Combustion Characteristics of a Direct-Injection Engine Fueled with

This phenomenon indicates that only when the hydrogen fraction in natural gas ... list of citations to this article, users are encouraged to perform a...
0 downloads 0 Views 349KB Size
Download PDF
540

Energy & Fuels 2006, 20, 540-546

Combustion Characteristics of a Direct-Injection Engine Fueled with Natural Gas-Hydrogen Mixtures Zuohua Huang,* Jinhua Wang, Bing Liu, Ke Zeng, Jinrong Yu, and Deming Jiang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong UniVersity, Xi’an, People’s Republic of China ReceiVed August 2, 2005. ReVised Manuscript ReceiVed December 22, 2005

In this article, we experimentally studied combustion characteristics of a direct-injection spark-ignited engine fueled with natural gas-hydrogen blends. For a specific operation mode, the results show that the heat release rate decreases with the increase of hydrogen fraction in the blends when hydrogen fraction is less than a certain volumetric fraction while the heat release rate increases with the increase of hydrogen fraction in the blends when hydrogen fraction is over a certain value. This phenomenon indicates that only when the hydrogen fraction in natural gas reaches a certain fraction can a large improvement in combustion be realized. Flame development duration, rapid combustion duration, and total combustion duration increase with the increase of hydrogen fraction in the blends when hydrogen fraction is less than a certain volumetric fraction, while they decrease with the increase of hydrogen fraction when hydrogen fraction is over the value. The crank angle of the center of heat release curve moves away from the top-dead-center with the increase of hydrogen fraction in the blends when the hydrogen fraction is less than a certain volumetric fraction, and it moves close to the top-dead-center when hydrogen fraction is over the certain value. Maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise, and maximum heat release rate decrease with the increase of hydrogen fraction when the hydrogen fraction is less than a certain volumetric fraction, and they increase with the increase of hydrogen fraction when hydrogen fraction is over the certain value. For fixed injection duration, the influence of hydrogen addition on natural gas-hydrogen mixture combustion is larger at low engine speed operation condition than that at high engine speed operation condition.

Introduction With increasing concern about energy shortage and environmental protection, research on improving engine fuel economy and reducing exhaust emissions has become the major research aspect in combustion and engine development. Due to limited reserves of crude oil, development of alternative fuel engines has attracted more and more concern in the engine community. Alternative fuels usually belong to clean fuels compared to diesel fuel and gasoline fuel in the engine combustion process. The introduction of these alternative fuels is beneficial to slowing down the fuel shortage and reducing engine exhaust emissions. Natural gas is considered to be one of the favorable fuels for engines, and the natural gas fueled engine has been realized in both the spark-ignited engine and the compression-ignited engine. However, due to the slow burning velocity of natural gas and the poor lean-burn capability, the natural gas sparkignited engine has the disadvantage of large cycle-by-cycle variations and poor lean-burn capability, and these will decrease the engine power output and increase fuel consumption.1-2 Due to these restrictions, a natural gas engine is usually operated at the condition of stoichiometric equivalence ratio with relatively low thermal efficiency. Traditionally, to improve the lean-burn capability and flame burning velocity of the natural gas engine * Corresponding author. E-mail: [email protected]. (1) Rousseau, S.; Lemoult, B.; Tazerout, M. Combustion Characteristics of Natural Gas in a Lean Burn Spark-Ignition Engine. Proc. Inst. Mech. Eng., Part D 1999, 213, 481-489. (2) Ben, L.; Dacros, N. R.; Truquet, R.; Charnay, G. Influence of Air/ Fuel Ratio on Cyclic Variation and Exhaust Emission in Natural Gas SI Engine. SAE Tech. Pap. Ser. 1999, No. 992901.

under lean-burn conditions, an increase in flow intensity in cylinder is introduced, and this measure always increases the heat loss to the cylinder wall and increases the combustion temperature as well as the NOx emission.3 One effective method to solve the problem of slow burning velocity of natural gas is to mix the natural gas with the fuel that possesses fast burning velocity. Hydrogen is regarded as the best gaseous candidate for natural gas due to its very fast burning velocity, and this combination is expected to improve the lean-burn characteristics and decrease engine emissions.4 Blarigan and Keller investigated the port-injection engine fueled with natural gas-hydrogen mixtures,5 Wong and Karim studied engine performance fueled by various hydrogen fractions in natural gas-hydrogen blends,6 and Bauer and Forest conducted an experimental study on natural gas-hydrogen combustion in a CFR engine.7 Furthermore, studies on lean combustion capability of natural gas-hydrogen combustion and natural (3) Das, A.; Watson, H. C. Development of a Natural Gas Spark Ignition Engine for Optimum Performance. Proc. Inst. Mech. Eng., Part D 1997, 211, 361-378. (4) Akansu, S. O.; Dulger, A.; Kahraman, N. Internal combustion engines fueled by natural gas-hydrogen mixtures. Int. J. Hydrogen Energy 2004, 29, 1527-1539. (5) Blarigan P. V.; Keller, J. O. A hydrogen fuelled internal combustion engine designed for single speed/power operation. Int. J. Hydrogen Energy 2002, 23, 603-609. (6) Wong, Y. K.; Karim, G. A. An analytical examination of the effects of hydrogen addition on cyclic variations in homogeneously charged compression-ignition engines. Int. J. Hydrogen Energy 2000, 25, 12171224. (7) Bauer, C. G.; Forest, T. W. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on S.I. engine performance. Int. J. Hydrogen Energy 2001, 26, 55-70.

10.1021/ef0502453 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/27/2006

Direct-Injection Engine Fueled with Natural Gas-Hydrogen

Energy & Fuels, Vol. 20, No. 2, 2006 541

Table 1. Engine Specifications bore (mm) stroke (mm) displacement (cm3) compression ratio combustion chamber injection pressure (MPa) ignition source

100 115 903 8 bowl-in-shape 8 spark plug

gas-hydrogen combustion with turbo-charging and/or exhaust gas recirculation were also conducted,8,9 and these studies showed that the exhaust HC, CO, and CO2 concentrations could be decreased when exhaust concentrations from an engine operated on natural gas-hydrogen blends were compared to those of natural gas engine. However, NOx may increase for natural gas-hydrogen combustion at rich mixture condition as the improvement of lean-burning ability and increased flame propagation speed; NOx concentration can be greatly decreased through lean combustion and retarding of the ignition advance angle. The previous work mainly concentrated on homogeneous mixtures where fuels are introduced from the port, and few articles were found for direct-injection engine using natural gashydrogen blends.10 Shudo et al. investigated the combustion and emissions of an engine with port-injected hydrogen and incylinder injection natural gas.11 This type of engine needs two separate fueling systems, and this makes the system complicated. This article will investigate the performance and emissions of a direct-injection engine fueled with various fractions of natural gas-hydrogen mixtures and expect to clarify the behaviors of engine fueled with natural gas-hydrogen mixtures. Experimental Procedures A single cylinder engine was modified into a natural gas directinjection engine. The specifications of the engine are listed in Table 1. The injector used in the study is a modified version of a gasoline direct-injection engine made by the manufacturer (Hitachi Co.). To increase the flow rate for natural gas application, the swirler near the tip of the nozzle was removed. The calibration of the pulse width with the injection amount was made by the manufacturer as well as by the authors.12 The flow rate of the injector under 9 MPa was 193 L/min. In addition to installing the natural gas high-pressure injector, a spark plug was also installed at the center of the combustion chamber as the ignition source. Natural gas was injected into cylinder at a constant pressure of 8 MPa. Hydrogen with purity of 99.995% was used, and natural gas constitutions are given in Table 2. The fuel properties of natural gas and hydrogen are given in Table 3. Different fractions of natural gas-hydrogen mixtures were prepared in advance in fuel bomb and were supplied to the fuel injector. Sonic flow of injected gases was presented due to the choke flow during injection. It is estimated that an 18% volume fraction of hydrogen corresponds to a 2% mass fraction of hydrogen in mixture, and thus the influence in sonic velocity is small and the influence in volumetric flow rate is limited. (8) Sierens, R.; Rosseel, E. Variable Composition Hydrogen/Natural Gas Mixtures for Increased Engine Efficiency and Decreased Emissions. Trans. ASME: J. Eng. Gas Turbines Power 2000, 122, 135-140 (9) Larsen, J. F.; Wallace, J. S. Comparison of emissions and efficiency of a turbocharged lean-burn natural gas and hythane-fueled engine. Trans. ASME: J. Eng. Gas Turbines Power 1997, 119, 218-226. (10) Allenby, S.; Chang, W. K.; Megaritis, A.; Wyszynski, M. L. Hydrogen enrichment: a way to maintain combustion stability in a natural gas fuelled engine with exhaust gas recirculation, the potential of fuel reforming. Proc. Inst. Mech. Eng., Part D 2001, 215, 405-418. (11) Shudo, T.; Shimamura, K.; Nakajima, Y. Combustion and emissions in a methane DI stratified charge engine with hydrogen pre-mixing. JSAE ReV. 2000, 21, 3-7. (12) Huang, Z. H.; Shiga, S.; Ueda, T.; Jingu, N.; Nakamura, H.; Ishima, T.; Obokata, T.; Tsue, M.; Kono, M. A basic behavior of CNG DI combustion in a spark-ignited rapid compression machine. JSME Int. J., Ser. B 2002, 45, 891-900.

Table 2. Compositions and Properties of Natural Gas items

volume fraction

items

volume fraction

CH4 96.16 C2H6 1.096 iC4H10 0.021 nC4H10 0.021 nC5H12 0.005 N2 0.001 H2S 0.0002 H2O 0.006 volumetric higher heating value: 36.588 MJ/m3a volumetric lower heating value: 32.970 MJ/m3a a

items

volume fraction

C3H8 iC5H12 CO2

0.136 0.006 2.54

Normal temperature and pressure. Table 3. Fuel Properties of Natural Gas and Hydrogen fuel properties (kg/m3)

density in 1 atm, 300 K stoichiometric air-to-fuel ratio (vol %) stoichiometric air-to-fuel ratio (wt %) laminar flame speed (m/s) quenching distance (mm) mass lower heating value (MJ/kg) volumetric heating value (MJ/m3) octane number C/H ratio

natural gas

hydrogen

0.754 9.396 0.062 0.38 1.9 43.726 32.97 120 0.2514

0.082 2.387 0.029 2.9 0.6 119.7 10.22 0

Table 4. Parameters of Operating Modes mode number

speed (r/min)

injection advance angle (CA deg BTDC)

injection duration (ms)

ignition advance angle (CA deg BTDC)

1 2 3 4 5 6 7 8 9 10

1200 1200 1200 1200 1200 1800 1800 1800 1800 1800

166 170 180 184 190 230 236 236 250 270

16.56 15.96 16.56 17.56 18.76 15.40 15.00 15.80 16.36 19.36

27.5 29.0 32.5 34.5 34.5 42.5 43.0 43.5 44.0 44.0

Thus, the volumetric flow rate of natural gas-hydrogen mixtures in this study is assumed to be unchangeable and can be regarded as a function of injection duration. Table 4 shows the setting of operation modes for the study. Different modes represent different combinations of injection advance angles and fuel injection durations. Five modes are for the engine speed of 1200 rpm, and the other five modes are for the engine speed of 1800 rpm. The difference among the different modes is the setting of injection advance angle and injection duration. Long injection duration corresponds to a rich mixture (more fuel injected), and to make the end timing of injection unchangeable before ignition, the injection advance angle must be advanced along with the increase of injection duration. The ignition timing is set at the timing for achieving the maximum brake torque determined by the experiments. The injection pressure is kept constant, and thus the density of fuel will be constant. Combining with the volumetric flow rate of natural gas-hydrogen mixtures in this study is assumed to be unchangeable, and it is a function of injection duration. Thus, the mass of fuel injection can be regarded as a constant value for various natural gas-hydrogen mixtures under specific injection duration. Four types of fuels were prepared for the experiment: pure natural gas, fuel blend with 95% of natural gas and 5% of hydrogen in volume, fuel blend with 90% of natural gas and 10% of hydrogen in volume, and fuel blend with 82% of natural gas and 18% of hydrogen. Ten modes were selected and shown in Table 4, and four types of fuels were experimentally investigated under the condition of each mode. The comparisons were made among those of different fuels under the specific condition, that is, at the same engine speed, the same injection duration, the same injection advance angle, and the same ignition advance angle. In the experiment, the opening of the throttle valve was fixed at 70%. Figure 1 gives the volumetric heat value of natural gashydrogen-air mixtures versus the hydrogen fractions at the

542 Energy & Fuels, Vol. 20, No. 2, 2006

Huang et al. The gas-state equation is: pV ) mRT

(2)

A variation of the gas-state equation with crank angle is given by: p

dp dT dV + V ) mR dφ dφ dφ

(3)

Heat release rate dQB/dφ can be derived from eqs 1 and 3 as follows: dQB CV‚V dp CP‚p dV dQW ) + + dφ R dφ R dφ dφ Figure 1. Volumetric heating values of natural gas-hydrogen-air mixtures at stoichiometric equivalence ratio.

where heat transfer rate is given by: dQW ) hc‚A‚(T - TW) dφ

Figure 2. H/C ratio of natural gas-hydrogen blends.

stoichiometric equivalence ratio. It can be seen that the volumetric heat value of natural gas-hydrogen-air mixtures will decrease with the increase of hydrogen fraction in the fuel blends, and this would be due to the low volumetric heat value of hydrogen-air mixture compared to that of natural gas-air mixture at the stoichiometric equivalence ratio condition. Thus, for a given fuel injection duration, the amount of heat release will decrease with the increase of hydrogen fraction in the fuel blends. To maintain the same equivalence ratio, more fuel by volume needs to be injected for the natural gas-hydrogen mixture combustion compared to that of pure natural gas injection, and the amount of injected fuel blends will increase with the increase of hydrogen fraction in the blends. Figure 2 shows the hydrogen/carbon (H/C) ratio of natural gashydrogen blends versus the hydrogen fractions. H/C ratio increases linearly with the increase of hydrogen fraction in the fuel blends, and this will be beneficial to the reduction of carbon-related emissions such as HC, CO, and CO2. Instrumentation and Method of Calculation. The cylinder pressure was recorded by a piezoelectric transducer with a resolution of 10 Pa, and the dynamic top-dead-center (TDC) was determined by motoring. The crank angle signal was obtained from an anglegenerating device mounted on the main shaft. The signal of cylinder pressure was acquired for every 0.5 °CA, and the acquisition process covered 254 completed cycles, the average value of these 254 cycles was outputted as the pressure data used for the calculation of combustion parameters. A thermodynamic model is used to calculate the thermodynamic parameters in this article. The model neglects the leakage through the piston rings,13 and thus the energy conservation in the cylinder is written as follows: dQB dQW d(mu) dV dV dT ) + p ) mCV +p dφ dφ dφ dφ dφ dφ

(1)

(13) Heywood, J. B. Internal Combustion Engine Fundamentals; McGrawHill: New York, 1988.

(4)

(5)

Heat transfer coefficient hc uses the correlation formula given by Woschni in ref 13. Cp and CV are temperature-dependent parameters; their formulas are given in ref 13. The primary source is cylinder pressure-crank angle data. Using those primary data and the above equations, we can calculate peak pressure pmax, mean gas temperature T, maximum mean gas temperature Tmax, rate of pressure rise and heat release (dp/dφ), (dQB/dφ), and its maximum value (dp/dφ)max or (dQB/dφ)max. Flame development duration is the angle interval from ignition start to the angle where 10% of accumulated heat release is reached; rapid combustion duration is the angle interval from 10% of accumulated heat release to the angle of 90% of accumulated heat release; and total combustion duration is the angle interval from the beginning of heat release to the ending of heat release. The crank angle of the center of the heat release curve is determined by the following equation:

φc )

∫

dQB ‚φ‚dφ dφ φe dQB dφ φs dφ

φe

φs

∫

(5)

in which φs is the crank angle at the beginning of heat release and φe is the crank angle at the end of heat release.

Results and Discussion Figure 3 gives the excess air ratio versus hydrogen fractions in natural gas-hydrogen blends. For a specific engine operation mode, the excess air ratio increases linearly with the increase of hydrogen fraction in the fuel blends. For the same mode, the amount of fresh air into the cylinder and the volumetric amount of fuel is the same due to the same engine speed and same fuel injection duration. As shown in Table 3, the stoichiometric air/ fuel ratio of hydrogen is just one-fourth to that of natural gas; thus, the excess air ratio will increase with the increase of hydrogen fraction in the fuel blends. For hydrogen fractions of 5, 10, and 18% in fuel blends, the excess air ratio will increase by 4, 8, and 15%, respectively, compared to that of natural gas. For a specific hydrogen fraction, the excess air ratio will decrease with the increase of fuel injection duration. Figure 4 gives the heat release rate of mixture combustion for various hydrogen fractions at a specific operation mode and two engine speeds. At low engine speed and low load, as shown in Figure 4a, when the hydrogen fraction is less than 10%, the heat release rate shows a decrease with the increase of hydrogen fraction in the fuel blends. As described above, excess air ratio increases (mixture dilution) with the increase of hydrogen

Direct-Injection Engine Fueled with Natural Gas-Hydrogen

Figure 3. Excess air ratio of natural gas-hydrogen blends.

fractions, and the volumetric heat value of natural gashydrogen-air mixtures decreases with the increase of hydrogen fraction in the fuel blends. Fuel injection with the fixed injection duration and increased hydrogen fractions will dilute the mixture (increase in excess air ratio) and decrease the heat value of the mixture. Meanwhile, the enhancement of burning velocity of the mixture is not great in the case of low hydrogen fraction, and this has been demonstrated in the previous burning velocity measurement;4 thus, the effect of burning velocity on combustion improvement by hydrogen addition is limited. The combined influences of these two factors make the heat release rate decrease with the increase of hydrogen fraction. When the hydrogen volumetric fraction is larger than 10%, the heat release

Figure 4. Heat release rate of natural gas-hydrogen blends.

Energy & Fuels, Vol. 20, No. 2, 2006 543

rate will increase with the increase of the hydrogen fraction, and a large increase in burning velocity of the mixture by hydrogen addition will contribute to the improvement of combustion. At low engine speed and high engine load (Figure 4b), a similar behavior is presented in the heat release rate, but the differences in the heat release rate and their timing phase for various hydrogen fractions become small compared to those at low engine load. This suggests that combustion at decreased excess air ratio (mixture enrichment) can decrease the combustion difference among mixtures of various hydrogen fractions. At high engine speed (Figure 4c,d), a similar behavior in heat release rate is presented, but the shifting point where the heat release rate is increased will move to 5% of hydrogen fraction, and the difference in the heat release rate among different hydrogen fractions becomes small compared to that at low engine speed. This reveals that strong turbulence at high engine speed can greatly increase the burning velocity of a natural gashydrogen-air mixture, which is larger than the influence from mixture dilution by hydrogen addition (increase of excess air ratio). Figure 5 shows the flame development duration versus hydrogen fractions for various engine operation modes. Flame development duration will increase with the increase of the hydrogen fraction when the hydrogen fraction is less than a certain value (10% at 1200 r/min and 5% at 1800 r/min). In this case, the effect from mixture dilution by hydrogen addition is larger than burning velocity enhancement. When the hydrogen fraction is over this value, flame development duration will decrease with the increase of hydrogen addition, and in this case, the influence of burning velocity enhancement from hydrogen addition is larger than that from mixture dilution (increase in excess air ratio) by hydrogen addition, resulting in fast flame propagation and decreasing in flame development duration. Figure 6 illustrates the rapid burning duration versus hydrogen fractions for various engine operation modes, and that for the total combustion duration is plotted in Figure 7. The results show a behavior similar to that of flame development duration.

544 Energy & Fuels, Vol. 20, No. 2, 2006

Figure 5. Flame development duration of natural gas-hydrogen blends.

Figure 6. Rapid burning duration of natural gas-hydrogen blends.

This can also be explained by the combined influence from mixture dilution and burning velocity enhancement. When the hydrogen fraction is less than a certain value, the mixture dilution by hydrogen addition shows a larger influence than burning velocity enhancement. When the hydrogen fraction is over this value, the burning velocity enhancement shows larger influence than mixture dilution by hydrogen addition. The position of the crank angle of the center of the heat release curve reflects the compactness of the whole heat release process and energy-conversion efficiency of engine combustion. Figure 8 shows the crank angle of the center of the heat release curve φc versus hydrogen fractions at various engine operation

Huang et al.

Figure 7. Total combustion duration of natural gas-hydrogen blends.

Figure 8. Crank angle of the center of heat release curve.

modes. In the case of low engine speed (1200 r/min), φc will move away from the TDC with the increase of hydrogen fraction when hydrogen fraction is less than 10% in the fuel blends, while φc will move close to the TDC with further increase of hydrogen fraction when the hydrogen fraction is larger than 10%. They are consistent with the behaviors of combustion durations, that is, a small value of φc corresponds to the short combustion durations as demonstrated in Figures 5-7. At high engine speed, the difference becomes small, and this would be due to the effect of strong turbulence, which speeds up the combustion process. Figures 9 and 10 give the maximum cylinder gas pressure pmax and the maximum mean gas temperature Tmax versus

Direct-Injection Engine Fueled with Natural Gas-Hydrogen

Figure 9. Maximum cylinder gas pressure of natural gas-hydrogen blends.

Figure 10. Maximum mean gas temperature of natural gas-hydrogen blends.

hydrogen fractions at various engine operation modes, respectively. pmax and Tmax show similar behavior with the variation of the hydrogen fraction. Increase in excess air ratio will decrease the high value of pmax and Tmax, while fast combustion will make the heat release process within a small cylinder volume and decrease the heat loss to the coolant, and this brings the high value of pmax and Tmax. The combined influences of these two factors decrease the values of pmax and Tmax with the increase of the hydrogen fraction when the hydrogen fraction is less than a certain value, and increase the values of pmax and

Energy & Fuels, Vol. 20, No. 2, 2006 545

Figure 11. Maximum rate of pressure rise of natural gas-hydrogen blends.

Figure 12. Maximum rate of heat release of natural gas-hydrogen blends.

Tmax with further increase of the hydrogen fraction when the hydrogen fraction is over this value. Figures 11 and 12 give the maximum rate of pressure rise (dp/dφ)max and the maximum rate of heat release (dQB/dφ)max versus hydrogen fractions at various engine operation modes, respectively. The similar behavior to those of combustion durations and pressure is presented for (dp/dφ)max and (dQB/ dφ)max, that is, (dp/dφ)max and (dQB/dφ)max decrease with the increase of the hydrogen fraction when the hydrogen fraction is less than a certain value (10% at 1200 r/min and 5% for 1800 r/min), and they will increase with the increase of the hydrogen

546 Energy & Fuels, Vol. 20, No. 2, 2006

fraction when the hydrogen fraction is over this value. As explained in the above interpretations, the combined influence from burning velocity enhancement and mixture dilution (increase in excess air ratio) by hydrogen addition makes this behavior. Conclusions Combustion characteristics and the heat release process of a direct-injection spark-ignited engine fueled with natural gashydrogen blends was investigated, and the main results are summarized as follows: (1) Heat release rate decreases with the increase of the hydrogen fraction in natural gas-hydrogen blends when the hydrogen fraction is less than a certain volumetric fraction while the heat release rate increases with the increase of the hydrogen fraction when the hydrogen fraction is over this certain volumetric value. This phenomenon indicates that only when the hydrogen fraction in the blends is over a certain fraction can large improvement in combustion be realized. (2) Flame development duration, rapid combustion duration, and total combustion duration increase with the increase of the hydrogen fraction in the blends when the hydrogen fraction is less than a certain volumetric fraction, and they decrease with the increase of hydrogen fraction when hydrogen fraction is over this value. The crank angle of the center of the heat release curve moves apart from the TDC with the increase of the hydrogen fraction when the hydrogen fraction is less than this certain volumetric fraction, and it moves close to the TDC when the hydrogen fraction is over this certain fraction. (3) Maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise, and maximum heat release rate decrease with the increase of the hydrogen fraction when the hydrogen fraction is less than this certain volumetric fraction, and they increase with the increase of the hydrogen fraction when the hydrogen fraction is over this certain fraction. (4) For fixed injection duration, the influence of hydrogen addition on natural gas-hydrogen mixture combustion is larger at low engine speed operation condition than that at high engine speed operation condition. Acknowledgment. This study was supported by the National Natural Science Foundation of China (50422261), National Basic

Huang et al. Research Project (2003CB214501), and State Key Laboratory Award Fund from the Natural Science Foundation of China (50323001). We acknowledge the students of Xi’an Jiaotong University for their help with the experiment and preparation of the manuscript. We also express our thanks to our colleagues at Xi’an Jiaotong University for their helpful comments and advice during the manuscript preparation.

Nomenclature A ) wall area ATDC ) after top-dead-center BTDC ) before top-dead-center Cp ) constant pressure specific heat (kJ/kg‚K) CV ) constant volume specific heat (kJ/kg‚K) C wt % ) mass fraction of carbon in fuel blend (dp/dφ)max ) maximum rate of pressure rise with crank angle (dQB/dφ) ) heat release rate with crank angle (dQB/dφ)max ) maximum rate of heat release with crank angle (dQw/dφ) ) heat transfer rate with crank angle hc ) heat transfer coefficient (J/m2‚s‚K) Hu ) lower heating value (MJ/kg) m ) mass of cylinder gases (kg) p ) cylinder gas pressure (MPa) pmax ) maximum cylinder gas pressure (MPa) R ) gas constant (J/kg‚K) T ) mean gas temperature (K) Tmax ) maximum mean gas temperature (K) Tw ) wall temperature (K) TDC ) top-dead-center V ) cylinder volume (m3) φ ) crank angle (deg) φc ) crank angle of the center of heat release curve (CA deg ATDC) φe ) crank angle of heat release ending (CA deg ADTC) φs ) crank angle of heat release beginning (CA deg BTDC) θfd ) flame development duration (CA deg) θrd ) rapid burning duration (CA deg) θtd ) total combustion duration (CA deg) φ ) equivalence ratio (equals to 1/λ) φign ) ignition advance angle (CA deg BTDC) φinj ) injection advance angle (CA deg BTDC) λ ) excess air ratio (m2)

EF0502453