An Experimental Study on the Two-Stage Combustion Characteristics

Energy & Fuels 2005, 19, 393-402

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An Experimental Study on the Two-Stage Combustion Characteristics of a Direct-Injection-Type HCCI Engine Kihyung Lee* Department of Mechanical Engineering, Hanyang University, 1271 Sa-1 Dong, Sangrok-gu Ansan-si, Gyenggi-do, 426-791, Korea

Changsik Lee Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdong-gu, Seoul, 133-791, Korea

Jeaduk Ryu and Hyungmin Kim Graduate School of Hanyang University, Department of Mechanical Engineering, Hanyang University 1271 Sa 1 Dong, Ansan, Kyungki Do, 425-791, Korea Received July 7, 2004. Revised Manuscript Received November 25, 2004

The purpose of this study was to investigate the combustion characteristics of a direct-injectiontype homogeneous charge compression ignition (HCCI) engine. From this experimental study, we found that the diesel HCCI combustion phenomenon occurred in two stages of a combustion pattern, which are the cool flame and the hot flame. To investigate the combustion and emission characteristics of the HCCI engine, we evaluated the influence of intake air temperature, pressure, and an additive on HCCI combustion and emission performance characteristics; in particular, we focused on those characteristics of the cool and hot flame, the auto-ignition time, and the indicated mean effective pressure (IMEP) under various engine running conditions. This research showed that, as the intake temperature was increased and the additive was used, the onset angle of cool and hot flames and the starting time of auto-ignition were advanced; moreover, the influence of intake conditions (pressure, temperature) affected the cool flame and the hot flame simultaneously, whereas the additive mainly affected the cool flame more than the hot flame. In the higher-speed regions, the rate of the hot flame varied according to the air:fuel ratio; yet, in the lower-speed regions, an inverse trend occurred. This result was determined based on the time needed to reach a critical temperature for H2O2 decomposition. In the rich-mixture region, the ignition delay was inversely proportional to the intake temperature; however, in the leanmixture region, an inverse trend occurred. An advancement of the auto-ignition time increased the HCCI engine output; however, excessive advancement decreased the IMEP and also increased the NOx emissions, because of knocking.

1. Introduction As environmental problems such as greenhouse gases, ozone layer decay, and acid rain continue to grow, technologies based on the reduction of exhaust emissions and better fuel consumption have become the major focus of engine research. Proposals for the regulation of exhaust emissions have been strengthened by the demands that engines meet super ultralow emissions vehicle (SULEV), zero emissions vehicle (ZEV) (moreso than low emissions vehicle, LEV), and ultralow emissions vehicle (ULEV) standards in the United States. Diesel engines have better thermal efficiency than gasoline engines; however, because the fuel is directly injected during the end of a compression stroke with a lean air:fuel ratio and under high-pressure and * Author to whom correspondence should be addressed. E-mail: [email protected].

Table 1. Engine Specifications specification

value

engine type bore stroke compression ratio IVO/IVC EVO/EVC

four-stroke, single-cylinder 95 mm 95 mm 18 BTDC 20°/ABDC 44° BBDC 44°/ATDC 44°

high-temperature conditions, serious problems in exhaust emission performance result, primarily from the NOx emissions, which are caused by a steep heat release in the premixed flame region, and from the particulate matter (PM) emissions, which are caused by the heterogeneous air-fuel mixture in the diffusion flame region. Therefore, research and development of a cleancombustion, low-emission engine that could satisfy the more-stringent regulations for exhaust emissions was needed. Improvements in diesel combustion were at-

10.1021/ef0498420 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/12/2005

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Figure 1. Schematic diagram of the homogeneous charge compression ignition (HCCI) diesel engine system.

Figure 2. Mechanism of the 2-ethylhexyl nitrate (EHN) additive.

Figure 4. Definition of the combustion parameter.

Figure 3. Effect of additive on the cetane improver. Table 2. Engine Test Conditions property

value

engine speed air:fuel ratio intake air temperature injection time charge air pressure fuel

900-1800 rpm 43, 52, 64, 72 353 K, 393 K, 433 K ATDC 30°-120° 1-1.4 bar diesel, diesel + EHN (0.5%)

tempted by promoting air-fuel mixtures that would utilize an optimal design for the intake port and combustion chamber, as well as by trying to control the diesel combustion through the use of exhaust gas recirculation (EGR), injection timing, and multiple injections. Besides this combustion control method, after-treatment methods were tried, using oxidation and a lean NOx catalyst, ultrahigh-pressure injections, and alternative fuels. Still, these developments failed to satisfy the tightened emission regulations. To solve the problem, a premixed combustion system that could fully and homogeneously mix a fuel injected

during intake and compression stroke with air and still could be auto-ignited at the same time during lean homogeneous conditions, such as homogeneous charge combustion ignition (HCCI), began to attract attention. A concept of HCCI combustion was initially applied by Noguchi et al.1 and Onishi et al.2 in a two-stroke engine. Najt et al. applied HCCI combustion to a fourstroke engine and conducted research that involved the chemical reactions of low-temperature and high-temperature combustion in HCCI combustion.3 Thring et al. achieved gasoline HCCI, using a spark ignition method under low load conditions.4 As HCCI combustion began to utilize an auto-ignition method that differed in diesel and gasoline engines, the characteristics of auto-ignition and the ignition delay of fuel could be seen to have important roles in HCCI combustion; therefore, Curran et al. investigated the low-temperature and high-temperature reaction theory using isooctane and (1) Noguchi, M.; Tanaka, Y.; Tanaka, T.; Takeuchi, Y. A Study on Gasoline Engine Combustion by Observation of Intermediate Reactive Products during Combustion. SAE Tech. Pap. Ser. 1979, 790840, 1-13. (2) Onishi, S.; Jo, S. H.; Shoda, K.; Jo, P. D.; Kato, S. Active Thermoatmosphere Combustion (ATAC)sA New Combustion Process for Internal Combustion Engines. SAE Tech. Pap. Ser. 1979, 790501, 1-10. (3) Najt, P. M.; Foster, D. E. Compression-Ignited Homogeneous Charge Combustion. SAE Tech. Pap. Ser. 1983, 830264. (4) Thring, R. H. Homogeneous-Charge Compression-Ignition (HCCI) Engine. SAE Tech. Pap. Ser. 1989, 892068.

Two-Stage Combustion in a HCCI Engine

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Figure 5. Comparison of the onset angle for the cool flame ((a) air:fuel ratio of A/F ) 43, engine speed of 1200 rpm; (b) A/F ) 72, 1800 rpm) and the hot flame ((c) air:fuel ratio of A/F ) 43, engine speed of 1200 rpm; (d) A/F ) 72, 1800 rpm).

n-heptane (similar to that of diesel fuel), so that the auto-ignition and ignition delay and emission characteristics could be evaluated and compared to the rapid compression machine (RCM) experiment result5,6 Minetti et al. investigated ignition delay at various initial pressures and temperature conditions using RCM.7 Heywood et al. investigated the auto-ignition characteristics, according to the fuel components (such as paraffins, cyclic paraffins, olefins, cyclic olefins, and aromatic hydrocarbon), using RCM and, as a consequence, proposed a combustion control method using various fuels and additives, according to the running conditions of a HCCI engine.8 (5) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. A Comprehensive Modeling Study of Iso-octane Oxidation. Combust. Flame 2002, 129, 253-280. (6) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. A Comprehensive Modeling Study of n-Heptane Oxidation. Combust. Flame 1998, 114, 149-177. (7) Minetti, R.; Carlier, M.; Ribaucour, M.; Therssen, E.; Sochet, L. R. A Rapid Compression Machine Investigation of Oxidation and Autoignition of n-Heptane: Measurement and Modeling. Combust. Flame 1995, 102, 298-309.

Moreover, because the HCCI combustion was ignited in both single-stage and two-stage combustions, which differed between diesel and gasoline engines, the ignition delay and the start time could be observed to have an important role in controlling HCCI combustion. This research was achieved using a direct-injection-type diesel method during the intake stroke in real singlecylinder engines, and observations were made regarding the cool and hot flame characteristics, according to the air:fuel ratio and engine speed (given in units of rpm), an additive that influences the auto-ignition and the start times, and the combustion and emission characteristics, according to these times. These results indicate that the ignition delay, the auto-ignition timing, and the combustion onset angle for HCCI combustion can be controlled through the engine running conditions (i.e., the air:fuel ratio, engine speed, and the use of additives), and the HCCI engine (8) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Two-Stage Ignition in HCCI Combustion and HCCI Control by Fuels and Additives. Combust. Flame 2003, 132, 219-239.

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Figure 6. Characteristics of cool and hot flames, relative to the air:fuel ratio (Ti ) 353 K, Pi ) 1 bar): (a) 1200 rpm and (b) 1800 rpm.

output as well as NOx and soot emissions were improved using these control parameter methods. 2. Experimental Apparatus and Procedures 2.1. Experimental Apparatus. The single-cylinder engine used in this study was modified from a direct-injection-type diesel engine and was controlled by changing the engine speed, using a dc motor. Table 1 and Figure 1 show the specifications and schematic diagram of the HCCI engine, respectively. The air:fuel ratio in the engine was changed by controlling the injection timing with a universal electronic control unit (ECU) system (model M4); to investigate the effects of intake air temperature, which could improve fuel atomization by controlling the auto-ignition timing, the intake heater was used to change the intake air temperature during engine combustion. To observe the effect of the charged air pressure, a supercharger that was operating off an ac motor and an inverter was used. Fuel was directly injected into the combustion chamber at a pressure of 50 bar, using a low-pressure-type common rail injector for the purpose of preventing a spray collision and fuel wall flow, which were produced when the fuel was injected during the intake stroke and at the beginning of the compression stroke. To observe the HCCI combustion, in terms of the air:fuel ratio and injection timing, a pressure sensor was installed on the cylinder head. 2.2. Fuel Additive. A fuel additive, which is defined as a chemical substance not composed of carbon and hydrogen (which prevents and eliminates deterioration of the engine performance, fuel consumption, exhaust emissions, and knock-

Figure 7. Comparison of the cool and hot flames, according to operating conditions: (a) effect of intake pressure, (b) effect of intake temperature (without additive), and (c) effect of intake temperature (with additive). ing), was used. As shown in Figure 2, a cetane improver that was among the fuel additives easily decomposed at an appropriate temperature (∼473 K), which improved the rate of the chain reaction and reduced the auto-ignition temperature, thereby improving the auto-ignition rate during diesel fuel combustion. The selected cetane improver was 2-ethylhexyl


Figure 8. Comparison of ignition delay values. nitrate (EHN), which has been referenced in various papers.8,9 Generally, although the improved range of the cetane number differs according to fuel property, the papers stated that, if 1000 ppm of EHN was used, the cetane number would be increased by ∼4-6. However, as shown in Figure 3, cetane improvement with increasing additive quantity was considerably greater for high-cetane-number fuel than for low-cetanenumber fuel. 2.3. Experimental Conditions and Procedures. The conditions of the experiment are shown in Table 2, in which a coolant temperature of 353 ( 2 K was maintained during the experiment. This experiment was conducted with a specified injection timing, intake air temperature, and engine speed for each air:fuel rate and charged air pressure. The combustion parameters such as the cool flame, hot flame, and ignition delay were obtained using the HCCI combustion pressure and heat-release data, as shown in Figure 4. From the rate of heat release (ROHR) data profile, the autoignition time was defined from the onset angle of the hot flame, whereas the ignition delay was defined from the onset angle of the cool flame to the onset angle of the hot flame.7,8,10 The onset angle was defined as the crank angle when the heat release first exceeded 10% of the maximum heat-release rate for both cool and hot flames.11

3. Experimental Result and Discussion 3.1. Auto-ignition Characteristics in HCCI Engines. Figure 5 shows the result of the influence of the intake air temperature, intake air pressure, and additive on the onset angle for the cool and hot flames. From these figures, the onset angle for cool and hot flames was affected by the intake air pressure, intake air temperature, and additive. On the other hand, the influence of intake air pressure was smaller than others. Namely, the onset angle for the cool and hot flames were (9) Higgins, B.; Siebers, D.; Mueller, C. Effects of 2-Ethylhexyl Nitrate on Diesel-Spray Process, Sandia Report SAND 98-8243, Sandia National Laboratories, Albuquerque, NM, 1998, pp 3-13. (10) Ciezki, H. K.; Adomeit, G. Shock-Tube Investigation of Selfignition of n-Heptane-Air Mixtures under Engine Relevent Conditions. Combust. Flame 1993, 93, 421-433. (11) Kim, K.-O.; Azetsu, A.; Oikawa, C. A Study on the Control of Ignition and Combustion Ether in Homogeneous Charge Compression Ignition Engine, COMODIA 2001, pp 453-460.


advanced, according to increasing the intake air temperature and the amount of additive. This result was in accord with Heywood’s experiments.8 Therefore, if it was controlled by the auto-ignition time in the HCCI engine, the intake air temperature control was an effective method. However, this method is not realistic, in terms of real engine conditions; therefore, it was concluded that the method of using the additive was realistic. Panels a and b in Figure 6 show the characteristics of both the cool and hot flame, in terms of the air:fuel ratio at 1200 and 1800 rpm. As shown in Figure 6a and 6b, the fuel injection time did not affect the characteristics of auto-ignition in the HCCI engine, because the fuel injection time occurred during the intake stroke. Because the air:fuel ratio was rich, the rates of the hot and cool flame displayed a tendency to increase, because of an increase in the chemical reaction rate.12 We found that the effectiveness in the rising temperature from the cool flame influences the thermal explosion of the hot flame. Moreover, the increasing rate of the hot flame was uniform regardless of the engine speed overall. On the other hand, the cool flame is a fairly slow process and exists only over a rather narrow temperature range, so the extent and amount of temperature increase of the cool flame will inversely with engine speed. That is because faster compressional heating at high speed will pass through the cool flame window more rapidly than at lower engine speeds. The additive allows the cool flame to begin at earlier crank angle degrees than without the additive, because of earlier radical production, and the cool flame ends at the same temperature as without the additive, so the additive widens the window over which cool flames can occur and also opens the window earlier in the cycle. This also shows why the additive affects mostly the cool flame, because the additive decomposition is completed long before the hot ignition occurs.13 Figure 7 shows the effects of the intake pressure, temperature, and additive on the cool and hot flame, in terms of the air:fuel ratio at 1200 and 1800 rpm. In the three cases, the intake conditions influenced the cool flame more than the hot flame. As shown in Figure 7a, as the combustion chamber pressure before and after auto-ignition was increased and the quantity of the airfuel mixture in the combustion chamber was also increased, the intake pressure remained proportional to the cool and hot flames. As shown in Figure 7b, the intake temperature was proportionally inverse to the cool flame, yet proportional to the hot flame. Notably, the cool flame reacted sensitively in the low-speed, lean air-fuel mixture region, whereas the hot flame reacted sensitively in the high-speed, rich air-fuel mixture region. In Figure 6, we see that the hot flame reaction increased and the cool flame reaction decreased; as the intake temperature increased, the time to reach a critical temperature shortened. As shown in Figure 7c, both the cool flame (12) John, E. Computational Study of the Effects of Low Fuel Loading and EGR on Heat Release Rates and Combustion Limits in HCCI Engine. SAE Tech. Pap. Ser. 2002, 2002-01-1309. (13) Westbrook, C. K. Chemical Kinetics of Hydrocarbon Ignition in Practical Combustion Systems. Proc. Combust. Inst. 2000, 28, 15631577.

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Figure 9. Contour of the ignition delay: (a) A/F ) 43 and (b) A/F ) 72.

and the hot flame increased through the use of the additive, compared to that observed by not using it. However, when the additive and the intake heating were used at the same time, the auto-ignition time advanced in excess and the cool flame and the hot flame decreased. These results indicate that the intake pressure and temperature may affect the cool flame and hot flame simultaneously, whereas an additive mainly affects the cool flame. Figures 8 and 9 display the ignition delay, namely, the time from the onset angle for the cool flame to the hot flame under various engine operating conditions. The pressure and temperature of the auto-ignition time (defined as the point at which the hot flame began) were calculated using the adiabatic compression process. As the air:fuel ratio went into the rich region, the ignition delay decreased under all tested engine running conditions. Figure 9 outlines the ignition delay via the temperature and pressure at the time of auto-ignition. The ignition delay is inversely proportional to the autoignition temperature within the rich-mixture region; however, within the lean-mixture region, there is no such inverse proportionality. Within the rich-mixture region, this result is in agreement with Minetti and Ciezki’s results; however, within the lean-mixture region, the reverse is observed.7,10 Minetti and Ciezki’s results were obtained through RCM experimentation and the use of a Shock tube with n-heptane fuel at a steady stoichiometric air:fuel ratio. Moreover, this result was obtained from a real engine that was using diesel fuel at a relatively lean air-fuel ratio, and we concluded that a variation occurred with the air:fuel ratio to the ignition delay. 3.2. Output and Combustion Characteristics in HCCI Engines. Figure 10 shows the effects of the engine speed, air:fuel ratio, and additive on the combus-

tion onset angle. The combustion onset angle is defined as the crank angle at which the burning rate is 10%. As shown in Figure 5, the combustion onset angle was advanced through the onset angle of the cool and hot flames, which were advanced because of the influence of the intake temperature and the additive. Therefore, it was confirmed that the intake temperature and the additive effect on the combustion in the HCCI engine form these results. Figure 11a details the effects of the air:fuel ratio on the combustion pressure and the ROHR profile at an intake temperature of 353 K, engine speed of 1200 rpm and a BTDC 120° injection time, as well as the effects of the intake pressure on the combustion pressure profile with an air:fuel ratio of 74, an intake temperature of 353 K, an engine speed of 1200 rpm, and a BTDC 120° injection time. As panels a and b in Figure 11 indicate, the crank angle that started the HCCI combustion was ∼330° in all the cases. The starting crank angle of HCCI combustion was defined as an onset crank angle of the hot flame, as shown in the ROHR profile. In the other hand, at a relatively high air:fuel ratio (43) or relatively high intake pressure (1.4 bar), the chemical-kinetic rates were very fast, resulting in a rapid rise of pressure due to combustion and a shorter ignition delay (see Figure 9) and a higher hot flame (see Figure 6). In short, as the air:fuel ratio was increased, the rate of pressure increase became progressively slower. Figure 12 shows the effects of the combustion onset angle on the indicated mean effective pressure (IMEP) at various intake pressures, temperatures, and set additive conditions in the HCCI engine. As the intake pressure was increased, the combustion onset angle advanced in the rich air-fuel mixture region. However, as the combustion onset angle advanced beyond 340°,



Figure 11. Combustion pressure and rate of heat release (ROHR) profile: (a) effect of the air:fuel ratio and (b) effect of the intake air temperature.

Figure 10. Comparison of the combustion onset angle and burning duration.

the IMEP decreased. The reason for this trend was caused by the knocking phenomenon that is due to excessive advancement of the auto-ignition time. Therefore, to improve performance in the HCCI engine, it is necessary to implement an HCCI combustion control method wherein the auto-ignition time is controlled so as not to excessively advance under rich air:fuel ratio conditions. In addition, as the start time advanced ∼3° and the IMEP increased, because of the additive region, the effect of the additive on the auto-ignition time and start time was confirmed to improve the HCCI engine performance. 3.3. Emission Characteristics in HCCI Engines. Figures 13 and 14 indicate the effect of the combustion onset angle on the NOx and soot emissions per output (presented in terms of kilowatts, kW) at various air:fuel ratios. From this figure, we found that the NOx and soot emissions were increased in rich air:fuel ratio regions.

The reason for this trend is that this region was not yet completely mixed and that locally rich or almost stoichiometric regions still exist at the time of combustion and produce NOx locally. In addition, both NOx and soot emissions decrease as the combustion is retarded, which means that delaying combustion from 340° to 350° allows more time for mixing and, therefore, reduces the NOx and soot production. Therefore, it is necessary to control the intake temperature, to improve the fuel evaporation and atomization, which have a great effect on the air-fuel mixing and HCCI combustion performance. Figures 15 and 16 specify the effect of the combustion onset angle on the NOx and soot emissions per output (given in units of kW) using the additive. Compared to Figures 13 and 14, the NOx and soot emissions are at similarly low levels as in the case of the lean air:fuel ratio region without an additive; on the other hand, the soot emissions showed a tendency toward considerable reduction in the rich air:fuel ratio region, and the NOx emissions showed a contrast tendency. This result was due to the influence of the additive, which acted to improve combustion efficiency (as shown in Figure 12) in soot emissions and to advance the combustion onset angle (as shown in Figure 12) in NOx emissions. From these results of HCCI emission performance, the auto-ignition crank angle directly influence a NOx

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Figure 12. Comparison of the indicated mean effective pressure (IMEP) for the combustion onset angle: (a) A/F ) 43 and (b) A/F ) 74.

and soot emission characteristic. Reitz et al. explained the effect of a fuel injection time on the amount of fuel film, which influences the HCCI emission characteristics and also showed that the wall film amount is affected not only by the injection timing, but also by the ambient conditions.14 Therefore, an injection time is able to evaporate most of injected fuel, but the dispersion (14) Hruby, E.; Ra, Y.; Reitz, R. D. Parametric Study of Combustion Characteristics in a DI Diesel HCCI Engine with a Low-Pressure Fuel Injector. Presented at ILASS Americas, 17th Annual Conference on Liquid Atomization and Spray Systems, 2004.

and air-fuel mixing time of the evaporated fuel is not sufficient, because both NOx and soot emissions decrease as the combustion onset angle is retarded. We estimated that the combustion control method is necessary to reduce NOx and soot emissions simultaneously, because the auto-ignition control method, which is, compression ratio control and exhaust gas recirculation, could retard a combustion onset angle. Therefore, we look forward to improving the combustion and emission performance of direct-injection-type HCCI engines.


Figure 13. NOx emissions (without additive).


Figure 15. NOx emissions (with additive).

Figure 14. Soot emissions (without additive). Figure 16. Soot emissions (with additive).

4. Conclusions An experimental study of a two-stage combustion and its emission characteristics of a direct-injection-type homogeneous charge compression ignition (HCCI) engine was performed. The influence of the intake temperature, pressure conditions, and cetane number on the combustion and emission characteristics was investigated. From this investigation, the following conclusions were drawn: (1) As the intake temperature increased and the additive used, the onset angle for the cool and hot flames and the start time of auto-ignition were advanced. Within the high-speed region, the rate of the hot flame varied according to the air:fuel ratio, and within the lowspeed region, an inverse trend occurred. This result was based on the time needed to reach a critical temperature for H2O2 decomposition.

(2) The influence of the intake condition and the additive to the cool flame and the flame quantity indicated that the intake pressure and temperature affected the cool flame and hot flame simultaneously, while an additive primarily affected the cool flame. (3) The influence of additives and intake air temperature revealed the largest onset angle for both the cool and hot flames. The ignition delay appeared within the rich air:fuel ratio region, which is similar to existing research that shows a decreasing tendency if the temperature increases, but with a lean air:fuel ratio region (an air:fuel ratio of 72), there is a contrastive tendency. (4) Advancing the auto-ignition time increased the HCCI engine output; however, excessive advancement

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led to a decrease of the IMEP and an increase in NOxemissions due to knocking. Therefore, to improve the HCCI output and emissions performance, it is necessary to set a suitable start time control for the engine load conditions.

Lee et al.

Acknowledgment. This work was supported by the “Development of techniques on the fundamental and practical use of a HCCI” project at Korea Automotive Technology Institute, 2004. EF0498420

An Experimental Study on the Two-Stage Combustion Characteristics

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