An Experimental Study on the Combustion and Emission

Energy & Fuels 2007, 21, 1901-1907

1901

An Experimental Study on the Combustion and Emission Characteristics of a Stratified Charge Compression Ignition (SCCI) Engine C. H. Lee and K. H. Lee* Department of Mechanical Engineering, Hanyang UniVersity, 1271 Sa 1-dong, Sanrog-gu Ansan-ci, Kyunggi-do, 425791, Korea ReceiVed December 14, 2006. ReVised Manuscript ReceiVed March 27, 2007

Stratified charge compression ignition (SCCI) combustion, which is developed to extend the homogeneous charge compression ignition operating range, has a good potential to improve fuel economy and reduce emissions. In this study, SCCI combustion and emissions characteristics were investigated in a single-cylinder gasoline engine with a direct-injection system. From the experimental results, we elucidated the effects of air-fuel ratio, intake temperature, and injection timing including early and late injection on the attainable SCCI combustion region. Injection timing during the intake process was found to be an important parameter that affects the extension of the SCCI operating region. The effects of stratified mixture formation can be utilized to extend the operating range for suitable SCCI combustion under different engine speeds and compression ratio conditions. In this study, we found that the higher intake temperature is very effective to evaporate the injected fuel, and this effect helps to enhance SCCI combustion. The intake temperature can be controlled by the compression ratio. Finally, the injection timing and stratified mixture formation played an important role in achieving stable combustion and extending the operating range.

1. Introduction In order to cope with intensified global emission standards such as ULEV (ultralow emission vehicle), CAFE (corporate average fuel economy), and the Kyoto Protocol, there are increasing demands in the automotive industry for better fuel efficiency and lower emissions levels. In turn, there have been substantial efforts to develop low-pollution engine technologies that can satisfy such emission regulations. Stratified-charge compression ignition (SCCI) combustion uses a new gasoline combustion concept of controlled auto ignition1 and creates a localized dense mixture based on stratified combustion to facilitate self-ignition. Such an approach enables the combustion of a stratified lean mixture. In this study, the SCCI combustion system was developed to enhance the operating region of homogeneous charge compression ignition2-5(HCCI) combustion. The only difference between two concepts is that HCCI combustion generates a uniform mixture for reignition and uses fuel with a high cetane number for compression-ignition. On the other hand, the SCCI * Corresponding author. Tel.: +82-31-418-9293. Fax: +82-31-4065550. E-mail: [email protected], [email protected]. (1) Lee, K.; Lee. C. An Experimental Study of the Extent of the Operating Region and Emission Characteristics of Stratified Combustion Using the Controlled Autoignition Method. Energy Fuels 2006, 20, 1862-1869. (2) Rudolf, H. S. Homogeneous Charge Compression Ignition (HCCI): Benefits, Compromises, and Future Engine Applications. SAE Tech. Pap. Ser. 1999, 1999-01-3682. (3) Toshio, S. HCCI Combustion of Hydrogen, Carbon Monoxide and Dimethylether (DME). SAE Tech. Pap. Ser. 2002, 2002-01-0112. (4) Tiegang, H.; Shenghua, L.; Longbao, Z.; Chi, Z. Combustion and Emission Characteristics of a Homogeneous Charge Compression Ignition Engine. Pro. IMechE, Part D: J. Automob. Eng. 2005, 219 (9), 11331139. (5) Sato, S.; Kweon, S. P.; Yamashita, D.; Iida, N. Influence of the Mixing Ratio of Double of Componential Fuels on HCCI Combustion. Int. J. Automot. Technol. 2006, 7 (3), 251-259.

Table 1. Engine Specifications engine class engine type bore × stroke volume CR IVO/IVC EVO/EVC max power (ps/rpm)

ND 130DI 4-stroke, single cylinder 95 mm × 95 mm 673 cm3 18 BTDC 20°/ABDC 50° BBDC 44°/ATDC 44° 13/2400

Table 2. Specifications of Fuel Injection System injection pressure spray geometry spray angle

5 MPa hollow cone/swirl type 60°

combustion method involves a stratified-mixture compression ignition, and it uses fuel with a relatively low cetane number like gasoline to form a heterogeneous mixture. Namely, the difference is the mixture distribution of homogeneity or heterogeneity. It is well-known that the intake temperature and the equivalence ratio are important factors to achieve SCCI combustion.6-10 Much research has been performed to optimize the combustion in a gasoline premixture compression-ignition (6) Aoyama, T.; Hattori, Y.; Mizuta, J.; Sato, Y. An Experimental Study on Premixed-Charge Compression Ignition Gasoline Engine. SAE Tech. Pap. Ser. 1998, 960081. (7) Choi, G. H.; Han, S. B.; Dibble, R. W. Experimental Study on Homogeneous Charge Ignition Compression Engine with Exhaust Gas Recirculation. Int. J. Automot. Technol. 2004, 5 (3), 195-200. (8) Lavy, J.; Dabadie, J.-C.; Angelberger, C.; Duret, P.; Juretzka, A.; Scha¨flein, J.; Ma, T. H.; Lendresse, Y.; Satre, A.; Schulz, C.; Kra¨mer, H.; Zhao, H.; Damiano, L. Innovative Ultra-Low NOx Controlled Auto-Ignition Combustion Process for Gasoline Engine. SAE Tech. Pap. Ser. 2000, 200001-1837. (9) Kaneko, M.; Morikawa, K.; Itoh, J.; Saishu, Y. Study on Homogeneous Charge Compression Ignition Gasoline Engine. COMODIA 2001, 441-446.

10.1021/ef060638h CCC: $37.00 © 2007 American Chemical Society Published on Web 05/17/2007

1902 Energy & Fuels, Vol. 21, No. 4, 2007

Lee and Lee

Figure 1. Schematic diagram of direct-injection-type SCCI engine system.

Figure 3. Comparison of pressure and ROHR with injection timing. Table 3. Engine Test Conditions engine speed compression ratio A/F intake air temperature injection pressure injection timing fuels

1200 rpm 18, 16.2, 14.2 40, 50, 60, 70 353 K, 393 K, 433 K 5 MPa late injection gasoline

the problem of the HCCI operating region still remains unsolved. This study attempts to achieve a higher ignition temperature by increasing the intake temperature, and the combustion and Figure 2. Comparison of pressure and ROHR with injection timing.

engine by changing the autoignition timing through internal exhaust gas recirculation, variable valve timing, and intake temperature.10 In addition, there have been studies to assess combustion characteristics by controlling compression ratio variation,15-16 intake temperature, and negative valve overlap11-14 to obtain stable HCCI combustion using gasoline fuel. However, (10) Urushihara, T. Expansion of HCCI Operating Region by the Combination of Direct Fuel Injection, Negative Valve Overlap and Internal Fuel Reformation. SAE Tech. Pap. Ser. 2003, 2003-01-0749. (11) Urushihara, T.; Hiraya, K.; Kakuhou, A.; Itoh, T. Combustion Characteristics and Exhaust Gas Emissions of Lean Mixture Ignited by Direct Diesel Fuel Injection with Internal EGR. SAE Tech. Pap. Ser. 1999, 1999-01-3265.

(12) Peng, Z.; Zhao, H.; Ladommatos, N. Visualization of the Homogeneous Charge Compression Ignition/Controlled Autoignition Combustion Process Using Two-Dimensional Planar Laser-Induced Fluorescence Imaging of Formaldehyde. Pro. IMechE, Part D: J. Automob. Eng. 2003, 217 (12), 1125-1134, . (13) Xu, H.; Rudolph, S.; Liu, Z.; Wallace, S.; Richardson, S. H.; Wyszynski, M.; Megaritis, A. An Investigation into the Operating Mode Transitions of a Homogeneous Charge Compression Ignition Engine Using EGR Trapping. SAE Tech. Pap. Ser. 2004, 2004-01-1911. (14) Urata, Y.; Awasaka, M.; Takanashi, J.; Kakinuma, T.; Hakozaki, T.; Umemoto, A. A Study of Gasoline-Fuelled HCCI Engine Equipped with an Electromagnetic Valve Train. SAE Tech. Pap. Ser. 2004, 2004-01-1898. (15) Haraldsson, G.; Tunestasl, P.; Johansson, B.; Hyvarnen, J. HCCI Combustion Phasing with Closed-Loop Combustion Control Using Variable Compression Ratio in a Multi Cylinder Engine. 2003, 2003-01-1830. (16) Christensen, M.; Hultqvist, A.; Johansson, B. Demonstrating the Multi Fuel Capability of Homogeneous Charge Compression Ignition Engine with Variable Compression Ratio. SAE Tech. Pap. Ser. 1999, 1999-013679.

A Stratified Charge Compression Ignition Engine

Energy & Fuels, Vol. 21, No. 4, 2007 1903

Figure 5. Comparison of pressure and ROHR with intake temperature. Figure 4. Comparison of pressure and ROHR with injection timing.

emission characteristics of a SCCI engine system were analyzed by varying injection timing, intake temperature, and compression ratio. 2. Experimental Apparatus and Procedure 2.1. Experimental Apparatus of test Engine. For this experiment, a new fuel supply system was installed in a single-cylinder diesel engine to realize a stratified charge compression-ignition engine system. Tables 1 and 2 present the engine and injector specifications, and Figure 1 shows the schematic diagram of the direct-injection-type SCCI engine. To realize direct-injection-type SCCI combustion, the mechanical nozzle of the cam-plunger was removed, and a low-pressure common-rail-type injector was attached at the center of the engine head to control the injection timing and injected fuel amount. A spark-ignition direct-injection injector was used as the low-pressure common-rail injector for this study, and the injection pressure was set at 5 MPa, which is lower than that of a diesel engine, to reduce the fuel impingement caused by collision of the injected fuel on the wall during the initial stages of the compression process. Since gasoline has a lower cetane number than diesel, a temperature controller was installed before the intake port to increase the intake temperature needed for the gasoline autoignition. 2.2. Experimental Procedure. The experimental conditions for this study are shown in Table 3. For the performance test, the coolant temperature was maintained at 80 ( 2 °C, and the intake temperature, compression ratio, injection timing, and air-fuel ratio

Figure 6. IMEP value according to various compression ratios.

were changed to investigate the performance and emission characteristics of a SCCI engine.

3. Results and Discussions 3.1. The Combustion Characteristics with the Injection Timing. The results of the combustion pressure and the heat release rate are shown in Figures 2 and 3 at an intake temperature of 353 K and compression ratios of 18 and 16.2, respectively. As the injection timing is retarded, the combustion pressure is increased and the peak pressure moves toward top dead center (TDC). The reason for this is because it makes a locally rich


Figure 7. HC, CO, and NOx emission with injection timing.

mixture distribution inside the piston bowl when the injection timing is retarded. On the other hand, as the injection timing is advanced, the fuel mixture distribution reaches a homogeneous state, and it results in lean conditions. As a result, the combustion will be unstable. In addition, as the autoignition temperature of a gasoline fuel increases, it leads to a long ignition delay time. In order to retard the injection timing, the in-cylinder temperature is increased during the compression stroke, and the injected fuel is easily stratified inside the piston bowl. Thus, the injected fuel is quickly evaporated and easily combusted. As can be seen in Figure 3, the combustion characteristics are improved because the injected fuel is burned in the locally rich mixture distribution in the piston bowl when the injection timing is retarded toward TDC. From the results of Figures 2 and 3, we found that the combustion pressure and temperature in the cylinder are strongly affected by reducing the compression

Lee and Lee

ratio, and the compression ratio is an important factor in controlling the evaporation and autoignition characteristics. For an intake temperature of 393 K, the characteristics of pressure and the heat release rate are shown in Figure 4. The pressure history and the rate of heat release showed more active combustion characteristics with an increase in the intake temperature. As the intake temperature is decreased, the autoignition timing is delayed due to poor evaporation of the injected fuel. This result also revealed that retarding the injection timing increased combustion pressure and the heat release rate and moved the peak value to TDC. This is caused by the stratified mixture formation. Therefore, mixture stratification has a great effect on the combustion characteristics. 3.2. Combustion Characteristics with the Intake Temperature. For an air-fuel ratio and a compression ratio of 60 and 16.2, respectively, the combustion pressure and the heat release rate corresponding to the changing intake temperature are shown in Figure 5. Although the intake temperature of 353 K and injection timing of 20° before top dead center (BTDC) constitute conditions for stratification, the low temperature of 353 K does not allow sufficient vaporization of the injected fuel. Thus, this temperature yields poor combustion characteristics. On the other hand, increasing the intake temperature creates conditions for sufficient vaporization of the injected fuel, which allows proper combustion. Increasing the intake temperature tends to enlarge the possible combustion region. From these characteristics, we found that the vaporization of gasoline is started by the increased intake temperature, which shortens the arrival time to the ignition temperature. 3.3. Indicated Mean Effective Pressure (IMEP) Characteristics with the Injection Timing and the Compression Ratio. The IMEP characteristics of the stratified charge

Figure 8. Emission characteristics of HC and NOx according to compression ratio.



Figure 9. Characteristics of soot according to compression ratio and intake temperature.

combustion according to the variation in the injection timing and the compression ratio are shown in Figure 6. The operating area of SCCI combustion is limited by the following two areas: (1) misfire area and (2) knocking limit area. The first boundary is the misfire area. Misfire occurs when lean conditions are formed, caused by a fuel film, because a large amount of injected fuel is impinged on the cylinder. This fuel film results in unstable combustion. The cycle-to-cycle variation due to this effect is further aggravated by the lean relative air-fuel ratio. As the air-fuel ratio moves to the lean region, the net heat release rate of fuel is also decreased. It is thought that the lower average combustion temperature caused

by lean conditions leads to more unburned HC and tends to increase the coefficient of variation of IMEP. Therefore, misfire occurs when the relative air-fuel ratio is too lean to allow the oxidation of the fuel. Knocking occurs when an excessively violent rise in heat release causes pressure fluctuation. In order to detect the knocking phenomenon, the data acquisition system is set to record the amplitude of the filtered pressure trace of each cycle when the air-fuel ratio closes to the knocking limit. From this result, as the injection timing retards toward TDC, the value of IMEP increases. In addition, a lower compression ratio makes the IMEP decrease because unstable combustion is occurring, owing to a lower in-cylinder temperature caused


Lee and Lee

Figure 11. Operating range by stratified mixture formation according to compression ratio and intake temperature.

Figure 10. Effect of compression ratio, injection timing, and intake temperature on the combustion stability.

by a lower compression ratio. This result also revealed that IMEP is affected by injection timing. For enhancing the stratified combustion, the compression ratio and the intake temperature are controlled by the injection timing. As the compression ratio decreases and injection timing advances, knock combustion may be observed in the misfire region. From these results, we found that the knocking and misfire regions can be controlled by intake temperature and injection timing. 3.4. Emission Characteristics with Injection Timing, Intake Temperature, and Compression Ratio. For a compression ratio and air-fuel ratio of 18 and 70, respectively, the emission characteristics with injection timing are shown in Figure 7. When the injection timing is retarded, the emission of NOx is increased. This is because the injected fuel is richly distributed on the bowl zone of the piston crown. It is expected that combustion will activate with knocking. On the other hand, when the injection timing advances, the emissions of HC and CO are increased because the injected fuel is attached to the cylinder wall as well as the top of the piston head, owing to lower ambient pressure. From Figure 7, we found that a BTDC of 70∼80° is the optimal injection timing for reducing both NOx and HC emissions. The emission characteristics with air-fuel ratio, compression ratio, and intake temperature are shown in Figure 8. As the compression ratio increases, the gasoline fuel is easily evaporated due to a higher in-cylinder temperature, and it results in proper combustion.

The emission characteristics of HC and NOx with the airfuel ratio at an intake temperature of 353 K are shown in Figure 8a. As injection timing retards toward TDC, the emission of HC is reduced. On the other hand, the emission of NOx increases rapidly in the case of the lean A/F region. By retarding the injection timing, it was found that this region is included in the knocking zone. Therefore, when the air-fuel ratio is rich, the knocking region is broadened. For a compression ratio of 16.2, the emission characteristics with air-fuel ratio and injection timing are shown in Figure 8b. Though the injection timing is retarded toward TDC, the emission of NOx is lower than that in Figure 8a. This is because the knocking region is narrowed due to the lower intake temperature and compression ratio. As the air-fuel ratio is increased, the knocking region and HC emissions are increased. For a compression ratio of 14.2, the emission characteristics versus the intake temperature are shown in Figure 8c and d. Figure 8c indicates that the emission characteristics of HC are exhausted regardless of the injection timing and air-fuel ratio. It is found that the large amount of HC emissions was exhausted due to frequent misfires compared with other conditions. For an intake temperature set to 434 K, the emission characteristics are shown in Figure 8d. It is found that the region of misfire is reduced due to increased intake temperature compared with Figure 8c. The result of Figure 8d is similar to that of Figure 8a. Compared with Figure 6, although the combustion performance decreases little compared with Figure 8a, the emission performance is improved more than 50% of that of Figure 8a. The characteristics of NOx and soot with the compression ratio, intake temperature, and injection timing are shown in Figure 9. The characteristics of the generated soot tend to coincide with the IMEP as shown in Figure 6. The soot is increased by forming a locally rich mixture distribution in the piston crown. However, as the stratified mixture distribution is formed in the bowl region and self-ignites, the rate of soot generation tends to be reduced. For compression ratios of 18 and 16.2 and an intake temperature of 353 K, the emission characteristics are shown in Figure 9a and b. As the emission of NOx increases, the emission of soot is also increased. As the air-fuel ratio increases, the NOx and soot tend to increase. The massive amount of soot causes late combustion, which occurred after TDC. For an intake temperature (393 K) higher than that in Figure 9b, the emission characteristics are shown in Figure 9c. As the air-fuel ratio is increased, NOx is increased and soot is decreased. For a compression ratio of 14.2 and intake temper-


atures of 393 and 433 K, the characteristics of the emission are shown in Figure 9d and e. As the intake temperature increases, NOx and soot are increased. From the result of Figure 9d, when the compression ratio is reduced, soot tends to decrease. As the air-fuel ratio is rich with the same compression ratio, the amount of soot increases. 3.5. Combustion Stability. The effect of compression ratio, injection timing, and intake temperature on the combustion stability is shown in Figure 10. Figure 10a shows the combustion stability for the case of an intake temperature of 353 K. It is found that combustion is unstable when the injection timing is advanced because the injected fuel wets the cylinder wall and the condition of the autoignition is inadequate. On the other hand, when the injection timing TDC is retarded, the combustion stability is improved. The combustion stability is also stable at a higher compression ratio condition than a lower compression ratio case. This is because the in-cylinder temperature improves the autoignition temperature. As the air-fuel ratio is increased, the combustion stability tends to improve as the injection timing is retarded. Figure 10b shows the combustion stability for the case of a compression ratio of 14.2 according to the intake temperature. Since the in-cylinder temperature is lower at this compression ratio, misfire easily occurs. As compared with the intake temperatures of 393 and 433 K, the combustion stability is improved in the higher temperature case (434 K) versus that in the lower temperature case (393 K). The operating areas corresponding to compression ratios and intake temperatures found in Figures 8 and 9 are shown in Figure 11. Kaneko et al.9 reported that the operating range where a uniform mixture can be created is around an air-fuel ratio of 35∼40. The results of this study revealed that, if the stratified mixture is formed, the operating range can be extended


compared to a uniform mixture. Even at a leaner air-fuel ratio compared to the operation range that forms a uniform mixture, it was found that combustion became active when a stratified mixture was generated. As for the operating range according to variation in the intake temperature, although the fuel consumption rate is disadvantageous compared to cases with high compression ratios, stratified combustion characteristics are welldisplayed over a wide operating range. 4. Conclusion The combustion characteristics according to variation in the injection timing revealed that, as the injection timing was advanced, the time for autoignition was delayed and combustion pressure was decreased. Combustion and emission characteristics according to variation in the compression ratio revealed that, as the compression ratio was increased, injection timing for obtaining stable combustion was moved toward TDC and the IMEP value increased. In cases where a stratified mixture was formed by changing the intake temperature, compression ratio, and injection timing, the operating region can be extended to a wider range compared to that of a uniform mixture in the case of an air-fuel ratio of 40∼70. Increasing the intake temperature created conditions for sufficient vaporization of the injected fuel, which allowed active combustion. The injection timing and induced air temperature played an important role in active stratified combustion. Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD; KRF-2006-214-D00020). EF060638H

An Experimental Study on the Combustion and Emission

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