Performance and emissions of a turbocharged, high-pressure common rail diesel engine operating on biodiesel/diesel blends

Transcription

1 127 Performance and emissions of a turbocharged, high-pressure common rail diesel engine operating on biodiesel/diesel blends X-G Wang, B Zheng, Z-H Huang*, N Zhang, Y-J Zhang, and E-J Hu State Key Laboratory of Multiphase Flow in Power Engineering, Xi an Jiaotong University, Xi an The manuscript was received on 31 March 2010 and was accepted after revision for publication on 21 July DOI: / JAUTO1581 Abstract: The effect of biodiesel addition to diesel on engine performance, combustion, and emissions were studied in a turbocharged, high-pressure common rail diesel engine. Biodiesel/ diesel blends with different biodiesel fractions were used and compared with neat biodiesel and diesel at different engine loads and speeds. The results show that the brake thermal efficiency increases slightly as biodiesel is added to diesel. Exhaust gas temperature is not significantly affected at low engine speeds and decreases gradually at high engine speeds with an increase in biodiesel fraction. Fuel injection includes both pilot and main injections. Diesel and biodiesel give a similar start to the heat release. The first peak in the heat release rate for biodiesel is lower than that of diesel, while the second peak is higher for biodiesel. The heat release rate curve for biodiesel indicates that the use of biodiesel increases thermal efficiency and NO x emission compared to that of diesel especially at high engine loads. Hydrocarbon and CO emissions maintain very low values and little variation is seen for the different fuels. CO 2 emission decreases with increasing biodiesel fraction in the blends. The level of NO x emission decreases slightly at low engine loads and increases at high engine loads with increasing biodiesel fraction. Biodiesel reduces particulate matter (PM) emission significantly and PM reduction effectiveness is increased at high engine loads and/or speed. The oxygen in biodiesel plays a key role in reducing PM emission. Biodiesel/diesel blends can improve performance and decrease emissions for turbocharged, high-pressure common rail diesel engines. Keywords: diesel engine, biodiesel, performance, emissions 1 INTRODUCTION The high fuel efficiency of diesel engines has led to their use in many areas including transportation and agricultural machinery. However, the exhaust emissions from diesel engines, especially NO x and particulate matter (PM), have a significant impact on the environment. Moreover, it is difficult to simultaneously reduce these emissions since there is a tradeoff between NO x and PM emission levels. Increases in the price of oil coupled with continuously tightening emission control regulations have led to significant *Corresponding author: State Key Laboratory of Multiphase Flow in Power Engineering, Xi an Jiaotong University, Xi an, , People s Republic of China. zhhuang@mail.xjtu.edu.cn interest in renewable energy sources. One such energy source is biodiesel; the use of which can lead to reductions in petroleum consumption and carbon dioxide emissions [1]. One advantage of biodiesel is that it contains almost 11 wt% oxygen and these oxygen molecules enhance the combustion process and inhibit soot formation in diesel engines [2]. There is an extensive literature on experimental investigations on the combustion and emission characteristics of biodiesel fuel in diesel engines [1 15]. From these papers it is clear that the use of biodiesel blends and neat biodiesel can decrease CO, hydrocarbon (HC), and soot emission levels compared to normal diesel fuels. Unfortunately, these decreases are accompanied by an increase in NO x emission levels [7 15]. The majority of the reports in the literature concern studies carried out on conventional pump-line-

2 128 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu injector injection engines with only limited interest being focused on high-pressure common rail engines [1, 16]. For engines equipped with conventional pump-line-injector injection systems, the increase in NO x emissions can be attributed to the earlier start of injection for biodiesel because of its higher bulk modulus of compressibility [16]. However, in a common rail injection system, such as found in modern turbocharged high-pressure common rail diesel engines, the actual injection timing between diesel and biodiesel can be ignored, which can remove the influence of injection timing for different fuels on the combustion process and NO x emissions. A high injection pressure is an effective method to improve direct injection diesel engine performance and decrease PM emission levels due to improved spray atomization and fuel air mixing [17 19]. Multiple injection strategies have been tested for emission reduction at different operating conditions and it has been reported that appropriate configurations could offer simultaneous soot and NO x reductions while maintaining a reasonable fuel economy [20]. Thus, the question must be asked: could the highpressure common rail injection system in a diesel engine be used to create a high injection pressure and realize multiple injection strategies, consequently reducing both NO x and PM emissions? The objectives of this study are to investigate the performance, combustion, and emission characteristics of biodiesel and biodiesel/diesel blends in a modern turbocharged, high-pressure common rail diesel engine, and to examine the possibility of improving engine performance and decreasing emissions simultaneously with biodiesel/diesel blends. The effects of biodiesel fraction on engine performance, combustion, and emissions are discussed and compared with those of diesel and biodiesel. Type Table 1 GW2.8TC engine specifications 2 EXPERIMENTAL SET-UP AND PROCEDURE 2.1 Test engine and apparatus In-line four-cylinder common rail injection, turbocharged diesel engine Combustion chamber v type Bore6stroke 93 mm6102 mm Displacement l Compression ratio 17.2:1 Rated power/speed 70 kw 3/3600 r/min Pump Bosch CP1H Common rail Bosch LWR Injector Bosch CR1P2, mm A commercial light-duty direct injection diesel engine GW2.8TC was used in this study. It is a four-stroke four-cylinder turbocharged high-pressure common rail diesel engine. The engine specifications are listed in Table 1. The diesel engine was coupled to an eddycurrent dynamometer, as shown in Fig. 1. The realtime engine speed, torque, and power, as well as exhaust gas and coolant temperatures and lubricating oil pressure were monitored by a Powerlink Engine Control System (Type FC2000). The glow plug of one cylinder was replaced by a Kistler piezoelectric transducer (Type 6055Csp), which has the same size as the glow plug and was used to record cylinder pressure with a resolution of 10 Pa. The dynamic top-dead-centre (TDC) was determined by motor operation. The crank angle signal was obtained from a Kistler crank angle encoder (Type 2614A) mounted on the main shaft. The temporal curves of the cylinder gas pressure and crank angle were recorded by a Yokogawa DL750 data acquisition system. The signal of the cylinder gas pressure was acquired for every 0.1u increment in crank angle over 100 completed cycles. The averaged value of the cylinder gas pressure was used to calculate the heat release rate Fig. 1 Schematic diagram of experimental system

3 Performance and emissions of a turbocharged common rail diesel engine 129 using the method in Heywood [21]. A high-precision electronic balance with an accuracy of 0.1 g was used to determine fuel consumption by weighing the fuel mass at the beginning and end of each test condition. For each fuel and test condition, fuel consumption was recorded over 5 min periods. Based on the power output for each test condition, the brake specific fuel consumption (BSFC) and brake thermal efficiency were calculated. A Horiba MEXA-700l analyser was used to measure the excess air ratio with an accuracy of 0.1. A Horiba MEXA-554JA analyser was used to measure unburned HC, CO, and CO 2 concentrations in the exhaust. The accuracies for HC, CO, and CO 2 are 12 ppm, 0.06 per cent, and 0.5 per cent respectively. A Horiba MEXA-720 NO x analyser was used to measure NO x concentration in the exhaust, with an accuracy of 30 ppm. An ELPI4.0 analyser was used to measure PM emission over a 150 s period when the engine was operating in steady state. Detailed information about ELPI can be found in Tsolakis [22]. 2.2 Test fuels The pure fuels used in this study were an ultra-low sulphur diesel fuel (, 50 ppm) and soybean-derived biodiesel with diesel fuel being used as basis for comparisons. The properties of the diesel and biodiesel fuels are given in Table 2. It can be seen that the biodiesel fuel has a lower low heating value and higher oxygen content than the diesel fuel. Five diesel/biodiesel blends, D90B10, D80B20, D60B40, D40B60, and D20B80, were used to study engine performance and improvement in emission levels with blended fuels, where D(X)B(100-X) denotes that the fuel blends are composed of X% diesel and (100- X)% biodiesel by volume. For consistency and convenience, the diesel and biodiesel fuels are denoted as D100B0 and D0B100 respectively in this paper. Oxygen contents and low heating values of various fuel blends as well as those of the pure diesel and biodiesel fuels are shown in Fig. 2. With the increase of biodiesel fraction in fuel blends, oxygen content is Table 2 Physical and chemical properties of the diesel and biodiesel fuels Diesel Biodiesel Low heating value (MJ/kg) Density (15 uc) (kg/m 3 ) Viscosity (30 uc) (mm 2 /s) Cetane number Carbon content (wt%) Hydrogen content (wt%) Oxygen content (wt%) (A/F) st Fig. 2 Oxygen content and low heating value of fuel blends increased and low heating value is decreased. The combination of different fuel compositions and properties for the biodiesel and diesel fuels may create the conditions to improve engine performance, reduce emissions, and lower the amount of consumed diesel. 2.3 Experimental procedure An extended warm-up period was used to ensure that the coolant reached approximately 80 uc. Then the engine was loaded to test the engine speed and torque. In the experiment, engine speed and torque variations were controlled within 10 r/min and 0.1Nm. Exhaust gas analyses were conducted during steady operating conditions. During this steady process, the cylinder gas pressure and crank angle were recorded simultaneously. In this study, the effect of fuel blends on engine performance and emissions were evaluated for each fuel at engine speeds of 1600 and 2600 r/min. Five engine torques of 34, 68, 101, 135, and 169 Nm, corresponding to brake mean effective pressure (BMEP) levels of 0.154, 0.308, 0.458, 0.612, and MPa were selected. The test matrix covers the main conditions that a diesel engine can achieve. During the completion of the engine test matrix, no adjustments were made to the engine operating parameters. 3 RESULTS AND DISCUSSION 3.1 Engine performance Figure 3 shows a comparison of BSFC levels for the investigated fuels as a function of engine load. At both 1600 and 2600 r/min, the BSFC decreases

4 130 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu Fig. 3 Brake specific fuel consumption monotonically with increasing BMEP. When the engine speed is increased from 1600 to 2600 r/min losses due to friction increase and this leads to the higher BSFC values observed at this engine speed. Compared to those of neat diesel, the BSFC levels of D90B10 and D80B20 are slightly decreased. As shown in Fig. 2, the low heating values of D90B10 and D80B20 are 98.6 and 97.2 per cent of that of diesel, respectively. Thus, improved combustion appears to compensate for the slightly decreased low heating values for D90B10 and D80B20. With further increasing biodiesel volume fraction in the blends, BSFC increases monotonically. The BSFC was compared under the same engine speed and BMEP. To maintain the same power output, more fuel needs to be consumed in the case of a decreased low heating value. The gradual decrease in low heating value, as shown in Fig. 2, is responsible for the monotonically increasing BSFC level as the biodiesel percentage exceeds 20 per cent by volume. The oxygen content in biodiesel can promote the combustion process in the combustion chamber of the diesel engine, and thus can decrease fuel consumption at low diesel concentrations. However, a large biodiesel fraction still results in high fuel consumption due to the lower heating value even though combustion is improved. Brake thermal efficiency can be used as a surrogate measure that reflects fuel economy when the engine is operated with different fuels. Figures 4(a) and (b) show the brake thermal efficiency versus engine load for different fuels. With the addition of biodiesel to diesel, the thermal efficiency is increased slightly. As previously discussed, the oxygen content in biodiesel promotes burning rate, and improves combustion efficiency and thermal efficiency. Figures 4(c) and (d) plot the thermal efficiency versus biodiesel fraction in the blends. The results clearly show that biodiesel and blends of biodiesel/diesel give slightly higher thermal efficiencies than those of pure diesel. The results on BSFC and thermal efficiency suggest that using diesel/ biodiesel blends does not lead to a decrease in the engine s thermal efficiency, and this provides the possibility to use the oxygen molecules in the biodiesel to create low emissions levels. Figure 5 plots the variation in the excess air ratio as a function of engine load for different fuels at engine speeds of 1600 and 2600 r/min. The results show that the excess air ratio is insensitive to engine speed and fuel type. This suggests that the oxygen levels taken from the air are the same under same engine speed and load for all fuels, thus the influence of this oxygen should be the same for all fuels. Thus, any differences in oxygen contribution to combustion and emissions for different diesel/ biodiesel blends are a result of the difference in oxygen content in the fuel blends. The excess air ratio is decreased as engine load is increased, and this will influence combustion and emissions under different loads. A comparison of measured exhaust gas temperatures for different fuels is shown in Fig. 6. When the engine speed increases from 1600 to 2600 r/min the exhaust gas temperature increases at a specific engine load. This is due to a decrease in heat loss to the coolant and postponed heat release. The exhaust gas temperature increases monotonically with the increase in engine load. More fuel is injected and more heat is released at high engine load, resulting in an increase in cylinder gas temperature and exhaust gas temperature. At the engine speed of 1600 r/min, little variation in exhaust gas temperature is observed

5 Performance and emissions of a turbocharged common rail diesel engine 131 Fig. 4 Brake thermal efficiency between the different fuels. However, at the engine speed of 2600 r/min, with the increase of biodiesel fraction, the exhaust gas temperature decreases gradually. Moreover, the difference in exhaust gas temperature among different fuels is obvious at high engine loads. For example, at the lowest engine load of 2600 r/min, the exhaust gas temperature is decreased from 227 to 220 uc when the fuel is changed from diesel to biodiesel, while the exhaust temperature changes from 473 to 432 uc at the highest engine load. The calculated adiabatic flame temperature and measured flame temperature of the biodiesel fuel are lower than those of the diesel fuel [4, 23, 24]. The lower temperature of the burning gas is responsible for the decreased exhaust gas temperature for biodiesel and biodiesel blends. Moreover, differences in the heat release rate will also influence the exhaust gas temperature. The decreased exhaust gas temperature caused by using biodiesel has also been reported in Ozsezen et al. [25]. 3.2 Combustion analysis The cylinder gas pressure and heat release rate of the diesel and biodiesel fuels are illustrated in Fig. 7. The cylinder pressure and heat release rate curves of the diesel/biodiesel blends are between those of pure diesel and biodiesel. Thus, Fig. 7 only plots the cylinder pressure and heat release rate for the pure diesel and biodiesel fuels. The heat release rate curve demonstrates two-stage heat release, which is different to the behaviour observed for premixed and diffusion combustion in a plunger-pump-injection diesel engine. The individual heat release rates reflect the pilot and main injections. From Fig. 7 it can be concluded that that the diesel and biodiesel fuels have similar initial heat release behaviours. In a common rail injection system, the difference between the actual injection timings of the diesel and biodiesel fuels can be ignored. This is different to the case of a conventional pump-lineinjector system, where the actual injection timing of biodiesel is earlier than that of diesel because of its

6 132 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu Fig. 5 Excess air ratio Fig. 6 Exhaust gas temperature higher bulk modulus [26]. Therefore, in the common rail diesel engine, the similar initial behaviour of the combustion for the diesel and biodiesel fuels (diesel/ biodiesel blends included as well) is the result of the injection timings being almost equivalent. (It should be noted that the diesel and biodiesel fuels used in this study give almost the same Cetane number, as shown in Table 2). Even though the initial combustion behaviour is similar, there is an obvious difference between the heat release rate curves for the biodiesel and diesel under each engine condition: the first peak in the heat release rate for biodiesel is lower than that of diesel, while the second peak is higher for biodiesel. This tendency becomes more obvious with increasing engine load and/or speed. The two heat release processes correspond to the two independent injections. The difference in the first peak in the heat release rate is due to differences in spray atomization. It is generally recognized that biodiesel has poor spray atomization characteristics due to its high surface tension and viscosity levels. Moreover, the first heat release is from the pilot injection, which has a short time scale. During this short pilot injection duration, the lift and fall of the needle in the injector occupy a high percentage of the injection time. Thus, the spray atomization is more prone to be affected by fuel properties. The poor spray atomization properties of biodiesel are responsible for the lower heat release rate. For the main injection duration, biodiesel gives a higher release rate which compensates for the lower heat release rate of the first heat release. With increasing engine load and/or speed, the differences in heat release rate between biodiesel and diesel become more obvious as the injected fuel amount is increased. Another tendency in heat release rate between biodiesel and diesel is that the second heat release curve of biodiesel moves closer to TDC than does the diesel and this tendency becomes more obvious at

7 Performance and emissions of a turbocharged common rail diesel engine 133 Fig. 7 Comparison of cylinder gas pressure and heat release rate for diesel and biodiesel high engine loads. The incompletely burned biodiesel from the pilot injection might advance the main combustion. This behaviour of the main heat release indicates that the overall heat release of biodiesel is more compact and thus a higher thermal efficiency will be created. This is consistent with the results in Fig. 4, that biodiesel has a higher thermal efficiency than diesel. Moreover, the heat release rate of biodiesel moves closer to TDC and thus the combustion process is finished earlier and a lower exhaust gas temperature is created in this case. Due to the advanced main heat release for biodiesel at high engine loads, the cylinder gas temperature of biodiesel is higher compared to that of diesel at high engine loads, as shown in Figs 7(c) and (f). This is reflected in a higher cylinder pressure for

8 134 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu biodiesel. This higher cylinder temperature may lead to the increased NO x formation in the cylinder. 3.3 Engine emissions Figure 8 shows the influence of biodiesel fraction on unburned HC emission from the exhaust. The results show that HC emissions for all fuels and loads have low values in this turbocharged common rail diesel engine. No observable difference in HC emission levels can be seen for any diesel/biodiesel blend or biodiesel. Generally, HC emissions from diesel engines are a result of poor fuel/air mixing and they consist of fuel droplets that are either completely unburned or only partially burned [21]. Due to the improved fuel/air mixing and combustion processes in the turbocharged, high-pressure common rail diesel engine, the effect of biodiesel addition on HC emissions is limited. This is different to HC emissions in a plunger-pump-injection diesel engine where biodiesel addition to diesel has been shown to significantly decrease HC emissions [7, 8]. This study indicates that HC emission levels are low and are not significantly influenced by fuel type in the high-pressure common rail injection diesel engine. Figure 9 shows CO emission versus engine load. CO emission levels are low for all fuels. The studied engine has low CO emission levels in diesel operation, thus the effect of biodiesel addition on CO levels is difficult to demonstrate. CO is generated by incomplete combustion processes [21], and these are strongly inhibited by the high excess air ratio and good combustion process properties in the turbocharged, high-pressure common rail diesel engine. Fig. 8 Brake specific HC emission Fig. 9 Brake specific CO emission CO 2 emission is regarded as a main factor in global warming. Figure 10 shows the exhaust CO 2 emission versus engine load for various fuels. With an increase in biodiesel fraction in the blends, CO 2 emission decreases monotonically. It is believed that CO 2 concentration has a strong relationship with the carbon hydrogen ratio in the fuel [25, 27]. Actually, biodiesel has a low carbon content and thus naturally produces less carbon dioxide in the exhaust gas. Figure 11 shows exhaust specific NO x emission versus engine load for different fuels. NO x concentration increases monotonically with increase in engine load except for the lowest engine load. The increase in NO x is attributed to the increased temperature of the burned gas. More fuel is injected and burnt at high engine loads, leading to an increased cylinder gas temperature. The results show little variation in NO x level as a function of engine speed. With increasing engine speed, more fuel is injected and a higher temperature of the burning gas is generated. This is beneficial for NO x generation. Meanwhile, the actual time that the burning gas is at the high temperature is reduced with increasing engine speed. The combination of these two competing effects results in the observed insensitivity of NO x emission with engine speed. It is also noted that, at the lowest engine load, NO x concentration decreases slightly when biodiesel is added to diesel; while at the highest engine load, NO x level clearly increases when biodiesel is added to diesel. Song et al. [28] thought that the combination of decreased low heating value and leaner overall mixtures with using oxygenated fuels was responsible for the slightly decreased NO x emission

9 Performance and emissions of a turbocharged common rail diesel engine 135 Fig. 10 Brake specific CO 2 emission levels. At low engine loads, the lower temperature of the biodiesel spray flame [4, 23, 24] results in slightly decreased level of NO x emission. As indicated in Fig. 7, biodiesel gives an obviously advanced and compact heat release rate curve compared to diesel at high engine loads, leading to an increased cylinder gas temperature of biodiesel operation and higher NO x emission levels. Moreover, at high loads, a very rich core is generated as more fuel is injected. NO x emission levels are likely to be highly influenced by the existence of this hightemperature fuel-rich core since the oxygen atom in biodiesel can be used to thermally generate NO x. These two factors contribute to the increased NO x concentration for biodiesel at high loads. Figure 12(a) shows the PM concentration versus engine load for different fuels. When the engine speed is increased from 1600 to 2600 r/min, PM emission levels decrease significantly. At high engine speeds, more fuel is injected and burned, thus the temperature of the burnt gas increases. Meanwhile, the excess air ratio remains almost constant as the engine speed is increased from 1600 to 2600 r/min. The high temperature of the burning gas in the cylinder might be beneficial to oxidize the already formed PM. The variation of PM emission versus engine load shows different characteristics to those observed in a naturally aspirated diesel engine, where PM emission levels generally increase with an increase in engine load [8]. The use of a turbocharger increases the intake air mass at high engine loads, which provides an opportunity to reduce PM emission at high engine loads. Biodiesel shows significant reduction in PM emission levels regardless of engine speed and load. The addition of biodiesel to diesel provides more oxygen to the combustion reaction and promotes complete combustion especially for those areas at the core of the fuel spray [7]. Moreover, the oxygen in biodiesel inhibits cyclic-carbon-molecule formation. Therefore, the addition of biodiesel decreases PM emission levels. The clear effect of oxygen content in a fuel on PM reduction has been previously reported in [7, 8]. Figure 12(b) is a plot of PM reduction rate versus biodiesel fraction for the blends. PM emission levels decrease with increasing biodiesel fraction. The influence of biodiesel fraction on PM reduction varies with engine speed and load. At a specific biodiesel fraction and engine load, PM reduction rate increases when the engine speed is increased from 1600 to 2600 r/min especially at low biodiesel fraction levels in the blends. Moreover, PM reduction rate is also increased at high engine loads. At low engine loads, where the overall mixtures are much leaner, the oxygen in biodiesel has a limited influence on PM emissions. While at high engine loads, more fuel is injected and burned, thus a relative rich core exists. In this high-temperature fuel-rich core, the oxygen atoms from biodiesel can consume the soot precursors through forming a OH radical [29]. This effect results in a significant reduction effect on soot formation. A similar effect can be used to explain the increased PM reduction rate when the engine speed is increased from 1600 to 2600 r/min. This behaviour of oxygenated fuel on PM reduction at high engine loads has previously been reported by Song et al. [28] 3.4 Discussion As previously described, at a specific engine speed and load, the excess air ratio is almost unchanged for different diesel/biodiesel blends, however, the PM

10 136 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu Fig. 11 Brake specific NO x emission concentration reduces significantly with an increase in biodiesel fraction in blends. This characteristic reflects the role of oxygen in biodiesel on PM reduction. The PM emission level versus excess air ratio is plotted in Fig. 13. The oxygen content of biodiesel is about 11 wt%, while the oxygen content of the charge is estimated to be six to fifteen times that of the injected biodiesel. Thus, from a quantitative viewpoint, the oxygen contribution from biodiesel compared to that from the intake air is small and could be ignored. This is the reason why a near constant excess air ratio was used for both diesel and biodiesel experiments. However, the small quantity of oxygen in biodiesel results in a significant reduction in the level of PM. This reveals that the oxygen in biodiesel plays an important role on PM reduction. Wang et al. [24] investigated soot formation in a biodiesel spray flame in a constantvolume combustion chamber, and their results indicate that the oxygen in biodiesel plays a significant role in the reduction of soot levels. This phenomenon can be used to garner valuable insight into PM emission reduction using biodiesel in a diesel engine. It might be argued that sulphur content in diesel fuel may also result in high PM emission. However, the diesel fuel used in this study has a low sulphur content. Therefore, PM reduction must be mainly attributed to the oxygen in biodiesel. The strong PM reduction effect by oxygen in biodiesel can be attributed to the fact that the oxygen in biodiesel finds it easy to participate in the combustion reaction. For the oxygen taken from the air, the fuel needs to be atomized and mixed with this air if it is to take part in combustion reaction. PM emission is reduced using biodiesel, while NO x emission is increased slightly at high engine loads. PM versus NO x emission using all the data in Figs 11 and 12 is plotted in Fig. 14. At the lowest engine load (a BMEP of MPa), with increasing biodiesel fraction in blends, PM emission reduces significantly

11 Performance and emissions of a turbocharged common rail diesel engine 137 Fig. 14 PM versus NO x emission Fig. 12 PM emission and PM reduction rate whereas NO x emission reduces only slightly. In other words, PM and NO x emissions reduce simultaneously using biodiesel/diesel blends and pure biodiesel. At a middle engine load (a BMEP of MPa), PM emission clearly reduces whereas the NO x emission level remains effectively constant with increasing biodiesel fraction. At high engine loads, PM emission decreases significantly whereas the NO x emission increases slightly as biodiesel fraction is increased. The significant reduction in PM emission indicates that the biodiesel/diesel blends (and biodiesel) contain oxygen and thus have a high exhaust gas recirculation (EGR) tolerance. This suggests that the combination of biodiesel/diesel blends and EGR could allow the simultaneous reduction of PM and NO x emissions. Fig. 13 PM emission versus excess air ratio 4 CONCLUSIONS Performance, combustion, and emissions of a highpressure common rail, turbocharged diesel engine fuelled with biodiesel/diesel blends as well as neat diesel and biodiesel have been investigated. The main conclusions that can be drawn from this work can be summarized as follows. 1. The brake thermal efficiency increases slightly as biodiesel is added to diesel. The exhaust gas temperature varies only to a small extent among the different fuels at low engine speeds, but decreases with an increase in biodiesel fraction in the blends at high engine speeds. 2. Biodiesel gives a low heat release rate at pilot injection and high heat release rate at main injection.

12 138 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu 3. HC and CO emissions vary only to a small extent among the different fuels. The level of NO x emission decreases slightly at low engine loads and increases at high engine speeds for biodiesel and biodiesel/diesel blends. Biodiesel and biodiesel/diesel blends significantly decrease PM emission. 4. The oxygen in biodiesel plays a key role in reducing PM emission. The combination of biodiesel/diesel blends and EGR could allow the simultaneous reduction of PM and NO x emissions. ACKNOWLEDGEMENTS This work was supported by the National Natural Foundation of China ( ). Technical support from Great Wall Motor Company Ltd is gratefully acknowledged. F Authors 2011 REFERENCES 1 Senatore, A., Cardone, M., and Buono, D. Combustion study of a common rail diesel engine optimized to be fueled with biodiesel. Energy Fuels, 2008, 22, Monyem, A. and Gerpen, J. H. V. The effect of biodiesel oxidation on engine performance and emissions. Biomass Bioenergy, 2001, 20, Canakci, M., Erdil, A., and Arcaklioglu, E. Performance and exhaust emissions of a biodiesel engine. Appl. Energy, 2006, 83, Nabi, N., Akhter, S., and Shahadat, Z. Improvement of engine emissions with conventional diesel fuel and diesel biodiesel blends. Bioresource Technol., 2006, 97(3), Laforgia, D. and Ardito, V. Biodiesel fueled IDI engines performances, emissions and heat release investigation. Bioresource Technol., 1995, 51(1), Schumacher, L. G., Borgelt, S. C., Fosseen, D., Goetz, W., and Hires, W. G. Heavy-duty engine exhaust emission tests using methyl ester soybean oil/diesel fuel blends. Bioresource Technol., 1996, 57(1), Lapuerta, M., Armas, O., and Rodriguez-Fernandez, J. Effect of biodiesel fuels on diesel engine emissions. Prog. Energy Combust. Sci., 2008, 34, Agarwal, A. K. Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines. Prog. Energy Combust. Sci., 2007, 33, Sharp, C. A., Howell, S. A., and Jobe, J. The effect of biodiesel fuels on transient emissions from modern diesel engines. Part I: regulated emissions and performance. SAE paper , Senatore, A., Cardone, M., Rocco, V., and Prati, M. V. A comparative analysis of combustion process in D.I. diesel engine fueled with biodiesel and diesel fuel. SAE paper , Akasaka, Y., Suzuki, T., and Sakurai, Y. Exhaust emissions of a DI diesel engine fueled with blends biodiesel and low sulfur diesel fuel. SAE paper , Hansen, K. F. and Jensen, M. G. Chemical and biological characteristics of exhaust emissions from a DI diesel engine fueled with rapeseed oil methyl ester (RME). SAE paper , McCormick, R. L., Ross, J. D., and Graboski, M. S. Effect of oxygenates on regulated emissions from heavy-duty diesel engines. Environ. Sci. Technol., 1997, 31, Choi, C. Y., Bower, G. R., and Reitz, R. D. Effects of biodiesel blended fuels and multiple injections on D. I. diesel engines. SAE paper , McCormick, R. L., Alvarez, J. R., Graboski, M. S., Tyson, K. S., and Vertin, K. Fuel additive and blending approaches to reducing NOx emissions from biodiesel. SAE paper , Karra, P. K., Veltman, M. K., and Kong, S. C. Characteristics of engine emissions using biodiesel blends in low-temperature combustion regimes. Energy Fuels, 2008, 22, Kato, T., Tsujimura, K., Shintani, M., Minami, T., and Yamaguchi, I. Spray characteristics and combustion improvement of DI diesel engine with high pressure fuel injection. SAE paper , Yokota, H., Kamimoto, T., Kosaka, H., and Tsujimura, K. Fast burning and reduced soot formation via ultra-high pressure diesel fuel injection. SAE paper , Varde, K. S. and Watanabe, T. Characteristics of high pressure spray and exhaust emissions in a single cylinder DI diesel engine. SAE paper , Hardy, W. L. and Reitz, R. D. An experimental investigation of partially premixed combustion strategies using multiple injections in a heavy-duty diesel engine. SAE paper , Heywood, J. B. Internal combustion engine fundamentals, 1998 (McGraw-Hill, New York). 22 Tsolakis, A. Effects on particle size distribution from the diesel engine operating on RME-biodiesel with EGR. Energy Fuels, 2006, 20, Sison, K., Ladommatos, N., Song, H., and Zhao, H. Soot generation of diesel fuels with substantial amounts of oxygen-bearing compounds added. Fuel, 2007, 86, Wang, X. G., Kuti, O. A., Zhang, W., Nishida, K., and Huang, Z. H. Effect of injection pressure on flame and soot characteristics of the biodiesel fuel spray. Combust. Sci. Technol., 2010, 182, 1 22.

13 Performance and emissions of a turbocharged common rail diesel engine Ozsezen, A. N., Canakci, M., and Sayin, C. Effects of biodiesel from used frying palm oil on the exhaust emissions of an indirect injection (IDI) diesel engine. Energy Fuels, 2008, 22, Kegl, B. and Hribernik, A. Experimental analysis of injection characteristics using biodiesel fuel. Energy Fuels, 2006, 20, Wang, Y., Zhou, L. B., and Wang, H. W. Diesel emission improvements by the use of oxygenated DME/diesel blend fuels. Atmos. Environ., 2006, 40, Song, J., Cheenkachorn, K., Wang, J., Perez, J., and Boehman, A. L. Effect of oxygenated fuel on combustion and emissions in a light-duty turbodiesel engine. Energy Fuels, 2002, 16, Dec, J. E. A conceptual model of DI diesel combustion based on laser-sheet imaging. SAE paper , 1997.