Heat Release Analysis on Combustion and Parametric Study on

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Energy & Fuels 2006, 20, 1870-1878

Heat Release Analysis on Combustion and Parametric Study on Emissions of HCCI Engines Fueled with 2-Propanol/n-Heptane Blend Fuels Lu¨ Xingcai,* Hou Yuchun, Ji Libin, Zu Linlin, and Huang Zhen School of Mechanic & Power Engineering, Shanghai Jiaotong UniVersity, Shanghai, People’s Republic of China ReceiVed March 27, 2006. ReVised Manuscript ReceiVed May 23, 2006

This article investigates the inhibition effects of a 2-propanol additive on n-heptane homogeneous charge compression ignition (HCCI) combustion, analyzes the relationship between the emissions with the combustion parameters and fuel component, and evaluates the influence of cold exhaust gas recirculation (EGR) on HCCI combustion and emissions. The experiments were conducted on a single-cylinder HCCI engine using neat n-heptane and 10∼60% (by volume) 2-propanol/n-heptane blend fuels at 1800 rpm. The experimental results reveal that the ignition timing of the low-temperature reaction (LTR) is retarded, and the peak values of heat release during the LTR are decreased with an increase of 2-propanol in blend fuels. As a result, the ignition timing of the high-temperature reaction is delayed, both the maximum and lowest indicated mean efficient pressures (IMEP) are increased, and the combustion efficiency is also decreased. Parametric studies on CO and HC emissions show that the fuel volume with a high cetane number plays a major role in HC emissions. While the main parameter which has an important influence on CO emission is the maximum combustion temperature, other parameters including pressure rising rate, IMEP, and ignition timing have an indirect effect on CO emission. Moreover, the fuel component shows little effect on CO emissions. In the case of the operation stability of HCCI combustion, at a fixed fuel supplied energy for each cycle, the cycle-to-cycle variations of the maximum combustion pressure and its corresponding crank angle and ignition timing deteriorated with the an increase of the 2-propanol additive. Furthermore, EGR illustrates a substantial influence on the combustion and emissions of n-heptane for HCCI combustion doped with a high volume of the 2-propanol additive.

1. Introduction Both spark-ignition (SI) engines and compression-ignition direct-injection engines have been faced with increasingly stringent emissions legislation for many years. Despite substantial improvements with engine electronic control and after treatment, new standards scheduled to take effect in US Tier 2 Bin 10 and in European EURO 5 require another order of magnitude reduction in nitrogen oxide (NOx), particulate matter (PM), CO, and HC emissions beyond the already stringent emissions standards.1 Furthermore, the greenhouse effect of carbon dioxide on the global environment and the foreseeable future depletion of worldwide petrol reserves provide strong encouragement to explore new high-thermal-efficiency combustion systems for automobile engines. Therefore, developing an advanced combustion system to reduce engine-out emissions and improve thermal efficiency is the key project for engine researchers. Homogeneous charge compression ignition (HCCI) combustion is one of the most promising internal combustion engine concepts for the future and features substantial reductions in both NOx and PM emissions, while still providing high diesellike efficiency. HCCI is not a recent discovery. The first efforts to characterize HCCI combustion were done on two-stroke * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Kima, D. S.; Lee, C. S. Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR. Fuel 2006, 85, 695704.

gasoline engines,2,3 and the primary reason was to reduce the unburned hydrocarbon emissions at part load and to decrease fuel consumption. Shortly after that, the HCCI concept was also implemented in four-stroke engines.4 At the end of the past decade, there was continued interest in developing a better understanding of the mechanism of HCCI combustion over a wide range of operating conditions with different fuels. Recently, a large number of papers related to this new combustion mode were published.5-13 (2) Onishi, S.; Jo, S. H.; Shoda, K. ActiVed thermo-atmosphere combustion (ATAC)sA new combustion process for internal combustion engines; SAE 790501; Society of Automotive Engineering: Warrendale, PA, 1979. (3) Noguchi, M.; Tanaka, T.; Takeuchi, Y. A study on gasoline engine combustion by observation of intermediate reactive products during combustion. SAE 790840. (4) Najt, P. M.; Foster, D. E. Compression-ignited homogeneous charge combustion. SAE 830264. (5) Hildingsson, L.; Persson, H.; Johansson, B.; Collin, R.; Nygren, J.; Richter, M.; Alden, M.; Hasegawa, R.; Yanagihara, H. Optical Diagnostics of HCCI and UNIBUS Using 2-D PLIF of OH and Formaldehyde. SAE Paper 2005-01-0175; Society of Automotive Engineering: Warrendale, PA, 2005. (6) Santoso, H.; Matthews, J.; Cheng, W. K. Managing SI/HCCI DualMode Engine Operation. SAE Paper 2005-01-0162; Society of Automotive Engineering: Warrendale, PA, 2005. (7) Aceves, S. M.; Flowers, D. L.; Espincisco-Loza, F.; Babajimopoulos, A.; Assanis, D. N. Analysis of Premixed Charge Compression Ignition Combustion With a Sequential Fluid Mechanics-Multizone Chemical Kinetics Model. SAE Paper 2005-01-0115; Society of Automotive Engineering: Warrendale, PA, 2005. (8) Sjober, M.; Dec, J.; Cernansky, N. P. Potential of Thermal Stratification and Combustion Retard for Reducing Pressure-Rise Rates in HCCI Engines, Based on Multi-Zone Modeling and Experiments. SAE Paper 200501-0113; Society of Automotive Engineering: Warrendale, PA, 2005.

10.1021/ef0601263 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/20/2006

Heat Release Analysis on Combustion

However, its commercial implementation has been impeded by difficulties in the control of ignition timing, high peak cylinder pressures, and power density limitations. HCCI ignition processes are extremely sensitive to the intake charge temperature. Improper ignition timing can lead to damaging cylinder pressures, excessive rates of increased cylinder pressure, and efficiency losses. It is widely accepted that HCCI combustion is dominated by chemical kinetics. Accordingly, different fuels with different chemical properties exhibit different HCCI combustion characteristics and emissions. In HCCI combustion, for most of the hydrocarbon fuels exhibiting two-stage ignition, the temperature-pressure history of the fuel/air mixture during the compression process has profound effects on the main combustion event. The timing and magnitude of the low-temperature reaction significantly influence the later high-temperature reaction.14,15 Therefore, controlling the low-temperature reactions is essential to realizing a wide operating range with HCCI combustion. For high-octane-number fuels, control may be achieved by introducing enhancing additives that increase the temperature or produce active radicals during the compression stroke.16,17 For highcetane-number fuels, control may be achieved by introducing suppression additives that reduce the temperature or consume radicals during the compression stroke. A few reports have addressed ignition control of HCCI combustion with high-cetane-number fuels using reaction suppression by low-ignitability fuel. Chen et al. tried to control the ignition timing of HCCI combustion by changing the dimethyl ether (DME)/natural gas ratio according to engine load.18 Sahashi et al. introduced N2 and CO2 gas into DME HCCI combustion to control the ignition timing and combustion duration over a wide range of engine speeds and loads.19 Yamada et al. investigated the controlling mechanism of DME (9) Chang, J.; Guralp, O.; Filipi, Z.; Assanis, D. New Heat Transfer Correlation for an HCCI Engine DeriVed from Measurements of Instantaneous Surface Heat Flux. SAE Paper 2004-01-2996; Society of Automotive Engineering: Warrendale, PA, 2004. (10) Kong, S. C.; Reitz, R. D.; Christensen, M.; Johansson, B. Modeling the Effects of Geometry Generated Turbulence on HCCI Engine Combustion. SAE Paper 2003-01-1088; Society of Automotive Engineering: Warrendale, PA, 2003. (11) Peng, Z.; Zhao, H.; Ma, T. Ladommatos, Nicos. Characteristics of Homogeneous Charge Compression Ignition (HCCI) combustion and emissions of n-heptane. Combust. Sci. Technol. 2005, 177 (11), 21132150. (12) Yap, D.; Karlovsky, J.; Megaritis, A.; Wyszynski, M. L.; Xu, H. An investigation into propane homogeneous charge compression ignition (HCCI) engine operation with residual gas trapping. Fuel 2005, 84 (18), 2372-2379. (13) Seref, S. Examination of combustion characteristics and phasing strategies of a natural gas HCCI engine. Energy ConVers. Manage. 2005, 46, 101-119. (14) Lu¨, X.-C.; Chen, W.; Huang, Z. A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR, Part 1: The basic characteristics of HCCI combustion. Fuel 2005, 84, 1074-1083. (15) Lu¨, X.-c.; Chen, W.; Huang, Z. A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR, Part 2: Effect of operating conditions and EGR on HCCI combustion. Fuel 2005, 84, 1084-1092. (16) Lu¨, X.-c.; Chen, W.; Ji, L.; Huang, Z. The effects of external exhaust gas recirculation and cetane number improver on the gasoline homogenous charge compression ignition engines. Combust. Sci. Technol. 2006, 178, 1237-1249. (17) Tanaka, K.; Ferrann, C. J.; Heywood, J. B. Two-stage ignition in HCCI combustion and HCCI control by fuels and additives. Combust. Flame 2003, 132 (1-2), 219-239. (18) Chen, Z.; Konno, M.; Oguma, M.; Yanai, M. T. Experimental Study of CI Natural Gas/DME Homogeneous Charge Engine. SAE Paper 200001-0329; Society of Automotive Engineering: Warrendale, PA, 2000. (19) Sahashi, W.; Azetsu, A.; Oikawa, C. Effects of N2 and CO2 mixing on ignition and combustion in a homogeneous charge compression ignition engine operated on dimethyl ether. Int. J. Engine Res. 2005, 6, 423-431.

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premixed HCCI combustion by adding enhancing (O3) or suppressing (methane) additives.20 Christensen and Johansson used port injection of water to control the ignition timing of HCCI combustion.21 Ogawa et al. attempted to control the ignition timing and suppress the excessively rapid combustion in an HCCI engine with light naphtha as fuel and the direct in-cylinder injection of water and low-ignitability fuels for reaction suppression.22 n-Heptane is a primary reference fuel for octane rating in internal combustion engines and has a cetane number of 56, which is often used as an auto ignition surrogate for diesel fuel. n-Heptane can be easily operated in an HCCI engine, although it ignites too readily and combustion phasing is chronically overadvanced. Limited by early ignition and knock combustion at higher fuel/air equivalence ratios, the HCCI operating regime with neat n-heptane is extremely narrow. To broaden the HCCI operating range of high-cetane-number fuels, an inhibition additive may be added to the fuel. Recently, lower-alcohol fuels including methanol and ethanol have been widely used as vehicle alternative fuels, octane number improvers of gasoline, and inhibition additives of HCCI combustion. However, relatively little work has been reported on higher-alcohol fuels such as propanol, butanol, pentanol, and so forth being used as alternative or additives of auto fuel.23-25 All of these alcohols can be produced from coal-derived syngas. Given the abundant coal reserves in China, the use of such higher-alcohol fuels offers an attractive option to alleviate the country’s needs for transportation fuels. Some investigations have revealed that higher-alcohol fuels can be used to depress the combustion rate of hydrocarbon fuels.26,27 On the basis of this background, the main purpose of this article is to clarify the effects of 2-propanol addition on n-heptane HCCI combustion on the basis of a heat release analysis, to reveal the main factors which predominate HC and CO emissions, and to evaluate the influence of cold exhaust gas recirculation (EGR) on n-heptane HCCI combustion doped with a 2-propanol additive. 2. Experimental System and Test Fuels A single-cylinder, four-stroke, high-speed direct-injection (DI) diesel engine, with a bore of 95 mm, a stroke of 105 mm, and a compression ratio of 18.5, was employed as a prototype engine. (20) Yamada, H.; Ohtomo, M.; Yoshii, M.; Tezaki, A. Controlling mechanism of ignition enhancing and suppressing additives in premixed compression ignition. Int. J. Engine Res. 2005, 6, 331-339. (21) Christensen, M.; Johansson, B. Homogeneous charge compression ignition with water injection. SAE Paper 1999-01-0182; Society of Automotive Engineering: Warrendale, PA, 1999. (22) Ogawa, H,; Miyamoto, N.; Kaneko, N.; Ando, H. Combustion control and operating range expansion in an homogeneous charge compression ignition engine with direct in-cylinder injection of reaction inhibitors. Int. J. Engine Res. 2005, 6, 341-359. (23) Kelkar, A. D.; Hooks, L. E.; Knofzynski, C. Comparative study of methanol, ethanol, isopropanol, and butanol as motor fuels, either pure or blended with gasoline. Proceedings of the 33rd Intersociety Energy ConVersion Engineering Conference, Colorado, August 2-6, 1988; Vol. 4, pp 381-386. (24) Saeed, M. N.; Henein, N. A. Combustion phenomena of alcohols in C.I. engines. J. Eng. Gas Turbines Power 1989, 111 (3), 439-444. (25) Gautam, M.; Martin, D. W., II. Combustion characteristics of higheralcohol/gasoline blends. Proceedings of the Institution of Mechanical Engineers, Part A: J. Power Energy 2000, 214, 497-511. (26) Azatyan, V. V.; Borisov, A. A.; Merzhanov, A. G.; Kalachev, V. I.; Masalova, V. V.; Mailkov, A. E.; Troshin, K. Y. Inhibition of various hydrogen combustion regimes in air by propylene and isopropanol. Combust., Explos. Shock WaVes 2005, 41, 1-11. (27) Sinha, A.; Thomson, M. J. The chemical structures of opposed flow diffusion flames of C3 oxygenated hydrocarbons (isopropanol, dimethoxy methane, and dimethyl carbonate) and their mixtures. Combust. Flame 2004, 136 (4), 548-556.

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Table 1. Chemical and Physical Properties of n-Heptane and 2-Propanol

mole weight (g/mol) density (g/mL @ 298 K) lower heating value (MJ/kg) latent heat (MJ/kg) oxygen content % RON purity %

n-heptane

2-propanol

100.16 0.692 44.5 0.317 0 0 >99.7

60.10 0.785 34.0 0.66 26.7% 107 >99.9

n-Heptane was used as the baseline test fuel, and 2-propanol was selected as a suppression additive. Table 1 shows the physicalchemical properties of two fuels. During the experiments, n-heptane and 10∼60% 2-propanol/n-heptane blend fuels (by volume) were injected into the intake pipe at a location of approximately 0.35 m upstream of the inlet port; this provides sufficient time for premixed fuel evaporation and air mixing. The injection timing was fixed at 285° crank angle before top dead center (CA BTDC) for all measuring points. To ensure the repeatability and comparability of the measurements for different fuels and operating conditions, the intake charge temperature was fixed at 20 °C, held accurately to within (1 °C. The coolant-out temperature remains at 85 °C, held accurately to within (2 °C. The oil temperature was kept at 90∼95 °C. The engine speed was fixed at 1800 rpm. The cylinder pressure was measured using a Kistler model 6125A pressure transducer. The charge output from this transducer was converted to an amplified voltage using a Kistler model 5015 amplifier. The 1440 pulses per rotation (four pulses per crank angle) from a shaft encoder on the engine crankshaft were used as the data acquisition clocking pulses to acquire the cylinder pressure data. Pressure data were recorded using high-speed memory. For each operating condition, the cylinder pressures recorded at each crank angle were averaged over 40 consecutive cycles for the experiment. For all data presented, the 0° crank angle (CA) is defined as top dead center (TDC) at the compression stroke. CO, HC, and NOx emissions were measured with an AVL gas analyzer. In the experiments, the air flow rate and fuel consumption rate at each testing point were measured, and then, the n-heptane and 2-propanol consumption rates can be obtained according to the density and 2-propanol volume of each blend fuel. As a result, the equivalence ratio of the total fuel, partial equivalence ratio of n-heptane, and partial equivalence ratio of 2-propanol can be obtained according to following equations: φn-heptane ) (Gn-heptane × AFn-heptane)/Gair φ2-propanol ) (G2-propanol × AF2-propanol)/Gair φfuel ) (Gn-heptane × AFn-heptane + G2-propanol × AF2-propanol)/Gair where Gair is the mass flow rate of air, G2-propanol and Gn-heptane are the mass flow rates of 2-propanol and n-heptane, and AFn-heptane and AF2-propanol represent the stoichiometric A/F ratio of n-heptane and 2-propanol, respectively. In this paper, the start of combustion (SOC) is defined as the crank angle corresponding to 10% of the magnitude of the peak of heat release on the rising side of the curve, θPKL is the crank angle corresponding to the peak point of the heat release curve during the low-temperature reaction (LTR), and θEND is the specific crank angle corresponding to 10% of the magnitude of the peak of heat release on the falling side of the curve. Therefore, the NTC region can be defined as the interval angle between θPKL and SOC; the burn duration is the interval angle between SOC and θEND.

3. Experimental Results and Discussion 3.1. Operation Ranges and Combustion Efficiency. HCCI operating ranges are limited by “knock” combustion and misfire

Figure 1. HCCI operation ranges for n-heptane and 2-propanol/nheptane blend fuels.

Figure 2. HCCI combustion efficiency as a function of IMEP for n-heptane and blend fuels.

or partial combustion. Knock is the name given to the noise which is transmitted through the engine structure when autoignition of the mixture only partly occurs. The engine “knock” in HCCI combustion is different from that in an SI engine, because there is no flame propagation. During the experiments, the authors found that the pressure oscillation at TDC is very audible, combustion noise increased substantially, and “metal” noise radiated from in-cylinder can be heard when the rate of increasing cylinder pressure exceeds 15 bar/°CA. On the other hand, engine operation was unstable when a misfire or partial combustion occurred; this leads to much higher HC emissions. In this paper, the upper boundary is limited by a rate of increasing cylinder pressure of 15 bar/°CA, and the lower limit is defined as a coefficient of variation of the indicated mean efficient pressure (IMEP) of less than 10%. Figure 1 shows the effects of the 2-propanol volume in blend fuels on the stable operating ranges at 1800 rpm. For neat n-heptane, the maximum attainable IMEP of HCCI combustion without “knock” combustion is only about 3.38 bar. For blend fuels, the maximum attainable IMEP is increased with increasing 2-propanol volume, with the increase scaling linearly with addition. For example, the maximum engine load of 60% 2-propanol/n-heptane blend fuel is increased up to 5.2 bar, but the lowest stable IMEP is also increased substantially when the 2-propanol volume in a blend fuel exceeds 40%. Particularly, misfiring was observed from cylinder pressure when the 2-propanol volume in the blend fuel was higher than 50% at conventional environmental conditions (without any other accessory method). As can be seen in the following sections, the most advanced ignition timing of neat n-heptane occurs on the knocking boundary. However, the ignition timing is obviously retarded by 2-propanol addition. As a result, the acceptable operation range was expanded to a higher IMEP. Figure 2 displays the HCCI combustion efficiency as a function of IMEP for neat fuel and blend fuels. In general, the combustion efficiency increases almost linearly with an increase



Figure 3. Effects of partial equivalence ratio of n-heptane on the combustion characteristics with the same equivalence ratio of total fuel.

of the IMEP, and the combustion efficiency attains a high level when the IMEP is larger than 4 bar. At a lower engine load, the combustion efficiency at the same IMEP is deteriorated with 2-propanol addition. Furthermore, the IMEP close to complete combustion varied with the fuel component. For example, the combustion efficiency of n-heptane at the upper critical point is nearly 99%, while the combustion efficiency for 60% 2-propanol/n-heptane at the critical point, with IMEP equal to 5.2 bar, is only about 92.7%. The decrease of combustion efficiency for blend fuels can be attributed to the following reasons. First, the cetane number is decreased and the autoignitability is deteriorated when 2-propanol is added to the n-heptane; then, incomplete oxidation and combustion of the fuel/air mixture in the boundary layer occurred. Second, the main combustion events of blend-fuel HCCI combustion are accomplished during the expansion stroke because of the delay in ignition timing; this leads to a decrease in the in-cylinder gas temperature. As a result, CO and HC emissions could not be further oxidized completely. At last, it can be seen from Table 1 that the latent heat of evaporation of 2-propanol is much higher than that of n-heptane; this leads to a lower temperature at the beginning of the compression stroke. 3.2. Heat Release Analysis on the Inhibition Effects of 2-Propanol Addition. Figure 3 illustrates the comparison of the gas pressure traces and heat release curves of HCCI combustion for different partial equivalence ratios of n-heptane and 2-propanol, while maintaining the same equivalence ratio of total fuel. In Figure 3a, the ignition timing and combustion rate are clearly delayed and depressed by decreasing the n-heptane and increasing the 2-propanol volumes; the peak values of the heat release rate are also decreased substantially. For Φn-heptane equal to 0.287, light knock combustion may be observed from the in-cylinder gas pressure curve, while when the partial equivalence ratio of n-heptane further decreased, such as Φn-heptane equal to 0.233, the ignition timing is observed during the expansion process. Particularly, misfire occurred for Φn-heptane equal to 0.217. Figure 3b displays the effect of n-heptane and 2-propanol quantities on the combustion characteristics while keeping a lean fuel/air mixture. With the decrease of the partial equivalence ratio of n-heptane from 0.211 to 0.193, the ignition timing is

delayed obviously, while the largest values of maximum gas pressure and heat release rate are obtained when the ignition timing occurred near the TDC. In general, the ignition timing is delayed with a decrease of the partial equivalence ratio of n-heptane and an increase of the partial equivalence ratio of 2-propanol. The excessively late combustion occurred when the n-heptane quantity further decreased; consequently, the peak values of heat release are decreased substantially. Once the partial equivalence ratio increased up to a certain value, misfire occurred. Figure 4 shows the comparison of in-cylinder pressure traces and heat release curves at the same partial equivalence ratio of n-heptane while with different 2-propanol addition quantities which were selected from all testing points. For a higher partial equivalence ratio of n-heptane, such as in Figure 4a, with an increase of the 2-propanol quantity, that means the equivalence ratio of total fuel also increases; the ignition timing is retarded clearly, whereas the maximum gas pressure and peak value of the heat release rate are increased. In Figure 4b, because of the lean fuel/air mixture, the combustion rate is very low. Despite the excessively advanced ignition timing for the minor addition of 2-propanol, the heat release was completed near the TDC. When the partial equivalence ratio of 2-propanol is increased up to 0.089, the ignition timing is observed about 7° CA ATDC, and the overall combustion event is accomplished during the expansion process; this will lead to an incomplete combustion. Another interesting thing that can be seen from the above figures is that the small heat-producing reactions occurred before the high-temperature reaction. These low-temperature reactions are termed cool flame reactions. Many experimental results reveal the HCCI combustion using hydrocarbon fuels exhibiting two-stage combustion including low-temperature heat release and high-temperature heat release and a negative temperature coefficient region; these phenomena can be verified from the above analysis. Moreover, ignition timing and heat release during the low-temperature reaction have a large impact on the high-temperature reaction. Figure 5 shows the enlarged low-temperature heat releases which correspond to Figure 3a. For a fixed equivalence ratio of total fuel, the ignition timing is advanced and the maximum


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Figure 4. Effect of 2-propanol addition quantity on the combustion characteristics at the same partial equivalence ratio of n-heptane.

Figure 5. Effect of partial equivalence ratio of n-heptane on the lowtemperature heat release with the same equivalence ratio of total fuel (Φtotal ) 0.312).

Figure 6. Effect of 2-propanol addition on the low-temperature heat release with the same partial equivalence ratio of n-heptane (Φn-heptane ) 0.263).

value of LTR is increased with the increase of n-heptane and decrease of 2-propanol. Consequently, the advancing of the firststage combustion directly influences the second-stage reaction and combustion. Figure 6 provides detailed information about the effects of the partial equivalence ratio of 2-propanol on cool flame heat release. It can be found that the occurrence timing of cool flame is delayed obviously; the peak values of low-temperature heat release are decreased substantially with 2-propanol addition. In particular, the low-temperature heat release almost diminished when the partial equivalence ratio of 2-propanol increased up

Figure 7. Effects of 2-propanol volume in blend fuels on the crank angle of the peak point of LTR.

to 0.089; this resulted in a noticeable change of the hightemperature reaction and the combustion characteristics. On the basis of above results, it can be speculated that, with the addition of 2-propanol, a low-temperature chemical reaction is suppressed; the free radical concentrations of the LTR decreased and resulted in a slightly slower initiation of lowtemperature heat release; accordingly, the rate of increasing temperature during the compression stroke is slow, and the oxidation velocity of the fuel/air mixture is depressed. As a result, the reaction rate of the high-temperature combustion is reduced, and the ignition timing is delayed. Figure 7 shows the effect of 2-propanol volume on the crank angle corresponding to the peak point of low-temperature heat release for different fuels. It is very interesting to note that the crank angle of the peak point delays linearly with an increase of the 2-propanol volume in blend fuels. For n-heptane, the lowtemperature reaction occurs around 20° CA BTDC regardless of the engine load, whereas the low-temperature reaction occurs around 11.5-13.5° CA BTDC for 50% 2-propanol/n-heptane. 3.3. Cycle-to-Cycle Variation. Unlike for the SI and CI engines, there is no direct combustion timing control for the HCCI engine. Ignition occurs when the homogeneous charge has reached its autoignition condition. The ignition timing is very sensitive to the air/fuel equivalence ratio, inlet temperature, compression ratio, residual gases, the cylinder wall temperature, and the oxidation chemical kinetics of the air/fuel mixture. As



Figure 8. Combustion stability of neat n-heptane and 2-propanol/n-heptane blend fuels (40 cycles).

a result of the temperature-sensitive nature of HCCI combustion, unstable operation may be observed. At high loads, because of the higher temperature of the residual gas and cylinder wall, early combustion and fast combustion velocity will lead to pressure oscillation, and then, the knock combustion may destroy the engine parts. At light loads, a lower initial in-cylinder temperature and lean fuel/air mixture often result in cyclic variations such as misfire and partial combustion. Then, the deteriorated combustion leads to high CO and HC emissions. According to the in-cylinder gas pressure, this part gives a short discussion on the cycle-to-cycle variations of the n-heptane and 2-propanol/n-heptane blend fuels. Figure 8 shows the combustion stability of HCCI combustion using different fuels. In Figure 8a, light knock combustion is observed; while the ignition timing, maximum gas pressure, and crank angle corresponding to the peak of in-cylinder pressure show excellent repeatability, the cycle-to-cycle variation is very small. Both 10% and 20% blend fuels show a slight cycle-tocycle variation; the maximum gas pressure is almost the same value, but the ignition timing and the crank angle corresponding to the peak of in-cylinder pressure begin to vary. For 30% and 40% blend fuels, distinct cycle-to-cycle variation behaviors are observed, the maximum gas pressure fluctuated obviously, and the ignition timing and the crank angle corresponding to the peak of in-cylinder pressure go back and forth to a certain point. Furthermore, partial combustion occurred in some cycles. With 50% blend fuels, strong cycle-to-cycle variations and misfire were observed. 3.4. Parametric Analysis on the HC and CO Emissions. A major technique obstacle of HCCI combustion is much higher CO and HC emissions. In recent years, a great deal of emphasis has been put on the combustion mechanism and control of ignition timing, while there has been less reported on the HC and CO formation mechanism and control strategy. In general, it is widely accepted that HC emissions mainly come from the wall quench layer of the combustion chamber, ring-crevice storage, and the absorption-desorption of fuel from oil layers and surface deposits. However, CO emissions from internal combustion engines are controlled primarily by the fuel/air equivalence ratio. For fuel-rich mixtures, CO concentrations increase steadily with an increase of the equivalence ratio. For lean fuel mixtures, the equivalence ratio has a weak effect on

CO emissions. CO emissions from HCCI engines are controlled by chemical kinetics. CO formation is one of the basic reaction steps in the hydrocarbon oxidation mechanism, which may be summarized as follows:28

RH f R f RO2 f RCHO f RCO f CO where R stands for the hydrocarbon radical. CO is a result of incomplete combustion in the intermediate temperature regions where the OH radical concentration becomes significantly diminished, resulting in less conversion of CO to CO2. Because of the decisive effects of combustion characteristics on emissions, the author presents a parametric study to evaluate the effects of combustion parameters and fuel components on CO and HC emissions. The maximum gas temperature is the major factor which determines the engine emissions. Figure 9a shows the influence of the maximum gas temperature on HC and CO emissions. It can be seen from the figure that HC levels improved with an increase of the gas temperature, particularly for the 60% 2-propanol/n-heptane blend fuel. CO emission as a function of gas temperature shows another tendency compared to HC emissions. CO emissions are very low for a combustion temperature of about 1000 K and then increase with an increase of combustion temperature and reach peak values at 1200 K for all fuels. After that, CO emissions begin to improve with an increase of combustion temperature. When the combustion temperature reaches 1500 K, CO emissions have improved to a much lower level. This phenomenon can be explained by the following reasons. CO emissions begin to form during the lowtemperature oxidation process and can only be further oxidized into CO2 at high-temperature conditions. Once the combustion temperature reaches the critical temperature level, for example 1500 K, CO emissions will decrease substantially. In the lowertemperature region, a large part of the fuel/air mixture was not oxidized. This leads to a lower level of CO and a high level of HC emitted from the cylinder. Because HCCI combustion is dominated by chemical kinetics, the fuel components or fuel chemical properties play an (28) Heywood, J. B. Internal Combustion Engine Fundamentals; McGrawHill Book Company: New York, 1988.


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Figure 9. Effects of combustion parameters and fuel component on HC and CO emissions.

important role in HC emissions. Figure 9b shows that the HC and CO emissions vary with the equivalence ratio of n-heptane to the total fuel. It is obvious that HC emissions decrease substantially with an increase in the n-heptane volume in blend fuels. Particularly, CO emissions of n-heptane are maintained at a very low level at a range of overall operating values regardless of the engine load. On the contrary, the n-heptane percentage in blend fuels shows no direct effect on CO emissions. Figure 9c displays the effects of the maximum rate of increasing pressure on CO and HC emissions. In actuality, the combustion process of the HCCI engine occurs by multipoint autoignition in the core zone but not by simultaneous autoignition within the cylinder and subsequently reacts homogeneously with no flame propagation. Once the combustion occurs simultaneously, the maximum rate of increasing pressure will tend toward infinity. Therefore, the maximum values of the rate of increasing pressure can be used to denote the volume of the core area which is subject to multipoint ignition at first. It can be found that CO emissions improve clearly with an increase

of the rate of increasing pressure, but HC emissions slightly improved with an increase of the rate of increasing pressure. In addition, it is worth noting that CO emissions begin to improve clearly when the maximum rate of increasing pressure is larger than 0.5 MPa/°CA regardless of the fuel properties. Figure 9d shows the effects of hot ignition timing on CO and HC emissions. In the figure, ignition timing is shown to have an important influence on HC emissionsshot ignition occurred during the compression stroke, and the HC emissions increased slowly with the delaying of the ignition timing. Once the hot ignition occurred during the expansion process, the HC emissions increased notably with the delaying of the ignition timing. Ignition timing has a moderate effect on CO emissions. The relationship between the CO and HC emissions with IMEP is shown in Figure 9e. It can be seen that HC emissions of n-heptane and 10∼30% 2-propanol/n-heptane blend fuels are very low, but HC emissions of 40% and 50% 2-propanol/nheptane blend fuels increase substantially at lower engine loads. CO emissions are very high at 1.5∼2.5 bar for all fuels. When the IMEP further increased, CO emissions decreased with the



Figure 10. NOx emissions of neat n-heptane and 2-propanol/n-heptane blend fuels in HCCI combustion.

increase of combustion temperature. But, it should be noted that CO emissions also decrease at very low engine loads. This is mainly due to the misfire of the fuel/air mixture; a large part of the fuel was discharged to the exhaust pipe directly without being oxidized. According to the above analysis, the major factor which determines HC emission levels is the fuel component or cetane number of the test fuel; other parameters including the maximum gas temperature, peak values of the rate of increasing pressure, hot ignition timing, and IMEP have moderate effects on HC emission. While the decisive parameter which influences CO emission is the maximum gas temperature, other factors such as the peak values of the rate of increasing pressure, hot ignition timing, and IMEP also have some indirect influence. To produce lower CO and HC emissions, the maximum gas temperature should be higher than 1500 K, the maximum rate of increasing pressure larger than 0.5 MPa/°CA, and the IMEP above 3 bar. Moreover, a high cetane number fuel is preferred. NOx emissions of HCCI combustion are very low for the lean fuel/air mixture and low-temperature combustion. Figure 10 shows the NOx emissions of n-heptane and 2-propanol/nheptane blend fuels for HCCI combustion. For all stable operation points, NOx emissions are lower than 10 ppm. 3.5. Effect of Cold EGR on Combustion and Emissions. EGR is widely used as the main method to depress NOx emissions from diesel engines. Currently, EGR is also used as the basic method to control ignition timing and the burn rate of HCCI combustion. In this section, the effects of cold EGR on HCCI combustion using different blend fuels were evaluated. The intake chare temperature (the temperature of the mixture of EGR, fresh air, and fuel) was kept between 28 and 32 °C. Figure 11 shows the effects of the cold EGR rate on 30% 2-propanol/n-heptane blend fuel HCCI combustion at the same fuel delivery rate. It can be seen from the figure that the hot ignition timing is gradually delayed with the introduction of the cold EGR rate. Because of the heat capacity effect, dilution effect, and chemical effect, the combustion rate is suppressed. As a resul, the maximum gas pressure and peak value of the heat release rate are clearly reduced. Figure 12evaluates the effects of the same EGR rate on different blend fuels in HCCI combustion under the same energy input for each cycle. With an increase in 2-propanol addition in blend fuels, the EGR shows more severe effects on combustion. Figure 13 illustrates the EGR rate on HC and CO emissions of 10∼40% 2-propanol/n-heptane blend fuels. During the experiment, the maximum EGR rate was 55%. The energy input per cycle was kept at 660 J for four test fuels. Under these conditions, the EGR rate shows little effect on the CO emissions

Figure 11. Effects of cold EGR rate on HCCI combustion using 30% 2-propanol/n-heptane blend fuels at the same fuel delivery rate.

Figure 12. 30% cold EGR on HCCI combustion using the different blend fuels under the same energy input.

of 10% and 20% blend fuels, while the CO emissions begin to increase with the introduction of EGR for 30% blend fuel, and CO increases substantially with an increase of EGR rate for 40% blend fuel. In regard to HC emissions, it can be seen from the figure that HC emissions increase slightly with the introduction of EGR. 4. Conclusions With an increase of 2-propanol addition in n-heptane, both the maximum and lowest engine loads increase linearly. Because of the deterioration of ignitability of 2-propanol/n-heptane blend fuels, the combustion efficiency at the same IMEP decreases with the introduction of 2-propanol. For the same partial equivalence ratio of n-heptane, both the low-temperature and high-temperature reactions are delayed


Lu¨ et al.

Figure 13. Effects of cold EGR rate on HC and CO emissions for different fuels (E ) 660 J/cycle).

because of the 2-propanol addition. For the same equivalence ratio of total fuel, both LTR and HTR advance with an increase of the n-heptane mole percentage. The maximum gas pressure and peak values of heat release reach the largest values when the hot ignition occurred at top dead center. With the addition of 2-propanol, a low-temperature chemical reaction is suppressed; the free radical concentrations of the LTR decreased and resulted in a slightly slower initiation of lowtemperature heat release, and accordingly, the rate of increasing temperature during the compression stroke is slow, and the oxidation velocity of the fuel/air mixture is depressed. As a result, the reaction rate of the hot combustion is suppressed, and the ignition timing is delayed. At a constant energy input, the cycle-to-cycle variation of ignition timing, the maximum gas pressure, and the crank angle corresponding to the maximum gas pressure deteriorated with an increase of 2-propanol addition. Moreover, partial combustion for 30% and 40% 2-propanol/n-heptane was observed, and misfire behavior was found in the 50% 2-propanol/n-heptane fuel.

The major factor which determines the HC emissions is the fuel component or cetane number of the test fuel; other parameters including the maximum gas temperature, peak values of the rate of increasing pressure, hot ignition timing, and IMEP have moderate effects on HC emission. While the decisive parameter which influences CO emission is the maximum gas temperature, other factors such as the peak values of the rate of increasing pressure, hot ignition timing, and IMEP also have some indirect influence. Cold EGR has a substantial effect on n-heptane HCCI combustion and emissions doped with high volume of 2-propanol additive. The general tendency is to depress the combustion rate, and as a result, the CO and HC emissions deteriorated. Acknowledgment. This work was supported by the National Basic Research Program of China (Grant No. 2001CB209208) and Shanghai Basic Research Program (Grant No. 05DJ14002). EF0601263

Heat Release Analysis on Combustion and Parametric Study on

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