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Applied Thermal Engineering 48 (2012) 97e104

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Influences of pilot injection and exhaust gas recirculation (EGR) on combustionand 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

Article history:Received 25 July 2011Accepted 11 March 2012Available online 21 March 2012

Keywords:HCCI-DICombustionEmissionsPilot injection quantityEGR

Abbreviations: HCCI, homogeneous charge compmixed charge compression ignition; DI, direct injectiodirect injection; TDC, top dead center; BTDC, beforespecific fuel consumption; IMEP, indicated mean effecombustion; CoVIMEP, coefficient of variation of IMEPparticulate matter; NOx, nitrogen oxides; HC, hydrecirculation; BMEP, brake mean effective pressure; DCA, crank angle; CoVppeak, coefficient of variation of t* Corresponding author. 502, Building A, School

Shanghai Jiao Tong University, 800, Dong Chuan road,21 34206859; fax: þ86 (0) 21 34205553.

E-mail address: z-huang@mail.sjtu.edu.cn (Z. Huan

1359-4311/$ e see front matter � 2012 Published bydoi:10.1016/j.applthermaleng.2012.03.021

a b s t r a c t

The HCCI-DI combustion mode was achieved in a heavy-duty diesel engine using the early pilot injectionin the intake stroke and the main injection around compression top dead center (TDC). The effects ofpilot injection quantity and EGR rate on HCCI-DI combustion and emissions were investigated. NOxemission has a dramatically decrease as the pilot injection quantity increases, but it is still in a high levelthat needs to be reduced. The smoke emission has a slight increase with the rise of pilot quantity, but it isalways in a low level. The increasing HC and CO emissions can be easily removed by the diesel oxidationcatalyst (DOC). The HCCI-DI combustion with low level of EGR is an effective method to reduce NOxemission further, and smoke emission increases as EGR rate increases, but it is in an acceptable level. TheHCCI-DI combustion mode operating range is limited by the peak of cylinder pressure, the pressure riserate and the cycle-to-cycle variations. There are optimal EGR rates and pilot quantities to achieve lowNOx and low smoke emissions.

� 2012 Published by Elsevier Ltd.

1. Introduction

Under the influence of increasingly stringent emission regula-tions, the new combustion modes were investigated to simulta-neously reduce NOx and soot emissions in diesel engine.Homogeneous Charge Compression Ignition (HCCI) is a promisingalternative combustion technology with high efficiency and lowNOx and soot emissions. Many studies of HCCI combustion showa potential for very low NOx and PM emissions [1e3]. However,there are several problems to be solved before the commercialapplication in automotive. Especially, it is difficult to control theignition timing and extend the load range of HCCI combustion [4].The conventional DI combustion doesn’t have uncontrollable

ression ignition; PCCI, pre-n; CIDI, compression ignitiontop dead center; BSFC, brakective pressure; SOC, start of; CO, carbon monoxide; PM,rocarbon; EGR, exhaust gasOC, diesel oxidation catalyst;he peak of cylinder pressure.of Mechanical Engineering,Shanghai, China. Tel.: þ86 (0)

g).

Elsevier Ltd.

ignition timing and small load range problems, but the NOx and PMemissions need to be reduced.

The HCCI-DI or PCCI-DI concept could be considered asa compromise between HCCI combustion and conventional CIDIcombustion. Several studies have revealed the advantages and thedisadvantages of similar combined combustion mode. Shakal andMartin studied the effect of auxiliary fuel injection (pilot, manifold,and port injection) on emissions and combustion in a two-strokediesel engine [5]. They found a decrease in NOx and increases inHC and smoke. Osses et al. performed an experimental study of thepotential of diesel fumigation partial premixing to reduce the sootfraction of PM emissions on a naturally aspirated DI diesel engine[6]. They reported improved soot and NOx emissionswith up to 20%port fumigation, but increased fuel consumption, CO, HC andvolatile organic fraction of the PM emissions. Simescu et al. con-ducted an experimental investigation of PCCI-DI combustioncoupled with cooled and uncooled EGR in a heavy-duty dieselengine [7]. The study showed significant NOx reductions at lightload conditions with up to 20% port fuel injection (PFI). The studyhowever showed that early PCCI combustion could adversely affectNOx emissions by increasing in-cylinder temperatures at the startof diffusion combustion. The PCCI-DI combustion also showedincreased brake specific fuel consumption (BSFC) and HC, CO, andPM emissions. Kim et al. investigated the effects of premixedgasoline fuel and direct injection timing on partial HCCI [8,9]. Maet al. carried out an experimental study of HCCI-DI combustion and

Table 1Engine specifications.

Parameter Value

Number of cylinders 4Bore/Stroke (mm/mm) 110/125Compression ratio 16.8:1Connecting rod (mm) 195Displacement (L) 4.751Nozzle number � orifice diameter (mm) 7 � 0.16Swirl ratio 1.5Fuel injection system Common rail

Q. Fang et al. / Applied Thermal Engineering 48 (2012) 97e10498

emissions in a diesel engine with dual fuel [10]. They found thatNOx emission decreased dramatically when premixed rate was lowand HCCI-DI could effectively improve the thermal efficiency at lowand medium loads. Wang et al. added another fuel system in airinlet pipe in diesel engine fueled with dimethyl ether (DME) [11].The results showed that the HCCI-DI combustion mode could alsobe achieved in the DME engine. However, all these experimentsneed another fuel supply system to form premix fuel.

The diesel engine with common-rail system using multipleinjection strategy to control NOx and PM emissions attracts moreand more attention. Nehmer et al. investigated the effect of rate-shaped and split injections on soot and NOx emissions ina heavy-duty diesel engine with an electronically-controlled, high-pressure common-rail injection system [12]. The results showedthat rate-shaped injection didn’t appreciably affect pressure rise.Split injections allowed peak pressure to be reduced and NOxemission also had a decrease trend without increase of PM. Yokotaet al. performed a combined experimental and computationalstudy of homogenous charge intelligent multiple injectioncombustion system (HiMICS), in which the quasi-homogeneousmixture was generated by very early, direct injection [13]. Theyalso showed the potential to improve both NOx and PM emissionsover some operating conditions, at the expense of significantincreases in HC emissions. Park et al. investigated the effect of pilot-, post- and multiple-fuel injection strategies on engine perfor-mance and emissions [14]. They found that the pilot-injectionreduced the ignition delay of main injection and the post-injection was effective to reduce PM emission. A.P. Carlucci et al.tested the effects of several injection parameters of multipleinjection strategy in a direct injection diesel engine [15]. The resultsshowed that NOx and soot both decreased performing the early andpilot before the main injection, but UHC levels remained constant.Okude et al. studied the effects of pilot injection fuel quantity andpilot injection timing on diesel emissions and combustion [16]. Leeet al. investigated the single-pilot injection and double-pilotinjection strategies with a wide injection timing range, variousinjection quantity ratios, and various dwell times [17]. The resultsshowed that single-pilot injection resulted in a dramatic reductionin NOx and smoke emissions when the pilot injection wasadvanced over 40�CA before the start of main injection and thedouble-pilot injection could improve the HC emission. The dieselengine with common-rail system using multiple injection strategyto achieve the HCCI-DI combustion is another trial. Based onprevious studies that have investigated and characterized multipleinjection strategies, the timing of multiple injection, the quantity offuel injected, and the dwell time between each injection are themain parameters to take into account when attempting to reducethe emission of NOx and PM. Nevertheless, there is still room forfurther studies of multiple injection strategies, owing to the veryhigh degree of freedom of fuel injection schedules offered by thecommon-rail system. Furthermore, there are few analyses aboutthe effect of EGR on HCCI-DI combustion and emissions withmultiple injection strategies.

The objective of this study was to achieve the HCCI-DIcombustion using a very early pilot injection in intake stroke andthe main injection around compression top dead center (TDC). Theeffects of EGR and pilot injection quantity on HCCI-DI combustionand 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 inTable 1. The schematic diagram of the experimental setup is shownin Fig. 1. The common-rail system allowed for the variation of railpressure (up to 160 MPa), timing (�360e360�CA), and number ofinjections. The fuel injection timing was controlled by a magneticsensor mounted on camshaft.

The cylinder pressure was measured with a pressure transducer(Model 6125B). The charger output from this transducer was con-verted to an amplified voltage with an amplifier. A magnetic sensormounted on flywheel was used as the clocking pluses to acquire thecylinder pressure data. The cylinder pressure was recorded at every0.5 crank angle (CA) using the instrument of engine combustionanalysis (Osiris). For eachmeasuring point, the pressure data of 200consecutive cycles were sampled and recorded. The pressure tracefor a specific condition was obtained by averaging the sampledpressure data. The exhaust emissions weremeasured by an AVL CEBgas analyzer. The smoke opacity was measured using an AVL 439Opacimeter analyzer. The heat release rate and the mean gastemperaturewere calculated using thefirst-lawheat-releasemodel.

2.2. Experimental method

In this study, double injection techniquewas applied in DI dieselengine using common-rail injection system. The following strategywas examined in order to achieve the HCCI-DI combustion. Thepilot fuel was injected in the intake stroke of engine cycle to formthe premixed fuel-air mixing. The pilot injection timing and pilotinjection quantity were controlled to achieve HCCI combustion. Themain injection fuel was injected around compression TDC tocontrol the ignition timing. The main injection timing was varied tooptimize the emissions and efficiency.

The test conditions are summarized in Table 2. For all datapresented, 0�CA is defined as the top dead center (TDC) atcompression stroke. To ensure the repeatability and comparabilityof the measurements for operating conditions, the temperatures ofintake air, oil and coolant water were held accurately stable duringthe experiments. The engine speed was kept at 1450 rpm duringthis experiment. The injection pressures were 80 MPa for 0.15 MPaand 0.3 MPa BMEP, 85 MPa for 0.45 MPa and 0.6 MPa BMEP. Theinjection pressures were kept constant at the same load. The intakepressures were 116 kPa, 120 kPa, 128 kPa and 140 kPa at four loadswithout EGR and pilot injection, respectively. The intake pressureswere influenced by the EGR and pilot injection. All the emissionswere continuously measured for 3 min and the average resultspresented here. Each test was repeated twice to ensure that theresults 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 andperformance were investigated over a range of engine speeds and

Fig. 1. Schematic of engine with common rail system.

Q. Fang et al. / Applied Thermal Engineering 48 (2012) 97e104 99

loads. The speed, load and main injection timing were heldconstant for each pilot injection quantity sweep and the EGR ratewas zero. The pilot injection quantity is defined the amount of onecycle from one injector.

Fig. 2 shows the observed changes in-cylinder pressure and heatrelease rate as the pilot quantity increases from 0 to 10 mm3 at0.3 MPa and 0.6 MPa BMEP with pilot injection timing at 340�CABTDC. The ratios of pilot injection quantity to total injectionquantity from 0 to 45% are in the brackets. The effects of pilotquantity on cylinder pressure and heat release rate are quite similarfor different loads. With the rise of pilot quantity, there is a corre-sponding increase in the maximum cylinder pressure. That isbecause the increasing HCCI combustion results in highertemperature at start of combustion (SOC) as the pilot quantityincreases. It is found that the heat release rate with pilot injectionconsists of the cool flame stage, the thermal flame stage of HCCIcombustion and diffusion combustion of CIDI combustion in Fig. 2a.This is thought to achieve HCCI-DI combustion with pilot injectiontiming at 340�CA BTDC [10,18]. The premixed fuel injected intocylinder in intake stroke enters the cool flame combustionapproximately 30�CA BTDC for all pilot quantity. It is noted thatpilot quantity had little impact on ignition timing of the cool flame

Table 2Engine test conditions.

Parameter Value

Engine speed (rpm) 1450Pilot injection timing (�CA BTDC) 360e20Pilot injection quantity (mm3) 0e10Main injection timing (�CA BTDC) 10.6e�5Intake air temperature (�C) 36 � 2Coolant temperature (�C) 80 � 2Lubricant oil temperature (�C) 90 � 2EGR rate 15%, 25%

combustion, showing the strong temperature dependence of thestart of cool flame reactions [19,20]. The peak of the cool flameregime increases slightly as the pilot quantity increases. That isbecause the reaction rates are proportional with the fuel concen-tration. The thermal flame reaction of HCCI combustion is alsoobserved with presence of the pilot injection and starts atapproximately 18�CA BTDC. The start of thermal flame combustionis found to be advanced and the maximum heat release rate ofthermal flame combustion increases with the rise of pilot quantity.When the pilot quantity increases up to certain value, the thermalflame combustion starts early even before the end of cool flamecombustion. That is possible that the thermal flame combustionand the cool flame combustion occur simultaneously because ofhigh temperature. The ignition timing of direct injected fuel ischanged slightly as the pilot quantity increases because the ignitiondelay of the direct fuel is minimal which is generated from the in-cylinder mean temperature increases due to the HCCI combustionof pilot injection. The similar results were found in [10,18]. The peakof heat release rate of the diffusion combustion decreases as thepilot quantity increases.

Fig. 3 shows the observed changes in emissions as a function ofpilot injection quantity for four different loads with pilot injectiontiming at 340�CA BTDC and main injection timing at 4.4�CA BTDC.Smoke emission increases with the rise of pilot quantity when pilotquantity is smaller than 8 mm3, and then has a little drop whenpilot quantity is bigger than 8 mm3, as shown in Fig. 3a. However,the smoke emission is always in a low level. The increasing smoke isbecause the increase of the pilot quantity would have consumedmore in-cylinder oxygen, and thus less oxygen is available andresults in increased smoke. Another reason could be the shortmixing time for the diesel fuel of the main injection.

NOx emission shows the similar trends in all loads, as shown inFig. 3b. NOx emission decreases with the rise of pilot quantity andthen increases as the pilot quantity is bigger than certain value. Thisis because of the trade-off relationship between the reduction of

Fig. 2. Effect of pilot injection quantity on cylinder pressure and heat release rate. a) 0.3 MPa BMEP; b) 0.6 MPa BMEP.

Q. Fang et al. / Applied Thermal Engineering 48 (2012) 97e104100

NOx during HCCI combustion and the increase of NOx duringdiffusive combustion. On one hand, the HCCI combustion producesvery little NOx and this part increases with the rise of pilot quantity,which is positive to reduce NOx emission. On the other hand, theincreasing HCCI combustion results in higher temperature at startof diffusion combustion, which is negative to the reduce NOxemission. Therefore, there are optimal pilot quantity values to getlow NOx emissions for different loads. However, the NOx emissionis also in a high level that needs to be reduced further.

The effect of pilot quantity on HC and CO emissions at fourdifferent loads are presented in Fig. 3c and d. HC and CO emissionsincrease as the pilot quantity increases. HC and CO emissions aremajor problems of HCCI combustion. In general, it is widelyaccepted that HC and CO emissions increase with the rise ofpercentage of HCCI combustion due to incomplete combustion.Another reason is possible due to the impingement of fuel againstthe cylinder wall and was also observed by Okude et al. [16,17].

Fig. 3. Effect of pilot injection quantity on em

3.2. HCCI-DI combustion with EGR

3.2.1. Combustion characteristicFig. 4 shows that the effects of EGR and pilot quantity on the in-

cylinder pressure and heat release rate of HCCI-DI combustion at0.3 MPa and 0.6 MPa BMEP with pilot injection timing at 340�CABTDC. It can be seen that the ignition timing of diffusion combus-tion stage with 15% and 25% EGR is plainly later than that of w/oEGR in the same pilot quantity. Because of the reduction in theoxygen content available for combustion and the increase in thespecific heat capacity of the gas mixture in the cylinder, the ignitiondelay becomes longer with EGR [21]. However, the diffusioncombustion is advanced with the rise of pilot quantity because thecylinder temperature increases due to the HCCI combustion of pilotinjection. The effect of EGR on retarding the ignition timing of thediffusion combustion weakens in high load. The maximum pres-sure of HCCI-DI combustion with EGR is obviously lower than that

issions. a) Smoke; b) NOx; c) HC; d) CO.

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.

Q. Fang et al. / Applied Thermal Engineering 48 (2012) 97e104 101

of w/o EGR. Owing to the later combustion of the EGR case, thecombustion is away from the TDC, leading to a reduction in the in-cylinder temperature. Meanwhile, the increase in the specific heatcapacity of gas mixture in the cylinder also results in lowertemperature.

3.2.2. Emission characteristicThe effects of EGR and pilot injection on NOx emission of HCCI-

DI combustion are shown in Fig. 5. NOx emission increases obvi-ously with increasing engine load, because of the higher combus-tion temperature at high loads. NOx emission is greatly reducedwith 15% EGR and 25% EGR compared with that of w/o EGR at twoloads. It is believed that the major factors affecting NOx formationare the combustion temperature, the local oxygen concentration,and the residence time in the high-temperature zone [22]. NOxemission exhibits the expected trends, because of the lowertemperature and decreasing oxygen concentration with increasingEGR level. Moreover, NOx emission is also reduced by using thepilot injection, which has already studied in pilot quantity sweep.

The effects of EGR and pilot injection on smoke opacity of HCCI-DI combustion are shown in Fig. 6.The smoke opacity increasesobviously with increasing engine load, because more fuel is injec-ted and burned in the diffusion mode. Smoke emission increasesslightly with the presence of EGR rate. The combustion tempera-tures decrease due to the lower oxygen concentration and higherheat capacity of the work gas. Smoke opacity increases firstly and

Fig. 5. Effect of EGR and Pilot injection on NOx em

then decreases asmain injection timing is retarded. However, whenthe pilot injection is applied, smoke opacity increases mono-tonically as main injection timing is retarded. This is possible thatthe increasing HCCI combustion results in higher temperature,leading to shorter ignition delay, higher smoke emission. In thispaper, NOx emission and the smoke opacity are limited below120 ppm and 0.5m�1, which is thought to achieve lowNOx and lowsmoke 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 workdue to split combustion stage are the main reasons. Second, thesignificant increase in HC and CO emission observed is indicative offuel energy losses due to incomplete combustion. Moreover, thepilot fuel injected in the intake stroke is possible towet the cylinderliner because lower cylinder pressure allows to the long spraydistance. So the proper pilot quantity is needed and the fuelconsumption is a limit to higher pilot quantity. Therefore, morecomplete oxidation of the HC and CO is desired, not only for lowemission, but also for low fuel consumption. It also found that BSFChas a slight increasewith the rise of EGR, which is mainly due to off-phasing of the combustion process and decrease of combustionefficiency because of incomplete combustion [23,24].

issions. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.

Fig. 6. Effect of EGR and Pilot injection on smoke opacity. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.

Q. Fang et al. / Applied Thermal Engineering 48 (2012) 97e104102

3.2.4. The peak of pressure and pressure rise rateThe 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 exceeds1.0 MPa/�CA. Fig. 8 shows the effects of EGR and pilot injection onthe peak of pressure and the maximum pressure rise rate.

It can be found that the peak of pressure and the maximumpressure rise rate are lower with EGR than that of w/o EGR in allpilot quantity. That is because of lower in-cylinder temperature andlower in-cylinder pressure when EGR is applied [21]. It can be alsofound that the peak of pressure increases with the rise of pilotquantity. The maximum pressure rise rate displays a decrease aspilot quantity increases. This trend is similar to that of Ma et al. [10].So the pilot quantity is limited to control the peak of cylinderpressure and pressure rise rate for low noise and stable engineoperation.

3.2.5. Cycle-to-cycle variationIn this paper, the cycle-to-cycle variations are defined as follow:

Fig. 7. Effect of EGR and Pilot injection on fuel ef

CoVppeak ¼ sppeak=ppeak

CoVIMEP ¼ sIMEP=IMEP

Where CoVppeak and CoVIMEP represent the coefficient of variationof the peak of cylinder pressure and the indicated mean effectivepressure (IMEP) in 200 cycles; sppeak and sIMEP are the standarddeviations of the peak of cylinder pressure and the IMEP in 200cycles; ppeak and IMEP are the average values of the peak of cylinderpressure and the IMEP in 200 cycles.

Fig. 9 shows the effects of EGR and pilot quantity on the cycle-to-cycle variations of maximum gas pressure and IMEP. The coef-ficient of variation of IMEP is bigger than the coefficient of variationof the peak of cylinder pressure, so CoVIMEP is the main limit to theengine stable. The coefficient of variation of the peak of cylinderpressure has a slight increasewhen the pilot quantity increases. Thecoefficients of variation of the IMEP decrease firstly and thenincrease with the rise of pilot quantity. Therefore, there are theoptimum pilot quantity values for the stable combustion of engine

ficiency. a) 0.3 MPa BMEP, b) 0.6 MPa BMEP.

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.

Q. Fang et al. / Applied Thermal Engineering 48 (2012) 97e104 103

at different loads. And the coefficient of variation of the peak ofcylinder pressure and the IMEP are the limits of the HCCI-DIcombustion.

4. Conclusion

(1) HCCI-DI combustion mode is achieved using the very earlypilot injection in the intake stroke and the main injectionaround compression TDC. The heat release rate of HCCI-DIcombustion exhibits three stages: the cool flame combustion,the thermal flame combustion and the diffusion combustion.

(2) NOx emission decreases with rise of pilot quantity. Aftera 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 ofincomplete combustion.

(3) NOx emission is reduced greatly with low level of EGR thanthat of w/o EGR, but smoke opacity emission increases. NOxemission and smoke emission are limited below 120 ppm and0.5 m�1, which is thought to achieve low NOx and smokeemission combustion.

(4) Fuel consumption increases slightly as EGR and pilot quantityincrease, but the increase is very small.

(5) The peak of cylinder pressure and the pressure rise rateincrease slightly as pilot quantity increase. The cycle-to-cyclevariation decreases with the rise of pilot quantity. When thepilot quantity increase further, the cycle-to-cycle variationstarts to increase. But the cycle-to-cycle variations have anobviously decrease when EGR is added into.

Q. Fang et al. / Applied Thermal Engineering 48 (2012) 97e104104

(6) HCCI-DI combustion is limited by the peak of cylinder pressure,the pressure rise rate and the cycle-to-cycle variation. There areoptimal EGR rates and pilot quantity values for low emissionsand low fuel consumptions.

Acknowledgements

This work is supported by Centre for Combustion and Envi-ronmental Technology of Shanghai Jiao Tong University.

References

[1] M. Christensen, B. Johansson, P. Amneus, F. Mauss, Supercharged homoge-neous 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 enginesby homogeneous charge with compression ignition part1: experimentalinvestigation of combustion and exhaust emission behavior under pre-mixedhomogeneous charge compression ignition method, SAE Paper No. 970313.

[4] M.F. Yao, Z.L. Zheng, H.F. Liu, Progress and recent trends in homogeneouscharge 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 forred particulate soot fraction emissions, SAE paper No. 980532.

[7] S. Simescu, T.W. Ryan, G.D. Neely, A.C. Matheaus, B. Surampudi, Partial pre-mixed combustion with cooled and uncooled EGR in a heavy-duty dieselengine, SAE Paper No. 2002-01-0963.

[8] D.S. Kim, M.Y. Kim, C.S. Lee, Effect of premixed gasoline fuel on the combus-tion characteristics of compression ignition engine, Energy Fuels 18 (2004)1213e1219.

[9] D.S. Kim, M.Y. Kim, C.S. Lee, Combustion and emission characteristics ofa partial homogeneous charge compression ignition engine when using two-stage 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 combustionand 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 emissionsin a DME engine, Fuel 88 (2009) 2255e2261.

[12] D.A. Nehmer, R.D. Reitz, Measurement of the effect of injection rate and splitinjections 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 forlow emission diesel combustion, SAE paper No. 970891.

[14] C. Park, S. Kook, C. Bae, Effects of multiple injections in a HSDI diesel engineequipped with common-rail injection system, SAE Paper No. 2004-01-0127.

[15] A.P. Carlucci, A. Ficarella, D. Laforgia, Control of the combustion behaviour ina 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 multipleinjections on diesel emissions and combustion characteristics, SAE paper No.2007-01-4178.

[17] Jinwoo Lee, Jinwoog Jeon, Jungseo Park, Choongsik Bae, Effect of multipleinjection strategies on emission and combustion characteristics in a singlecylinder 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-DIcombustion 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 HCCIcombustion and emissions by fuel design concept combined with controllableEGR, 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 HCCIcombustion and emissions by fuel design concept combined with controllableEGR, part 2: effect of operating conditions and EGR on HCCI combustion, Fuel84 (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 DMEmeet US 2007/2010 emissions standard with a simplified after treatmentsystem?, SAE paper No. 2006-01-0053.

[23] M. Zheng, Y.Y. Tan, M.C. Mulenga, M.P. Wang, Thermal efficiency analyses ofdiesel low temperature combustion cycles, SAE paper No. 2007-01-4019.

[24] R. Kumar, M. Zheng, Fuel efficiency improvements of low temperaturecombustion diesel engines, SAE paper No. 2008-01-0841.