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Influences of pilot injection and exhaust gas recirculation (EGR) on combustion and emissions in a HCCI-DI combustion engine
Applied Thermal Engineering 48 (2012) 97e104
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Influences of pilot injection and exhaust gas recirculation (EGR) on combustion and emissions in a HCCI-DI combustion engine Qiang Fang, Junhua Fang, Jian Zhuang, Zhen Huang* Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2011 Accepted 11 March 2012 Available online 21 March 2012
The HCCI-DI combustion mode was achieved in a heavy-duty diesel engine using the early pilot injection in the intake stroke and the main injection around compression top dead center (TDC). The effects of pilot injection quantity and EGR rate on HCCI-DI combustion and emissions were investigated. NOx emission has a dramatically decrease as the pilot injection quantity increases, but it is still in a high level that needs to be reduced. The smoke emission has a slight increase with the rise of pilot quantity, but it is always in a low level. The increasing HC and CO emissions can be easily removed by the diesel oxidation catalyst (DOC). The HCCI-DI combustion with low level of EGR is an effective method to reduce NOx emission further, and smoke emission increases as EGR rate increases, but it is in an acceptable level. The HCCI-DI combustion mode operating range is limited by the peak of cylinder pressure, the pressure rise rate and the cycle-to-cycle variations. There are optimal EGR rates and pilot quantities to achieve low NOx and low smoke emissions. Ó 2012 Published by Elsevier Ltd.
Keywords: HCCI-DI Combustion Emissions Pilot injection quantity EGR
1. Introduction Under the influence of increasingly stringent emission regulations, the new combustion modes were investigated to simultaneously reduce NOx and soot emissions in diesel engine. Homogeneous Charge Compression Ignition (HCCI) is a promising alternative combustion technology with high efficiency and low NOx and soot emissions. Many studies of HCCI combustion show a potential for very low NOx and PM emissions [1e3]. However, there are several problems to be solved before the commercial application in automotive. Especially, it is difficult to control the ignition timing and extend the load range of HCCI combustion [4]. The conventional DI combustion doesn’t have uncontrollable
Abbreviations: HCCI, homogeneous charge compression ignition; PCCI, premixed charge compression ignition; DI, direct injection; CIDI, compression ignition direct injection; TDC, top dead center; BTDC, before top dead center; BSFC, brake specific fuel consumption; IMEP, indicated mean effective pressure; SOC, start of combustion; CoVIMEP, coefficient of variation of IMEP; CO, carbon monoxide; PM, particulate matter; NOx, nitrogen oxides; HC, hydrocarbon; EGR, exhaust gas recirculation; BMEP, brake mean effective pressure; DOC, diesel oxidation catalyst; CA, crank angle; CoVppeak, coefficient of variation of the peak of cylinder pressure. * Corresponding author. 502, Building A, School of Mechanical Engineering, Shanghai Jiao Tong University, 800, Dong Chuan road, Shanghai, China. Tel.: þ86 (0) 21 34206859; fax: þ86 (0) 21 34205553. E-mail address: [email protected] (Z. Huang). 1359-4311/$ e see front matter Ó 2012 Published by Elsevier Ltd. doi:10.1016/j.applthermaleng.2012.03.021
ignition timing and small load range problems, but the NOx and PM emissions need to be reduced. The HCCI-DI or PCCI-DI concept could be considered as a compromise between HCCI combustion and conventional CIDI combustion. Several studies have revealed the advantages and the disadvantages of similar combined combustion mode. Shakal and Martin studied the effect of auxiliary fuel injection (pilot, manifold, and port injection) on emissions and combustion in a two-stroke diesel engine [5]. They found a decrease in NOx and increases in HC and smoke. Osses et al. performed an experimental study of the potential of diesel fumigation partial premixing to reduce the soot fraction of PM emissions on a naturally aspirated DI diesel engine [6]. They reported improved soot and NOx emissions with up to 20% port fumigation, but increased fuel consumption, CO, HC and volatile organic fraction of the PM emissions. Simescu et al. conducted an experimental investigation of PCCI-DI combustion coupled with cooled and uncooled EGR in a heavy-duty diesel engine [7]. The study showed significant NOx reductions at light load conditions with up to 20% port fuel injection (PFI). The study however showed that early PCCI combustion could adversely affect NOx emissions by increasing in-cylinder temperatures at the start of diffusion combustion. The PCCI-DI combustion also showed increased brake specific fuel consumption (BSFC) and HC, CO, and PM emissions. Kim et al. investigated the effects of premixed gasoline fuel and direct injection timing on partial HCCI [8,9]. Ma et al. carried out an experimental study of HCCI-DI combustion and
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emissions in a diesel engine with dual fuel [10]. They found that NOx emission decreased dramatically when premixed rate was low and HCCI-DI could effectively improve the thermal efficiency at low and medium loads. Wang et al. added another fuel system in air inlet pipe in diesel engine fueled with dimethyl ether (DME) [11]. The results showed that the HCCI-DI combustion mode could also be achieved in the DME engine. However, all these experiments need another fuel supply system to form premix fuel. The diesel engine with common-rail system using multiple injection strategy to control NOx and PM emissions attracts more and more attention. Nehmer et al. investigated the effect of rateshaped and split injections on soot and NOx emissions in a heavy-duty diesel engine with an electronically-controlled, highpressure common-rail injection system [12]. The results showed that rate-shaped injection didn’t appreciably affect pressure rise. Split injections allowed peak pressure to be reduced and NOx emission also had a decrease trend without increase of PM. Yokota et al. performed a combined experimental and computational study of homogenous charge intelligent multiple injection combustion system (HiMICS), in which the quasi-homogeneous mixture was generated by very early, direct injection [13]. They also showed the potential to improve both NOx and PM emissions over some operating conditions, at the expense of significant increases in HC emissions. Park et al. investigated the effect of pilot, post- and multiple-fuel injection strategies on engine performance and emissions [14]. They found that the pilot-injection reduced the ignition delay of main injection and the postinjection was effective to reduce PM emission. A.P. Carlucci et al. tested the effects of several injection parameters of multiple injection strategy in a direct injection diesel engine [15]. The results showed that NOx and soot both decreased performing the early and pilot before the main injection, but UHC levels remained constant. Okude et al. studied the effects of pilot injection fuel quantity and pilot injection timing on diesel emissions and combustion [16]. Lee et al. investigated the single-pilot injection and double-pilot injection strategies with a wide injection timing range, various injection quantity ratios, and various dwell times [17]. The results showed that single-pilot injection resulted in a dramatic reduction in NOx and smoke emissions when the pilot injection was advanced over 40 CA before the start of main injection and the double-pilot injection could improve the HC emission. The diesel engine with common-rail system using multiple injection strategy to achieve the HCCI-DI combustion is another trial. Based on previous studies that have investigated and characterized multiple injection strategies, the timing of multiple injection, the quantity of fuel injected, and the dwell time between each injection are the main parameters to take into account when attempting to reduce the emission of NOx and PM. Nevertheless, there is still room for further studies of multiple injection strategies, owing to the very high degree of freedom of fuel injection schedules offered by the common-rail system. Furthermore, there are few analyses about the effect of EGR on HCCI-DI combustion and emissions with multiple injection strategies. The objective of this study was to achieve the HCCI-DI combustion using a very early pilot injection in intake stroke and the main injection around compression top dead center (TDC). The effects of EGR and pilot injection quantity on HCCI-DI combustion and emissions were investigated. 2. Experimental setup 2.1. Experimental engine and apparatus The engine used in this study was a turbocharged, four-cylinder, and four-stroke heavy-duty diesel engine equipped with common
rail injection system. The main engine specifications are listed in Table 1. The schematic diagram of the experimental setup is shown in Fig. 1. The common-rail system allowed for the variation of rail pressure (up to 160 MPa), timing (360e360 CA), and number of injections. The fuel injection timing was controlled by a magnetic sensor mounted on camshaft. The cylinder pressure was measured with a pressure transducer (Model 6125B). The charger output from this transducer was converted to an amplified voltage with an amplifier. A magnetic sensor mounted on flywheel was used as the clocking pluses to acquire the cylinder pressure data. The cylinder pressure was recorded at every 0.5 crank angle (CA) using the instrument of engine combustion analysis (Osiris). For each measuring point, the pressure data of 200 consecutive cycles were sampled and recorded. The pressure trace for a specific condition was obtained by averaging the sampled pressure data. The exhaust emissions were measured by an AVL CEB gas analyzer. The smoke opacity was measured using an AVL 439 Opacimeter analyzer. The heat release rate and the mean gas temperature were calculated using the first-law heat-release model. 2.2. Experimental method In this study, double injection technique was applied in DI diesel engine using common-rail injection system. The following strategy was examined in order to achieve the HCCI-DI combustion. The pilot fuel was injected in the intake stroke of engine cycle to form the premixed fuel-air mixing. The pilot injection timing and pilot injection quantity were controlled to achieve HCCI combustion. The main injection fuel was injected around compression TDC to control the ignition timing. The main injection timing was varied to optimize the emissions and efficiency. The test conditions are summarized in Table 2. For all data presented, 0 CA is defined as the top dead center (TDC) at compression stroke. To ensure the repeatability and comparability of the measurements for operating conditions, the temperatures of intake air, oil and coolant water were held accurately stable during the experiments. The engine speed was kept at 1450 rpm during this experiment. The injection pressures were 80 MPa for 0.15 MPa and 0.3 MPa BMEP, 85 MPa for 0.45 MPa and 0.6 MPa BMEP. The injection pressures were kept constant at the same load. The intake pressures were 116 kPa, 120 kPa, 128 kPa and 140 kPa at four loads without EGR and pilot injection, respectively. The intake pressures were influenced by the EGR and pilot injection. All the emissions were continuously measured for 3 min and the average results presented here. Each test was repeated twice to ensure that the results are repeatable within the experimental uncertainties. 3. Results and discuss 3.1. Effect of pilot injection on HCCI-DI combustion and emissions The effect of pilot injection quantity on engine emissions and performance were investigated over a range of engine speeds and Table 1 Engine specifications. Parameter
Value
Number of cylinders Bore/Stroke (mm/mm) Compression ratio Connecting rod (mm) Displacement (L) Nozzle number orifice diameter (mm) Swirl ratio Fuel injection system
4 110/125 16.8:1 195 4.751 7 0.16 1.5 Common rail
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Fig. 1. Schematic of engine with common rail system.
loads. The speed, load and main injection timing were held constant for each pilot injection quantity sweep and the EGR rate was zero. The pilot injection quantity is defined the amount of one cycle from one injector. Fig. 2 shows the observed changes in-cylinder pressure and heat release rate as the pilot quantity increases from 0 to 10 mm3 at 0.3 MPa and 0.6 MPa BMEP with pilot injection timing at 340 CA BTDC. The ratios of pilot injection quantity to total injection quantity from 0 to 45% are in the brackets. The effects of pilot quantity on cylinder pressure and heat release rate are quite similar for different loads. With the rise of pilot quantity, there is a corresponding increase in the maximum cylinder pressure. That is because the increasing HCCI combustion results in higher temperature at start of combustion (SOC) as the pilot quantity increases. It is found that the heat release rate with pilot injection consists of the cool flame stage, the thermal flame stage of HCCI combustion and diffusion combustion of CIDI combustion in Fig. 2a. This is thought to achieve HCCI-DI combustion with pilot injection timing at 340 CA BTDC [10,18]. The premixed fuel injected into cylinder in intake stroke enters the cool flame combustion approximately 30 CA BTDC for all pilot quantity. It is noted that pilot quantity had little impact on ignition timing of the cool flame
Table 2 Engine test conditions. Parameter
Value
Engine speed (rpm) Pilot injection timing ( CA BTDC) Pilot injection quantity (mm3) Main injection timing ( CA BTDC) Intake air temperature ( C) Coolant temperature ( C) Lubricant oil temperature ( C) EGR rate
1450 360e20 0e10 10.6e5 36 2 80 2 90 2 15%, 25%
combustion, showing the strong temperature dependence of the start of cool flame reactions [19,20]. The peak of the cool flame regime increases slightly as the pilot quantity increases. That is because the reaction rates are proportional with the fuel concentration. The thermal flame reaction of HCCI combustion is also observed with presence of the pilot injection and starts at approximately 18 CA BTDC. The start of thermal flame combustion is found to be advanced and the maximum heat release rate of thermal flame combustion increases with the rise of pilot quantity. When the pilot quantity increases up to certain value, the thermal flame combustion starts early even before the end of cool flame combustion. That is possible that the thermal flame combustion and the cool flame combustion occur simultaneously because of high temperature. The ignition timing of direct injected fuel is changed slightly as the pilot quantity increases because the ignition delay of the direct fuel is minimal which is generated from the incylinder mean temperature increases due to the HCCI combustion of pilot injection. The similar results were found in [10,18]. The peak of heat release rate of the diffusion combustion decreases as the pilot quantity increases. Fig. 3 shows the observed changes in emissions as a function of pilot injection quantity for four different loads with pilot injection timing at 340 CA BTDC and main injection timing at 4.4 CA BTDC. Smoke emission increases with the rise of pilot quantity when pilot quantity is smaller than 8 mm3, and then has a little drop when pilot quantity is bigger than 8 mm3, as shown in Fig. 3a. However, the smoke emission is always in a low level. The increasing smoke is because the increase of the pilot quantity would have consumed more in-cylinder oxygen, and thus less oxygen is available and results in increased smoke. Another reason could be the short mixing time for the diesel fuel of the main injection. NOx emission shows the similar trends in all loads, as shown in Fig. 3b. NOx emission decreases with the rise of pilot quantity and then increases as the pilot quantity is bigger than certain value. This is because of the trade-off relationship between the reduction of
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Fig. 2. Effect of pilot injection quantity on cylinder pressure and heat release rate. a) 0.3 MPa BMEP; b) 0.6 MPa BMEP.
NOx during HCCI combustion and the increase of NOx during diffusive combustion. On one hand, the HCCI combustion produces very little NOx and this part increases with the rise of pilot quantity, which is positive to reduce NOx emission. On the other hand, the increasing HCCI combustion results in higher temperature at start of diffusion combustion, which is negative to the reduce NOx emission. Therefore, there are optimal pilot quantity values to get low NOx emissions for different loads. However, the NOx emission is also in a high level that needs to be reduced further. The effect of pilot quantity on HC and CO emissions at four different loads are presented in Fig. 3c and d. HC and CO emissions increase as the pilot quantity increases. HC and CO emissions are major problems of HCCI combustion. In general, it is widely accepted that HC and CO emissions increase with the rise of percentage of HCCI combustion due to incomplete combustion. Another reason is possible due to the impingement of fuel against the cylinder wall and was also observed by Okude et al. [16,17].
3.2. HCCI-DI combustion with EGR 3.2.1. Combustion characteristic Fig. 4 shows that the effects of EGR and pilot quantity on the incylinder pressure and heat release rate of HCCI-DI combustion at 0.3 MPa and 0.6 MPa BMEP with pilot injection timing at 340 CA BTDC. It can be seen that the ignition timing of diffusion combustion stage with 15% and 25% EGR is plainly later than that of w/o EGR in the same pilot quantity. Because of the reduction in the oxygen content available for combustion and the increase in the specific heat capacity of the gas mixture in the cylinder, the ignition delay becomes longer with EGR [21]. However, the diffusion combustion is advanced with the rise of pilot quantity because the cylinder temperature increases due to the HCCI combustion of pilot injection. The effect of EGR on retarding the ignition timing of the diffusion combustion weakens in high load. The maximum pressure of HCCI-DI combustion with EGR is obviously lower than that
Fig. 3. Effect of pilot injection quantity on emissions. a) Smoke; b) NOx; c) HC; d) CO.
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Fig. 4. Effect of EGR and Pilot injection on the pressure and heat release rate. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.
of w/o EGR. Owing to the later combustion of the EGR case, the combustion is away from the TDC, leading to a reduction in the incylinder temperature. Meanwhile, the increase in the specific heat capacity of gas mixture in the cylinder also results in lower temperature. 3.2.2. Emission characteristic The effects of EGR and pilot injection on NOx emission of HCCIDI combustion are shown in Fig. 5. NOx emission increases obviously with increasing engine load, because of the higher combustion temperature at high loads. NOx emission is greatly reduced with 15% EGR and 25% EGR compared with that of w/o EGR at two loads. It is believed that the major factors affecting NOx formation are the combustion temperature, the local oxygen concentration, and the residence time in the high-temperature zone [22]. NOx emission exhibits the expected trends, because of the lower temperature and decreasing oxygen concentration with increasing EGR level. Moreover, NOx emission is also reduced by using the pilot injection, which has already studied in pilot quantity sweep. The effects of EGR and pilot injection on smoke opacity of HCCIDI combustion are shown in Fig. 6.The smoke opacity increases obviously with increasing engine load, because more fuel is injected and burned in the diffusion mode. Smoke emission increases slightly with the presence of EGR rate. The combustion temperatures decrease due to the lower oxygen concentration and higher heat capacity of the work gas. Smoke opacity increases firstly and
then decreases as main injection timing is retarded. However, when the pilot injection is applied, smoke opacity increases monotonically as main injection timing is retarded. This is possible that the increasing HCCI combustion results in higher temperature, leading to shorter ignition delay, higher smoke emission. In this paper, NOx emission and the smoke opacity are limited below 120 ppm and 0.5 m1, which is thought to achieve low NOx and low smoke combustion. 3.2.3. Brake specific fuel consumption (BSFC) The effects of EGR and pilot quantity on BSFC are shown in Fig. 7. It can be found that BSFC increases with the rise of pilot quantity. The main factors contributing to the increasing BSFC are as follows. First, the off-phasing of combustion process and the negative work due to split combustion stage are the main reasons. Second, the significant increase in HC and CO emission observed is indicative of fuel energy losses due to incomplete combustion. Moreover, the pilot fuel injected in the intake stroke is possible to wet the cylinder liner because lower cylinder pressure allows to the long spray distance. So the proper pilot quantity is needed and the fuel consumption is a limit to higher pilot quantity. Therefore, more complete oxidation of the HC and CO is desired, not only for low emission, but also for low fuel consumption. It also found that BSFC has a slight increase with the rise of EGR, which is mainly due to offphasing of the combustion process and decrease of combustion efficiency because of incomplete combustion [23,24].
Fig. 5. Effect of EGR and Pilot injection on NOx emissions. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.
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Fig. 6. Effect of EGR and Pilot injection on smoke opacity. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.
3.2.4. The peak of pressure and pressure rise rate The maximum pressure rise rate is usually adopted as an index to describe the intensity of combustion roughness. In this paper, the “knock combustion” is defined as the maximum value exceeds 1.0 MPa/ CA. Fig. 8 shows the effects of EGR and pilot injection on the peak of pressure and the maximum pressure rise rate. It can be found that the peak of pressure and the maximum pressure rise rate are lower with EGR than that of w/o EGR in all pilot quantity. That is because of lower in-cylinder temperature and lower in-cylinder pressure when EGR is applied [21]. It can be also found that the peak of pressure increases with the rise of pilot quantity. The maximum pressure rise rate displays a decrease as pilot quantity increases. This trend is similar to that of Ma et al. [10]. So the pilot quantity is limited to control the peak of cylinder pressure and pressure rise rate for low noise and stable engine operation. 3.2.5. Cycle-to-cycle variation In this paper, the cycle-to-cycle variations are defined as follow:
CoVppeak ¼ sppeak =ppeak CoVIMEP ¼ sIMEP =IMEP Where CoVppeak and CoVIMEP represent the coefficient of variation of the peak of cylinder pressure and the indicated mean effective pressure (IMEP) in 200 cycles; sppeak and sIMEP are the standard deviations of the peak of cylinder pressure and the IMEP in 200 cycles; ppeak and IMEP are the average values of the peak of cylinder pressure and the IMEP in 200 cycles. Fig. 9 shows the effects of EGR and pilot quantity on the cycleto-cycle variations of maximum gas pressure and IMEP. The coefficient of variation of IMEP is bigger than the coefficient of variation of the peak of cylinder pressure, so CoVIMEP is the main limit to the engine stable. The coefficient of variation of the peak of cylinder pressure has a slight increase when the pilot quantity increases. The coefficients of variation of the IMEP decrease firstly and then increase with the rise of pilot quantity. Therefore, there are the optimum pilot quantity values for the stable combustion of engine
Fig. 7. Effect of EGR and Pilot injection on fuel efficiency. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.
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Fig. 8. Effect of EGR and Pilot injection on the peak of pressure and maximum pressure rise rate. a) peak of pressure, b) maximum pressure rise rate.
Fig. 9. Effect of EGR and pilot quantity on the COVppeak and CoVIMEP. a) COVppeak; b) CoVIMEP.
at different loads. And the coefficient of variation of the peak of cylinder pressure and the IMEP are the limits of the HCCI-DI combustion.
4. Conclusion (1) HCCI-DI combustion mode is achieved using the very early pilot injection in the intake stroke and the main injection around compression TDC. The heat release rate of HCCI-DI combustion exhibits three stages: the cool flame combustion, the thermal flame combustion and the diffusion combustion. (2) NOx emission decreases with rise of pilot quantity. After a certain pilot quantity, NOx emission has a slight increase. Smoke is always in a low level. CO and HC emissions increase
monotonously as pilot quantity increases, which is because of incomplete combustion. (3) NOx emission is reduced greatly with low level of EGR than that of w/o EGR, but smoke opacity emission increases. NOx emission and smoke emission are limited below 120 ppm and 0.5 m1, which is thought to achieve low NOx and smoke emission combustion. (4) Fuel consumption increases slightly as EGR and pilot quantity increase, but the increase is very small. (5) The peak of cylinder pressure and the pressure rise rate increase slightly as pilot quantity increase. The cycle-to-cycle variation decreases with the rise of pilot quantity. When the pilot quantity increase further, the cycle-to-cycle variation starts to increase. But the cycle-to-cycle variations have an obviously decrease when EGR is added into.
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(6) HCCI-DI combustion is limited by the peak of cylinder pressure, the pressure rise rate and the cycle-to-cycle variation. There are optimal EGR rates and pilot quantity values for low emissions and low fuel consumptions. Acknowledgements This work is supported by Centre for Combustion and Environmental Technology of Shanghai Jiao Tong University. References [1] M. Christensen, B. Johansson, P. Amneus, F. Mauss, Supercharged homogeneous charge compression ignition, SAE paper No. 980787. [2] R.H. Stanglmaier, C.E. Roberts, Homogeneous charge compression ignition (HCCI): benefits compromises, and future engine applications, SAE paper No. 1999-01-3682. [3] H. Suzuki, N. Koike, H. Ishii, M. Odaka, Exhaust purification of diesel engines by homogeneous charge with compression ignition part1: experimental investigation of combustion and exhaust emission behavior under pre-mixed homogeneous charge compression ignition method, SAE Paper No. 970313. [4] M.F. Yao, Z.L. Zheng, H.F. Liu, Progress and recent trends in homogeneous charge compression ignition (HCCI) engines, Prog. Energ. Combust. Sci. 35 (2009) 398e437. [5] J. Shakal, J.K. Martin, Effects of auxiliary injection on diesel engine combustion, SAE paper No. 900398. [6] M. Osses, G.E. Andrews, J. Greenhough, Diesel fumigation partial premixing for red particulate soot fraction emissions, SAE paper No. 980532. [7] S. Simescu, T.W. Ryan, G.D. Neely, A.C. Matheaus, B. Surampudi, Partial premixed combustion with cooled and uncooled EGR in a heavy-duty diesel engine, SAE Paper No. 2002-01-0963. [8] D.S. Kim, M.Y. Kim, C.S. Lee, Effect of premixed gasoline fuel on the combustion characteristics of compression ignition engine, Energy Fuels 18 (2004) 1213e1219. [9] D.S. Kim, M.Y. Kim, C.S. Lee, Combustion and emission characteristics of a partial homogeneous charge compression ignition engine when using twostage injection, Combust. Sci. Technol. 179 (2007) 531e551.
[10] J.J. Ma, X.C. Lü, L.B. Ji, Z. Huang, An experimental study of HCCI-DI combustion and emissions in a diesel engine with dual fuel, Int. J. Thermal Sci. 47 (2008) 1235e1242. [11] Y. Wang, L. He, J. Zhou, L.B. Zhou, Study of HCCI-DI combustion and emissions in a DME engine, Fuel 88 (2009) 2255e2261. [12] D.A. Nehmer, R.D. Reitz, Measurement of the effect of injection rate and split injections on diesel engine soot and NOx emissions, SAE paper No. 940668. [13] H. Yokota, Y. Kudo, H. Nakajima, T. Kakegawa, T. Suzuki, A new concept for low emission diesel combustion, SAE paper No. 970891. [14] C. Park, S. Kook, C. Bae, Effects of multiple injections in a HSDI diesel engine equipped with common-rail injection system, SAE Paper No. 2004-01-0127. [15] A.P. Carlucci, A. Ficarella, D. Laforgia, Control of the combustion behaviour in a diesel engine using early injection and gas addition, Appl. Therm. Eng. 26 (2006) 2279e2286. [16] K. Okude, K. Mori, S. Shiino, K. Yamada, Y. Matsumoto, Effects of multiple injections on diesel emissions and combustion characteristics, SAE paper No. 2007-01-4178. [17] Jinwoo Lee, Jinwoog Jeon, Jungseo Park, Choongsik Bae, Effect of multiple injection strategies on emission and combustion characteristics in a single cylinder direct-injection optical engine. SAE paper No. 2009-01-1354 (2009). [18] S. Simescu, B.F. Scott, G.D. Lee, An experimental investigation of PCCI-DI combustion and emissions in a heavy-duty diesel engine, SAE paper No. 2003-01-0345. [19] X.C. Lü, W. Chen, Z. Huang, A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR, part 1: the basic characteristics of HCCI combustion, Fuel 84 (2005) 1074e1083. [20] X.C. Lü, W. Chen, Z. Huang, A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR, part 2: effect of operating conditions and EGR on HCCI combustion, Fuel 84 (2005) 1084e1092. [21] J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, Inc., 1988. [22] H. Teng, J.C. McCandless, Can heavy-duty diesel engines fueled with DME meet US 2007/2010 emissions standard with a simplified after treatment system?, SAE paper No. 2006-01-0053. [23] M. Zheng, Y.Y. Tan, M.C. Mulenga, M.P. Wang, Thermal efficiency analyses of diesel low temperature combustion cycles, SAE paper No. 2007-01-4019. [24] R. Kumar, M. Zheng, Fuel efficiency improvements of low temperature combustion diesel engines, SAE paper No. 2008-01-0841.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Influences of pilot injection and exhaust gas recirculation (EGR) on combustion and emissions in a HCCI-DI combustion engine Qiang Fang, Junhua Fang, Jian Zhuang, Zhen Huang* Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2011 Accepted 11 March 2012 Available online 21 March 2012
The HCCI-DI combustion mode was achieved in a heavy-duty diesel engine using the early pilot injection in the intake stroke and the main injection around compression top dead center (TDC). The effects of pilot injection quantity and EGR rate on HCCI-DI combustion and emissions were investigated. NOx emission has a dramatically decrease as the pilot injection quantity increases, but it is still in a high level that needs to be reduced. The smoke emission has a slight increase with the rise of pilot quantity, but it is always in a low level. The increasing HC and CO emissions can be easily removed by the diesel oxidation catalyst (DOC). The HCCI-DI combustion with low level of EGR is an effective method to reduce NOx emission further, and smoke emission increases as EGR rate increases, but it is in an acceptable level. The HCCI-DI combustion mode operating range is limited by the peak of cylinder pressure, the pressure rise rate and the cycle-to-cycle variations. There are optimal EGR rates and pilot quantities to achieve low NOx and low smoke emissions. Ó 2012 Published by Elsevier Ltd.
Keywords: HCCI-DI Combustion Emissions Pilot injection quantity EGR
1. Introduction Under the influence of increasingly stringent emission regulations, the new combustion modes were investigated to simultaneously reduce NOx and soot emissions in diesel engine. Homogeneous Charge Compression Ignition (HCCI) is a promising alternative combustion technology with high efficiency and low NOx and soot emissions. Many studies of HCCI combustion show a potential for very low NOx and PM emissions [1e3]. However, there are several problems to be solved before the commercial application in automotive. Especially, it is difficult to control the ignition timing and extend the load range of HCCI combustion [4]. The conventional DI combustion doesn’t have uncontrollable
Abbreviations: HCCI, homogeneous charge compression ignition; PCCI, premixed charge compression ignition; DI, direct injection; CIDI, compression ignition direct injection; TDC, top dead center; BTDC, before top dead center; BSFC, brake specific fuel consumption; IMEP, indicated mean effective pressure; SOC, start of combustion; CoVIMEP, coefficient of variation of IMEP; CO, carbon monoxide; PM, particulate matter; NOx, nitrogen oxides; HC, hydrocarbon; EGR, exhaust gas recirculation; BMEP, brake mean effective pressure; DOC, diesel oxidation catalyst; CA, crank angle; CoVppeak, coefficient of variation of the peak of cylinder pressure. * Corresponding author. 502, Building A, School of Mechanical Engineering, Shanghai Jiao Tong University, 800, Dong Chuan road, Shanghai, China. Tel.: þ86 (0) 21 34206859; fax: þ86 (0) 21 34205553. E-mail address: [email protected] (Z. Huang). 1359-4311/$ e see front matter Ó 2012 Published by Elsevier Ltd. doi:10.1016/j.applthermaleng.2012.03.021
ignition timing and small load range problems, but the NOx and PM emissions need to be reduced. The HCCI-DI or PCCI-DI concept could be considered as a compromise between HCCI combustion and conventional CIDI combustion. Several studies have revealed the advantages and the disadvantages of similar combined combustion mode. Shakal and Martin studied the effect of auxiliary fuel injection (pilot, manifold, and port injection) on emissions and combustion in a two-stroke diesel engine [5]. They found a decrease in NOx and increases in HC and smoke. Osses et al. performed an experimental study of the potential of diesel fumigation partial premixing to reduce the soot fraction of PM emissions on a naturally aspirated DI diesel engine [6]. They reported improved soot and NOx emissions with up to 20% port fumigation, but increased fuel consumption, CO, HC and volatile organic fraction of the PM emissions. Simescu et al. conducted an experimental investigation of PCCI-DI combustion coupled with cooled and uncooled EGR in a heavy-duty diesel engine [7]. The study showed significant NOx reductions at light load conditions with up to 20% port fuel injection (PFI). The study however showed that early PCCI combustion could adversely affect NOx emissions by increasing in-cylinder temperatures at the start of diffusion combustion. The PCCI-DI combustion also showed increased brake specific fuel consumption (BSFC) and HC, CO, and PM emissions. Kim et al. investigated the effects of premixed gasoline fuel and direct injection timing on partial HCCI [8,9]. Ma et al. carried out an experimental study of HCCI-DI combustion and
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emissions in a diesel engine with dual fuel [10]. They found that NOx emission decreased dramatically when premixed rate was low and HCCI-DI could effectively improve the thermal efficiency at low and medium loads. Wang et al. added another fuel system in air inlet pipe in diesel engine fueled with dimethyl ether (DME) [11]. The results showed that the HCCI-DI combustion mode could also be achieved in the DME engine. However, all these experiments need another fuel supply system to form premix fuel. The diesel engine with common-rail system using multiple injection strategy to control NOx and PM emissions attracts more and more attention. Nehmer et al. investigated the effect of rateshaped and split injections on soot and NOx emissions in a heavy-duty diesel engine with an electronically-controlled, highpressure common-rail injection system [12]. The results showed that rate-shaped injection didn’t appreciably affect pressure rise. Split injections allowed peak pressure to be reduced and NOx emission also had a decrease trend without increase of PM. Yokota et al. performed a combined experimental and computational study of homogenous charge intelligent multiple injection combustion system (HiMICS), in which the quasi-homogeneous mixture was generated by very early, direct injection [13]. They also showed the potential to improve both NOx and PM emissions over some operating conditions, at the expense of significant increases in HC emissions. Park et al. investigated the effect of pilot, post- and multiple-fuel injection strategies on engine performance and emissions [14]. They found that the pilot-injection reduced the ignition delay of main injection and the postinjection was effective to reduce PM emission. A.P. Carlucci et al. tested the effects of several injection parameters of multiple injection strategy in a direct injection diesel engine [15]. The results showed that NOx and soot both decreased performing the early and pilot before the main injection, but UHC levels remained constant. Okude et al. studied the effects of pilot injection fuel quantity and pilot injection timing on diesel emissions and combustion [16]. Lee et al. investigated the single-pilot injection and double-pilot injection strategies with a wide injection timing range, various injection quantity ratios, and various dwell times [17]. The results showed that single-pilot injection resulted in a dramatic reduction in NOx and smoke emissions when the pilot injection was advanced over 40 CA before the start of main injection and the double-pilot injection could improve the HC emission. The diesel engine with common-rail system using multiple injection strategy to achieve the HCCI-DI combustion is another trial. Based on previous studies that have investigated and characterized multiple injection strategies, the timing of multiple injection, the quantity of fuel injected, and the dwell time between each injection are the main parameters to take into account when attempting to reduce the emission of NOx and PM. Nevertheless, there is still room for further studies of multiple injection strategies, owing to the very high degree of freedom of fuel injection schedules offered by the common-rail system. Furthermore, there are few analyses about the effect of EGR on HCCI-DI combustion and emissions with multiple injection strategies. The objective of this study was to achieve the HCCI-DI combustion using a very early pilot injection in intake stroke and the main injection around compression top dead center (TDC). The effects of EGR and pilot injection quantity on HCCI-DI combustion and emissions were investigated. 2. Experimental setup 2.1. Experimental engine and apparatus The engine used in this study was a turbocharged, four-cylinder, and four-stroke heavy-duty diesel engine equipped with common
rail injection system. The main engine specifications are listed in Table 1. The schematic diagram of the experimental setup is shown in Fig. 1. The common-rail system allowed for the variation of rail pressure (up to 160 MPa), timing (360e360 CA), and number of injections. The fuel injection timing was controlled by a magnetic sensor mounted on camshaft. The cylinder pressure was measured with a pressure transducer (Model 6125B). The charger output from this transducer was converted to an amplified voltage with an amplifier. A magnetic sensor mounted on flywheel was used as the clocking pluses to acquire the cylinder pressure data. The cylinder pressure was recorded at every 0.5 crank angle (CA) using the instrument of engine combustion analysis (Osiris). For each measuring point, the pressure data of 200 consecutive cycles were sampled and recorded. The pressure trace for a specific condition was obtained by averaging the sampled pressure data. The exhaust emissions were measured by an AVL CEB gas analyzer. The smoke opacity was measured using an AVL 439 Opacimeter analyzer. The heat release rate and the mean gas temperature were calculated using the first-law heat-release model. 2.2. Experimental method In this study, double injection technique was applied in DI diesel engine using common-rail injection system. The following strategy was examined in order to achieve the HCCI-DI combustion. The pilot fuel was injected in the intake stroke of engine cycle to form the premixed fuel-air mixing. The pilot injection timing and pilot injection quantity were controlled to achieve HCCI combustion. The main injection fuel was injected around compression TDC to control the ignition timing. The main injection timing was varied to optimize the emissions and efficiency. The test conditions are summarized in Table 2. For all data presented, 0 CA is defined as the top dead center (TDC) at compression stroke. To ensure the repeatability and comparability of the measurements for operating conditions, the temperatures of intake air, oil and coolant water were held accurately stable during the experiments. The engine speed was kept at 1450 rpm during this experiment. The injection pressures were 80 MPa for 0.15 MPa and 0.3 MPa BMEP, 85 MPa for 0.45 MPa and 0.6 MPa BMEP. The injection pressures were kept constant at the same load. The intake pressures were 116 kPa, 120 kPa, 128 kPa and 140 kPa at four loads without EGR and pilot injection, respectively. The intake pressures were influenced by the EGR and pilot injection. All the emissions were continuously measured for 3 min and the average results presented here. Each test was repeated twice to ensure that the results are repeatable within the experimental uncertainties. 3. Results and discuss 3.1. Effect of pilot injection on HCCI-DI combustion and emissions The effect of pilot injection quantity on engine emissions and performance were investigated over a range of engine speeds and Table 1 Engine specifications. Parameter
Value
Number of cylinders Bore/Stroke (mm/mm) Compression ratio Connecting rod (mm) Displacement (L) Nozzle number orifice diameter (mm) Swirl ratio Fuel injection system
4 110/125 16.8:1 195 4.751 7 0.16 1.5 Common rail
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Fig. 1. Schematic of engine with common rail system.
loads. The speed, load and main injection timing were held constant for each pilot injection quantity sweep and the EGR rate was zero. The pilot injection quantity is defined the amount of one cycle from one injector. Fig. 2 shows the observed changes in-cylinder pressure and heat release rate as the pilot quantity increases from 0 to 10 mm3 at 0.3 MPa and 0.6 MPa BMEP with pilot injection timing at 340 CA BTDC. The ratios of pilot injection quantity to total injection quantity from 0 to 45% are in the brackets. The effects of pilot quantity on cylinder pressure and heat release rate are quite similar for different loads. With the rise of pilot quantity, there is a corresponding increase in the maximum cylinder pressure. That is because the increasing HCCI combustion results in higher temperature at start of combustion (SOC) as the pilot quantity increases. It is found that the heat release rate with pilot injection consists of the cool flame stage, the thermal flame stage of HCCI combustion and diffusion combustion of CIDI combustion in Fig. 2a. This is thought to achieve HCCI-DI combustion with pilot injection timing at 340 CA BTDC [10,18]. The premixed fuel injected into cylinder in intake stroke enters the cool flame combustion approximately 30 CA BTDC for all pilot quantity. It is noted that pilot quantity had little impact on ignition timing of the cool flame
Table 2 Engine test conditions. Parameter
Value
Engine speed (rpm) Pilot injection timing ( CA BTDC) Pilot injection quantity (mm3) Main injection timing ( CA BTDC) Intake air temperature ( C) Coolant temperature ( C) Lubricant oil temperature ( C) EGR rate
1450 360e20 0e10 10.6e5 36 2 80 2 90 2 15%, 25%
combustion, showing the strong temperature dependence of the start of cool flame reactions [19,20]. The peak of the cool flame regime increases slightly as the pilot quantity increases. That is because the reaction rates are proportional with the fuel concentration. The thermal flame reaction of HCCI combustion is also observed with presence of the pilot injection and starts at approximately 18 CA BTDC. The start of thermal flame combustion is found to be advanced and the maximum heat release rate of thermal flame combustion increases with the rise of pilot quantity. When the pilot quantity increases up to certain value, the thermal flame combustion starts early even before the end of cool flame combustion. That is possible that the thermal flame combustion and the cool flame combustion occur simultaneously because of high temperature. The ignition timing of direct injected fuel is changed slightly as the pilot quantity increases because the ignition delay of the direct fuel is minimal which is generated from the incylinder mean temperature increases due to the HCCI combustion of pilot injection. The similar results were found in [10,18]. The peak of heat release rate of the diffusion combustion decreases as the pilot quantity increases. Fig. 3 shows the observed changes in emissions as a function of pilot injection quantity for four different loads with pilot injection timing at 340 CA BTDC and main injection timing at 4.4 CA BTDC. Smoke emission increases with the rise of pilot quantity when pilot quantity is smaller than 8 mm3, and then has a little drop when pilot quantity is bigger than 8 mm3, as shown in Fig. 3a. However, the smoke emission is always in a low level. The increasing smoke is because the increase of the pilot quantity would have consumed more in-cylinder oxygen, and thus less oxygen is available and results in increased smoke. Another reason could be the short mixing time for the diesel fuel of the main injection. NOx emission shows the similar trends in all loads, as shown in Fig. 3b. NOx emission decreases with the rise of pilot quantity and then increases as the pilot quantity is bigger than certain value. This is because of the trade-off relationship between the reduction of
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Fig. 2. Effect of pilot injection quantity on cylinder pressure and heat release rate. a) 0.3 MPa BMEP; b) 0.6 MPa BMEP.
NOx during HCCI combustion and the increase of NOx during diffusive combustion. On one hand, the HCCI combustion produces very little NOx and this part increases with the rise of pilot quantity, which is positive to reduce NOx emission. On the other hand, the increasing HCCI combustion results in higher temperature at start of diffusion combustion, which is negative to the reduce NOx emission. Therefore, there are optimal pilot quantity values to get low NOx emissions for different loads. However, the NOx emission is also in a high level that needs to be reduced further. The effect of pilot quantity on HC and CO emissions at four different loads are presented in Fig. 3c and d. HC and CO emissions increase as the pilot quantity increases. HC and CO emissions are major problems of HCCI combustion. In general, it is widely accepted that HC and CO emissions increase with the rise of percentage of HCCI combustion due to incomplete combustion. Another reason is possible due to the impingement of fuel against the cylinder wall and was also observed by Okude et al. [16,17].
3.2. HCCI-DI combustion with EGR 3.2.1. Combustion characteristic Fig. 4 shows that the effects of EGR and pilot quantity on the incylinder pressure and heat release rate of HCCI-DI combustion at 0.3 MPa and 0.6 MPa BMEP with pilot injection timing at 340 CA BTDC. It can be seen that the ignition timing of diffusion combustion stage with 15% and 25% EGR is plainly later than that of w/o EGR in the same pilot quantity. Because of the reduction in the oxygen content available for combustion and the increase in the specific heat capacity of the gas mixture in the cylinder, the ignition delay becomes longer with EGR [21]. However, the diffusion combustion is advanced with the rise of pilot quantity because the cylinder temperature increases due to the HCCI combustion of pilot injection. The effect of EGR on retarding the ignition timing of the diffusion combustion weakens in high load. The maximum pressure of HCCI-DI combustion with EGR is obviously lower than that
Fig. 3. Effect of pilot injection quantity on emissions. a) Smoke; b) NOx; c) HC; d) CO.
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Fig. 4. Effect of EGR and Pilot injection on the pressure and heat release rate. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.
of w/o EGR. Owing to the later combustion of the EGR case, the combustion is away from the TDC, leading to a reduction in the incylinder temperature. Meanwhile, the increase in the specific heat capacity of gas mixture in the cylinder also results in lower temperature. 3.2.2. Emission characteristic The effects of EGR and pilot injection on NOx emission of HCCIDI combustion are shown in Fig. 5. NOx emission increases obviously with increasing engine load, because of the higher combustion temperature at high loads. NOx emission is greatly reduced with 15% EGR and 25% EGR compared with that of w/o EGR at two loads. It is believed that the major factors affecting NOx formation are the combustion temperature, the local oxygen concentration, and the residence time in the high-temperature zone [22]. NOx emission exhibits the expected trends, because of the lower temperature and decreasing oxygen concentration with increasing EGR level. Moreover, NOx emission is also reduced by using the pilot injection, which has already studied in pilot quantity sweep. The effects of EGR and pilot injection on smoke opacity of HCCIDI combustion are shown in Fig. 6.The smoke opacity increases obviously with increasing engine load, because more fuel is injected and burned in the diffusion mode. Smoke emission increases slightly with the presence of EGR rate. The combustion temperatures decrease due to the lower oxygen concentration and higher heat capacity of the work gas. Smoke opacity increases firstly and
then decreases as main injection timing is retarded. However, when the pilot injection is applied, smoke opacity increases monotonically as main injection timing is retarded. This is possible that the increasing HCCI combustion results in higher temperature, leading to shorter ignition delay, higher smoke emission. In this paper, NOx emission and the smoke opacity are limited below 120 ppm and 0.5 m1, which is thought to achieve low NOx and low smoke combustion. 3.2.3. Brake specific fuel consumption (BSFC) The effects of EGR and pilot quantity on BSFC are shown in Fig. 7. It can be found that BSFC increases with the rise of pilot quantity. The main factors contributing to the increasing BSFC are as follows. First, the off-phasing of combustion process and the negative work due to split combustion stage are the main reasons. Second, the significant increase in HC and CO emission observed is indicative of fuel energy losses due to incomplete combustion. Moreover, the pilot fuel injected in the intake stroke is possible to wet the cylinder liner because lower cylinder pressure allows to the long spray distance. So the proper pilot quantity is needed and the fuel consumption is a limit to higher pilot quantity. Therefore, more complete oxidation of the HC and CO is desired, not only for low emission, but also for low fuel consumption. It also found that BSFC has a slight increase with the rise of EGR, which is mainly due to offphasing of the combustion process and decrease of combustion efficiency because of incomplete combustion [23,24].
Fig. 5. Effect of EGR and Pilot injection on NOx emissions. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.
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Fig. 6. Effect of EGR and Pilot injection on smoke opacity. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.
3.2.4. The peak of pressure and pressure rise rate The maximum pressure rise rate is usually adopted as an index to describe the intensity of combustion roughness. In this paper, the “knock combustion” is defined as the maximum value exceeds 1.0 MPa/ CA. Fig. 8 shows the effects of EGR and pilot injection on the peak of pressure and the maximum pressure rise rate. It can be found that the peak of pressure and the maximum pressure rise rate are lower with EGR than that of w/o EGR in all pilot quantity. That is because of lower in-cylinder temperature and lower in-cylinder pressure when EGR is applied [21]. It can be also found that the peak of pressure increases with the rise of pilot quantity. The maximum pressure rise rate displays a decrease as pilot quantity increases. This trend is similar to that of Ma et al. [10]. So the pilot quantity is limited to control the peak of cylinder pressure and pressure rise rate for low noise and stable engine operation. 3.2.5. Cycle-to-cycle variation In this paper, the cycle-to-cycle variations are defined as follow:
CoVppeak ¼ sppeak =ppeak CoVIMEP ¼ sIMEP =IMEP Where CoVppeak and CoVIMEP represent the coefficient of variation of the peak of cylinder pressure and the indicated mean effective pressure (IMEP) in 200 cycles; sppeak and sIMEP are the standard deviations of the peak of cylinder pressure and the IMEP in 200 cycles; ppeak and IMEP are the average values of the peak of cylinder pressure and the IMEP in 200 cycles. Fig. 9 shows the effects of EGR and pilot quantity on the cycleto-cycle variations of maximum gas pressure and IMEP. The coefficient of variation of IMEP is bigger than the coefficient of variation of the peak of cylinder pressure, so CoVIMEP is the main limit to the engine stable. The coefficient of variation of the peak of cylinder pressure has a slight increase when the pilot quantity increases. The coefficients of variation of the IMEP decrease firstly and then increase with the rise of pilot quantity. Therefore, there are the optimum pilot quantity values for the stable combustion of engine
Fig. 7. Effect of EGR and Pilot injection on fuel efficiency. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.
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Fig. 8. Effect of EGR and Pilot injection on the peak of pressure and maximum pressure rise rate. a) peak of pressure, b) maximum pressure rise rate.
Fig. 9. Effect of EGR and pilot quantity on the COVppeak and CoVIMEP. a) COVppeak; b) CoVIMEP.
at different loads. And the coefficient of variation of the peak of cylinder pressure and the IMEP are the limits of the HCCI-DI combustion.
4. Conclusion (1) HCCI-DI combustion mode is achieved using the very early pilot injection in the intake stroke and the main injection around compression TDC. The heat release rate of HCCI-DI combustion exhibits three stages: the cool flame combustion, the thermal flame combustion and the diffusion combustion. (2) NOx emission decreases with rise of pilot quantity. After a certain pilot quantity, NOx emission has a slight increase. Smoke is always in a low level. CO and HC emissions increase
monotonously as pilot quantity increases, which is because of incomplete combustion. (3) NOx emission is reduced greatly with low level of EGR than that of w/o EGR, but smoke opacity emission increases. NOx emission and smoke emission are limited below 120 ppm and 0.5 m1, which is thought to achieve low NOx and smoke emission combustion. (4) Fuel consumption increases slightly as EGR and pilot quantity increase, but the increase is very small. (5) The peak of cylinder pressure and the pressure rise rate increase slightly as pilot quantity increase. The cycle-to-cycle variation decreases with the rise of pilot quantity. When the pilot quantity increase further, the cycle-to-cycle variation starts to increase. But the cycle-to-cycle variations have an obviously decrease when EGR is added into.
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(6) HCCI-DI combustion is limited by the peak of cylinder pressure, the pressure rise rate and the cycle-to-cycle variation. There are optimal EGR rates and pilot quantity values for low emissions and low fuel consumptions. Acknowledgements This work is supported by Centre for Combustion and Environmental Technology of Shanghai Jiao Tong University. References [1] M. Christensen, B. Johansson, P. Amneus, F. Mauss, Supercharged homogeneous charge compression ignition, SAE paper No. 980787. [2] R.H. Stanglmaier, C.E. Roberts, Homogeneous charge compression ignition (HCCI): benefits compromises, and future engine applications, SAE paper No. 1999-01-3682. [3] H. Suzuki, N. Koike, H. Ishii, M. Odaka, Exhaust purification of diesel engines by homogeneous charge with compression ignition part1: experimental investigation of combustion and exhaust emission behavior under pre-mixed homogeneous charge compression ignition method, SAE Paper No. 970313. [4] M.F. Yao, Z.L. Zheng, H.F. Liu, Progress and recent trends in homogeneous charge compression ignition (HCCI) engines, Prog. Energ. Combust. Sci. 35 (2009) 398e437. [5] J. Shakal, J.K. Martin, Effects of auxiliary injection on diesel engine combustion, SAE paper No. 900398. [6] M. Osses, G.E. Andrews, J. Greenhough, Diesel fumigation partial premixing for red particulate soot fraction emissions, SAE paper No. 980532. [7] S. Simescu, T.W. Ryan, G.D. Neely, A.C. Matheaus, B. Surampudi, Partial premixed combustion with cooled and uncooled EGR in a heavy-duty diesel engine, SAE Paper No. 2002-01-0963. [8] D.S. Kim, M.Y. Kim, C.S. Lee, Effect of premixed gasoline fuel on the combustion characteristics of compression ignition engine, Energy Fuels 18 (2004) 1213e1219. [9] D.S. Kim, M.Y. Kim, C.S. Lee, Combustion and emission characteristics of a partial homogeneous charge compression ignition engine when using twostage injection, Combust. Sci. Technol. 179 (2007) 531e551.
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