Effect of Compression Ratio and Spray Injection Angle on HCCI

Energy & Fuels 2006, 20, 69-76

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Effect of Compression Ratio and Spray Injection Angle on HCCI Combustion in a Small DI Diesel Engine Myung Yoon Kim,† Jee Won Kim,† Chang Sik Lee,*,‡ and Je Hyung Lee§ Graduate School and Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, and Research & DeVelopment DiVision for Hyundai Motor Company & Kia Motors Corporation, Jangduk-dong, Whasung-si, Gyunggi-do, 445-706, Korea ReceiVed June 10, 2005. ReVised Manuscript ReceiVed NoVember 5, 2005

An experimental investigation was performed on a small direct injection (DI) diesel engine equipped with a common-rail injection system to find the optimal operating conditions of a homogeneous charge compression ignition (HCCI) engine. It is generally agreed that NOx is formed within the stoichiometric diffusive flame and may continue to form in the hot combustion products and that particulate matter is formed under a rich fuel/air mixture condition. Therefore, if an adequately diluted fuel/air mixture is formed before the start of ignition by prolonging the ignition delay, a homogeneous lean mixture is burned at a low temperature. In this way, simultaneous reduction of NOx and soot can be achieved. To realize this fundamental concept and find the optimal operating conditions, injection timing was varied from top dead center (TDC) to 80° before TDC and up to 45% of exhaust gas recirculation (EGR) was tested. The features of the base engine were modified as follows. First, the geometric compression ratio was reduced from 17.8 of the base diesel engine to 15 for expanding ignition delay by modification of the piston shape. Second, the injection angle was reduced from 156° of the base engine to 60° to reduce fuel deposition on the wall of the combustion chamber when using early injection timing. Experimental results showed that clean operating conditions existed either at an early start of injection timing with/without EGR or at a very late start of injection timing near TDC with high rates of EGR.

1. Introduction Although recent improvements in the low emissions design and fuel injection system resulted in large reductions in noise and both NOx and soot emissions, they are still too high to meet future emissions regulations. For direct injection (DI) diesel engines, soot forms throughout the fuel vapor volume very near the start of combustion at equivalence ratio from 2 to 5.1 NOx forms in the regions in the cylinder that are at a high temperature, and its formation becomes important at charge temperatures above 2000 K.2,3 To reduce simultaneously the formation of NOx and soot in the combustion process, sufficient mixing time is required before the start of combustion, which allows lean, low-temperature combustion. In a general way, exhaust gas recirculation (EGR) has been regarded as the simplest and most effective method for reducing NOx emissions for every kind of internal combustion engine. The induced exhaust gases of the EGR system slow the combustion rate, lower local peak temperature, and inhibit NOx formation.4-7 But * To whom correspondence should be addressed. Phone: +82-2-22200427. Fax: +82-2-2281-5286. E-mail: [email protected]. † Graduate School, Hanyang University. ‡ Department of Mechanical Engineering, Hanyang University. § Research & Development Division for Hyundai Motor Company & Kia Motors Corporation. (1) Dec, J. E.; Kelly-Zion, P. L. SAE Tech. Pap. Ser. 2000, 2000-010238. (2) Dec, J. E. SAE Tech. Pap. Ser. 1997, 970873. (3) Dec, J. E. SAE Tech. Pap. Ser. 2002, 2002-01-1309. (4) Kim, M. Y.; Kim, D. S.; Lee, C. S. Energy Fuels 2003, 17, 755761. (5) Bhave, A.; Kraft, M.; Mauss, F.; Oakley, A.; Zhao, H. SAE Tech. Pap. Ser. 2005, 2005-01-0161. (6) Abd-Alla, G. H. Energy ConVers. Manage. 2002, 43, 1027-1043.

the induction of EGR results in increasing soot emissions, because it decreases the concentration of oxygen in the charge. The use of EGR does not noticeably increase the maximum in-cylinder soot formation and inhibits soot oxidation.1 Therefore, if soot is not formed in the first stage of the combustion process, the reduced oxidation rate will not be a problem. To achieve simultaneous reduction of NOx and soot emissions while maintaining a high thermal efficiency, alternative combustion processes are being investigated. One promising approach is homogeneous charge compression ignition (HCCI) combustion. In an HCCI engine, the diluted fuel/air mixture is ignited during the compression or expansion stroke. As combustion takes place throughout the bulk of the lean mixture, NOx emissions are much lower than those from conventional diesel combustion with a diffusive flame. These NOx emissions originate from thermal dissociation due to high flame temperature and soot formation in the fuel vapor volume of a high equivalence ratio. The effective preparation of a homogeneous lean mixture and the avoidance of fuel deposition on the combustion chamber wall are important problems for achieving high thermal efficiency, reducing exhaust emissions, and preventing dilutions of lubricant oil. HCCI engines could be classified by the two methods of fueling: port injection (fumigation) and in-cylinder (direct) injection.8 The port injection method of introducing the mixture into the cylinder during the intake stroke is the simplest way to form a homogeneous in-cylinder fuel/air mixture. This method ensures (7) Zheng, M.; Reader, G. T.; Hawley, J. G. Energy ConVers. Manage. 2004, 45, 883-890. (8) Stanglmaier, R. H.; Roberts, C. E. SAE Tech. Pap. Ser. 1999, 199901-3682.

10.1021/ef0501694 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/14/2005

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effective premixing, because sufficient time for evaporation of premixed fuel is allowed. However, the premixed fuel/air mixture is exposed to the entire time-temperature history of the cylinder, and thus ignition control by varying the timing of injection is impossible, whereas injection timing can be controlled with the direct injection method. However, with this method it is difficult to form a homogeneous mixture, because insufficient time is allowed for the evaporation of fuel before the start of ignition. The direct injection method for HCCI combustion can also be classified into early and late in-cylinder injection. For the early in-cylinder injection method, a homogeneous fuel/air mixture is formed by the fuel injection during the intake or compression process. This can be done through the same nozzle used for diesel combustion. However, in this condition, deposition of fuel on the wall of the combustion chamber due to inordinate spray penetration because of low in-cylinder pressure and temperature can cause lower thermal efficiency and increase incomplete combustion products such as hydrocarbons (HC) and carbon monoxide (CO). Several modifications to injector design are thus commonly used to reduce fuel deposition, including reducing the nozzle diameter, increasing the number of nozzle holes, and using collision spray.9-11 A multiple injection at an early injection timing is a simple approach to reduce spray tip penetration.12 In the late in-cylinder injection method, similarly to a conventional DI diesel engine, the fuel is directly injected into the combustion chamber, but the ignition delay is prolonged by the application of high rates of cooled EGR, a lower compression ratio, and a high swirl ratio. The results show that the combustion reaction begins well after the end of injection and that a diffusive combustion phase is not established. Despite the significant inhomogeneous fuel/air mixture, the reduction in combustion temperature lowers NOx emissions.13 The approach adopted in this work was to operate DI diesel engines in HCCI combustion mode with early or late in-cylinder injection strategies. To find the optimal operating conditions, experimental conditions including injection timing and EGR ratio were varied with the different engine configurations. The features of the test engine were modified as follows, and their effects were examined. First, the geometric compression ratio was reduced from 17.8 of that of base diesel engine to 15, expanding ignition delay to achieve a diluted mixture before ignition. Second, the injection angle was reduced from 156° of the base engine injector to 60° to reduce fuel deposition on the wall of the combustion chamber when fuel is injected at early timing.

Kim et al. Table 1. Specifications of Test Engine engine type bore × stroke displacement volume injection system number of nozzle holes IVO IVC valve timing EVO EVC

single cylinder DI diesel engine 75.5 × 83.5 373.6 cm3 common rail 6 BTDC 8° ABDC 52° BBDC 8° ATDC 38°

Configurations of Conventional Diesel Engine compression ratio (rc) 17.8 injection angle (θinj) 156° Configurations of Reduced Compression Ratio compression ratio (rc) 15 spray angle (θinj) 60°

fuel/air mixture and to fit with an injector of reduced spray angle. In this work, two compression ratios of 17.8 (baseline engine) and 15 were investigated. The detailed specifications of the test engine are listed in Table 1. Figure 1 gives schematics of both configurations of the combustion chamber tested in this work. 2.2. EGR System and Measurement of Injection Rate. In an HCCI engine, EGR has been proposed as a promising method to control the ignition delay and combustion phase. Therefore, up to 45% of EGR was carried out to delay the timing of the start of ignition and to limit the combustion rate. The rate of exhaust gas recirculation (EGR%) is defined as the percentage of the total intake mixture that is reduced by inducted exhaust gas, EGR% )

Qo - QEGR × 100 Qo

where Qo is the air mass flow rate with 0% EGR, and QEGR is the air mass flow rate while including exhaust gas recirculation. To reduce the fluctuation of exhaust pressure and to maintain an even exhaust pressure, a settling chamber was used as the exhaust surge chamber with 0.028 m3 volume. EGR was attained by a direct link between the exhaust surge chamber and intake pipe. The reinducted exhaust gas was cooled by a water-cooled heat exchanger, and the subsequent intake air temperature rise with EGR operating was kept lower than 5 °C. Details of EGR control, intake, and exhaust systems are illustrated in Figure 2. Fuel injection parameters including injection pressure, injection timing, and amount of fuel injected were controlled by a timing pulse generator with a universal injector driver. Exhaust gas measurements from the engine were obtained using a NOx and soot analyzer and HC-CO analyzer. The in-cylinder pressure was measured via a piezoelectric pressure sensor (6052B1, Kistler) coupled to a charge amplifier (5011, Kistler). The pressure history

2. Experimental Apparatus and Procedures 2.1. Experimental Configuration. The experiments were carried out with a single-cylinder DI diesel engine with a displacement volume of 373.6 cm3. To avoid fuel penetration toward the corner between the liner and the piston top surface when the fuel is injected at an early timing, a reduced spray angle (θinj ) 60°) was chosen. The geometric compression ratio (rc) was also lowered by modification of the piston shape to prolong auto-ignition timing of the (9) Walter, B.; Gatellier, B. Oil Gas Sci. Technol. 2003, 58, 101-114. (10) Nordgren, H.; Hultqvist, A.; Johansson, B. SAE Tech. Pap. Ser. 2004, 2004-01-2990. (11) Shimazaki, N.; Tsurushima, T.; Nishimura, T. SAE Tech. Pap. Ser. 2003, 2003-01-0742. (12) Helmantel, A.; Denbratt, I. SAE Tech. Pap. Ser. 2004, 2004-010935. (13) Yokota, H.; Kudo, Y.; Nakajima, H.; Kakegawa, T.; Suzuki, T. SAE Tech. Pap. Ser. 1997, 970891.

(1)

Figure 1. Schematic of the combustion chamber and fuel spray.

Compression Ratio and Injection Angle in Combustion

Energy & Fuels, Vol. 20, No. 1, 2006 71

Figure 2. Schematic diagram of intake and exhaust system.

Figure 4. Effect of injection angle on the injection rate (mass of injected fuel ) 8 mg).

Figure 3. Definitions of symbols related with HCCI combustion. Table 2. Experimental Conditions engine speed injection pressure coolant temperature mass of fuel injection intake air pressure EGR rate

1500 rpm 50 and 100 MPa 70 °C 8 mg/stroke 0.1 MPa (naturally aspirated) 0 and 45%

was recorded using a data acquisition board and a combustion analysis program. The pressure histories were ensemble-averaged from the engine cycles, and the rate of heat release was calculated from the cylinder pressure data by applying the first law of thermodynamics to the control volume of the cylinder gas. An optical angular encoder allowed synchronization of the injection timing with a resolution of 0.1 crank angle degrees. Detailed experimental conditions are outlined in Table 2. A Bosch-type injection rate meter was used to provide the timeresolved injection profile and injection delay of the two injectors, which have different spray angles. This apparatus was based on the pressure variation of a tube filled with fuel when the fuel is injected into the tube.14 While measuring the injection rate, the pressure of the tube was set to 4 MPa. The fuel pressure in the tube was measured by a piezoelectric sensor, and injection pressure was typically ensemble-averaged over 300 injection pressure histories for a given test condition. 2.3. Definitions of Symbols Related to HCCI Combustion. In this work, to characterize a diesel HCCI combustion process that consisted of a sequential combination of a low-temperature reaction (LTR) and a high-temperature reaction (HTR), parameters indicating these two combustion timings and the maximum rates of heat release were introduced. The start of LTR (θLTR) was defined as the time when the LTR rises through a zero level, and the start of HTR (θHTR) was defined as the timing when the heat release (14) Bosch, W. SAE Tech. Pap. Ser. 1966, 660749.

rate just rises after the decrease of LTR. Figure 3 illustrates the definitions of the symbols related with HCCI combustion. A combustion was regarded as “two-stage” if a negative temperature coefficient (NTC) region was observed between LTR and HTR. Therefore, if the two humps of LTR and HTR were not observed in the heat release curve, the combustion was classified as a singlestage combustion.

3. Experimental Results 3.1. Injection Rate Profiles. In an HCCI engine, the injection parameters such as injection flow rate profile, injection delay, and injection duration have great influence on combustion characteristics. In this work, injection pressures with various injection angles were measured to analyze the effect of these injection parameters on combustion characteristics. Figure 4 shows the injection rate profiles at different injection pressures with injection angles of 60° and 156°, which were adopted in this experiment. The overall shape of the injection rate with different injectors was very similar at a constant injection pressure. The injection rate of the fuel with both types of injector was less than 2.0 mg/deg at Pinj ) 100 MPa and 1.20 mg/deg at Pinj ) 50 MPa. The injection delays were 270 µs at Pinj ) 100 MPa and 310 µs at Pinj ) 50 MPa. As the injection pressure increased from 50 to 100 MPa, the peaks of injection rate increased at least 50% and injection duration was shortened accordingly. A higher injection pressure promoted an increase of injection rate and atomization of fuel. These effects mean that higher injection pressure offered proper conditions for a homogeneous fuel/air mixture to form before the ignition of the mixture, due to its fast injection rate and improved fuel atomization. From these observations, 100 MPa was chosen as the injection pressure of the baseline test engine in this investigation. 3.2. Effect of Early Injection Timing. Figure 5 shows the effect of injection timing on the indicated mean effective pressure (IMEP) according to the start of energizing (SOE) with the configuration of the baseline diesel engine (θinj ) 60°, rc ) 15). As injection timing advanced further, IMEP decreased


Kim et al.

Figure 5. Effect of injection timing on indicated mean effective pressure with a baseline diesel engine configuration.

Figure 7. Effect of injection timings and EGR rates on hydrocarbon and carbon monoxide emissions.

Figure 6. Effect of injection timing on NOx emissions with a baseline diesel engine configuration.

almost linearly. In the case of early injection with an SOE of 30-50° before TDC, the IMEP was approximately half compared to that attained under the conventional diesel combustion with an injection timing near TDC. Two possible reasons for this are fuel wetting on the wall of combustion and deterioration of combustion efficiency. The amount of excess fuel film resulting from the spray colliding on the wall increased with fuel consumption and unburned hydrocarbon emissions and led to the dilution of lubricant oil. These trends are regarded as typical problems of early injection HCCI engines.8,11,12 At the SOE timing of 30° before TDC, the IMEP reached a local minimum and increased as SOE timings were either retarded or advanced. One possible reason is that the distance from the nozzle tip to the surface of the piston head was suddenly shortened because spray penetrates toward the protruding surface of the piston head. Therefore, wall wetting was increased at the time of injection, and this led to incomplete combustion of the injected fuel. Consequently, the IMEP was lowered at the SOE timing. Figure 6 shows the effect of injection timing on NOx emissions for a baseline diesel engine. The NOx emissions showed a very strong dependence on the SOE timing. It reached a maximum with a start of energizing of 10° before TDC. At this SOE timing (SOE ) 10° BTDC), the start of combustion took place close to TDC with a short ignition delay as shown in Figure 10. Accordingly, the combustion progressed at a high temperature, and combustion gases spent a long time at the high temperature, where NOx formation rates are high. Outside this range, the concentration of NOx emissions decreased as the timing was either retarded or advanced. As the timing of SOE was further advanced beyond 30° BTDC, NOx emissions at a near-zero level appeared, although the start of ignition took place during the compression stroke. At the early SOE timing, the

gas temperature in the cylinder at the time of fuel injection was decreased, increasing the ignition delay for air/fuel mixing. When enough mixing time was allowed before combustion, a uniform lean fuel/air mixture was formed at the ignition timing and burned at a low temperature. In this situation, NOx emissions decreased rapidly. Figure 7 illustrates the effect of engine configurations and EGR rates on hydrocarbon and carbon monoxide emissions according to SOE timings. As the SOE was advanced, the HC and CO emissions increased because of the mixture trapped in the crevice volume during the compression stroke. When the charge temperatures were too low for combustion, the unburned fuel/air mixture in the crevice was outgassed to the exhaust manifold during the cycle. In the case of late SOE timing between 20° BTDC and TDC, the measured HC and CO emissions were low, because the fuel spray did not penetrate the corner between the liner and the piston top surface. The increased HC and CO emissions due to incomplete combustion were influenced by the decrease in IMEP as shown in Figure 5. A modified diesel engine with a reduced compression ratio and narrow spray angle showed relatively low HC emissions compared to that of the conventional diesel engine. This may be because fuel vapor spread along the curved interior of the combustion chamber wall, and therefore the mixture did not disperse to the squish area in the case of the modified configuration. The application of 45% of the EGR rate led to slightly increased HC and CO emission levels with all SOE timings. 3.3. Effect of Compression Ratio and Injection Angle. Figure 8a,b shows the effect of compression ratio and injection angle on combustion characteristics for conventional diesel combustion at near-TDC injection and HCCI combustion at early injection timing, respectively. In the case of injection timing of 10° BTDC, as shown in Figure 8a, the shape of the heat release curve appeared to indicate premixed, dominant combustion, as opposed to the classical premixed, diffusive combustion typically seen in diesel engines. In the case of conventional diesel configuration with diesel combustion (baseline), the IMEP was 0.58 MPa. As the compression ratio was reduced to 15, peak combustion pressure was lowered, and mean effective pressure was reduced from 0.58 to 0.55 MPa.


Figure 8. Effect of compression ratio and injection angle on combustion characteristics.

Comparing the cases of these two compression ratios, the ignition delays were increased according to decreasing compression ratio because of low compression pressure and charge temperature. On the other hand, for the case of early injection timing (SOE ) 50° BTDC), as shown in Figure 8b, the maximum combustion pressures are reversed (Figure 8a). Accordingly, the IMEP measured for the case of rc ) 15 and θinj ) 60° was 0.52 MPa, which was much higher than the case with rc ) 17.8 and θinj ) 156° (IMEP ) 0.27 MPa). This can be explained by spray penetrating toward the cylinder liner and the piston surface outside of the piston bowl. Therefore, combustion performance rapidly deteriorated for the case of a conventional diesel engine (rc ) 17.8 and θinj ) 156°) with early injection. On the other hand, the fuel/air mixture was distributed in the piston bowl for the modified configuration (rc ) 15 and θinj ) 60°), even though fuel is injected at an early timing. 3.4. Effect of EGR on the Engine with Modified Configuration (rc ) 15 and θinj ) 60°). The effects of SOE timings on the rate of heat release with different EGR rates are given in Figure 9. With the SOE timings near TDC, the rate of heat release showed typical features of DI diesel combustion with high pressure injection. In this case, premixed dominant combustion was seen that generally appears in a small DI diesel engine with high pressure injection. For the late injection, the actual injection duration was only about 4.77 crank angle degrees, and it ended just before or after the premixed heat release began. As SOE timing advances before 30° BTDC, the rate of heat release pattern shows diesel HCCI combustion that


generally appears as two humps. The first hump was attributed to LTR, after which there was a short delay due to the NTC ignition behavior of the mixture. The second hump, which was due to HTR, was subsequently observed.15,16 As SOE timing advanced, the peaks of heat release decreased, because a leaner mixture was formed as ignition delays were prolonged due to low charge temperature at the time of injection. Figure 9a,b shows that the ignition timing of the LTR and HTR was greatly affected by EGR, which caused the dilution of exhaust gases. In addition, the higher heat capacity of the exhaust gas reduced the charge temperature at the end of the compression stroke, which would tend to retard ignition as well.5,7 These results suggest that EGR can be chosen as the controlling factor for the start of HCCI combustion. The great influence of EGR is observed at the late SOE timings near TDC, because the start of combustion was delayed to the expansion stroke, where in-cylinder temperature rapidly decreased, and reaction rate slowed. These combustion characteristics showed that typical diesel HCCI combustion with late injection timing and simultaneous reductions of NOx and soot can be expected. Kimura et al.17 suggested a new combustion concept, namely modulated kinetics combustion, wherein low-temperature combustion is accomplished by using heavy EGR with a lowered compression ratio to increase ignition delay. At the same time, sufficient mixing is allowed by prolonging the ignition delay as well as promoting the dispersion of injected fuel. Figures 10 and 11 show the peaks of LTR and HTR (LTRmax, HTRmax) and appearance timings of LTR and HTR (θLTR,θHTR) according to different SOE. To clarify the effect of EGR on the combustion process, the cases for 0% of EGR (Figure 10) and 45% of EGR (Figure 11) are compared. For an operating condition without EGR, as shown in Figure 10, the SOE timings are divided into two regions by their combustion characteristics. At SOE timings before 25° BTDC, two-stage combustions with LTR and HTR, the general characteristics of HCCI combustion due to a slow reaction rate were observed. At SOE timings after 25° BTDC, only singlestage combustions (HTR) were observed. At the SOE timing range where single HTR was observed, it can be seen that combustion (HTR) began almost at the time when fuel injection ended. This means that the ignition and combustion processes took place before the fuel/air premixed adequately, and therefore the local equivalence ratio was expected to be high at the ignition timing. High levels of NOx emissions could also be expected. From the upper panel of Figure 10, which shows the maximum rate of heat release of LTR and HTR, it was observed that HTRmax was more sensitive to SOE timing compared to LTRmax. HTRmax obtained its highest value between 20 and 5° BTDC. When SOE timing was retarded more, it dropped rapidly, because θHTR was retarded beyond TDC, and combustion occurred during the expansion stroke, when the in-cylinder temperature dropped rapidly. As the SOE timings were advanced before 70° BTDC, HTRmax became lower than LTRmax. It seems that combustion efficiency decreased sharply due to an increased deposition of fuel that was injected into cold, low-pressure air as well as the fuel/air mixture being located to outside of the piston bowl. As previous research by Nordgren et al.10 and Helmantel and Denbratt12 describes, if diesel fuel is injected at an early timing, the initial air temperature is low, so that the (15) Kim, D. S.; Kim, M. Y.; Lee, C. S. Energy Fuels 2004, 18, 12131219. (16) Kim, D. S.; Kim, M. Y.; Lee, C. S. Combust. Sci. Technol. 2005, 177, 107-125. (17) Kimura, S.; Aoki, O.; Ogawa, H.; Muranaka, S.; Enomoto, Y. SAE Tech. Pap. Ser. 1999, 1999-01-3681.


Kim et al.

Figure 9. Effect of injection timing on the rate of heat release with different EGR rates (rc ) 15, θinj ) 60°).

Figure 10. Start of LTR and HTR without EGR (rc ) 15, θinj ) 60°).

Figure 11. Start of LTR and HTR with 45% of EGR (rc ) 15, θinj ) 60°).

vaporization rate decreases and wall wetting increases by long spray penetration due to low air pressure. The combustion characteristics with 45% of EGR are shown

in Figure 11. The variation of HTRmax was very small for the range of SOE timings between 50 and 10° BTDC. Outside this SOE range, HTRmax rapidly dropped. For SOE timings earlier



Figure 14. Combustion pressure, rate of heat release, and fuel injection rate for SOE of 5° BTDC, injection pressure of 100 MPa.

Figure 12. Effect of injection timing and EGR rate on the IMEP and coefficient of variance of IMEP.

Figure 15. Effect of injection timing on soot emissions for different EGR rates.

Figure 13. Effect of injection timing and EGR rate on NOx emissions.

than 50° BTDC, it is supposed that excessive amounts of fuel film lowered the combustion efficiency. On the other hand, at the SOE of 5° BTDC, HTRmax rapidly decreased as θHTR was delayed, until the expansion stroke. Two-stage combustion appeared at all injection timings, unlike the case with 0% EGR. Under this operating condition, the delay between the end of injection and the beginning of HTR (θHTR) was longer than the actual injection duration. For the SOE of 5° BTDC, actual injection started at 2.57° BTDC and ended at 2.2° ATDC, and the beginning of HTR occurred at 10° ATDC. This means there was a delay of about 7.8° between the end of injection and the beginning of HTR. Therefore, a major portion of the fuel/air mixture was adequately premixed, and it can be supposed that the lean premixture was formed at the time of ignition due to the long ignition delay. Consequently, a very low emission level can be inferred due to the elimination of the region with locally high temperature and equivalence ratio where NOx and soot formation rates are high. A comparison of Figures 10 and 11 shows that, as expected, the application of EGR retarded θHTR and θLTR at equal injection timings. The effects of SOE on the IMEP and coefficient of variance of IMEP (COVIMEP) for different EGR rates are shown in Figure 12. It can be seen that the application of a narrow angle injector and modified piston head led to an increase in IMEP at earlier SOE timings compared to the results of the base diesel engine configuration shown in Figure 5. Although SOE is advanced

by 40-50° before TDC, the IMEP almost maintained the high value of a near-TDC injection with conventional diesel combustion. This result revealed that a modified injection angle and combustion chamber geometry are appropriate for an injection with early timing. The case of 45% of EGR showed a slightly increased IMEP; however, the COVIMEP was not affected by EGR at the SOE range from TDC to 60° BTDC. The effects of SOE timing on NOx emissions for different EGR rates are shown in Figure 13. As expected, dramatic NOx reductions were observed by adopting 45% of the EGR rate. It is well-known that EGR lowers NOx formation as a result of lowering local flame temperature and reducing oxygen concentrations in the charge air. In the case of 45% of EGR, it is interesting that a near-zero NOx level region also appeared at the late SOE timing between 5-30° BTDC. This can be explained by the combined effects of EGR on lowering combustion temperature and increasing ignition delay that allowed a more diluted lean mixture to form before the start of combustion and the achievement of HCCI combustion. The combustion pressure, rate of heat release, and fuel injection rate for an injection timing of 5° before TDC for EGR rates of 0 and 45% are shown in Figure 14. The employment of EGR greatly affected the combustion characteristics in this configuration. Exhaust gas that was substituted for fresh air lowered combustion temperature and reduced the oxygen concentration of the charge, leading to a slowing of the reaction rate. Furthermore, for late injection timing near TDC, combustion characteristics were more sensitive with EGR because EGR retards the start of ignition. Ignition occurred during the expansion stroke, and the charge temperature, which is directly related to the chemical reaction rate, was gradually lowered during the


expansion stroke. Consequently, when sufficient mixing time before ignition was allowed, the reaction rate was lowered, and oxygen concentration of charge was reduced. As these results suggest, low NOx and soot emissions can be expected. Figure 15 presents the effect of EGR on NOx emissions with various injection timings and shows that similar trends are observed for all SOE timings, regardless of EGR rate. Dec2 showed that the application of EGR does not increase the soot formation rate, but that it decreases the soot oxidation rate. The low soot emission results for 45% of EGR are expected for this reason. The application of EGR increased ignition delay and offered sufficient mixing time for fuel before the start of combustion. This effect increased soot formation but maintained the same levels of soot emission compared to 0% of EGR, despite the decreased soot oxidation rate. 4. Conclusions The concept of HCCI combustion for reducing engine emissions for a direct injection diesel engine was experimentally investigated in a small single-cylinder diesel engine. The injection angle was narrowed to reduce fuel deposition when fuel was injected at an early timing, and piston head shape was also formed to fit with the narrowed injection angle and to increase ignition delay. The following conclusions can be drawn based on experimental results and associated data analysis. 1. For the conventional diesel engine, NOx emissions are greatly lowered as injection timing advances beyond 30° before

Kim et al.

TDC, while indicated mean effective pressure decreases almost linearly due to the increase in fuel deposition of fuel injected early. 2. The modification of the combustion chamber shape and injection angles fitted for early timing injection resulted in a high IMEP for an early timing injection. 3. The modified engine configuration offered very low levels of NOx and soot emission for early injection timing while maintaining a high IMEP. Also, the low emission with high IMEP can be achieved in the case of late injection timing with high rates of EGR (45%). 4. Heat release data show that early injected fuel (before 30° BTDC) burns with HCCI combustion characteristics of a lowtemperature reaction followed by a high-temperature reaction. Moreover, two-stage combustion is observed for all injection timings with 45% of EGR due to increased ignition delay. Reduced compression ratio and high EGR rates resulted in a slow reaction rate. For the late injection timing with 45% of EGR, low NOx and soot emissions are expected, because sufficient mixing time for the fuel/air is allowed before the start of combustion by the prolonged ignition delay. Acknowledgment. This work was supported by the CEFV (Center for Environmentally Friendly Vehicle) of the Eco-STAR project from the MOE (Ministry of Environment, Republic of Korea). EF0501694

Effect of Compression Ratio and Spray Injection Angle on HCCI

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