Cyclic variations on LPG and gasoline-fuelled lean burn SI engine

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Cyclic variations on LPG and gasoline-fuelled lean burn SI engine M.A. Ceviz, F. Yu¨ksel Department of Mechanical Engineering, Faculty of Engineering, University of Atatu¨rk, Erzurum 25240, Turkey Received 15 January 2005; accepted 12 September 2005

Abstract Lean operation is an attractive operational condition; it is known as one of the methods to increase thermal efficiency, and to decrease exhaust emissions and fuel consumption. However, as the mixture leans, cyclic variations increase. Cyclic variations are usually attributed to the result of random fluctuations, excess air ratio and flow field due to the turbulent nature of the flow in the cylinder that limits the range of operating conditions of the spark ignition engine. Gaseous fuels as clean, economical and abundant fuels can improve the lean operating limits and decrease the cyclic variations. Therefore, the purpose of this research is to investigate the use of liquefied petroleum gas (LPG) as a fuel for spark ignition engine in terms of lean operation, and focuses on the cyclic variations and exhaust emissions. The results of this study showed that use of LPG decreased the coefficient of variation in the indicated mean effective pressure, and emission. r 2005 Elsevier Ltd. All rights reserved. Keywords: LPG; Cyclic variations; S.I. engines; Exhaust emissions

1. Introduction Cyclic variations in internal combustion engines have long been recognized as a limiting factor in engine performance, fuel efficiency, and emissions [1,2]. In particular, cyclic variations are a major factor during lean operation. Lean operation is desired to reduce nitrogen oxides and hydrocarbon emissions as well as to improve fuel efficiency. If cyclic variations are large during engine operation near the lean combustion limit, combustion Corresponding author. Fax: +90 442 235 44 93.

E-mail address: [email protected] (M.A. Ceviz). 0022-5096/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2005.09.016

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may not occur resulting in misfires, drivability problems, and increased hydrocarbon emissions. Minimization of cycle-to-cycle variations is a key factor in effectively operating near to or extending the effective lean limit [3]. Generally, it can be stated that the effect of the relative air–fuel ratio of the mixture on the engine cyclic variation is through the laminar burning speed, and that the highest burning speed is achieved in stoichiometric or slightly enriched mixtures [1,2]. Therefore, any deviation from the stoichiometric ratio leads to decrease in the laminar burning velocity with consequent increase in the ignition delay time and the level of cyclic variations [1]. To achieve the demand of minimization of cyclic variations, liquefied petroleum gas (LPG) among the gaseous fuels can be used. A stoichiometric LPG-fueled engine has limited applications due to excessively high exhaust gas temperatures, which cause durability problems and a lower thermal efficiency. However, the lean burn strategy may also be implemented to overcome these shortcomings. Additionally, the level of engine modifications required to convert a conventional spark ignited (SI) engine to an LPG lean burn engine is low enough to make lean burn operation a cost-effective way to achieve better emissions and fuel efficiency [4]. LPG is an environmentally friendly fuel for spark ignition engine which has potential emission advantages over gasoline [5]. LPG is liquefied under pressure and compressed and stored in steel tanks under pressure that varies from 1.03 to 1.24 MPa. It is used for heating, cooking, and can be used as engine fuel. The fuel is liberated from lighter hydrocarbon fraction produced during petroleum refining of crude oil and from heavier components of natural gas. It is also a by-product of oil or gas mining. Poulton [6] presents a comprehensive review of LPG. Experiences show that LPG has some advantages over gasoline due to the following: (1) LPG produces lower exhaust emissions than gasoline, (2) it reduces engine maintenance, (3) it offers faster cold starting and (4) it provides overall lower operational cost. On the other hand, LPG displaces 15–20% greater volume than gasoline. Thus the power output decreases by 5–10%. This reduction can reach up to 30% at very lean conditions [7]. There are many reports about the effects of the LPG on the engine performance and emission characteristics. However, it is apparent that very little information is available about the cyclic variability when using LPG as a fuel. There are some studies about the lean operating performance of LPG-used spark ignition engines. Badr et al. [8] carried out a parametric study on the lean operating limits of an SI engine using propane and LPG as fuels, and the effects of engine speed, spark timing, compression ratio, intake temperature, intake pressure, and relative humidity of intake air on engine operational limits were examined. Alasfour [9] investigated the use of commercial LPG-fuelled spark ignition engine in terms of lean misfire limits. The results showed that ignition timing strongly affected the overall engine lean misfire limit. The experimental results show that as the fuel/ air equivalence ratio leaned, the MBT ignition timing needed to be advanced, where lean mixtures required advancing in ignition timing to provide more time for reaction due to the excess presence of oxygen in the fuel/air mixture. The engine lean misfire limit increases as the engine speed increases, with the same effect of preheating inlet air temperature, and as the engine load increases, the lean misfire limit increases. The objective of the study reported in this paper is to compare the cyclic variability and emission characteristics of LPG and gasoline-fuelled spark ignition engine at lean

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operating conditions. Cylinder pressure, indicated mean effective pressure (imep), mass fraction burned (MFB) and combustion duration are presented in relation to cyclic variability. Variations in the CO, CO2 and HC emissions are also discussed. 2. Testing procedure The experimental apparatus used in the measuring of the operating parameters and the in-cylinder pressure were explained in another study by the authors of this paper [10]. All the experiments were conducted at 1800(715) rpm, minimum spark advance for best torque (MBT), and wide open throttle (WOT). Relative air–fuel ratio was changed by varying the pressure of the space existing above the fuel in the carburetor float chamber when gasoline was used. To obtain leaner fuel–air mixture on the LPG experiments, LPG flow rate was decreased by closing the fuel valve. The experiments were performed at four different relative air–fuel mixture values from stoichiometric to lean: 1.001, 1.090, 1.195 and 1.299 for gasoline; and five different relative air–fuel mixture values: 1.000, 1.092, 1.212, 1.293 and 1.410 for LPG using. In the gasoline experiments, the engine could not be operated at leaner mixture from the point of about 1.3 relative air–fuel ratio because of the excessive misfiring and uneven operating conditions. Coefficient of variation in imep was used to investigate the cyclic variability. It is the standard deviation in imep divided by the mean imep [11] and usually expressed in percent as simep COVimep ¼
100. (1) imep The imep is easy to calculate and provides a measure of the work produced for an engine cycle; imep is defined as imep ¼

Wc , Vd

(2)

where V d is the engine displacement volume and W c is the work per cycle, defined as I W c ¼ P dV . (3) Additionally, Rassweiler and Withrow method [12,13] was used for estimating the MFB profile from cylinder pressure and volume data. In this method, the MFB is given by Pi¼0 i¼ign Dpc;i MFBy ¼ Pi¼N , (4) i¼ign Dpc;i

where MFBy is the MFB at crank angle y, Dpc the corrected pressure rise due to the combustion, i the integer crank angle location, ign the ignition crank angle location, EEOC the crank angle for estimated end of combustion. The corrected pressure rise due to combustion is calculated from the difference between the incremental measured pressure rise and the pressure rise corresponding to a polytropic compression/expansion process, and then referenced to the cylinder volume at TDC: Dpc;i ¼ ½pi  ðV i1  V i Þn pi1 ðV i1  V r Þ,

(5)

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where n is the assumed polytropic index, V is the cylinder volume and V r is the reference volume at TDC. 3. Results and discussion Fig. 1–4 show the engine in cylinder pressure from the start of compression stroke to the end of expansion, and various parameters (MFB, imep, and return maps of imep) calculated from the cylinder pressure operating with LPG and gasoline at different relative air–fuel ratios from stoichiometric to lean for 80 successive cycles. The cyclic variations are intensive at combustion process since the pressure development is uniquely related to this period. It can be seen from Fig. 1 that cylinder pressure traces are different, especially in the combustion period. Fig. 2 represents the MFB calculated with Rassweiler and Withrow method. As the relative air–fuel ratio increases, dispersion on the MFB traces becomes large both on the LPG and gasoline operating conditions. Fig. 3 shows the imep for 80 consecutive cycles, and Fig. 4 shows the return maps of these values, which indicates the correlations between each individual pair of events in the data set without any averaging, and an alternative method for examining interactions between combustion events. It can be seen from these figures that as the relative air–fuel ratio increases, the cluster of points expand. Figs. 5–7 illustrate the effect of the relative air–fuel ratio and gasoline and LPG on the CO, CO2 and HC emissions, respectively. It can be seen from these figures that as the relative air–fuel ratio increases CO emissions decreases, and there is no serious change over the lean burn operating condition. CO2 emission decreased with lower C number of fuel (LPG) and leaner mixture (Fig. 6). The lean operation decreases the flame speed and the burning rate, and the reduction in burning rate results in an increase in the overall combustion duration, which in turn leads to increased heat transfer losses to the cylinder walls and a decrease in the overall thermal efficiency. Since the increased combustion duration under lean conditions necessitates an earlier spark timing, it was advanced both on using LPG and gasoline. Fig. 7 shows the increase in the combustion duration with the relative air–fuel ratio, as can be seen from Fig. 2. However, the increase in the combustion duration when in operation with LPG, is lower than that of gasoline despite working on more lean conditions. The reason for the lower combustion duration is the higher laminar burning velocity of LPG (0.46 m/s, stoichiometric) when compared with gasoline (0.42 m/s, stoichiometric) (Fig. 8). Fig. 9 indicates the variation of coefficient of variation in imep with relative air–fuel ratio. It can be seen that the increase in COVimep of gasoline is more pronounced than that of LPG, which is due to the higher laminar burning velocity of LPG and the differences of the fuel–air mixture formation techniques between the LPG and gasoline. It can be deduced from Fig. 9 that cyclic variation operating with LPG as a fuel is lower than that of gasoline. Because LPG is vaporized before the intake manifold and it is well mixed with air as a gaseous fuel, local air–fuel ratio near the sparking plug region is more equal for successive cycles. This effect causes to decrease the cyclic variations. 4. Conclusions The present study showed that using LPG as a fuel for lean-operated spark ignition engine had an effect on the cyclic variations end emissions. It can be deduced that the

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Fig. 1. Variation of in-cylinder pressure for 80 consecutive cycles: (a) l ¼ 1:000, (b) l ¼ 1:092, (c) l ¼ 1:212, (d) l ¼ 1:293 (e) l ¼ 1:410 for gasoline experiments and (f) 1.001, (g) 1.090, (h) 1.195 and (i) 1.299 for LPG experiments.

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Fig. 2. Variation of mass fraction burned for 80 consecutive cycles: (a) l ¼ 1:000, (b) l ¼ 1:092, (c) l ¼ 1:212, (d) l ¼ 1:293 (e) l ¼ 1:410 for gasoline experiments and (f) 1.001, (g) 1.090, (h) 1.195 and (i) 1.299 for LPG experiments.

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5 4 CO, %Vol.

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increase in the relative air–fuel ratio increases the coefficient of variations in imep; however, LPG decreases the cyclic variations and emissions, and it is a more suitable fuel for lean combustion engine when compared with gasoline.

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It can be concluded that the higher laminar flame speed of LPG and good mixing of gaseous fuels with air causes a decrease in cyclic variations, and higher H/C ratio of LPG decreases the engine emissions.

References [1] Ozdor N, Dulger M, Sher E. Cyclic variability in spark ignition engines: A literature survey. SAE paper no: 940987, 1994. [2] Young MB. Cyclic dispersion in homogeneous charge spark ignition engine. SAE paper no: 810020, 1981. [3] Wagner RM, Drallmeier JA, Daw CS. Prior-cycle effects in lean spark ignition combustion-fuel air charge considerations. SAE paper no: 981047, 1998. [4] Lee D, Shakal J, Goto S, Ishikawa H, Uneo H, Harayama N. Observation of flame popagation in an LPG lean burn SI engine. SAE paper no: 1999-01-0570, Presentation at the Society of Automotive Engineers, International congress and exposition, Detroit, Michigan, March 1–4, 1999. [5] Lutz BR, Stanglmaier RH, Matthews RD, Cohen J, Wicker R. The effects of fuel composition, system design, and operating conditions on in-system vaporization and hot start of a liquid-phase LPG injection system. SAE paper no: 981388, Presentation at the Society of Automotive Engineers, International spring fuels and lubricants meeting and exposition Dearborn, Michigan, May 4–6, 1998.

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[6] Poulton ML. Alternative fuels for road vehicles. Bellerica, MA: Computational Mechanics Publications; 1994. [7] Baljit D, David C. Tailpipe emissions comparison between propane and natural gas forklifts, SAE Paper No: 2000-01-1865, Presentation at the Society of Automotive Engineers, International spring fuels and lubricants meeting and exposition, Paris, France, June 19–22, 2000. [8] Badr O, Alsayed N, Manaf M. A parametric study on the lean misfiring and knocking limits of gas-fueled spark ignition engines. Appl Therm Eng 1998;18:579–94. [9] Alasfour FN. Lean misfire limits of LPG fuelled S.I. Engine. SAE paper no: 2000-01-1960, Presentation at the Society of Automotive Engineers, International spring fuels and lubricants meeting and exposition, Orlando, Florida, May 7–9, 2001. [10] Ceviz MA, Yu¨ksel F. Effects of ethanol-unleaded gasoline blends on cyclic variability and emissions in an SI engine. Appl Therm Eng 2005;25/5–6:917–25. [11] Heywood JB. Internal combustion engine fundamentals. New York: Mc-Graw-Hill, Inc.; 1988. [12] Rassweiler GM, Withrow L. Motion pictures of engine flames correlated with pressure cards. SAE paper no: 380139, 1938. [13] Brunt MFJ, Emtage AL. Evaluation of burn rate routines and analysis errors. SAE paper no: 970037, Presentation at the Society of Automotive Engineers, International congress and exposition, Detroit, February 24–27, 1997.