Influence of Water Content Ratio on Combustion Fluctuation of Diesel Engine Using Emulsified Fuel

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Influence of Water Content Ratio on Combustion Fluctuation of Diesel Engine Using Emulsified Fuel*

Yasufumi Yoshimoto**, Toshinori Kuramoto***, Ziye Li***, Minoru Tsukahara *** The application of water-in-oil type emu1sified fuel to marine diesel engines is one of the attractive methods from a viewpoint of NOx reduction. Considering with actual application, it must be known how the water content and environment temperature influence the stability of combustion and engine performance. However, the combustion stability of diesel engine operated with emulsified fuel have not been reported. Therefore, the authors examined the influences of water content ratio (water to gas oil ratio: Gw = 0- 1. 19 mass) and intake air temperature (25-6C゜) on the stability of combustion. As a result, the sufficient combustion stability was obtained when the engine was operated under the conditions with higher load ratios than 2/4 (BMEP = 0.26 Mpa) and lower water ratios than Gw = 0.51.

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Introduction

Nitrogen oxide (NOx) emitted from diesel engines is listed as one of the causative substances to cause the extensive environmental pollution such as the acid rain, and introduction of the international regulation has also been considered in the field of ships navigating on the sea, and it will be implemented. As widely known, NOx emission is generated under the excellent combustion condition, and it is difficult to simultaneously reduce NOx together with the fuel consumption and particulates. Many methods have been considered1) as the measures to reduce NOx in diesel engines. Though the method to use the water emulsified fuel is not in the practical use, but NOx can be greatly reduced without sacrificing the engine performance, and it has been attended again focusing in the field of ships2). The authors have made a series of studies on the diesel engines of direct injection type in which the emulsified fuel is used, and as a result, reported that NOx can be greatly reduced without deterioration of the smoke density and the specific fuel consumption (BSFC) can be simultaneously improved under appropriate conditions3)11). When the water content ratio is increased to im-

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Translated from Journal of MESJ Vo1. 32, No. 6 (Manuscript received Nov. 4, 1996) Lectured May 1 5, 1 996 ' Niigata Institute of Technology (Kashiwazaki City) Joetsu University of Education (Joetsu City) (16)

prove the NOx reduction ratio in the emu1sified fuel, it is expected that the ignition lag is increased, and the stability of combustion is deteriorated. No examples of the quantitative examination can be found on how the low temperature atmosphere (the temperatures of in-take air and cooling water) affect the stability of combustion and performance with the emulsified fuel operation. Thus, in this study, the diesel engine was operated using the emulsified fuel in which the water content ratio is widely changed, and the stability of combustion was examined, under the steady state of engine operation. As a result, it was found that the stable operation is possible with the emulsified fuel whose water content ratio (water/gas oil) < 0.51 mass irrespective of the fuel injection timing and the temperature of the intake air in the operation range above 2/4 load. The test is reported below in detail. 2.

Experimental Apparatus and Method

2.l Engine and Fuel Injection System The engine used in the test is the 4-cycle direct injection type diesel engine having a spherical combustion chamber on the piston of the vertical, water-cooled, and single cylinder, and the principal specifications of the engine and its fuel injection system are shown in Table 1. The section of the combustion chamber and the direction of fuel injection are shown in Fig.1. Fig. 2 indicates the outline of the experimental apparatus. The temperature of the intake air Ta in-cludes 3 conditions, i.e.,the normal temperature (25゜C),

Bulletin of the M.E.S.J., Vol. 26, No.1

Influence of Water Content Ratio on Combustion Fluctuation of Diesel Engine Using Emulsified Fuel

15゜C, and 6゜C in the case where the heat exchanger for air cooling is installed in the middle of the air intake pipe. Under the normal temperature condition of Ta = 25゜C, the outlet temperature of the cooling water Two = 80゜C, and the lubricating oil temperature To = 54゜ C,which are respectively constant, but in the test with low temperature air (Ta = 15゜C and 6゜C), Two = 13 ゜ C and To = 22 ゜ C which are also constant respectively. To measure the output, an AC type dynamometer was used, and the engine was operated at a constant engine speed of 1200 rpm. As for the fuel injection timing, 2 kinds of fuel injection timings were selected,i.e., the fuel injection timing (21 ゜CA.B TDC )7)in which the best BSFC is obtained with gas oil operation,and the case ( 15゜CA.BTDC) in which the fuel injection timing is retarded to reduce NOx emission. The BSFC in using the emulsified fuel for the test engine was reduced compared with the case using gas oil over the whole range of the set fuel injection timing of 24- 10゜CA.BTDC under the rated output. The fuel injection timing at which the BSFC is best is likely to be retarded by 3-4゜CA compared with gas oilh. 2.2 Test Fuel Gas oil for automobile on the market (specific gravity of 0.841, and Cetane index of 57.5), and the February 1998

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emulsified fuel of this gas oil and water were used for the test fuel. The preparing method and the stability of the emulsion are indicated in the previous paper3). The surfactant (Ionet S-2, HLB = 6) of 1% mass is added to gas oil, and mixed using an in pipe mixer to prepare the emulsion of water-in-oil type, and the stable emulsion whose water particle diameter is 1-2 μ m can be obtained. The water content ratio Gw is defined by the mass ratio to gas oil, and the test was carried out with the emulsion of Gw = 0-1.19 mass. As for the fuel injection rate in using the emulsified fuel, the fuel injection duration becomes longer corresponding to the water content compared with gas oil. Fig. 3 shows6) the fuel injection rate of gas oil and the emulsified fuel of water content ratio Gw = 0.51 by the long tube method by Bosch. The emulsified fuel is injected using the same injection system as gas oil, and the total injection amount of the emulsified fuel is increased proportional to the water content ratio Gw. As indicated in Fig. 3, the peak value of the fuel injection rate is not changed much when the same injection system is used, and the injection duration of the emulsified fuel is increased by the water content. It means that the fuel injection rate (the heat supply ratio)

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Yasufumi Yoshimoto, Toshinori Kuramoto, Ziye Li, Minoru Tsukahara

of only gas oil decreases. On the other hand, the atomizing property of the case where injection is made with the same injection amount (volume) of gas oil and the emulsified fuel was examined. As a result, it is found12) that compared with gas oil, the spray angle reduces due to the increase in the viscosity in the emulsified fuel, and in addition, the mean droplet size of the spray increases whereas the spray penetration increases. Though the injection duration increases, and the atomizing characteristics deteriorate, the BSFC of the emulsified fuel reduces remarkably under an appropriate condition7). 2.3 Measuring Apparatus To measure the indicator diagram, the pressure pick-up of strain gage type was used, the injection valve lift and the crank angle were detected by the differential transformer type displacement meter and the photo-interrupter, and the detected signals were received by the 4-channel synchroscope for observation and re-cording. The cycle to cycle variation in maximum combustion pressure Pmax is taken as the index to express the combustion stability, and measured using the device in Fig. 2. The maximum combustion pres-sure Pmax was recorded during 360 continuous cycles using the combustion analyzer, and the combustion fluctuation rate was defined as the standard deviation in the distribution of Pmax divided by the mean value of Pmax. The ignition lag was defined as the time difference (crank angle) between the start points of injection and pressure rise due to ignition. The fluctuation rate of the ignition lag is similarly defined as the value of the standard deviation in the distribution divided by its mean value. The CLA analyzer was used for measurement of NOx in the exhaust gas, FIA for HC, NDIR for CO respectively. The smoke density was measured using Bosch smoke meter. 3. 3.1

Experimental Results, and Discussion Combustion Fluctuation and Performance at N orm al A ir T em perature

Figs. 4(a), (b) show the combustion fluctuation rate with BMEP = 0.52 Mpa (rated output) and 0.13 Mpa under the constant condition of the temperature of the intake air Ta = 25゜C, and the engine performance and the combustion characteristic value to the water content ratio Gw. Here, the NOx concentration is indicated by the reduction ratio to the emission concentration of gas oil in each operating condition. As indicated in Fig. 4, the fluctuation rate per cycle of the

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maximum combustion pressure Pmax shows the value which is little changed from that with the regular fuel even with the emulsion of Gw = 1.0 under the rated output. However, the combustion fluctuation rate is rapidly increased when Gw exceeds 0.8 under the low load operation (BMEP = 0.13 Mpa). In particular, the combustion fluctuation is large with Gw = 1.19, and no data could be obtained. The specific fuel consumption BSFC is remark-ably reduced by water emulsification under the high load operation. The BSFC seems to be improved by ( 1 ) rapid volumetric expansion of water particles in the water-in-oil emulsion, i.e., secondary atomization of the sprayed oil droplets due to micro-explosion effect,(2) inclusion effect of air into the spray flux due to increase in the spray momentum, (3) increase in the premixed combustion due to longer ignition lag, and (4) decreased cooling loss due to lower combustion temperature and a less luminous flame3). The water content ratio at which the BSFC becomes best is around Gw = 0.5. On the other hand, the BSFC under the low load operation is deteriorated compared with gas oil. Thus, it is necessary to take into consideration the property in the practical application of the emulsification combustion method. NOx decreases monotonously as the water content ratio increases, and reduction of about 40% is obtained in the emu1sified fuel of Gw = 0.5 compared with gas oil. The smoke density is also reduced remarkably, and the BSFC, NOx, and smoke can be simultaneously reduced by using the emulsified fuel. CO is not greatly increased/decreased compared with gas oil when the emulsified fuel is used, but HC increases when the water content ratio increases. 3.2

Combustion Fluctuation and Performance at L ow A ir T em perature

In Fig. 5, the fluctuation rate of the maximum combustion pressure Pmax under the rated output with the constant temperature of the intake air Ta = 6゜C, the ignition lag and its fluctuation rate the BSFC, NOx concentration, and smoke density are plotted to the water content ratio. The data on the combustion fluctuation rate at the normal temperature (= 25゜C) are also shown for comparison. As shown in the figure, the BSFC is remarkably improved compared with gas oil up to Gw = 0.4 even when the engine is in the atmosphere of low temperature (Ta = 6゜C, Two = 13゜C, To = 22゜C). In comparison with Fig. 4(a), the water content ratio Gw at which the BSFC is best is shown to be dropped from 0.5 to 0.3 in the atmosphere of lower temperature .

Bulletin of the M.E.S.J., Vo1. 26, No.1

Influence of Water Content Ratio on Combustion Fluctuation of Diesel Engine Using Emu1sified Fuel

On the other hand, the combustion fluctuation rate shows the increasing trend by dropping the intake air temperature from 25゜C to 6゜C. In particular, this trend is remarkable in the retarded injection timing with the water content ratio Gw exceeds 0.5. Though the fluctuation rate of the ignition lag in this case are dispersant, it shows good correlation with the fluctuation rate of Pmax. In such an emulsified fuel, it is a general trend that the ignition lag increases as the water content ratio increases, and this was confirmed by a simple calculation. Fig. 6 shows the relationship between the mean gas temperature in the combustion chamber at the ignition and the ignition lag with various water content ratios. Various equations have been proposed to pre-

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dict the ignition lag, and here, the ignition lag was determined by Wolfer's Eq. ( 1)13). As the assumption of the calculation, the fuel is instantaneously introduced in the combustion chamber at the crank angle at the ignition, and the fuel is instantaneously evaporated. The mass of the evaporated gas is neglected, and the incylinder gas temperature is dropped from the temperature T1 before injection to the uniform temperature T. The Eq. (2) indicates the heat balance in the in-cylinder gas before and after the fuel injection.

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Yasufumi Yoshimoto, Toshinori Kuramoto, Ziye Li, Minoru Tsukahara

In calculating the in-cylinder gas temperature T1 before the fuel injection, the polytropic exponent of the gas in the compression stroke is 1.35, and the injection amount Mw and Mo during the ignition lag period are obtained from the measured injection rate. The crank angle at the ignition is given by the measured value of gas oil. (7, 8 and 9゜CA.BTDC at Ta = 6, 15, and 25゜ C respectively) As indicated in Fig. 6, the calorie to be absorbed by evaporation of the injected fuel together with the increase in the water content ratio is increased, the mean temperature of the gas in the combustion chamber is dropped. The temperature of the emulsified fuel of Gw = 0.5 1 is lower than gas oil by about 30゜C. The change in the mean gas temperature at ignition to the change in the temperature of the intake air Ta has no difference due to the difference in the water content ratio, and for

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Bulletin of the M.E.S.J., Vo1. 26, No.1

Influence of Water Content Ratio on Combustion Fluctuation of Diesel Engine Using Emu1sified Fuel

example, the mean gas temperature at ignition at Ta = 6゜C show the lower value than that at Ta = 25゜C by about 30゜C. Drop of the incylinder gas temperature due to the increase in the water content ratio or drop of the temperature of the intake air brings the increase in the ignition lag as a result. The measured value of the ignition lag is also shown in the figure, and they agree well with each other. Fig. 7 shows the correlation between the fluctuation rate of Pmax and the fluctuation rate of the ignition lag. They are in the linear relationship, indicating the strong positive correlation. From these findings, the

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increase in the fluctuation rate of Pmax accompanied by the increase in the water content ratio when the emulsified fuel is used in the low temperature atmosphere seems to be attributable to the fluctuation in the ignition lag. Fig. 8 shows one example of the distribution of the position of the crank angle where the peak pressure is generated. The left side in Fig. 8 shows the case of the air at normal temperature, and the right side shows the case of the air at low temperature, and X-axis expresses the crank angle, and Y-axis expresses the frequency of appearance of Pmax at the position. Pmax is distributed in a narrow range around 7゜CA.ATDC irrespective of the difference in the water content ratio in the atmosphere of normal temperature, but when the water content ratio is high in the atmosphere of low temperature, the position of Pmax retards greatly and the variance increases. Fig. 9 shows the position of crank angle and the combustion fluctuation rate when Pmax appears to the water content ratio at the intake air temperature of 6゜ C.As clearly indicated in the figure, the position of Pmax is likely to be delayed in angle as the combustion fluctuation rate is increased. When Pmax is in the range of 5-12゜CA.ATDC, the combustion can be in the stable condition.

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Yasufumi Yoshimoto, Toshinori Kuramoto, Ziye Li, Minoru Tsukahara

3.3 Region of Stable Combustion Fig. 10 shows the influence of the intake air temperature on the stability of combustion with the water content ratio at BMEP = 0.52 Mpa (rated output)/ and 0.26 Mpa (2/4 load). In the condition of the low temperature (Ta = 6゜C), and 1/4 load, the combustion fluctuation is large, and no test was carried out. In the operation of the rated output, the fluctuation rate of Pmax is relatively small even when the emulsified fuel of Gw = 0.79 is used, but it is clearly shown that the combustion fluctuation rate is greatly increased as the temperature of the intake air is dropped with 2/4 load. On the other hand, in the case of the emulsified fuel of Gw = 0.51, no problems Me raised in the combustion stability in the operation of around 2/4 1oad. In summarizing the results of this test, the map on the combustion stability of the emu1sified fuel was

prepared. The results are shown in Fig. 11. The water content ratio Gw is plotted on X-axis, the load BMEP is plotted in the depth direction, and the combustion fluctuation rate (the fluctuation rate of Pmax) is plotted in the height direction, and comparison is made for the intake air temperatures Ta = 25゜C and 6゜C. The boundary is set at the location of 0.04 in the combustion fluctuation rate, intending that the combustion is sufficiently stable if the fluctuation rate of Pmax is below this value. As indicated in the figure, the range of the stable combustion of the emulsified fuel is narrowed in the atmosphere of the low temperature. However, when the emulsified fuel of the water content ratio of < 0.5 is used, the stable operation is possible irrespective of the difference in the fuel injection timing and the temperature of the intake air in the operation range of BMEP > 0.26 Mpa (2/4 1oad). 4. Conclusions The combustion stability was examined by per-forming the operation of the direct injection type diesel engine using the emulsified fuel whose water content ratio is extensively changed, and examining the fluctuation of the maximum combustion pressure Pmax in the steady state of engine operation. The results of the present investigation are summarized as fol1ows. (1) It is found that the fluctuation in the maximum combustion pressure Pmax is sufficiently small irrespective of the load and the fuel injection timing when the emulsified fuel with the mass ratio Gw of water/gas oil is not more than 0.51 in the atmosphere of normal temperature (tempera-ture of the intake air of 25゜C). (2) The NOx concentration, smoke density, and specific fuel consumption are simultaneously reduced in the high load operation by using the emulsified fuel of around 0.3 mass in Gw even in a case of the atmosphere of low temperature (temperature of

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Influence of Water Content Ratio on Combustion Fluctuation of Diesel Engine Using Emulsified Fue1

the intake air of 6゜C). (3) The stable operation is possible irrespective of the fuel injection timing and the intake air temperature in the operation range of BMEP > 0.26 Mpa (2/4 load) when the emulsified fuel of water content ratio Gw is not more than 0.51 mass. (4) The strong positive correlation was found between the fluctuation of the maximum combustion pres-sure Pmax and the fluctuation of the ignition lag. The increase in the combustion fluctuation when the emulsified fuel of high water content ratio is used in the atmosphere of low temperature seems to be attributable to the fluctuation in the ignition lag. The authors express their gratitude to those concerned for the subsidy from the Research and Study Committee on Purification of Emissions by Emulsified Fuel of the Marine Engineering Society in Japan. References

1) S. Kobayashi, Journal of the SAEJ, 50- 1, 1996, p. 71 (in Japanese).

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2) For instance, N. Nakayama, et al., Journal of the MESJ, 30-9, 1995, p.661 (in Japanese). 3) T. Murayama, et al., SAE Paper 780224; SAE Tmns., 87, 1979, p. 946. 4) M. Tsukahara, et al., Bulletin of the JSME, 25202, 1982, p.612. 5) M. Tsukahara, et al., Trans. of the JSME, 48-426 (B), 1982, p. 381 (in Japanese). 6) M. Tsukahara, et al., Trans. of the JSME, 54-506 (B), 1988, p. 2955 (in Japanese). 7) M. Tsukahara, et aI., SAE Paper 890449; SAE Trans., 98, 1990, p. 777. 8) M. Tsukahara, et al., SAE Paper 891841 ; SAE Trans.,98, 1990, p. 1795. 9) M. Tsukahara, et al., Trans. of the JSME, 57-542 (B), 1991, p. 3584 (in Japanese). 10) Y. Yoshimoto and M. Tsukahara, Joumal of the N4ESJ, 28-3, 1993, p. 228 (in Japanese). 11) M. Tsukahara, et al.,Trans. of the JSME,61-590 (B), 1995, p. 3561 (in Japanese). 12) Y. Yoshimoto and M. Tsukahara, Trans. of the JSME,54-503 (B), 1988, p. 1866 (in Japanese). 13) H.H. Wolfer, VDI-Forsch., 392 ( 1938-9/ 10).

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