s dream!” Yes, it really is! Armed with this enthusiasm, I have attempted to create as complete a picture of truck engineering as possible. Commercial vehicle developers are living in exiting times right now. The diesel engine that was propelling our trucks for more than a century is now getting challenged. New environmentally friendly drivetrains are needed: This booklet describes possible technical solutions for powertrains of the future that are currently being discussed and written about. It is certain that the diesel powered internal combustion engine will be around for quite some time. But it will not be the solution of the future. Supplements like hybrid systems or waste heat utilization might be used to improve the diesel engine systems but will ultimately not solve the CO2-problem. Alternative fuels are being considered. The most probable path of the future however, are electric drives that might be battery powered or fuel cell powered. A lot of interesting development paths to be discovered! Readers who are studying the subject will find this text to be a good introduction, and I hope that they may be encouraged by the text to find out more about commercial vehicle technology as an exciting field of professional activity. In addition, I am convinced that this booklet will provide added value for technical specialists from related disciplines who would like see the bigger picture and are looking for a compact and easy-to-understand summary of the subjects in question. Finally, I have a personal favor to ask. It is my intention to maintain continual further development of this text. Dear readers, I would greatly welcome your help in this regard. v

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Preface to the Second Edition

Please send any technical comments and suggestions for improvements to the following email address: [email protected]. The more specific your comments are, the easier it will be for me to grasp their implications, and possibly incorporate them in future editions. If you discover any inconsistencies in the content or you would like to express your praise, please let me know via the same email address. And now, I hope you have lots of fun reading, skimming and browsing this booklet. April 2022

Michael Hilgers Weinstadt-Beutelsbach Beijing Aachen

Contents

1 Alternative Powertrains and Extensions to the Conventional Powertrain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Electromobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Hydrogen Based Solutions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 4

2 The Electric Drive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Concept Design of the Electric Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Components of the Electric Powertrain. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Power Electronics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Electric Motors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Energy Storage Device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Brake Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Auxiliary Loads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Vehicle Layout of an Electrically Driven Commercial Vehicle. . . . . . . . . . 2.3.1 Thermomanagement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Commercial Vehicles with Battery-Powered Electric Drives . . . . . . . . . . . 2.5 Charging Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Battery Swap Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Overhead Catenary Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Fuel Cell as Part of the Electric Drive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 Alternatives to the Direct Hydrogen Fuel Cell . . . . . . . . . . . . . . . .

5 6 6 7 8 8 11 12 12 13 14 15 16 17 18 20

3 Hybrid Vehicles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Control and Operating Strategy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Functions of a Hybrid Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Hybrid Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Serial Hybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Parallel Hybrid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Plug-In Hybrid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Brake Energy Recovery in a Full Trailer or Semitrailer . . . . . . . . .

21 22 22 26 28 32 39 40

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Contents

3.3.5 Classification of the Hybrid Concepts on the Basis of the Installed Electrical Capacity . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Technical Comparison of the Hybrid Concepts. . . . . . . . . . . . . . . . . . . . . . 3.5 Evaluation of the Hybrid Concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Fuel Savings by Means of Hybrid Drives . . . . . . . . . . . . . . . . . . . .

40 41 42 42

4 Other Supplements to the Conventional Drive . . . . . . . . . . . . . . . . . . . . . . . . 4.1 The Rankine Cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Thermoelectric Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

45 45 46

5 Hydrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Fuel Cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Some Physical Parameters of Fuel Cells. . . . . . . . . . . . . . . . . . . . . 5.2 Differen‑t Reactions for Fuel Cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Direct Methanol Fuel Cell, DMFC . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Alkaline Fuel Cell, AFC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Proton Exchange Membrane Fuel Cell, PEMFC. . . . . . . . . . . . . . . 5.3 Thermomanagement of a Fuel Cell Vehicle. . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Hydrogen as Fuel for Combustion Processes . . . . . . . . . . . . . . . . . . . . . . .

47 50 50 52 52 52 52 56 57

6 Alternative Fuels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Alternative Fuels and the CO2 Problem. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Biogenic Fuels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Overview on Alternative Fuels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Fuels for Combustion Processes on the Diesel Engine Principle . . . . . . . . 6.3.1 Non-Esterified Vegetable Oils. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Esterified Vegetable Oils – Biodiesel – FAME . . . . . . . . . . . . . . . . 6.3.3 Hydrotreated Vegetable Oil—HVO. . . . . . . . . . . . . . . . . . . . . . . . . 6.3.4 Synthetic Fuels, X to Liquid, XTL . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.5 Dimethyl Ether – DME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.6 Gas-Diesel Mixed-Fuel Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Combustion Processes that Work with a Spark Ignition Engine (Otto-process) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Natural Gas: CNG and LNG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 LPG – Liquefied Petroleum Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.3 Ethanol and Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 60 61 62 64 65 66 66 69 69

Comprehension Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

73

Abbreviations and Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

79

Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

83

70 70 71 71

1

Alternative Powertrains and Extensions to the Conventional Powertrain

The conventional drivetrain with an internal combustion engine that burns diesel fuel from fossil oils has been propelling trucks for many decades. The first truck with a diesel engine first appeared in the 1920s. It is expected that the diesel engine will continue to play a dominant role in propelling commercial vehicles for the next few years especially if you look at the truck population on a global scale. However, the quest to find alternatives to the conventional diesel drivetrain is more important than ever. The most important impetus to find extensions or alternatives to the conventional diesel drivetrain is resulting from the concern about CO2 emission and climate change. The pollutants that a diesel engine emits are already low at the moment and will be reduced further in the future. However, it remains inevitable that CO2 will be released in the diesel combustion process. If the diesel originates from fossil deposits, it is associated with an increase of the CO2 concentration in the atmosphere. Finite reserves of mineral oil and the desire to not have to rely on individual supplier countries that may be politically unpredictable for the supply of energy are two more reasons to search for supplements and alternatives to the established sources of energy. Three approaches are under discussion to alter the currently known drivetrain and reduce the release of CO2 from fossil deposits. Firstly, an attempt is being made to more efficiently use diesel fuel from fossil deposits. Operating commercial vehicles on as little fuel as possible has always been an important goal. Long before the CO2 problem was even known about, the aim of the manufacturers and operators of commercial vehicles was to make do with as little fuel as possible in order to provide transport services as cost-efficiently as possible. [1] is devoted to the fuel efficiency of the conventional vehicle. Further supplements to the traditional diesel-combustion drivetrain are being developed that will further increase the utilization efficiency of the chemical energy of diesel. One prominent technology that supplements the conventional drivetrain is the electric drive part of the hybrid vehicle. In the case of the hybrid drive, some of the primary © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Alternative Powertrains and Extensions to the Conventional Powertrain, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-65570-2_1

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1  Alternative Powertrains and Extensions …

energy continues to come from diesel fuel. The hybrid drive increases the efficiency with which this primary energy is utilized and, when configured as an electric plug-in hybrid, allows additional energy sources to be used for mobility.1 Obviously increasing the efficiency of today’s Diesel technology will ultimately not solve the CO2 problem. It can be a way of reducing the problem in an intermediate phase. Secondly, alternative fuels in combustion engines are being sought that allow the consumption of diesel from fossil deposits to be reduced by means of blending, or avoided altogether. Diesel fuel sold today already has an admixture of hydrocarbons from biomass. As an alternative fuel, combustion engines that use hydrogen have been a topic of public discussion for many years and are currently (again) being evaluated in technical research and testing. Whether this path will offer a solution to the CO2 problem largely depends on how this fuel is going to be produced. Thirdly, work is being performed on genuine alternative drives that will completely replace the classic internal combustion engine. The usual suspects in this case are an electric drive with a fuel cell and an electric drive with batteries as an energy storage device. Figure 1.1 outlines the various paths that could be taken to conserve our mineral oil resources: • supplements to the diesel engine, • alternative fuels, and • departure from the internal combustion engine. Technological progress, the problem of global warming and the possible long-term increase in diesel prices will, in the long run, lead to technologies being used that are not yet economically attractive at the present time.

1.1 Electromobility Some of the developmental Electromobility paths shown in Fig. 1.1 are strongly related to electric drives (i.e. electromobility): the battery-electric drive, the fuel-cell drive and increased efficiency via hybrid all contain electric drivetrain solutions. Increasing the efficiency of the conventional internal combustion vehicles via the electric hybrid has been a widespread solution in passenger cars. As this is a complex solution that only offers moderate benefits for world climate a shift to full electric drives is expected. Full electric drives can obtain their energy either from a battery, a fuel cell or an overhead contact wire. 1 Hybrid

technology in itself is fuel-independent. A hybrid drive can be implemented regardless of whether the primary energy source of the vehicle is diesel, gasoline or any other fuel.

1.1 Electromobility

3

Fig. 1.1   Diagram of the basic possibilities of supplementing or replacing conventional fossil diesel

To what extent electric drives will contribute to effectively minimizing the CO2 emissions depends decisively on how the electricity for electric vehicles is produced. In many countries, power plants that run on fossil fuels are currently contributing significantly to the electricity mix, meaning the CO2 emissions of electricity production have to be taken into consideration in the CO2 footprint of an electric drive. Electromobility is particularly attractive when the electricity can be renewably produced. Charging Electric vehicles (see Chap. 2) and plug-in hybrids (see Sect. 3.3.4) need charging or refueling stations. In principle electricity is available everywhere, so as long as plug-in electric powertrain vehicles and battery-electric vehicles are niche products—we can be sure that the infrastructure will not be a problem. The costs of a charging station, unlike those for the hydrogen drive, will also remain within manageable limits. Solutions for chargers are available for home use and for refueling stations since quite a long time, as [35] and other manufacturers of infrastructure technology show. However, if electric powertrains are meant to bear a major portion of our mobility and transport sector, the electric infrastructure might become an issue in most countries. The Effects of Electromobility on the Automotive Industry Electric drives, whether as stand-alone drives or in the form of hybrid drives, will bring about profound changes to the vehicle industry. High-voltage technology must

4

1  Alternative Powertrains and Extensions …

be mastered in the production and service areas. Many thousands of repair and service workshops will have to face the challenge of high-voltage technology. In addition, there will be a shift in value-added proportions for the suppliers of vehicle parts. The value-added proportion of established branches of technology, such as internal combustion engines and exhaust systems, will decline while new industries will emerge. In this case, especially battery technology and the production of e-machines and power electronics must be mentioned. Study [2] is looking intensively at this change. It attributes the same importance to the change from the internal combustion engine to electromobility as the technological leap from the horse-drawn carriage to the vehicle with an internal combustion engine in the late nineteenth century. The stride to electromobility might also affect the business model of refueling stations and parking lots.

1.2 Hydrogen Based Solutions For hydrogen as the energy source the same holds true as for electricity. The positive impact on world climate and the CO2 footprint depends on the CO2 emission of the production chain: only hydrogen that is produce with low carbon emissions is a real solution to the CO2 problem. There are two hydrogen based solutions in Fig. 1.1: the hydrogen combustion engine and the fuel cell (see Chap. 5). Both require a hydrogen infrastructure. Fuel Cell vehicles and fuel cell infrastructure, namely hydrogen fuel stations and hydrogen production facilities might face a hen and egg problem: not only the vehicle technology but also the infrastructure, the hydrogen production and the refueling station technology has to be developed and established and needs investment. As long as there is no sufficient infrastructure, fuel cell vehicles are not attractive for a lot of applications. At the same time investing into hydrogen production and hydrogen fuel stations is commercially not attractive if the number of vehicles is low.

2

The Electric Drive

Currently the two major development directions for the environmentally friendly drive of the future are battery electric vehicles and vehicles with fuel cells. Both approaches have in common that the drive function is an electric drive. In the case of a battery electric vehicle the electricity is generated in the infrastructure whereas the fuel cell vehicle needs fuel—most commonly used candidate is hydrogen—and the electricity is generated onboard by the fuel cell. Catenary systems are another variant of an electric vehicle. Forerunner for electric drivetrains in commercial vehicle technology are Busses [36]. Especially public urban busses have taken the lead to bring this new technologies to the road due to the fact the municipal bus operations do not only decide on mere economical considerations (as most commercial freight forwarding companies have to do) but take societal  trends into account. At the moment electric drivetrains—both fuel cell driven and battery driven—cannot compete with the conventional drivetrain with combustion engine in terms of production cost. Despite the higher cost, the concern about CO2 emission and climate change are paving the way for the electric drivetrain. Battery technology as well as fuel cell technology are improving so cost reductions are expected. A helpful advantage of the electric powertrain is that electrical components, when mass produced on an industrial scale, give rise to the expectation of fairly low-maintenance costs. In the passenger car sector serious serial products with electric drives have been available for quite a long time—see [17] for example. And important players in the passenger car sector already announced that they consider battery-electric vehicles as the path into the future [41, 42]. For road-using commercial vehicles battery electric drives are an attractive solution for small trucks or distribution transport with short journeys and with daily return to the trucking depot. For longhaul trucks battery electric concepts are worked on as well. But those trucks are operated for several hours and do not return to a fixed place. So the battery with a very limited energy content as energy source is not ideal. Future faster © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Alternative Powertrains and Extensions to the Conventional Powertrain, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-65570-2_2

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charging might remedy this problem. Overhead contact wire systems, although they seem exotic at the present time, are discussed as possible electricity source—at least on busy routes, see Sect. 2.5. The long-term solution might be the fuel cell generating the necessary electricity on board as used—see Sect. 2.6. In principle, electric drivelines in vehicles are easy to implement. Electric motors are robust and easy to handle. In the early days of the automobile, electric drives were studied on equal terms alongside drives with an internal combustion engine [4]. However the combustion engine did prevail for more than one century. One of the reasons was the high mass and the poor energy density of batteries in those days.

2.1 Concept Design of the Electric Drive The starting point to specify the drivetrain of a truck is to understand the usage conditions of the vehicle. What total weight is planned? What are the requirements on acceleration, startability, gradebility and top speed? From that the power demand of the e-machine is determined. Beside that, energy consumption will be considered. Understanding the maximum driving distance until recharging and the driving conditions while driving gives the total energy that needs to be on board. Some additional energy must be foreseen for other energy consumers—see Fig. 2.1. If the ePTO will serve a big consumer this might play an important role. On top some reserve will be added to account for detours, traffic jams or for the reduced battery capacity if the battery is aging. Power demand and total energy consumption of the vehicle between two charging cycles determine (part of) the battery specification. Other HV components like wiring, inverters, switching have to be designed according to the energy flow in the HV system. Based on the specification of battery, motor and the other HV components a cooling system must be designed that keeps all components in their best operating temperature range. Besides the requirements from the driving function other requirements (as always in automotive industry) must be considered such as cost, weight and serviceability.

2.2 Components of the Electric Powertrain An electric powertrain requires a lot of specific components. First of all an electric motor and an energy supply are  needed. In the battery electric case the battery is the onboard energy supply. In the fuel cell a hydrogen tank, the fuel cell and on top a (smaller) battery is needed. Converters convert direct current (DC) into alternating current (AC) and vice versa or change the voltage level of the DC current (DC/DC converters). Power electronics are required to direct electrical energy to flow between the different components.

2.2  Components of the Electric Powertrain

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Fig. 2.1   Generic diagram of the HV architecture of a truck. Some components might be either placed in the High Voltage (HV) or Low Voltage (LV) net

The power electronics, the electric machine and, depending on the technology, also the batteries require active cooling. A control logic is required to take over the energy management function. The control logic can be distributed in different ECUs. The control must ensure that all electrical components are protected against overloads and must control the system so that the components age as little as possible (service life optimization). A low aging effect is an important optimization criterion, especially for the batteries. Figure 2.1 shows a generic and simplified diagram of the HV architecture for a Truck. Some components that are directly mechanical driven by the combustion engine in a vehicle with ICE such as the air compressor, the fan of the cooling system or the water pump of the engine cooling system must be electrically driven in an battery electric drivetrain.

2.2.1 Power Electronics The power electronics ensure that the electrical energy of the generator is fed into the battery or that, if needed, the electric motor is supplied with electricity from the battery.

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As the charge status of the battery affects the battery voltage, a DC/DC converter ensures that a stable voltage is available to the DC electric motor. If an AC motor is used, a DC/AC converter converts the DC current into a three-phase AC current to operate the motor. The electric vehicle will have a 24 V on-board electrical system similar to trucks with combustion engine. To supply the 24 V system a suitable DC/DC converter transforms the high voltage of the traction battery or the fuel cell system battery to the voltage level of the on-board electrical system (step-down converter).

2.2.2 Electric Motors In many configurations, the electric motor is also used as a generator for energy recuperation. It is therefore often referred to as the electric machine, or simply, e-machine. The properties of electric motors are highly suitable for vehicle drives. High-quality electric motors are reliable, have a long service life and require little maintenance compared to the internal combustion engine. Electric motors with sufficient power require a relatively small space. Another feature required in vehicle design is low noise emission and very low vibration operation. Electric drives that are suitable for vehicle drives already provide a high torque at a rotational speed of 0 and a constant torque over a broad rotational speed range. This behavior provides for a very pleasant vehicle ride. In addition there are tangible design advantages: the clutch and transmission can be simplified for an electric-only drive and, in some cases, can even be completely dropped. A complex six to twelve gear transmission is a necessity for the internal combustion engine as well as for the hybrid. It causes extra costs, additional weight and reduces the efficiency of the drivetrain due to its power loss.

2.2.3 Energy Storage Device The total energy that the energy storage device can accumulate is an important design parameter of the electric vehicle. In addition to a high level of total energy, the storage device must be able to cope with high power input—and provide high power output— in order to be able to store the high level of electrical energy that is suddenly available if brake energy is recuperated. The third important electrical requirement of the energy storage device is its operating lifetime. Another important electrical property to be considered is its behavior when it is overcharged or undercharged. Thermal tests, storage capability at cold and hot temperatures, resistance to salt water, mechanical strength, for example, in the event of vibrations and impact loads

2.2  Components of the Electric Powertrain

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and behavior in the presence of fire must be taken into account when designing the energy storage device. The self-discharge rate of the energy storage device must also be considered. The energy storage devices must, of course, also be assessed in terms of the criteria— always applied in vehicle design—of weight and cost. By today’s standards, the energy storage device is the component of an electric drive with the greatest necessity and the greatest potential for further improvement. Batteries The battery cell, in which electrical energy is generated from chemical energy is the smallest unit of a battery. The specific battery chemistry of a cell—there are many different chemical combinations that make up a cell—determines the voltage that will be delivered. There is the classic lead-acid battery, which has been used for decades as a starter battery, and there are countless other pairings of materials from which batteries can be formed. Table 2.1 shows several different battery technologies. The specific energy content and the specific power input of various storage systems are shown in Fig. 2.2. To provide practical voltages, many single cells are connected in series (so-called daisy-chained) to form so-called modules. Several modules are combined into a battery system or battery pack to provide the desired voltage and/or requisite capacity of the battery. Typical voltages for the traction batteries in vehicles are in the region of 400 to 800 V. The battery system is monitored and controlled by an electronic battery management system. The battery management system monitors the charge status and the temperature of the battery cells, providing compensation if the difference in charge status between the cells varies greatly. In addition, the battery management system controls the temperature management of the battery, if the selected technology requires it. It controls fans and pumps that may be necessary to keep the battery within the optimal temperature range. The complexity of battery management depends on the battery technology selected. Lithium-ion battery Lithium-ion batteries are currently the most widespread battery technology used for traction batteries in vehicles. The lithium-ion battery has a good gravimetric energy density—a lot of energy per unit of weight—compared to other battery technologies. It has a low self-discharge rate and exhibits no memory effect. The lithium-ion battery also is comparatively inexpensive today. Lithium-ion batteries are used largely in electronic devices, such as laptops and cell phones. Thermal management is an important factor in lithium-ion technology. Lithium-ion batteries require active automatic control of the battery temperature. If the temperatures are too high, the aging of the lithium-ion battery greatly accelerates, and if the temperatures are too low, the efficiency drops sharply. The battery must be operated within a defined temperature range, which means that cooling and possibly also heating of the battery is required. Approximately 40–45 °C is considered to be the upper temperature

2  The Electric Drive

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Table 2.1  Various battery technologies: the list cannot be complete because the number of technologies for battery cells is huge, which one can check in the relevant technical literature on battery technology or more easily, for example, on Wikipedia Battery technology

Comment

Lead-acid

The classic battery For decades, THE starter battery

Lithium-ion

The generic term for various batteries with lithium Good energy density Temperature management is important

Lithium-nickel-manganese-cobalt oxide

A variant of lithium-ion technology, in widespread use, frequently abbreviated to NMC or similar

Lithium ferric phosphate

An improved lithium-ion battery Less critical with regard to temperature behavior

Lithium polymer

An improved lithium-ion battery No liquid electrolyte

Lithium-sulfur

Very high energy density possible Cycle life problematical

Lithium-air

Research stage Theoretically high gravimetric energy density

Sodium-nickel chloride

High energy density Must be heated up in operation

Nickel metal hydride

Proven in serial use Moderate energy density Significant aging effects Self-discharge

Nickel-hydrogen

Used in spaceflight Expensive at the moment

limit in the cell. Also, no temperature difference of more than 5 °C should prevail inside the individual cell or between the different cells. The geometry of the individual cells and the inter-cell arrangement is therefore optimized from the temperature perspective [13]. Capacitors as energy storage devices In a battery, a chemical process is used to store energy. In a capacitor, the energy is stored in an electrical field. The absence of a chemical reaction makes capacitors very long-lasting and reliable.1 The cycle life of a capacitor is very long. Capacitors endure

1 Capacitors

also have no moving parts that are subject to wear, such as flywheel storage devices or hydraulic systems.

11

2.2  Components of the Electric Powertrain Specific energy [Wh/kg] Specific energy of diesel fuel

104

Specific energy (mechanical energy at the crankshaft, diesel fuel)

103

Li-ion

102 Lead-acid

101

NiMH

Flywheel

Double-layer capacitor EDLC Hydraulic accumulator

100

10-1 101

102

103

104 Specific power [W/kg]

Fig. 2.2   The so-called Ragone diagram shows the specific energy content, specific power input and power output of various energy storage media. The diagram is called Ragone diagram after an engineer who studied batteries for vehicles more than half a century ago [48]

hundreds of thousands of charging and discharging processes, practically wear-free. Furthermore, capacitors are characterized by a high efficiency. There are various capacitor technologies, such as polymer capacitors and double-layer capacitors. Supercapacitors, supercaps or ultracaps being the sales designations of individual manufacturers, have the advantage of a high power density. High-performance capacitors are expensive, though, and the energy density remains well below that of ­batteries (see Fig. 2.2).

2.2.4 Brake Resistor The e-motor of the electric drive serves as a wearfree endurance brake. And what is even better: If the electric drive system acts as an endurance brake it at the same time recuperates energy. Mechanical energy from the decelerated vehicle is converted into electrical energy that is stored in the battery. In very rare cases it might happen that the battery is already fully charged, but one probably still wants to use the e-motor as a wearfree endurance brake. What do to with the electricity produced when the e-motor is slowing down the vehicle and acting as a

12

2  The Electric Drive

generator but the electricity cannot be fed back to the battery? A possible answer is a brake resistor or waste resistor: the electricity is guided through a brake resistor that is converting the electricity into heat. The heat is disspated to the atmosphere. A high performance heat resistor needs active cooling (fan) and must be carefully packaged in the vehicle (gets hot!). In case the electricity is guided through the brake resistor the energy is obviously lost.

2.2.5 Auxiliary Loads The necessary auxiliary loads for a vehicle with electric drive are different from the auxiliary loads of a vehicle with combustion engine. The internal combustion engine needs a high-pressure fuel injection pump, it has a low-pressure system for fuel delivery and auxiliary loads in the exhaust after-treatment system such as the injection of AdBlue and the heating of the AdBlue lines. Those auxiliaries are not needed in a electric vehicles. Other auxiliaries are necessary in both kind of vehicles but might look differently: the lubrication and the oil pump differ and especially the cooling system differs a lot between vehicles with combustion engines and all electric vehicles. For other auxiliary loads that are driven by the internal combustion engine in vehicles with a conventional powertrain, electric solutions must be used on electrically driven vehicles. Such auxiliary loads are the air compressor, the power steering pump and the compressor of the air-conditioning system. Electrical operation of these auxiliary consumers reduces the amount of energy from the battery that can be used for driving. An additional, possibly even dominant in the case of buses, auxiliary load to be considered in electric drive vehicles, is the heating of the passenger compartment in winter. The classic internal combustion engine generates enough waste heat to heat the passenger compartment after a start-up phase. This waste heat is not available with the electric drive. The interior has to be electrically heated. As heating requires a great deal of energy, the energy storage device is subjected to a severe load.

2.3 Vehicle Layout of an Electrically Driven Commercial Vehicle If the conventional drive is replaced by an electric drive, many components are no longer required, for example, the internal combustion engine, transmission, air intake, fuel tank system, exhaust gas after treatment and some noise measures, that are required with the combustion engine, might become obsolete. The components of the electric drive take their place: the electric motor, energy storage device and power electronics.

2.3  Vehicle Layout of an Electrically Driven Commercial Vehicle

13

The conventional drive requires a relatively rigid arrangement of the components. For example, a corridor must be available between the engine/transmission and axle gear, through which the propeller shaft runs. The electric drive, however, allows greater freedom for arranging the components. Conventional architecture with a central motor, as with the internal combustion engine, is just as conceivable as motors on various axles or even one wheel hub motor per wheel. Some early attempts for battery electric trucks copied the layout of the Diesel truck: The e-motor was sitting in the front and a propshaft did connect the  e-motor  with the rear axle(s). More practical is to place the e-motor  next to the axle with the e-motor  axle being parallel to the drive axle. The losses between ring gear and pinion gear of a conventional axle are avoided. Wheel hub motors make it possible to embody the ESP function via the drive system, or to optimize the traction of the vehicle on difficult terrain through the targeted activation of individual wheels. However, wheel hub motors significantly increase unsprung masses and are therefore problematic in terms of comfort and durability. In the case of the battery-electric vehicle, the battery size and battery weight are an important challenge for successfully integrating the components into the vehicle. With the electric drive new safety requirements must be defined. The hazard and risk analysis of a electric vehicle will contain some risks that do not occur in Diesel vehicles. In an accident a damaged battery might expel hot substances or start burning. In designing the battery and the vehicle adaequate precautions are necessary. The high voltage itself is another risk that must be considered. In case of an accident a reliable switch-off scheme is required.

2.3.1 Thermomanagement An important task is the cooling system of the battery electric vehicle. Figure 2.3 shows schematically the basic principles of the cooling in the battery electric vehicle. Different cooling circuits are needed. The radiators are staggered behind each other. As all the radiators are cooled by the same air hitting the front face of the vehicle the temperature level in the different circuits influence each other. Circuit 1 in Fig. 2.3 is the cooling circuit of the battery. From a temperature management standpoint today’s Li-Ion batteries are demanding: they require heating and cooling. If ambient temperatures are too low the battery has to be heated. The maximum allowed temperature for today’s Li-Ion-battery is around 40 °C and requires high performance cooling. Passive cooling is not always (depending on outside temperature) sufficient: the battery electric vehicle needs a active cooling circuit with compressor and condensor to keep the battery in the desired range. The cooling circuit of the battery can be combined with the cooling circuit of the HVAC system for the cab (circuit 2 in Fig. 2.3). By the way, a homogeneous temperature distribution across the whole battery is required as well. Temperature differences between different cells should be low.

14

2  The Electric Drive

Fig. 2.3   Schematic diagram of the different cooling circuits in a battery-electric vehicle. The diagram focusses on some general principles. Valves etc. are not shown. See for example [47]

The e-motor  might be operated at around 60 °C. This adds a third cooling circuit (circuit 3 in Fig. 2.3). Power electronics that require cooling might be integrated in the battery’s or the e-motor's  circuit. Circuit 4 in Fig. 2.3 is a heating circuit. The heat of the combustion engine is no longer available in a battery electric truck so a electric heater is needed to heat the cab. Heat from the e-motor  circuit (circuit 3 in the figure) might be used as well. As electricity is a scarce resource in the battery electric truck a heat pump for heating the cab might be used to increase efficiency. This adds additional complexity to the temperature management system (not shown in Fig. 2.3). The brake resistor or waste resistor explained in Sect. 2.2.4 might need an additional cooling circuit not shown in Fig. 2.3.

2.4 Commercial Vehicles with Battery-Powered Electric Drives The batteries are the biggest technical problem of electric vehicles. The batteries are expensive, very heavy and the driving range that is available is low compared to a vehicle with an internal combustion engine and a decently sized tank.

2.5  Charging Process

15

A very light distribution truck (3.5 t permissible gross vehicle weight) with a battery capacity of 40 kWh allows a range of more than 100 km [29]. Vans are also offered ex works with similar battery capacities [30]. For distribution transport, a range of 100 km is often sufficient. For the mail and parcel distribution in conurbations, many of the vehicles travel less than 100 km per day. So the use of electric vehicles is somewhat easier than in other truck segments. Due to the high starting-stopping frequency, the electric drive with brake energy recovery is able to show its advantages in urban delivery driving profiles. But also heavier trucks with all-electric powertrain are offered in small quantities since quite a while. In [5] a forerunner (at this time) in a niche segment offers a 18-ton electric-only heavy distribution trucks. Soon after well-known market players were becoming increasingly involved in the market for battery-electric trucks and are offering corresponding products (in some cases in small series) [39]. In heavy long-haul transport with long routes and high average speeds, high tonnages and consequently high energy requirements, the use of a battery-aided electric drive is less obvious than in the distribution transport and garbage collection segments. However in some regions of the world like Europe and China the stringent CO2-limits for heavy commercial vehicles will give rise to a new market segment of battery electric trucks. At the same time availability of lower-cost components for electric drivetrains will improve the economic attractiveness of electrically powered commercial vehicles. It can be expected that the market for all-electric trucks will grow rapidly in the next years. The technical feasibility is beyond dispute. Weight and range targets require further technical progress. Range extender One way of counteracting the disadvantages of the limited range of battery-powered electric drive is provided by the so-called range extender: in principle, the vehicle operates in battery-electric mode. If the battery charge is running out, a generator is operated by a (small-sized) internal combustion engine that generates additional power. [31] presents such a concept as an experimental garbage collection truck (plenty of recuperation!). However, if the range extender steps in, the vehicle starts emitting CO2 and other pollutants. And the range extender adds additional cost, weight and complexity to the vehicle. Such concepts currently tend to be niche applications and studies.

2.5 Charging Process The battery electric drive needs frequent charging. The battery charging process has to be automatically controlled and monitored. Part of this control function must be implemented on the vehicle, because the charging process will have to be specifically adapted for different batteries.

16

2  The Electric Drive

The charging process inevitably comes with electric power losses. So in the charging process the whole charging system including the battery is heated up. For high power charging active (water) cooling is required. Plug-in charging The most obvious solution for the charging process calls for a charging cable and a corresponding connection on the vehicle. Various standards regulate, for example, plugs [11] and the charging system for electric vehicles [12]. Collector charging For vehicles that travel on fixed routes, for example, urban buses, an automated connection of the vehicle to the charging station makes sense: the vehicle has a current collector similar to a trolley bus or a tram. At the charging station, for example, at the last stop, this collector moves out and connects onto the pickup of an electricity source (see example [9]). The current collector can be configured simply because, unlike a trolley bus, it is not connected to the overhead wire during the journey. Inductive charging In addition, there are also solutions with inductive charging technologies, as known from electric toothbrushes; a primary coil is recessed into the base. The vehicle has a pickup coil that must be positioned sufficiently close to and exactly over the primary coil. As short a distance as possible between the coils is required to keep stray fields and the reactive power (losses) as low as possible. The advantage of inductive charging is that the connection and disconnection process for the charging cable is no longer required. (See for example [10]).

2.5.1 Battery Swap Systems Battery swap systems recharge the battery outside the vehicle. The idea of a battery swap system is to exchange mechanically the entire battery. The battery is no longer an integral part of the vehicle but is exchanged if needed in a deposit return scheme. Battery swap systems are especially being developed in China both for passenger cars and for trucks. An fully automated process takes out the battery and attaches a replacement battery to the vehicle. For passenger cars with integrated car bodies this must be done from the bottom. For trucks with a frame structure, replacement of batteries can be done from the side or even from above with overhead cranes. The basic advantage of battery swap systems is to refuel the vehicle in a very short time (comparable time to refuelling a conventional vehicle with Diesel) by exchanging the complete battery. The battery replacement process can take place in only a few minutes. The recharging of batteries then takes place outside the vehicle in the battery swap station. The recharging process of the batteries in the station that are momentarily not

2.5  Charging Process

17

mounted in a vehicle can be scheduled such that predominantly abundant and cheap electricity—maybe at night time—is used. Of course the vehicle itself (with battery) still can be connected to a normal plug-in charging station. It is not necessary to go to the battery swap station every time you want to recharge the vehicle. A positive side effect to a battery electric vehicle with battery swap function is, that— depending on the business model of the battery swap system—the initial price of the truck is lower. The battery might belong to an infrastructure company and is rented per use. Albeit it seems unclear whether such an infrastructure service can be economically provided. A battery swap system requires spare batteries (more than one!) at all battery swap stations. So much more batteries are required than vehicle that take part in the battery swap system. The battery swap station is a recharging station plus a battery storage station plus a battery exchange station. So it can be assumed that a battery swap station is more expensive than electric charging stations. Battery swap systems have to solve some technical challenges as well: To make a battery swap system feasible a standardized battery, or a few different standardized battery formats are required. A robust mechanical and electrical interface between battery and vehicle are necessary that can be disconnected and connected very reliably. As most batteries are liquid-cooled the cooling circuit of the battery must be connected to the vehicle. An automated system that avoids any coolant leakage even after years of hard operations is difficult to maintain. Moreover in the process of battery replacement no coolant should be spilled. It is to be seen whether battery swapping system will have a future especially if other charging standards advance and charging gets quicker and quicker.

2.5.2 Overhead Catenary Systems Overhead catenary systems are an alternative way to bring electric energy into the vehicle. In this technology the vehicle is drawing its power, similar to an electrified railway, from an overhead contact wire by means of an overhead collector (pantograph/pole). Unlike on the railway, the overhead contact wire for road vehicles is bipolar i.e. it has two poles because with road vehicles the return flow is not possible via the road surface due to the rubber tires on asphalt. And unlike railways a horizontal (y-direction) mobility of the vehicle relative to the overhead wires is necessary. The electricity of the catenary system and the HV-system of the truck are usually on different voltage level. So the truck needs an additional DC/DC converter to make use of the electricity from the overhead wire. If there are routes without an overhead contact wire, any faults in the overhead contact wire or, if passing or overtaking is performed that requires disconnection, the vehicle drives in battery electric mode. Up to now, overhead catenary systems for vehicles that are not track-guided (not on a railroad) have existed practically only for fixed route bus services, also known as trolley

18

2  The Electric Drive

buses [6]. Since quite some time systems are being investigated for trucks on a test track under laboratory conditions [8] and on real highways. Catenary systems can be integrated into the mobility system in two different ways: The catenary system can be used as additional charging opportunity for BEV trucks. The truck is mainly a battery-electric vehicle. As optional equipment a pantograph can be fitted to the vehicle if it fits to the specific usage. Some major highways might be equipped with a few miles of catenary system. If the trucks travels on such a section it can connect to the overhead wire and use the overhead wire as intermediate charging opportunity while travelling. Additional range can be attained. Stationary charging stations for trucks at rest area and motorway stations are necessary in huge numbers. The degree of road electrification significantly determines the chargeable additional range of the vehicle. The user’s potential benefit from catenary technology strongly depends on individual routes. In this scenario the main basis of the truck drivetrain is the battery electric vehicle. The fraction of roads with catenary systems can gradually be extended. An alternative scenario sees the catenary system as a dominant source of energy for the truck (like in the electric railway system). The trucks are equipped with an electric drivetrain and a small battery. In this scenario a high fraction of the roads needs catenary. So this scenario has a thresholds for realization both in time and cost. A major fraction of the road must offer catenary systems very quickly. And the line costs for the erection of these overhead contact wires are considerable. [7] specifies a lower limit of 3 Mio. Euros per kilometer for Germany. A big challenge comes from the necessary power required, if a big portion or all trucks on a highway use the catenary system: In rush hour there might be one truck after the other within a distance of 50 m. On an uphill slope each of those trucks might need a considerable electric power in the range of 200 to 250 kW. So the catenary system must be designed such that around 5 MW per kilometer overhead wire of electrical energy is available. Taking the permissible voltage in todays systems into account this is difficult to realize. The scenario with catenary as the main source of energy has additional disavantages: The catenary for trucks has very little synergies with other mobility systems and if there is a malfunction in the catenary system fallback ­solutions are difficult.

2.6 Fuel Cell as Part of the Electric Drive Storage systems for electricity, i.e. batteries are heavy and expensive. Therefore producing the electricity onboard as needed is an obvious idea. The fuel cell can do that: Electricity is generated onboard and used in an electric drive to propel the vehicle. The fuel cell vehicle is an electric vehicle with an onboard electricity generator see Fig. 2.4. Typically fuels with the composition CmHn or CxHyOz can be used. Electrical energy can be created by the proton release of fuel with the composition CmHn or CxHyOz. This is the principle of the fuel cell.

2.6  Fuel Cell as Part of the Electric Drive

19

Fig. 2.4   Very general sketch of the electric architecture of a fuel cell electric vehicle: The fuel cell vehicle is an electric vehicle with an onboard electricity generator—compare Fig. 2.1

The most direct fuel cell uses hydrogen, H2, directly as fuel. Hydrogen and air are  needed for the reaction shown in Eq. 2.1:

1 O2 + H2 → H2 O + electrical energy 2

(2.1)

Hydrogen fuel cell systems have an outstanding advantage: The only emisssion is water! Moreover hydrogen has a high gravimetric power density and hydrogen tanks on the vehicle can be refuelled in a short time. Vehicles with a fuel cell have a powerful high-voltage battery. This serves as a buffer battery to cover electrical load peaks and to realize brake energy recovery. As the required electrical components are already on board, it is obvious that fuel cell vehicles should be constructed with a recuperation function. Since the fuel cell of today is damaged most in the first minutes after the start, especially the membrane suffers, it is attractive to be able to operate the vehicle, at least for a short time, in battery-electric mode. Very short distances and maneuvering operations are then performed in battery electric-only mode to extend the service life of the fuel cell system. It is technically easy to combine electricity from different sources. So in order to realize a certain value of electrical power generation in a fuel cell vehicle it is possible to use multiple fuel cells and add their power to realize the desired system power. Parallel arrangement of independent fuel stacks or even fuel cell systems allows for an

20

2  The Electric Drive

easy realization of high power output. As a positive side effect one might achieve some redundancy. Fuel cell vehicles have long been considered as a possible solution of the future. The first fuel cell vehicles are being mass-produced in the passenger car sector. In the commercial vehicle sector, some fuel cell vehicles are being road tested. See chapter 5 for more information on Fuel Cells.

2.6.1 Alternatives to the Direct Hydrogen Fuel Cell As mentioned above as an alternative to refueling with hydrogen, there is the possibility of taking on board hydrocarbons as the fuel for fuel cell drives. With hydrocarbon on board, there are two ways of further utilization. Firstly, the hydrogen can be extracted from the hydrocarbons in a reformer and then fed to a fuel cell. As a result, though, the efficiency of the overall system drops severely and additional expensive technology has to be taken on board with the reformer. The other option is feeding the hydrocarbon compound, for example, methanol, directly to the fuel cell membrane. The membrane allows protons to pass out of the methanol molecule but is impermeable to other substances. This way the membrane generates a charge separation, which causes current to flow. This example is referred to as the direct methanol fuel cell. There are countless other manifestations of the fuel cell. In addition to the hydrogen fuel cell and the methanol fuel cell mentioned above, there are numerous other fuel cell concepts based on other fuels. However, it seems that direct hydrogen fuel cells are the prime path for fuel cell drives.

3

Hybrid Vehicles

In engineering, the word hybrid is used for solutions in which two things are combined. A hybrid vehicle is a vehicle in which two drive systems are combined. As a rule, hybrid vehicles have an internal combustion engine with a fuel tank as the energy storage device (mostly diesel in the case of commercial vehicles) and a second drive system. The most common hybrid variant—in addition to the internal combustion engine, which is the main drive—has an electric drive with battery storage. Electric hybrid technology is often seen as the preparation for the supposed next step, the fuel cell drive or battery-electric vehicles, because electric drivetrain components will be required just as much for battery-powered as for fuel cell powered vehicles. In addition to electric hybrids, there are also hybrid systems with flywheel storage devices and with hydraulic or pneumatic pressure accumulators.1 In conventional vehicles with internal combustion engines, the kinetic energy generated during braking is converted into heat and thereby lost. With the hybrid, the second drive system is selected so that it can store some of the vehicle’s kinetic energy when braking. This energy is used again to drive the vehicle when it accelerates. This process is called brake energy recovery or recuperation. So the main reason to use hybrid technology is fuel consumption reduction by doing so reducing the CO2 emission of the vehicle.

1 Bivalent,

also known as bi-fuel, vehicles must be distinguished from hybrids. Bivalent vehicles are vehicles whose engines can be operated with two different fuels. For example, gas and gasoline are often envisaged as possible fuels. Bivalent vehicles are frequently used when low-cost alternatives to conventional fossil oil-based fuel are to be used, but a complete refueling station network for the alternative fuel is not guaranteed. Such vehicles are usually not classified as hybrid vehicles. © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Alternative Powertrains and Extensions to the Conventional Powertrain, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-65570-2_3

21

22

3  Hybrid Vehicles

Hybrid technologies have been sporadically experimented with for decades. In classic road-based commercial vehicle design, the hybrid has occupied a niche in recent years. To achieve the current CO2 targets in Europe hybrid technology will not be sufficient. Other concepts such as battery electric vehicles and fuel cell vehicles will be required.

3.1 Control and Operating Strategy An important core element of the hybrid system is the control or operating strategy of the system. Firstly, a sophisticated operating strategy makes it possible to utilize the full potential of a hybrid system. On the basis of the driver’s performance wishes, current vehicle speed, charge status of the energy storage device and possible system restrictions (e.g. the permissible temperature), the operating strategy decides in which mode the internal combustion engine and electric motor/generator will be operated. Emission characteristic maps and catalytic converter temperatures can also be taken into consideration. Navigation data—enriched with altitude profiles of the road—will also be integrated into the operating strategy. If the vehicle knows that a downhill stretch is about to follow, the battery can be partially discharged in the stretch before the downhill stretch begins because the battery can be easily recharged on the downhill stretch. On the other hand, if the road is leading to an uphill gradient, the operating strategy of the hybrid system can attempt to provide a battery with the maximum charge at the foot of the gradient in order to be able to give electrical assistance on the gradient. Route-specific operating strategies are conceivable for fixed route buses. If the auxiliary loads, such as the air-conditioning system, air compressor and others are also operated electrically, the current and predicted energy requirements of the auxiliary loads must be integrated into the operating strategy.

3.2 Functions of a Hybrid Drive Recuperation is the most important and most original task of a hybrid system in the commercial vehicle sector. During recuperation, the vehicle’s mechanical energy (kinetic energy/potential energy) that is lost as brake energy with the conventional drive is used to fill an energy storage device of the vehicle. This energy is then used again at a later time to drive the vehicle. It also follows from this that hybrid technology promises a particularly high percentage of fuel consumption savings in vehicle uses that have a high proportion of braking procedures. Theses are, for example, fixed route buses and garbage collection vehicles. However, in addition to the brake energy recovery, there are other leverage points for reducing fuel consumption with the hybrid.

3.2  Functions of a Hybrid Drive

23

Most hybrids are designed as plug-in hybrid. This means that the battery can be charged from the infrastructure like a battery electric vehicle. There are different methods to do the charging—see Chap. 2. The batteries might be fully charged when the vehicle is started. The additional energy from the battery is supporting the propulsion of the vehicle and will lead to a reduced consumption of diesel fuel and—depending on how the electricity to charge the battery has been produced—to a reduced emission of CO2. The operating point of the internal combustion engine can be optimized by the hybrid drive: if it is favorable to operate the internal combustion engine at a higher load active charging of the energy storage device can be done. This way the generator function of the hybrid is used to increase the working load and feed energy into the hybrid storage device. On the other hand, the internal combustion engine can be supported by the hybrid drive, thus lowering the load of the internal combustion engine. Figure 3.1 explains the internal combustion engine’s load point shift by the hybrid component on the basis of a fictitious engine map. Another advantage of a hybrid drive can be that one has the opportunity to use a smaller internal combustion engine with the same performance ratings. This is called downsizing of the internal combustion engine. A smaller internal combustion engine has less internal friction. It also works more frequently at a higher load within the engine map, where the specific consumption is usually more favorable. With downsizing, the reduced power of the internal combustion engine is compensated by the additional power of the hybrid drive. On a long full-load journey, though,

Torque

Full-load curve of the engine Support of the internal combustion engine by the second drive unit

Curve of the lowest specific fuel consumption

Additional load for the internal combustion engine due to active charging of the energy storage device

Light: low spec. fuel consumption

Engine speed

Dark: high spec. fuel consumption

Fig. 3.1   The load point of the internal combustion engine can be shifted by the hybrid component so that the engine operates within the range of the lowest specific fuel consumption

24

3  Hybrid Vehicles

Hybrid

Full battery

Recuperation Start/ stop

Operation of the internal combustion engine at the optimum point Recuperation

Empty battery

Boosting

Internal combustion engine mode

Electrical acceleration

Fig. 3.2   Illustration of various hybrid functionalities that can contribute to fuel saving

the energy storage devices of present-day hybrid systems are exhausted quickly and the lower operating energy input of the internal combustion engine becomes noticeable. In addition to the mentioned leverage points for reducing fuel consumption: • brake energy recovery, • plug-in charging, • load-point shift, and • downsizing hybrid drives can facilitate other interesting product features. As a rule, the pursuit of fuel efficiency is dominant, so additional functions that can be obtained with hybrid technology should not reduce the fuel consumption effects of hybrid technology. The various functions that hybrids can achieve are described briefly below (See Fig. 3.2): Electric-only mode means an operating mode in which only the electric motor (or hydraulic drive) moves the vehicle. The internal combustion engine is completely switched off. The technical prerequisite for electric-only mode, though, is that the auxiliary loads of a vehicle are electrically driven. In particular, the air compressor (brakes) and the power steering must be electrified.2 Electrically powered consumers offer several advantages compared to mechanically operated ones: they can be easily switched on and off, they do not contribute to the idling

2 In

principle, the possibility also exists of allowing the internal combustion engine to continue to run in electric-only mode in order to supply the non-electrically driven auxiliary loads.

3.2  Functions of a Hybrid Drive

25

consumption of the vehicle and the auxiliary loads can be operated at the desired operating point regardless of the engine speed. Electric-only mode is an attractive option for vehicles traveling in busy cities, because in stop-and-go traffic, the diesel engine emits a particularly large amount of pollutants. A hybrid vehicle with an electric-only drive function provides relief from this and can be exempted from any driving restrictions. In addition, the vehicle moves almost silently, minimizing any noise nuisance for residents, especially from nighttime delivery vehicles. However, the very low noise generation with electric-only mode involves an additional hazard. The driving noise of vehicles with conventional internal combustion engines always serves as an indication and warning for other road users, especially for pedestrians and cyclists who, not being inside a vehicle, are not detached from the ambient noise. This natural warning is strongly reduced if a vehicle travels in electric mode. So electric vehicles might need a sound generator to catch attention of other road users. Electric maneuvering is the most moderate form of electric-only mode. In this case, the vehicle is briefly moved at low speeds only by the electric motor. Coasting mode is, in modern commercial vehicles, a so-called EcoRoll mode that is often used to save fuel: in rolling phases, the transmission is shifted to neutral to enable the longest possible rolling distance. The disengaged internal combustion engine continues to idle. If the vehicle speed drops below the cruise control set speed, a gear is re-engaged and the internal combustion engine propels the vehicle again. With a hybrid system, the EcoRoll phases can be electrically extended by the electric motor gently pushing the vehicle and the engagement of the internal combustion engine thereby being delayed for as long as possible. This can be called coasting mode. Reduction of the speed in the rolling phase is reduced by the support of the electric drive. In contrast to electric-only mode, the output of the electric motor can turn out to be moderate. No electrical auxiliary loads are required because the internal combustion engine continues to run. Electric power take-offs are easier to implement in hybrid vehicles because an efficient electrical energy supply is installed due to the hybrid system. Cooling units, lifting devices and hydraulic pumps that are installed in commercial vehicles can also be operated electrically. This can significantly reduce noise in cities, especially at night. Boost mode means that the output of the internal combustion engine and electric motor are combined in a targeted manner to temporarily provide the driver in certain driving situations with increased drive power, for example, for kickdown. With many electric motors, it is even possible to exceed the nominal power of the electric motor for a short time. Starter function: It is obvious that the drive’s electric motor can also be used to turn over the internal combustion engine. The starter—a component that is used only during a short portion of the truck's travel time, namely when the engine starts—can then be dispensed with. However, the cold-start capability is a challenge in this case: in extremely

26

3  Hybrid Vehicles

cold weather, the diesel engine is particularly unwilling to start and at the same time the cold battery provides less electricity than in normal driving conditions. The electrical equipment of the hybrid system can be used to supply the 24 V on-board electrical system. The generator and the classic starter battery are then no longer required. To do this, though, it is necessary to transform the high voltages of the hybrid system from several hundred volts to 12 or 24 V. With certain configurations of hybrid vehicles, the drive power of the electric motor can be used to perform gear shifts with virtually no interruption of the tractive power. When the clutch is disengaged to perform a gear shift, the electric motor continues to propel the vehicle. In recuperation mode, the electric motor of a high-performance hybrid system can contribute a noticeable braking torque. The hybrid system is integrated into the functionality of the braking system: if the driver brakes—or if he pulls the lever for the engine brake and/or retarder—on hybrid vehicles, in addition to the wheel brake, engine brake and retarder (if installed), the electric motor is additionally available in recuperation mode to build up braking torque. This way, the wear to the wheel brake can be reduced. Unfortunately, the braking torque of the hybrid system is not suitable for downsizing conventional retarder systems or even replacing them with a hybrid. A powerful retarder has a significantly higher braking capacity than a hybrid system. The recuperation function also has only limited availability: when the battery is fully charged, the braking torque of the hybrid system is no longer available. When driving downhill the battery storage device might be fully charged, with the result that the braking effect of the hybrid might not be available long enough. An overview of the various functions that hybrid systems can provide is summarized in Table 3.1. There is fluid transition between some of the functions discussed here. For example, electrical maneuvering and electric mode differ in terms of speed and duration. There is no clear definition. There are various basic concepts for the implementation of a hybrid drive. Not all concepts can offer all of the functions of Table 3.1. The technical complexity of the various hybrid concepts also differs.

3.3 Hybrid Concepts A few hybrid concepts are explained below. There are too many different variants to explain them all. The concept of power-split hybrid for example, which has become widespread in the passenger car hybrid segment, is not being pursued in the truck segment. A fundamental classification of hybrid concepts is their subdivision into serial hybrids and parallel hybrids. With the parallel hybrid, both drive systems can act simultaneously (parallel) on the wheels. Both the internal combustion engine and the secondary motor (electric motor, hydraulic motor, etc.) have a mechanical connection to the wheels.

3.3  Hybrid Concepts

27

Table 3.1  Various functions that a hybrid system can facilitate Function

Description

Recuperation—brake energy recovery

Transformation of the kinetic energy of the vehicle during braking into storable and later usable energy

Optimization of the operating point of the inter- The operating point at which the internal comnal combustion engine bustion engine is operated can be shifted by the hybrid system: if a higher internal combustion engine load is more efficient, the electric motor acts as a generator and generates an additional load. If a lower internal combustion engine load is rational (or the hybrid storage device is full), the internal combustion engine is supported Downsizing

Reduction in size of the internal combustion engine. If needed the combined internal combustion engine and the second drive (temporarily) intermittently allow the performance ratings of a larger internal combustion engine, resulting in cost and weight savings

Electric mode

The vehicle drives long distances with only the electric motor drive

Maneuvering operations without the internal combustion engine

The maneuvering operations (for a short time at low speeds) take place without the internal combustion engine

Driving off without the internal combustion engine

When driving off, the vehicle is accelerated by the second drive (e.g. starting off at traffic lights)

Electrical extension of the rolling phases

The rolling phase of the vehicle (e.g. EcoRoll) is extended by the support of the electric motor

Electrically operated power take-offs

The electric motor of the hybrid system drives a power take-off, so that the unit driven by the power take-off can be used even when the internal combustion engine is switched off

Boosting

Overlaying of the drive power of the internal combustion engine with the drive power of the second drive and therefore more dynamic handling characteristics than with the internal combustion engine alone

Starter function

The second drive of the hybrid vehicle (electric motor) turns over the internal combustion engine and takes on the task of the starter motor (continued)

28

3  Hybrid Vehicles

Table 3.1   (continued)

Function

Description

Generator function

The task of the alternator in the conventional vehicle is taken over by the hybrid system

Motor start/stop function

The internal combustion engine is automatically switched off when the vehicle comes to a halt and then restarted when the vehicle continues driving

Gear shifting with no interruption of tractive power

During a gear shift, the tractive power of the internal combustion engine is not available at the propeller shaft. This interruption of tractive power is smoothed by the electric motor

Electrically powered consumers

Consumers like the air-conditioning system are electrically powered and can therefore be operated even when the internal combustion engine is switched off

Electrically powered bodies

Bodies that are operated on conventional vehicles via the power take-off of the internal combustion engine, are operated electrically from the battery

Electrical contribution to the braking torque

The electric motor contributes to the braking torque in recuperation mode

With the serial hybrid, the drive components form a chain (in series): the internal combustion engine operates a generator. The energy that the generator delivers is then used in the second motor for the vehicle propulsion system.

3.3.1 Serial Hybrid Serial drive forms Serial drives have already been in use for a long time on specialized machines and large units of equipment, such as construction machinery, diesel locomotives, submarines and ships. A serial drive is a drive on which the internal combustion engine has no mechanical connection to the driven axle (or the ship’s propeller). The conventional drivetrain with a transmission and propeller shaft is obsolete. With the so-called diesel-electric drive, the internal combustion engine operates a generator that produces electricity. Electric motors that drive the wheels are fed with the generator’s electrical energy. With this design, the internal combustion engine is always able to work within the optimum range of effectiveness. The driving-off procedure and automatic speed control, which takes place in normal motor vehicles via the clutch and transmission, are very easy to control with diesel-electric systems. A diesel-electric drive

3.3  Hybrid Concepts

29

also allows interesting installation space arrangements, which a conventional drive with a mechanical connection between the internal combustion engine and the drive wheel does not facilitate. Hub motors make it possible to eliminate the propeller shaft and allow, for example, further lowering of the passenger compartment floor on buses. With the serial hydraulic drive (diesel-hydraulic), the internal combustion engine drives a hydraulic pump, which pressurizes the hydraulic fluid. Hydraulic motors that turn the wheels can be driven by the high-pressure fluid. Construction machines or forklift trucks are typical vehicles that have serial hydraulic drives. These vehicles need a powerful hydraulic system anyway to move excavator arms, lifting forks, etc. It is therefore obvious that the drive function and working function can both be driven hydraulically. The most significant disadvantage of the serial drive is that the entire drive power of the vehicle is installed three fold: firstly as an internal combustion engine, secondly as a generator or pump, and thirdly as a driving motor. It is therefore a correspondingly expensive and heavy system. The serial drive becomes more efficient if the kinetic energy of the vehicle when braking is recovered and fed into an energy storage device. The advantages of serial hybrids are the very high power input during recuperation and the possibility of operating the internal combustion engine at the optimum operating point even during dynamic operation of the vehicle. Serial hydraulic hybrid The hybrid function can be easily added to vehicles equipped with a serial hydraulic drive. The hydraulic motor must also be able to act as a pump. Two additional hydraulic pressure accumulators (one high-pressure and one low-pressure accumulator) allowing the energy to be buffered and several valves together with a controller make a hybrid out of the serial hydraulic drive. Figure 3.3 outlines a serial hydraulic hybrid. When the vehicle decelerates, the hydraulic motor functions as a pump: the kinetic energy of the vehicle is (partly) converted into potential energy in the vehicle’s pressure accumulator. Upon driving off, the energy of the pressure accumulator supports the hydraulic pump on the internal combustion engine. The advantages of a hydraulic hybrid compared to an electric hybrid are the high power input and power output that are possible. As a result, there are driving cycles in which a very high recuperation rate is possible with the hydraulic hybrid. The disadvantage is that the energy density of the storage medium, the pressure accumulator, is extremely low. In addition, hydraulic components that can deal with the pressures required here are heavy and expensive.3 Another advantage of hydraulic hybrids is that the energy storage device is subject to a slower aging process than with electrical systems. 3 The

components of the hydraulic hybrid are not expected to have a much lower cost in the future. Most experts envision a significantly higher cost-reduction potential for electric hybrids in the future.

3  Hybrid Vehicles

30 Hydraulic fluid reservoir

Internal combustion engine Hydraulic pump-motor unit Hydraulic pump

N2 High-pressure accumulator

Fig. 3.3   Diagram of a serial hydraulic hybrid

Serial electric hybrid Figure 3.4 shows a diagram of a serial hybrid with an electric motor and an internal combustion engine. Only the electric motor drives the wheels of the vehicle. The internal combustion engine drives a generator that generates the electricity to operate the electric motor. In braking mode, the drive motor also functions as a generator and generates electricity by converting some of the vehicle’s kinetic energy into electrical energy. The recuperated energy is stored in the battery. As the serial electric hybrid must have a very powerful electric machine, a large amount of the electric braking power can be used (if the battery allows it) for energy

Electric motor

Generator Internal combustion engine

Battery

Fig. 3.4   Diagram of a serial electrical hybrid

3.3  Hybrid Concepts

31

recuperation. This makes it possible to recover a high proportion of the brake energy and to buffer it in the electric storage device. However, in driving mode, the serial electric hybrid always requires costly conversion of the mechanical energy of the internal combustion engine into electrical energy and subsequent conversion of the electrical energy back into mechanical energy at the wheels. This is naturally less efficient than a conventional drivetrain. The efficiency losses of the conversion chain—mechanical rotation of the internal combustion engine-generator-electric motor-mechanical rotation of the wheels—must then be compensated by the advantages of the hybrid system during recuperation before the system can claim any fuel savings. The serial hybrid is therefore not suitable for application profiles in which driving is performed for a long period at a moderate to high speed, and the recuperation proportion tends to be low, for example, long-haul transports. Some advantages and disadvantages of the electric serial hybrid are outlined in Table 3.2. The potentials of a diesel-electrical serial hybrid in fixed route bus transport are shown in [15]. Fuel savings in excess of 30% are considered possible. In fixed route bus transport, the savings depend on the speed of the bus and on the distance between stops (i.e. the number of stops per kilometer). In addition, the topography of the route plays an important role. The liberty of component arrangement with the serial hybrid can be well utilized on buses. The battery can be located on the roof, wheel hub motors allowing space-friendly implementation of the electric drive.

Table 3.2  The most important advantages and disadvantages of the serial electric hybrid Advantages

Disadvantages

Arrangement of the drive components in the vehicle is very variable

High cost (three times the installed power)

High power recuperation possible

The total volume of the system is large

Starter and generator functions are feasible

Heavy weight (three times the installed power)

Electric-only mode possible

No overlaying of the power of the internal combustion engine and electric drive is possible

Battery charging is also possible when stationary Well suited for split drives, such as wheel hub motors A diesel engine can be operated at its best efficiency point

Efficiency losses due to multiple energy conversions

3  Hybrid Vehicles

32

3.3.2 Parallel Hybrid Parallel hybrid concepts have a conventional drivetrain and, in addition, an electric (or hydraulic) drive concept that can drive the wheels at the same time as the internal combustion engine. The conventional drivetrain delivers the mechanical energy of the internal combustion engine to the wheels with high efficiency. On vehicles where the internal combustion engine is operated for long distances under load and little braking is required, the parallel hybrid is preferred over the serial hybrid. Compared to serial systems, the electrical branch of a parallel hybrid is less powerful so that the recovery rate of the brake energy is usually less than with the serial electrical system. Especially when braking heavily, not all of the kinematic energy can be recovered. There are various types of parallel hybrids. The common designation for parallel hybrids is based on the installation location of the electric machine. P1 hybrid A P1 hybrid is a hybrid in which the electric machine is seated behind the internal combustion engine and in front of the clutch, see Fig. 3.5. The important function of recuperation is implemented only to a very limited extent with the P1 hybrid, because with the P1 hybrid the drag torque from internal combustion engine affects recuperation. Accordingly, the efficiency of recuperation mode is low. Originally, the arrangement of the components of the P1 hybrid was not even initially conceived as a hybrid. It was more about combining the starter and generator functions in an electric machine as a so-called Integrated Starter Generator (ISG), and optimally accommodating it in an installation space that does not require major changes to the vehicle layout. In this respect, the P1 hybrid is also only encountered in the form of a micro hybrid with comparatively low electric power. One could not call a P1 hybrid a fully-fledged hybrid solution.

Generator / electric motor Internal combustion engine

Clutch

Transmission

Battery

Fig. 3.5   Diagram of a P1 hybrid. The electric machine is located between the motor and the clutch

3.3  Hybrid Concepts

33

The advantages and disadvantages of the P1 hybrid are outlined in Table 3.3. P2 hybrid A P2 hybrid is a hybrid vehicle in which the electric machine is located between the clutch and the transmission, see Fig. 3.6. This rather simple version of a hybrid system enables many hybrid functionalities and has few disadvantages. P2 configurations are therefore often selected for hybrids in the truck segment. All European truck manufacturers favor the P2 hybrid in their studies and preproduction vehicles. See Table 3.4 for advantages and disadvantags of the P2 hybrid. [22, 24, 28] each present P2 hybrids for the truck sector. Increased convenience can be provided by the P2 hybrid, in which two clutches are provided: one clutch between

Table 3.3  The most important advantages and disadvantages of the P1 hybrid Advantages

Disadvantages

Starter and generator functions are feasible

Electric-only mode is not possible because the internal combustion engine cannot be disengaged

Overlaying of the internal combustion engine and electric motor is possible (boost mode)

With recuperation, the drag of the transmission and above all the engine has to be overcome, making the brake energy recuperation inefficient

Battery charging is also possible when stationary (with the clutch disengaged) The concept is comparatively simple

Internal combustion engine

Generator/ electric motor

Clutch

Transmission

Battery

Fig. 3.6   Diagram of a P2 hybrid. The electric machine is located between the clutch and the transmission

3  Hybrid Vehicles

34 Table 3.4  The most important advantages and disadvantages of the P2 hybrid Advantages

Disadvantages

The concept is implementable with a relatively small electric machine

Recuperation is power limited

A generator function is feasible

A starter motor is required to be able to engage the internal combustion engine when in electric-only mode under load (e.g. uphill)

Electric mode is possible Overlaying of the internal combustion engine and electric motor is possible (boost mode) Battery charging is also possible when stationary (transmission in neutral) The concept is comparatively simple

the internal combustion engine and the electric machine, and one clutch between the electric machine and the transmission. This configuration is considered to have a very promising future for passenger car applications. In the commercial vehicle sector, there is less focus on comfort and performance when launching from standstill, so a P2 hybrid with a single clutch is sufficient. P1-P2 hybrid A so-called P1-P2 hybrid has two electric machines. One is located before the clutch (as on the P1 hybrid) and a second electric machine is located between the clutch and the transmission (as on the P2 hybrid), see Fig. 3.7.

Generator/ electric motor

Generator/ electric motor

Internal combustion engine

Clutch

Transmission

Battery

Fig. 3.7   Diagram of a P1-P2 hybrid. An electric machine is located between the engine and the clutch, and a second electric machine between the clutch and the transmission

3.3  Hybrid Concepts

35

The P1-P2 hybrid also makes it possible to operate the system as a serial hybrid by disengaging the clutch. Table 3.5 provides an overview of the advantages and disadvantages of the P1-P2 hybrid. P3 hybrid On the P3 hybrid, the electric machine is located at the transmission output. The P3 hybrid is shown in Fig. 3.8. As the torque of the electric motor no longer undergoes any conversion by the transmission, the electric motor must provide high torques (at low rotational speeds). Even in braking mode (recuperation), the rotational

Table 3.5  The most important advantages and disadvantages of the P1-P2 hybrid Advantages

Disadvantages

Highly variable energy management is possible

Costly control system

Overlaying of the internal combustion engine and electric machines, even High cost with both electric motors, is conceivable (boost mode) Recuperation is possible with both electric machines

Large installed length

Electric mode is possible Gentle engagement of the internal combustion engine during electric mode Starter/generator functions are possible Battery charging is also possible when stationary

Generator/ electric motor

Internal combustion engine

Clutch

Transmission

Battery

Fig. 3.8   Diagram of a P3 hybrid. The electric machine is located at the transmission output

3  Hybrid Vehicles

36

speed tends to be low. The electric machine built into the P3 hybrid is therefore bigger, heavier and more expensive than the e-machine for other hybrid concepts. The P3 hybrid can provide gear shifting free from tractive power interruption: when the clutch is disengaged for gear shifting, the electric motor continues to propel the vehicle. Table 3.6 describes the advantages and disadvantages of this hybrid type. The electric machine for the P3 hybrid can be attached to the transmission in the same way as a power take-off. A P3 hybrid can therefore be easily implemented. If the power take-off can be engaged, it is possible to directly compare a hybridized drivetrain with a non-hybridized drivetrain in the same vehicle. P2-P3 hybrid A so-called P2-P3 hybrid offers a solution consisting of two electric machines. One is located between the clutch and the transmission (as with the P2 hybrid) and a second electric machine is located at the transmission output (as with the P3 hybrid), see Fig. 3.9. The two electric machines are of different designs because the electric motor must operate behind the transmission within the range of rotational speed of the axle (i.e. the speed spread of the vehicle), while the electric motor before the transmission must be able to deal only with speed spread of the engine. Some characteristics of the P2-P3hybrid are summerized in Table 3.7. Parallel hydraulic hybrid A parallel hybrid can also be implemented hydraulically. Figure 3.10 shows a diagram of a parallel hydraulic hybrid. The hydraulics enable—compared to electrical hybrids—a high power input but at the same time it is very limited in terms of the amount of storable energy (high power density, low energy density). Typical energy storage amounts are

Table 3.6  The most important advantages and disadvantages of the P3 hybrid concept Advantages

Disadvantages

Simple structure

An electric motor with a wide output range is required (expensive, heavy)

Can be easily integrated into the drivetrain, especially on commercial vehicles

No starter function possible

Electric mode is possible

Battery charging not possible when stationary

Tractive power interruptions due to gear shifts can be smoothed out by the electric drive Overlaying of the internal combustion engine and electric machine is possible (boost mode)

3.3  Hybrid Concepts

Internal combustion engine

37 Generator/ electric motor

Generator/ electric motor

Transmission

Clutch

Battery

Fig. 3.9   Diagram of a P2-P3 hybrid. An electric machine is located between the clutch and the transmission, and a second electric machine at the transmission output

Table 3.7  A list of the advantages and disadvantages of the P2-P3 hybrid Advantages

Disadvantages

Overlaying of the internal combustion engine and two electric machines is possible (boost mode)

Two different electric motors required—high cost

Electric mode is possible

Heavy weight

A generator function is feasible

A starter motor is required to be able to engage the internal combustion engine when in electric-only mode under load (uphill)

Electric motors can be used for synchronization when shifting gear Tractive power interruptions due to gear shifts can be smoothed out by the electric drive P3 Battery charging is also possible when stationary (transmission in neutral)

around a few hundred kilojoules. With 500 kJ of energy, an empty garbage truck (16 t) can be accelerated once from stationary to just under 30 km/h. The parallel hydraulic hybrid is therefore particularly suitable for applications in which the vehicle travels at moderate speeds but is braked and accelerated frequently and rapidly. The garbage collection operation is an ideal application for this. The vehicle usually does not travel all that fast, but is braked abruptly. As a result, a storage device is necessary, enabling a high power input.

3  Hybrid Vehicles

38 Hydraulic fluid reservoir

Internal combustion engine

Clutch

Transmission Hydraulic pump-motor unit

High-pressure accumulator

N2

Fig. 3.10   Diagram of a parallel hydraulic hybrid

[16] describes technical solutions for a serial and a parallel hydraulic hybrid, and explains the advantages of a parallel hydraulic hybrid like that of a garbage truck. However, the usage of hydraulic hybrids in road vehicles, will probably be very limited: parallel electrical systems have the great advantage that they can address a much wider range of applications and, therefore, be produced more in the long term. On the other hand, the parallel hydraulic hybrid provides good results only in very specific applications. Vehicle manufacturers will therefore have to give preference to the parallel electric hybrid. Through the road hybrid —P4 hybrid The through-the-road hybrid (or P4 hybrid) is a vehicle in which the electric machine motor acts on a different axle to the internal combustion engine (Fig. 3.11). In this way, a hybrid can be constituted without interfering in the conventional drivetrain. A P4 hybrid enables short-term driving with an additional driven axle and can be used as traction assistance. It is unfortunate, though, that the availability of the traction assistance is dependent on the charge status of the energy storage device. If the battery is low, the driver will wait in vain for the traction assistance of the additional driven axle. In this respect, for commercial vehicles that are used off-road, the all-wheel drive function of the P4 hybrid is not a reliable product feature (See Table 3.8). In passenger cars, there are series production vehicles that have implemented a P4 hybrid [26].

3.3  Hybrid Concepts

39

Internal combustion engine

Transmission

Clutch

Generator/ electric motor

Battery

Second driven axle

Fig. 3.11   Diagram of a through-the-road hybrid. With this form of hybridization, the electric machine is seated on a different axle to the internal combustion engine. In this way, brake energy can be recuperated and short-term four-wheel drive provided

Table 3.8  Through the road hybrid, P4 hybrid—advantages and disadvantages Advantages

Disadvantages

The conventional drive remains untouched (no conversions, no additional torque on the axle)

Drive for a second axle necessary (cost, installation space, weight)

Added value by means of all-wheel drive

No “real” traction assistance because its availability depends on the battery charge status

Overlaying of the internal combustion engine and electric motor is possible (boost mode)

No starter operation possible

Electric mode is conceivable

When charging, the forces at the electric axle change

Easily feasible, if a four-wheel variant of the vehicle exists anyway

Battery charging by the ICE not possible when stationary/idling

Tractive power interruptions due to gear shifts can be smoothed out by the electric drive

–

3.3.3 Plug-In Hybrid As mentinoned above a plug-in hybrid is a vehicle with an electric hybrid drive in which the vehicle’s power batteries can be charged from the infrastructure. The batteries might be fully charged when the vehicle is started. Plug-in hybrids are attractive, if electric-only mode is required and a large proportion of the mileage is over short distances.

40

3  Hybrid Vehicles

If the installed electric drive power is sufficiently large, the vehicle can initially run in electric-only mode. One can consider the vehicle as an electric vehicle that has an internal combustion engine as an additional drive source. The delineation between the electric vehicle and the plug-in hybrid is fluid. Depending on which vehicle characteristic is to be focused on, the serial electric hybrid with plug-in functionality can also be seen as an electric vehicle with a range extender. The vehicle is driven electrically. The energy for the electric motor is obtained from the battery. If the battery energy is running out, an internal combustion engine (the range extender) is started. This drives a generator so that on-board electricity is generated and the vehicle can continue to operate beyond the limited range of the battery capacity. The additional cost of the external charging port of the plug-in hybrid is particularly worthwhile, if the battery has a large storage capacity. If the vehicle body (e.g. a garbage truck) is operated electrically, it makes sense to fully charge the vehicle’s drive battery prior to use. Powerful plug-in hybrid systems with sufficient battery capacity could also interest tradesmen that have an electrical energy source available in the form of the (fully charged) battery of the hybrid system at the place of use or on the construction site.

3.3.4 Brake Energy Recovery in a Full Trailer or Semitrailer In the course of the general endeavors to brake energy recovery, concepts are also being presented in which electric machines in a trailer recuperate energy during braking and feed the energy into batteries [27]. The functions of a full trailer or semitrailer, such as a cooling unit or ventilation system, can be operated by means of the electrical energy. However, these must have a further source of energy in order to ensure function even when the battery is flat. To avoid the cost and weight of the battery in the trailer, this concept can be developed further and the recuperated energy can be fed into the (hybrid) battery of the traction vehicle. This approach requires a defined and standardized high-voltage interface between the traction vehicle and the towed unit, though. The high-voltage cable between the trailer and traction vehicle must meet the requirements of high voltage (HV) safety.

3.3.5 Classification of the Hybrid Concepts on the Basis of the Installed Electrical Capacity Hybrids are frequently also subdivided into different classes that result from the power output of the electric drives. Hybrid systems with low electrical power are designated as micro or mild hybrid drives. The main focus here is often on the start-stop function. Systems with an electric output that can appreciably support the internal combustion engine in drive mode are called full hybrids. In this case it is necessary to design the

3.4  Technical Comparison of the Hybrid Concepts

41

control of the hybrid system so that the internal combustion engine is kept within the optimum range in drive mode and the aim is to recover as much brake energy as possible. Conceptually, a serial hybrid is always a full hybrid because the electrical branch has the same output as the internal combustion engine.

3.4 Technical Comparison of the Hybrid Concepts The concepts presented for hybrid drivetrains are compared qualitatively in an overview in Table 3.9. This is, of course, an abbreviated representation of the advantages and disadvantages. Other evaluations may result for niche vehicles with special application profiles. Table 3.9  A simplified comparison of the different hybrid systems. The table provides indications of the merits of the individual systems. Other evaluations may result for special vehicles or application profiles. The relative weighting of the criteria listed below differs depending on the customer segment Function

P2

P1/2

P3

P2/3

P4

Serial electric

Serial hydraulic

Brake energy recovery

0

 + 

0

0

0

 ++

 +++

Optimization of the operating point of the internal combustion engine

0

0

0

0

–

 ++

 ++

Downsizing

 + 

 + 

 + 

 + 

–

xxa

xxa

Weight

 ++ 

–

0

––

 + 

––

––

Cost

 ++ 

–

0

––

 + 

––

––

Complexity

 ++ 

–

 ++ 

–

 + 

–

–

Electric mode

 + 

 + 

 + 

 + 

0

 ++

 + b

Charging when stationary  and ICE idling

 + 

 + 

–

 + 

–

 + 

 + 

Boost mode

 + 

 + 

 + 

 + 

 + 

––

––

Starter function

0

 + 

–

0

–

 + 

–

Generator function

 + 

 + 

–

 + 

 + 

 + 

––  ± c  +  + d

Installation space

 + 

Gear shifting with no inter- – ruption of tractive power a In

–

 ++ 

 + 

 + 

 ± c

–

 + 

 + 

–

 +  + d

the case of the serial hybrid, one may be able to use an internal combustion engine with lower power. The second engine (electric motor or hydraulic motor) is decisive for the driving performance bIn this case: hydraulic mode cThe installation space requirement for serial hybrids is large. However, advantages can result from the extensive degree of freedom available for arrangement of the various components dThe internal combustion engine is decoupled from the wheel speed

42

3  Hybrid Vehicles

Of the multitude of hybrid concepts, the P2 hybrid for trucks and the serial hybrid for urban buses are preferred in the commercial vehicle sector. Indications as to why this is the case follow from the same Table 3.9: the P2 hybrid is advantageous with regard to cost, weight and complexity, but still provides the possibility of embodying numerous functions. Like all parallel hybrid systems, however, it is inferior to the serial system in terms of braking energy recovery. The costly serial hybrid, with its large recuperation capacity, is attractive in urban buses.

3.5 Evaluation of the Hybrid Concept For quite some time, hybrid vehicles have been adjudged as making an important contribution to the mobility of the future [14]. In the passenger car segment, hybrid vehicles have been available for many years, some of which with respectable market success [34]. In the commercial vehicle sector, the numbers of hybrid vehicles sold are much less impressive. This is also due to the fact that, for hybrid vehicles, it was difficult to find a business case to date that is economical without government incentives. The fuel consumption savings and the corresponding CO2-reduction potential that are possible with hybrid technology are set out in Sect. 3.5.1. Currently it seems that—as the CO2reduction potential is limited—the commercial vehicle sector will largly leave out hybrid technology and develop into electric drives, be it battery–electric or fuel cell powered. Hybrid technology will not be sufficient to meet the CO2 targets of the future.

3.5.1 Fuel Savings by Means of Hybrid Drives THE decisive motivation behind installing hybrid technology in a vehicle is the desire to save fuel and reduce CO2 emissions. Table 3.10 provides some orientation points on how high the savings from hybrid drives can be. However, it should be reiterated that there are also application profiles for which practically no appreciable fuel consumption savings are achieved through hybridization. Braking phases and coasting phases in which the engine produces drag torque are necessary to be able to recuperate any energy at all. As a result, hybrid technology achieves the highest percentage of fuel savings in applications with frequent braking. The highest savings are achieved in urban traffic.

3.5  Evaluation of the Hybrid Concept

43

Table 3.10  Fuel consumption savings by means of hybrid drives in various applications. The values ​​cited are intended to be for orientation purposes. The exact configuration and application profile of the vehicle influence the possible fuel saving effects very heavily Area of application

Typical savings [%]

Remarks

Long-haul transport

4–7

[21]

Distribution transport

15–23

Central city deployment—see, for example, [23–25]

Garbage collection vehicle

up to 25

[16] Variants are also being tested, on which the body is electrically operated

Long-distance bus

approx. 6

Fixed route bus in urban traffic

up to 30

[15, 18, 19]

However, the absolute savings gained through hybrid technology over the entire service life of a vehicle result from the absolute consumption and total mileage of the vehicle. The comparatively small percentage of savings in long-haul transport multiplied by the high mileage that a long-haul transport vehicle travels results in considerable overall savings.

4

Other Supplements to the Conventional Drive

4.1 The Rankine Cycle With a conventional drive, a very large proportion of the chemical energy of diesel fuel is lost in the form of thermal energy [1]. The thermal energy is either lost to the exhaust gas or dispensed to the cooling system. It is evident that this is where supplements to the conventional drive to improve the efficiency of energy use in vehicles can be applied. Waste Heat Recovery (WHR1) can be approached in various ways: A promising approach is the so-called Rankine cycle: a working fluid is heated and evaporated in the EGR cooler and/or in the exhaust system. Water (with antifreeze) or a water/ethanol mixture can be used as the working fluid. The heated working fluid is fed into an expansion machine (turbine, piston engine, scroll expander2). The expanded working fluid is then re-condensed and circulates. The condenser is cooled via the engine’s cooling system. The mechanical movement generated by the aformentioned cycle can be added to the movement of the internal combustion engine and increases the efficiency of the drive. If the heat utilization benefits the mechanical drive, it is called mWHR.3 To be able to make an expansion machine that is small and efficient, it would have to operate at relatively high rotational speeds. A transmission is therefore required to translate the rotational speed of the expansion machine. In long-haul transport, for diesel combustion engines consumption reductions of 3–5% are considered to be realistic [33].

1 WHR =  Waste

Heat Recovery. If the thermal energy of exhaust gas is utilized, it is called EHR = Exhaust Heat Recovery. 2 The scroll expander consists of a stator and a rotor. Both have a spiral contour. The rotor is mounted eccentrically. The expansion causes the spiral rotor to move in the stator. 3 mWHR = mechanical Waste Heat Recovery. © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Alternative Powertrains and Extensions to the Conventional Powertrain, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-65570-2_4

45

46

4  Other Supplements to the Conventional …

The mechanical energy from waste heat recovery can also be used to generate electricity via a generator – appropriately called eWHR.4 The electricity can be used for the boosting function or for auxiliary loads, which are then not a load on the internal combustion engine. Currently unusable electrical energy is temporarily stored in the battery of the hybrid system. From a battery viewpoint, energy storage for exhaust heat recovery is less demanding than energy storage for brake energy recovery because the electrical power to be stored is lower (less energy per time) yet is more uniformly available than with the more sudden and power-intensive braking procedure. Due to the additional conversion of mechanical energy into electrical energy and then back again, the efficiency of electrical waste heat recovery (eWHR) is of course, reduced compared to mechanical exhaust heat recovery. Hybrid technology and the Rankine cycle The utilization of waste heat in a cyclic process as a supplement to the conventional drive and the hybrid as a further supplement to the conventional drive complement each other: hybrid technology creates fuel savings in transient drive situations  when the vehicle has to be braked or accelerated. In continuous drive mode (on the freeway), hybrid technology is less suited to achieve efficiency gains. However, this is where the strength of the Rankine cycle lies: the energy of diesel fuel is utilized better in continuous drive mode. However, putting both hybrid technology and a waste heat recovery system on a diesel combustion engine will result in a pretty expensive system.

4.2 Thermoelectric Generator In thermoelectric generators (TEGs), electricity is generated directly from heat. The so-called Seebeck effect is used to generate an electric voltage from a temperature differential.5 A high temperature differential can be created with the hot exhaust gas of the vehicle. Thermoelectric generators have the advantage that no moving, and therefore no wearing parts are required. However, the efficiency of thermoelectric generators is low, so their use in vehicles is not worthwhile.

4 eWHR = electrical

Waste Heat Recovery. Seebeck effect is, so to speak, the reverse process to the much better known Peltier effect, in which temperature differences are generated by means of electricity. 5 The

5

Hydrogen

Hydrogen, H2, is regarded as the visionary transportable energy source of the future. Hydrogen can be used for fuel supply in several ways: it can be used directly as fuel in internal combustion engines, hydrogen can be used to operate fuel cells and hydrogen gas is used to make synthetic fuels in a synthesis process with hydrocarbon compounds. Ideally, one could make environmentally-neutral synthetic fuel from the two raw materials of hydrogen and carbon dioxide (see Chap. 6). Figure 5.1 summarizes some properties of hydrogen. Hydrogen has a very low density and a very high specific energy, it burns in a wide range of mixtures with air and it needs a low ignition energy. Hydrogen creates some additional challenges for the materials used: Hydrogen embrittlement damages the materials and due to the fugacity of H2 wear of sealing surfaces is more critical. Special materials might be needed. Only water is created by the combustion of hydrogen in a combustion process1 or by using hydrogen in a fuel cell – see Fig. 5.2. The reaction  process is completely free of pollutants and releases no exhaust gases or CO2. If the hydrogen is obtained from water by means of solar energy, for example, by electrolysis, in principle it is the ideal energy source because it is emission free. In the production process, water is broken down, free of pollutants, into oxygen and hydrogen.

2H2 O → 2H2 + O2

(5.1)

The electrolysis of water to obtain hydrogen is very energy inefficient, though. And if the energy mix of present-day electricity generation is used as the basis for the electrolysis process, hydrogen drives are anything but CO2 neutral. At present, however, almost all

1 Unfortunately

for hydrogen in a combustion engine this is only true in theory: As combustion engines are using air to supply oxygen and as hydrogen is burning with a high temperature NOx will form. © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Alternative Powertrains and Extensions to the Conventional Powertrain, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-65570-2_5

47

5 Hydrogen

48

Fig. 5.1    Properties of hydrogen

Fig. 5.2   Hydrogen can be used in a fuel cell and in an internal combustion engine

of the total hydrogen requirement is met by fossil fuels, especially from natural gas. The most frequent process is steam reforming, in which hot steam catalytically reacts with methane:

CH4 + 2H2 O → CO2 + 4H2

(5.2)

5 Hydrogen

49

If fossil methane is used to generate hydrogen in the way shown in Eq. 5.2 , the ecological advantages of hydrogen are lost. In the future water electrolysis using electricity from renewable energy is a path for environmental friendly hydrogen [40]. The storage of hydrogen can take place in various ways. Hydrogen can be liquefied at extremely low temperatures (−253 °C). Storage tank and piping needs high performance insulation. Nevertheless, the hydrogen in the tank heats up slowly and the evaporating gas increases the pressure in the tank. Tank venting is required. In order to avoid/postpone venting a high pressure tank can be used that allows some pressure increase due to the evaporation gas. It must be noted that liquefication consumes an appreciable proportion of the energy of hydrogen (about 20%). A second possibility for storing hydrogen is the pressurized gas accumulator, in which hydrogen is compressed at a high pressure (700 bar). A significant amount of energy in relation to the tank content is also required for compression. The incorporation of hydrogen into solid matter is a third possibility of storing hydrogen. For example, hydrogen is incorporated into the crystal lattice of a metal. Such metal hydride storage systems are used in submarines. However, the technology is expensive and far from being a realistic solution for automotive applications. The range of a hydrogen vehicle with the same tank volume is significantly less than the range of a diesel vehicle. In the case of liquefied hydrogen, the energy density of the tank content is a lot less than with conventional fuels (see Fig. 6.2). When the hydrogen is finally in the tank, one is confronted with the problem of diffusion: hydrogen diffuses through many materials, so there might be an economically significant evaporation loss. In addition, ignitable mixtures due to the diffusion of hydrogen out of the tank must be prevented from forming. Unfortunately, air-hydrogen mixtures are ignitable over a very wide range of mixing ratios. This by the way poses an additional challenge to the below mentioned hydrogen combustion: Hydrogen has a high knocking tendency when used in a combustion process. The construction of a functioning hydrogen infrastructure is not only technically challenging but also economically demanding. The construction of hydrogen refueling stations is expensive. In addition, when the technical questions relating to hydrogen vehicles are solved, there could be a chicken-and-egg problem: as long as there is no nationwide network of hydrogen refueling stations, the sale of hydrogen-powered vehicles will only progress sluggishly. Conversely, nobody feels motivated to invest in hydrogen refueling stations as long as there is no appreciable population of hydrogen vehicles. For years there were good reasons for skepticism looking at hydrogen as an energy source for vehicles. [20] expects, for example, that the use of hydrogen will not be realistic before 2050. On the other hand climate change and the CO2-problem and the steadily rising willingness to tackle climate change on a broader scale give good reasons to believe that the long-lasting dream of emission free hydrogen powertrains finally come into the mass market.

5 Hydrogen

50

5.1 Fuel Cells A fuel cell is a device that produces electricity directly from a chemical reaction without the detour of converting the chemical energy to e.g. mechanical energy. Therefore, in this sense the fuel cell is similar to a battery. Different from batteries the reactants needed for the reaction are stored outside the device and can be constantly re-supplied to the reaction. At the same time the products of the reaction (the “exhaust”) are constantly removed from the device - again different from conventional batteries. Fuel Cells have been considered as a propulsion system for the future again and again – see for example [37, 38]. Reaching mass production of fuel cell vehicles however has been proven difficult. In the early days cold start and durability of fuel cells were among major challenges. It seems these challenges are mastered. The most important hurdles remaining seem to be system cost in the vehicle, hydrogen cost and the hydrogen infrastructure. With the growing concern about climate change and the CO2 problem hydrogen fuel cells are considered (again) as a very promising technology of the future especially for applications where battery-electric vehicles do not look too promising. This in particular is true for long haul trucking as the disadvantages of battery systems with their frequent charging needs and high weight play a major role in this application. Accordingly not only first passenger cars are available with fuel cells [43] but especially companies which target on the heavy duty long haul truck market are working on fuel cell propulsion systems in Europa, Asia and North America (E.g. [44–46]).

5.1.1 Some Physical Parameters of Fuel Cells The physical power P of an electric device is given by:

P =V ·I Where V is the voltage and I is the current. Energy E then is given by (with t being time):

E =P·t =V ·I ·t The current I is determined by the flow of fuel (reactants) into the reaction minus the fuel that leaves the cell unused. A fuel utilization coefficient µfuel is defined as follows:

µfuel =

mass of fuel taking part in the required reaction mass of fuel inputed to the fuel cell system

(5.3)

Obviously, it should be the target to bring µfuel close to 1. As there is always some slip, realistic numbers might be around 95%. Efficiency

5.1  Fuel Cells

51

The fuel cell is not a Carnot machine so the definition of a fuel cell efficiency is different from what one might remember from combustion engines (see [51]). As the current generated by the fuel cell is determined by the quantity of fuel used, the decisive quantity for the efficiency of a fuel cell is the voltage. Efficiency η for a fuel cell is defined by

η = µfuel ·

Vc Vr

Here µfuel from Eq. 5.3 is used, Vc is the actual voltage of the cell and Vr is the reversible voltage or open cell voltage of this particular fuel cell technology. Vr is a measure for the maximum voltage the cell technology in question can deliver. However, it should be noted that Vr depends on the temperature of the cell, it depends on the pressure and on the concentration of the reactants. Moreover Vr can be expressed based on the LHV or the HHV resulting in different values for Vr. To cut a long story short: there might be different values for the efficiency of the same fuel cell. So one should make sure to have a common definition and understanding of fuel cell efficiency before discussing efficiencies. For automotive applications not the efficiency of the cell but the efficiency of the overall fuel cell system is decisive: The system will consume some of the energy it is producing to run different pumps (cooling, hydrogen and air pump). If necessary, some energy is used up by heating the reactants and to humidify them. The system efficiency might be defined by the ratio of the electrical energy provided by the fuel cell system and the chemical energy that was delivered to the system with the fuel:

ηsystem =

Eelectrical output Efuel consumed

(5.4)

Again, the term Efuel consumed describing the energy content of the fuel might be expressed based on the LHV or the HHV. Using the lower heating value LHV obviously leads to a higher efficiency value. If the heat generated by the system is used – e.g. in a combined heat and power generation unit (CHP) - the enumerator of the equation Eq. 5.4 will contain an additional term for the usable thermal energy from the system. To compare different fuel cell designs the current per area of active electrodes is sometimes used. It is current I divided by area A and the frequently used unit is Ampere per square centimeters (A/cm2). Other important parameters of fuel cells are lifetime expectation and cost (as always in automotive development).

5 Hydrogen

52

5.2 Differen‑t Reactions for Fuel Cells There are many different reactions and materials that can be used to form a fuel cell. [50] gives a good and easy to understand overview. As the Hydrogen based Proton Exchange Membrane fuel cell (PEMFC) is the dominant choice for automotive applications this fuel cell technology will be explained in more detail in Sect. 5.3. In principle the chemical energy of a fuel with the composition CmHn or CxHyOz can be utilized in two ways. First of all, as a fuel for classic internal combustion engines, in which the fuel is combusted, the chemical energy is thereby converted into thermal energy and the thermal energy converted into mechanical energy. Secondly, electrical energy can be created from (many) fuels with the composition CmHn or CxHyOzwith a fuel cell. Some fuels can be directly used in a fuel cell, others might be used in a fuel reformer to create the fuel that goes into a fuel cell—e.g. hydrogen.

5.2.1 Direct Methanol Fuel Cell, DMFC An example for a fuel cell using hydrocarbon fuel is the Direct Methanol fuel cell DMFC. The sum reaction reads – see Eq. 5.5:

2CH 3 OH + 3O2 → 4H2 O + 2CO2

(5.5)

A similar reaction in a fuel cell is possible with Ethanol. As can be seen from the reaction the Methanol (and Ethanol) fuel cell is not CO2-neutral.

5.2.2 Alkaline Fuel Cell, AFC The Alkaline fuel cell uses Hydrogen as a fuel just like the PEMFC. The sum reaction is the same as in the PEMFC: Hydrogen and Oxygen form water and electricity is generated. The difference between the alkaline fuel cell and the PEM fuel cell is that in the alkaline fuel cell hydroxyl ions (OH–-ions) are moving through the electrolyte. One of the big problems of the AFC is that it is very sensitive to CO2. Even small quantities of carbon dioxide harm the fuel cell.

5.2.3 Proton Exchange Membrane Fuel Cell, PEMFC The most popular fuel cell for automotive applications is the PEM fuel cell using hydrogen, H2, as a fuel. At the anode, the hydrogen is split into protons (positively charged hydrogen nuclei) and electrons:

H2 → 2H+ + 2e−

(5.6)

5.2  Differen‑t Reactions for Fuel Cells

53

The electrons collect at the anode, while the protons migrate (through the membrane) to the cathode. There, water is created:

1 O2 + 2H+ + 2e− → H2 O 2

(5.7)

The entire reaction results as a summation of the two reaction Eqs. 2.1 and 2.2:

1 O2 + H2 → H2 O + electrical energy 2

(5.8)

The sum reaction 5.8 does not differ in terms of the chemical reaction equation from the hot combustion of hydrogen. The fuel cell is capable, though, of delivering a higher degree of efficiency than an internal combustion engine. Figure 5.3 illustrates the basic principle of the reaction in a Proton Exchange Membrane fuel cell PEMFC. Figure 5.4 shows schematically the components of the Proton Exchange Membrane fuel cell: The bipolar plates are the supporting structure and the wrapping of a cell. A bipolar plate is shared by two cells. On one side the bipolar plate delivers hydrogen to

Fig. 5.3   Principle of the Proton Exchange Membrane PEM fuel cell. Hydrogen and oxygen form water. The hydrogen is split up into positively charged protons and into electrons. The electron flow generates the usable electricity

54

5 Hydrogen

Fig. 5.4   a Schematic illustration of the components of a PEM fuel cell. b Multiple fuel cells are connected in series to form a so-called stack. The stack voltage is the sum of the cell voltages

one cell on the backside it delivers air to the adjacent cell. Moreover, the bipolar plate might contain water channels: cooling water is circulating through those water channels to cool down the cell (not shown in Fig. 5.4). The heart of the fuel cell is the membrane. Hydrogen is delivered to the gas diffusion layer on one side of the membrane and air is supplied to the gas diffusion layer on the other side of the membrane. The membrane is in contact with and separating the two gas  diffusion layers. To facilitate the desired reaction the membrane is coated with a catalyst (e.g. Platinum in the case of the PEMFC). The humidity and water management in the cell is very important. The membrane needs a certain humidity to function well. So hydrogen and air might be humidified. At the same time, water is generated on the air side. Some of the water might diffuse through the membrane to the hydrogen side. The water must be removed from both sides of the cell. A typical voltage of a hydrogen PEM fuel cell is around 0.65 V. To achieve a voltage in the 100 V range one puts many fuel cells in series to form a so-called stack. The stack voltage is essentially the sum of the voltages of the single cells - see Fig. 5.4b. The bipolar plate is the connection of two cells. It connects the cathode of one cell with the anode of the next cell (therefor: bipolar plate). To reduce the Ohmic resistance the bipolar plate is made from a material with high conductivity e.g. stainless steel or carbon.

5.2  Differen‑t Reactions for Fuel Cells

55

Hydrogen and air consumption With 33.3 kWh/kg energy content LHV (see Fig. 5.1) and assuming a conversion efficiency of 50% approximately 60g of hydrogen is needed to provide 1kWh of electrical energy. According to Eq. 5.3 and knowing that the molar mass of H2 is 2 and the molar mass of O2 is 32, we need 8 times more oxygen i.e. around 540g of oxygen. As air contains around 23% oxygen in weight we need around 2.35kg of air to produce 1kWh of electric energy inside the fuel cell. In reality for different reasons a surplus of oxygen is needed. The real value is above 3kg and up to 5kg of air per 1kWh of electric energy. Similar to the combustion engine an air number λ is defined. λ is defined as the actual mass of air flowing through the stack per time m ˙ air divided by the amount of air stoichiometrically needed to produce a current I per time. The stoichiometrically required air can be obtained as follows: The current I can be expressed as

I=

1 1 C = · nelectrons · F = · F · 2 · nH2 t t t

Here the first equation says that current is charge C per time t. The charge we get is the quantity of electrons (in mole) times the Faraday constant F.2 The number of electrons is twice the number of H2 molecules as each H2 molecule delivers 2 electrons – see Eq. 5.6. If we have z cells in series each cell needs the same amount of hydrogen. The flow of hydrogen per time is then

nH2 I ·z = = n˙ H2 t 2F The number of oxygen molecules we need is only half the number of hydrogen molecules – see Eq. 5.8.

n˙ O2 =

I ·z 1 n˙ H2 = 2 4F

If we want to determine how much air we need, we use the ratio between the quantity of air and the quantity of oxygen in the air xO2:

n˙ Air =

1 I ·z · xO2 4F

Now, we want to express the air needed as an air mass. The quantity of a substance n and the mass m are connected via the molar mass M :

m =n·M 2 The

and

m ˙ = n˙ · M

Faraday constant F is defined as the charge of 1 mole of electrons. As one mole is 1 mol = 6.022 · 1023 particles and each electron has a charge of 1.6022 · 10-19 C the Faraday constant is 9.6485 · 104 C/mol(e-).

5 Hydrogen

56

m ˙ Air =

I · z Mair · 4F xO2

The air number λ is then defined as

ℒ=

Actual m ˙ Air stoichiometric m ˙ Air

=(

Actual m ˙ Air I·z 4F

·

Mair xO2

)

As said a fuel cell operates with an excess of air. In “Diesel speech” one would say a hydrogen fuel cell is always operated with a lean mixture (air excess). [49] states that typical λ values for a hydrogen fuel cell are between 1.5 and 3. At idle (low throughput) the air excess is in particular high. At idle λ is higher than in full load operation to get rid of the humidity and the water in the stack. Fuel cells are subject to aging. The specifications in the literature (status 2010) fluctuate between a power loss of about 6% and 15% over 5000 operating hours. The cost of fuel cell systems still needs to be significantly reduced for fuel cell drives to be competitive with internal combustion engines. The present-day hydrogen fuel cells require very clean hydrogen.

5.3 Thermomanagement of a Fuel Cell Vehicle In Chap. 2 it was already mentioned that a fuel cell vehicle is an electric vehicle with an additional electricity generator onboard, the fuel cell. The cooling system of a fuel cell vehicle needs to take into account the specific needs of the fuel cell. Figure 5.5 shows schematically the basic principles of the cooling system. In many aspects, it resembles that of a battery electric vehicle (see Chap. 2): There is a cooling circuit for the battery. An active cooling circuit with compressor and condenser is needed for cooling the battery at high ambient temperatures and for the HVAC system of the cab. In Fig. 5.5 an additional cooling circuit serves the e-motor. In principle the e-motor  could be integrated into the battery circuit as well to reduce the numbers of independent circuits. The fuel cell operates at higher temperatures than battery and e-motor. A separate cooling circuit is foreseen for the fuel cell. Air and hydrogen must be preconditioned before entering the fuel cell. The temperature preconditioning (humidity preconditioning is required as well) can be done via the fuel cells cooling circuit. Heat from the fuel cell circuit also can be used for cab heating. The fuel cell needs a high performance cooling system. Additional radiators compared to the needs of a combustion engine with the same power rating are necessary. The efficiency of a fuel cell converting chemical energy to electricity and an ICE converting chemical energy into mechanical energy is similar. So and ICE and a fuel cell with the same power rating both produce approximately the same amount of waste heat. However, the ICE expels hot exhaust gas that takes a lot of heat with it. Moreover, the

5.4  Hydrogen as Fuel for Combustion Processes

57

Fig. 5.5   Schematic diagram of the cooling circuits in a fuel cell vehicle. The diagram focusses on some general principles. Valves etc. are not shown. See for reference [47]

ICE is operated at a temperature of around 100 °C (or more) whereas the fuel cell is operated at 70–80 °C. Therefore, the temperature difference to the ambient temperature is smaller for a fuel cell than for an ICE. This temperature difference (∆T) is very important for the size of the radiator required. The smaller ∆T the more radiator area is required. Those two reasons make it necessary to have huge radiators on fuel cell vehicles.

5.4 Hydrogen as Fuel for Combustion Processes Hydrogen fuel cell systems need hydrogen with high purity whereas a hydrogen ICE does not require the same hydrogen purity as a fuel cell. And fuel cells  are (still) expensive compared to combustion engines. Therefore, research is being performed on combustion processes for hydrogen internal combustion. Most approaches to using hydrogen in internal combustion engines are based on the spark-ignition process. Studies with diesel-like combustion processes have also been carried out in order to use the better

58

5 Hydrogen

efficiency of the diesel process for hydrogen propulsion. However, in [32] a pure autoignition mode for hydrogen combustion in a motor vehicle is classified as being impracticable for automotive application. There are specific challenge that distinguish a hydrogen ICE from conventional Diesel combustion engines: • Hydrogen H2 is highly flammable and ignites in a wide range of Hydrogen-Air mixtures. Combustion control is more difficult. Hydrogen has a high knock tendency. • Hydrogen burns at very high temperatures. Surfaces of the engine must withstand those temperatures. • Hydrogen combustion engines are in contrary to fuel cell systems not completely emission free. Due to the high temperatures, NOx will form. Burned lubrication oil might add additional emissions. • Hydrogen (different from Diesel fuel) does not contribute to lubrication.

6

Alternative Fuels

The directions considered most promising for the environmentally friendly—especially CO2 free—drive of the future are currently battery electric vehicles and vehicles with fuel cells running on hydrogen fuel. See the preceeding chapters. In those two technologies combustion of hydrocarbons that will result into carbondioxide emissions to the atmosphere is no longer needed.

6.1 Alternative Fuels and the CO2 Problem Alternative fuels try to reduce the dependence of the transport sector on fossil oil and reduce the carbondioxide emissions by using alternative chemical compounds. The currently known candidates for alternative fuels (except hydrogen!) all contain carbon and will result in substantial CO2 emissions if burned. Figure 6.1 shows the CO2 emissions of different carbon based fuels per energy content. The basic rule that applies to the fuels with regard to the CO2 emissions is: the higher the hydrogen content in the compound, the less CO2 is emitted per unit of energy. The from a Carbon-dioxide standpoint best candidate Methane contains less Carbon (about 25% less) on account of the more favorable hydrogen/carbon ratio of the methane molecules, than diesel with the same energy content. Reducing the CO2 emissions by around 25% obviously will not solve the climate change problem. Moreover Methane (and natural gas that mainly consists of Methane) is a greenhouse gas (GHG) by itself. The Gas-Diesel-Mixed-Fuel system (see Sect. 6.3.6) tries to combine the efficiency of the Diesel process with the favorable CO2 emission of LNG. However [60] shows that ultimately CO2 emission of the Diesel-LNG process (the report considers the HPDI process) is similar to the Diesel-only engine if both tank-to-wheel (TTW) emissions and well-to-tank (WTT) are considered in a well-to-wheel (WTW) consideration. So carbon based fuels from fossil deposits obviously cannot solve the CO2 problem. © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Alternative Powertrains and Extensions to the Conventional Powertrain, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-65570-2_6

59

60

6  Alternative Fuels

Fig. 6.1   Specific CO2 emissions of different fuels in the tank: The graph shows how much CO2 is emitted if fuel with an calorific content of 1 MJ is burned (TTW). Is is not considered that different fuels might result into different thermal efficiencies of the engine. CO2 emissions or CO2 capture from the atmosphere in the production process depends on the production process and is not considered here. Values for Diesel, gasoline, LPG and natural gas follow [61]. The value for Ethanol is taken from [62]

6.1.1 Biogenic Fuels Some interest is paid to biogenic fuels. As the carbon dioxide produced during the combustion process has been bound from the atmosphere in the production process, these fuels are ideally greenhouse gas neutral.1 The CO2 emissions of the different fuels are compared with one another in various studies [56, 57]. In theory those fuels solve the CO2 problem as the carbondioxide has been extracted from the atmosphere while producing the fuel. At the moment no process is in sight that can fullfil this requirement and can meet the energy demand of the different s­ ectors like transport sector, heating, industry and electricity generation. The biomass available worldwide is insufficient to meet the energy demands of these sectors. All sectors: transport, electricity generation, industry and heating, are currently resorting to a high proportion of fossil fuels and are therefore making a high contribution to the CO2 emissions. In addition, the land required for the production of biogenic energy carriers is very high. The energy yield per area is higher when electricity is produced on the same area.

1 As

a rule, biogenic fuels are also not 100% climate neutral. This means that substances from fossil resources are also used in the production of biofuels. The methanol necessary for the production of FAME might be obtained, for example, from fossil oil (Sect. 5.1.2).

6.2  Overview on Alternative Fuels

61

6.2 Overview on Alternative Fuels It will take quite some while until fuels based on fossil oil will be replaced by battery-electricity or fuel cells. And even if this is about to happen, a certain portion of propulsion systems will rely on hydrocarbon-based combustion engines. The specific advantages of hydrocarbon-fuels, namely the high energy density and very easy transportability and storage capability, play a great role in the transport sector. So especially in niches (vehicle is used in remote places, used not very often etc.) the hydrocarbon based combustion engine might remain for a long time.2 So in these (niche) applications alternative fuel—i.e. non fossil fuel based hydrocarbons—will find their place. Numerous approaches for alternatives to the diesel and gasoline fuels that currently dominate the automotive sector have been under discussion [52, 54]. Some of these alternatives have already been successful for several decades, often in regional niches. The fuels considered are, like diesel and gasoline, hydrocarbons of the form CxHy or carbon-hydrogen–oxygen compounds of the form CxHyOz. The various alternatives differ in terms of their exact chemical composition, their production and the extraction of the fuel. The physical properties of the different compounds also differ. Promising fuels will be evaluated according to various criteria: • The CO2 emissions compared to fuels based on fossil oil. The total CO2 balance of the fuel, including its production, must be taken into consideration in this case. This is called WTW (Well-to-wheel) evaluation. • The usability of the fuel in existing vehicles and the compatibility of the fuel with the existing infrastructure (refueling station network, pipelines, etc.). • The emission of other gases. • The infrastructure necessary for the fuel. • The ability to carry the fuel in the vehicle’s tank. • The availability of the raw material. • The committing of resource to produce the fuel, for example, the cultivated area of land required for the fuel. • The cost of the fuel. • The gravimetric energy density (energy content by mass [MJ/kg]) and the volumetric energy density (energy content by volume [MJ/l]) of the fuel and, therefore, the range that a sensibly-dimensioned tankful of the respective fuel enables. Figure 6.2 shows the comparative energy densities of several different fuels. • Other chemical-physical properties of the fuel, such as toxicity and explosiveness.

2 Not

to mention the countries that will stay with fossil oil as they consider their local development more important than contributing to the fight against global warming .

6  Alternative Fuels

62

Volumetric energy density [MJ/l] size of the fuel tank 36

34

33

32 25

LNG 19.3

21

For comparison:

21 CNG 7.4

Diesel

Rapeseed oil

Gasoline

FAME

LPG

Ethanol

27 37 43

DME

Natural gas

8.5

3.7

Liquid hydrogen

Li-ion battery

NiMH battery

0.7

0.2

0.6

28

37 44

46*

Gravimetric energy density [MJ/kg] weight of the fuel

46*

120*

* Additional costly and heavy fuel tanks required

Fig. 6.2   A comparison of the gravimetric and volumetric energy density of various fuels. Additional weight and an additional space requirement, necessary due to the costly fuel tank technology required for CNG, LNG, LPG and especially hydrogen, is NOT taken into consideration here. In addition, for batteries it also has to be taken into consideration that only some of the stored energy is drawn from batteries in order to extend the service life of the battery. The usable energy density is therefore smaller by a factor of 2 to 5 for batteries

Various alternative fuels are presented below that already have a certain importance or are considered in the available literature to be promising options. Table 6.1 provides an overview. Processes are also being discussed in which substances that are already usable as fuel in their own right will be further refined. One example is the Methanol to Gasoline (MTG) process in which methanol is further processed to produce gasoline.

6.3 Fuels for Combustion Processes on the Diesel Engine Principle The oils of numerous plants can be used as fuel. Figure 6.3 shows the various different approaches for replacing fossil diesel fuel with renewable alternatives. Fuel that is obtained from the fruit of plants is always in competition with the production of food on the same land and is sometimes also under question from the ethical perspective.

6.3  Fuels for Combustion Processes on the Diesel Engine Principle

63

Table 6.1  Some physical properties of various fuels. The data serves as an indication. In the case of fossil energy sources, the exact physical properties depend on the deposit and the refinery process. In the case of biogenic fuels, the properties vary due to the raw material and the further processing Property

Main constituent

Density

Viscosity

Calorific value

Cetane number

Unit

–

kg/l

mm2/s =  cSt

MJ/kg

–

Important for:

Combustion

Fuel tank volume

Carbonization Range by and mass lubrication

Ignition quality

0.84

2–5

43

 > 51

Vegetable oil CmHn0p (example: rape- e.g seed oil) C18H34 02

0.92

72

37

40

Esterified vegetable oils

0.88

70

37

40

HVO Hydrotreated Vegetable Oil

0.78

44

80–99

 > 40

High

28

55–60

Diesel engine principle Diesel

CmHn C10 to C23

BTL

CmHn

Dimethyl ether

CH3OCH3

0.74 at boiling point (-25 °C)

Spark ignition engine process Gasoline

CmHn

Octane rating 0.75

0.53

44

95

C5 to C12 LPG: propane and butane

C3H8 and C4H10

0.54 at 5–10 bar

–

46

110

Methane

CH4

0.00072 (gas)

–

50

120

CNG (high-energy natural gas)

CH4

0.16 at 200 bar

–

46

120

LNG (high-energy natural gas)

CH4

0.42 at -25 °C – (liquid)

46

120

Methanol

CH3OH

0.79

0.75

20

160

Ethanol

C2H5OH

0.79

1.5

27

 > 104

Hydrogen (continued)

6  Alternative Fuels

64 Table 6.1   (continued)

Property

Main constituent

Density

Viscosity

Calorific value

Hydrogen (gas)

H2

0.0000899

–

120

0.071

–

120

H2 (liqH2 uid)