Influence of Fuel and Operating Conditions on Combustion

1422

Energy & Fuels 2009, 23, 1422–1430

Influence of Fuel and Operating Conditions on Combustion Characteristics of a Homogeneous Charge Compression Ignition Engine Haifeng Liu,† Mingfa Yao,*,† Bo Zhang,‡ and Zunqing Zheng† State Key Laboratory of Engines, Tianjin UniVersity, Tianjin, 300072, China and Guangxi Yuchai Machinery Company Limited, Yulin 537005, China ReceiVed August 29, 2008. ReVised Manuscript ReceiVed December 30, 2008

The effect of fuel and operating conditions on the combustion process, load range, and exhaust emissions of a homogeneous charge compression ignition (HCCI) engine was investigated in a modified four-cylinder direct-injection diesel engine through an experimental study. Six fuels were used during the experiments: two primary reference fuels (PRF), two mixtures of PRF and ethanol, and two commercial unleaded gasoline fuels. All research octane numbers (RON) of these fuels are over 90. Six operating conditions were considered, including different intake temperatures (Tin), intake pressures (pin), and engine speeds (n). Experimental results indicate that autoignition of gasoline is earliest under low pin but the PRF is earliest under high pin. That is, the effect of fuel properties on the HCCI combustion process depends upon the operating conditions. It is beneficial to extend the load range with the sensitive fuels and suitable control strategies. For a sensitive fuel, a higher Tin is needed to extend the load range toward light load, while a lower Tin and higher pin is needed to extend the load range toward high load. In addition, the octane index (OI) does not show a correlation with autoignition, HC and CO emissions, and load range when the mixtures of PRF and ethanol are used in some operating conditions.

1. Introduction Homogeneous charge compression ignition (HCCI) is an autoignition combustion process in which a lean fuel/air mixture is allowed to autoignite. It can provide both good fuel economy and very low emissions of nitrogen oxides (NOx) and particulates.1-3 Therefore, it is considered to be one of the most promising internal combustion engine concepts for the future. However, it still poses some challenges that must be overcome before the HCCI combustion can be widely implemented in production engines. The HCCI combustion process is primarily controlled by chemical kinetics of the air-fuel mixture,4,5 which means that autoignition timing is difficult to control. In addition, the HCCI can only operate at partial load. It will misfire at low load and knock at high load. Therefore, for the HCCI, the major issues are controlling the autoignition process and extending the load range. To solve these issues, many methods have been * To whom correspondence should be addressed. Phone: 86-22-27406842 ext. 8014. Fax: 86-22-27383362. E-mail: [email protected]. † Tianjin University. ‡ Guangxi Yuchai Machinery Co. Ltd. (1) Kim, D. S.; Kim, M. Y.; Lee, C. S. Effect of premixed gasoline fuel on the combustion characteristics of compression ignition engine. Energy Fuels 2004, 18, 1213–1219. (2) 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, 2372– 2379. (3) Peng, Z.; Zhao, H.; Ma, T.; Ladommatos, N. Characteristic of homogeneous charge compression ignition (HCCI) combustion and emissions of n-heptane. Combust. Sci. Technol. 2005, 177, 2113–2150. (4) Lu, X.; Hou, Y.; Ji, L.; Zu, L.; Huang, Z. Heat release analysis on combustion and parametric study on emissions of HCCI engines fueled with 2-propanol/n-heptane blend fuels. Energy Fuels 2006, 20, 1870–1878. (5) 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.

proposed including in-cylinder fuel injection timing,6 intake air temperature control,7 exhaust gas recirculation (EGR),8 variable compression ratio (VCR),9 variable valve timing (VVT),10 and varying fuel properties. Among these, fuel properties is an important one. Many fuels have been tried in HCCI combustion to control the autoignition process and expand the load range, including diesel fuel,11,12 dimethyl ether (DME),13 gasoline,11,14 ethanol,15,16 methanol,13 (6) Kook, S.; Park, S.; Bae, C. Influence of early fuel injection timings on premixing and combustion in a diesel engine. Energy Fuels 2008, 22, 331–337. (7) Shibata, G.; Urushihara, T. The interaction between fuel chemicals and HCCI combustion characteristics under heated intake air conditions. SAE Tech. Pap. Ser. 2006, 2006-01-0207. (8) Zhao, H.; Xie, H.; Peng, Z. Effect of recycled burned gases on homogeneous charge compression ignition combustion. Combust. Sci. Technol. 2005, 177, 1863–1882. (9) Hyvonen, J.; Haraldsson, G.; Johansson, B. Operating conditions using spark assisted HCCI combustion during combustion mode transfer to SI in a multi-cylinder VCR-HCCI engine. SAE Tech. Pap. Ser. 2005, 2005-01-0109. (10) Li, Y.; Zhao, H.; Brouzos, N.; Ma, T. Parametric study on CAI combustion in a GDI engine with an air-assisted injector. SAE Tech. Pap. Ser. 2007, 2007-01-0196. (11) Kim, D. S.; Lee, C. S. Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR. Fuel 2006, 85, 695– 704. (12) Kim, M. Y.; Kim, J. W.; Lee, C. S.; Lee, J. H. Effect of compression ratio and spray injection angle on HCCI combustion in a small DI diesel engine. Energy Fuels 2006, 20, 69–76. (13) Yao, M.; Zheng, Z.; Chen, Z.; Zhang, B. Experimental study on homogeneous charge compression ignition operation by burning dimethyl ether and methanol. Int. J. Green Energy 2007, 4, 283–300. (14) Yeom, K.; Bae, C. Gasoline-di-methyl ether homogeneous charge compression ignition engine. Energy Fuels 2007, 21, 1942–1949. (15) Yap, D.; Megaritis, A. Applying forced induction to bioethanol HCCI operation with residual gas trapping. Energy Fuels 2005, 19, 1812– 1821.

10.1021/ef800950c CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

Influence of Fuel and Operating Conditions

liquefied petroleum gas,17 and natural gas.18 In addition, numerous experimental and numerical studies have been performed using the primary reference fuels (PRF), n-heptane, and iso-octane.19-22 These investigations have been very useful for probing the effect of the fuel properties on HCCI combustion. Recently, HCCI development efforts have tended to focus on the use of gasoline or diesel fuels due to their wide availability and because they provide the potential to revert to traditional spark ignition or compression ignition for high load operation. In addition, a more volatile fuel will have an advantage over a less volatile fuel in HCCI engines. Therefore, the unleaded gasoline fuels were used in this experiment. The gasoline fuels were bought from the Chinese market. Ethanol is considered by many as one of the most important alternatives to gasoline and diesel because it can offer substantial reductions in consumption of fossil fuels and emissions of greenhouse gases. There has been substantial research on blending ethanol with diesel fuel or gasoline.23-25 In addition, its application in HCCI combustion was also investigated.15,16,26 However, little information was investigated about using the mixture of PRF and ethanol in HCCI combustion. The properties of the PRF are known. Therefore, a study should start with fuels that are known to focus on the effect of ethanol on the combustion process of an HCCI engine. In this work, two mixtures of PRF and ethanol were used. Finally, in previous works, the effects of the research octane number (RON) of PRF on HCCI combustion have been investigated.21,27 These results have shown that the maximum indicated mean effective pressure (IMEP) of high-octane fuel is higher than that of the low-octane fuel. However, if the octane number is too high, the engine cannot run smoothly at low IMEP and the HCCI operating speed is also limited (e.g., RON ) 90). Therefore, to overcome the limit of previous works that the RON of the PRF cannot exceed 90, some PRF with higher RON were used in this paper. (16) Megaritis, A.; Yap, D.; Wyszynski, M. L. Effect of inlet valve timing and water blending on bioethanol HCCI combustion using forced induction and residual gas trapping. Fuel 2008, 87, 732–739. (17) Yeom, K.; Jang, J.; Bae, C. Homogeneous charge compression ignition of LPG and gasoline using variable valve timing in an engine. Fuel 2007, 86, 494–503. (18) Yap, D.; Peucheret, S. M.; Megaritis, A.; Wyszynski, M. L.; Xu, H. Natural gas HCCI engine operation with exhaust gas fuel reforming. Int. J. Hydrogen Energy 2006, 31, 587–595. (19) Zhao, H.; Peng, Z.; Ma, T. Investigation of the hcci/cai combustion process by 2-D plif imaging of formaldehyde. SAE Tech. Pap. Ser. 2004, 2004-01-1901. (20) Huang, C.; Lu, X.; Huang, Z. New reduced chemical mechanism for homogeneous charge combustion ignition combustion investigation of primary reference fuels. Energy Fuels 2008, 22, 935–944. (21) Yao, M.; Zhang, B.; Zheng, Z.; Chen, Z. Experimental study on homogeneous charge compression ignition combustion with primary reference fuel. Combust. Sci. Technol. 2007, 179, 2539–2559. (22) Kim, D. S.; Lee, C. S. Effect of n-Heptane premixing on combustion characteristics of diesel engine. Energy Fuels 2005, 19, 2240–2246. (23) Chandra, R.; Kumar, R. Fuel properties of some stable alcoholdiesel microemulsions for their use in compression ignition engines. Energy Fuels 2007, 21, 3410–3414. (24) Fernando, S.; Hanna, M. Development of a novel biofuel blend using ethanol-biodiesel-diesel microemulsions: EB-diesel. Energy Fuels 2004, 18, 1695–1703. (25) Liao, S. Y.; Jiang, D. M.; Cheng, Q.; Huang, Z. H.; Wei, Q. Investigation of the cold-start combustion characteristics of ethanol-gasoline blends in a constant-volume chamber. Energy Fuels 2005, 19, 813–819. (26) Hou, Y.; Lu, X.; Zu, L.; Ji, L.; Huang, Z. Effect of high-octane oxygenated fuels on n-heptane-fueled HCCI combustion. Energy Fuels 2006, 20, 1425–1433. (27) Yao, M.; Zhang, B.; Zheng, Z.; Chen, Z.; Xing, Y. Effects of exhaust gas recirculation on combustion and emissions of a homogeneous charge compression ignition engine fueled with primary reference fuels. Proc. Inst. Mech. Eng., Part. D: J. Automob. Eng. 2007, 221, 197–213.

Energy & Fuels, Vol. 23, 2009 1423

Figure 1. Experimental setup: 1, engine; 2, dynamometer; 3, air compressor; 4, air tank; 5, air flow meter; 6, electric heater; 7, pressure transducer; 8, encoder; 9, charge amplifier; 10, data acquisition system; 11, fuel tank; 12, fuel flow meter; 13, fuel pump; 14, fuel injector; 15, electronic control unit; 16, exhaust analyzer. Table 1. Engine Specifications displacement

1.3 L

Bore stoke compression ratio inlet valve open inlet valve close exhaust valve open exhaust valve close

112 mm 132 mm 17.5 13.5° BTDC 38.5° ABDC 56.5° BBDC 11.5° ATDC

In addition, some researchers have shown that the effects of fuel on the combustion process are dependent on the operating conditions. Kalghatgi et al.28 proposed an octane index, described as a function of research octane number (RON), motor octane number (MON), and operating condition (K value), and K value was affected by different operating conditions. Shibata and Urushihara7 investigated the interaction between fuel chemicals and HCCI combustion characteristics under heated intake air conditions. The authors showed that with more intake air heating the low-temperature heat release decreased and the difference in high-temperature heat release profiles between the fuels became smaller. Therefore, in the current work six fuels were used, including PRF, gasoline, and a mixture of PRF and ethanol. The purpose of this research is to gain a better understanding of the effect of fuel properties on the combustion process, load range, and exhaust emissions of an HCCI engine under different operating conditions, such as different intake temperatures (Tin), intake pressures (pin), and engine speeds (n). In addition, the correlation between the octane index (OI) and autoignition is also investigated using these six fuels. 2. Experimental Apparatus and Methods 2.1. Engine. A diesel engine (Yuchai 4112 series, YC4112ZLQ) was converted to run in HCCI mode. The four-cylinder engine was modified to operate in one cylinder only. This arrangement gives a robust and inexpensive single-cylinder engine but at the cost of the reliability of the brake-specific results. However, with a pressure transducer, indicated results can be used instead. The most important engine parameters are shown in Table 1. Figure 1 illustrates the experimental setup. The boost pressure was generated by an external air compressor that could be used to (28) Kalghatgi, G.; Risberg, P.; Angstrom, H. E. A method of defining ignition quality of fuels in HCCI engines. SAE Tech. Pap. Ser. 2003, 200301-1816.

1424 Energy & Fuels, Vol. 23, 2009

Liu et al.

Table 2. List of Fuels Tested composition (% v/v) fuel code

iso-octane

PRF94 PRF97 PRFE94.2 PRFE96.2 G94.1 G97.4

94 97 60 55

n-heptane 6 3 20 20 gasoline gasoline

ethanol

RON

MON

20 25

94 97 94.2 96.2 94.1 97.4

94 97 88.9 88.3 79 90

control the inlet pressure up to 3 bar. A large tank (∼150 L) was added as a pressure stabilizer in the intake system to reduce the effect of inlet air pressure pulsation on the measurement of the air flow rate. The air flow meter was mounted at the outlet of the tank. An electric heater was installed in the intake system upstream of the injector to heat the intake air. The intake temperature was measured with a K-type thermocouple at a distance of 25 cm from the intake port. A feedback control system maintained the intake air temperature within 1 °C of the chosen temperature. An electronic port fuel injector, typical of those used in modern SI engines, was installed in the inlet pipe at a location approximately 20 cm from the intake port. This injector was used to inject fuel into the inlet air for operation of the engine in HCCI mode. An injector controller was used to drive the injector, controlling both the injection timing and the fuel quantity per cycle (Qcyc) by the pulse width of the injection event. A pressure transducer (Kistler 6125A) was fitted flush with the wall of the cylinder head, connected via a charge amplifier (Kistler 5011) to a data acquisition board (National Instruments) fitted in a compatible PC. The cylinder pressure data was recorded in halfcrank-angle increments, triggered by an optical shaft encoder. The cylinder pressure data was analyzed using a single-zone heat-release model with an assumption that the mixture of air and fuel is homogeneous in the whole cylinder volume. In addition, there is assumed to be no mass leakage from the cylinder. The heat-transfer coefficient was obtained from the Woschni correlation. The rate of heat release (ROHR) is calculated in this way, as used in previous research.21,27,33 The concentrations of CO2, CO, O2, NOx, and THC in the exhaust gas were measured by the exhaust analyzer (Horiba MEXA7100DEGR), which measures THC by the flame ionization method, CO and CO2 by the nondispersive infrared method, and NOx by the chemiluminescent method. 2.2. Fuels. The fuels used in this work are listed in Table 2, including PRF, mixtures of PRF and ethanol, and two commercial unleaded gasoline fuels purchased from the Chinese market. Codes G94.1 and G97.4 were used as the marker of the unleaded gasoline, codes PRFE94.2 and PRFE96.2 as the marker of the mixtures of PRF and ethanol, and codes PRF94 and PRF97 as the marker of the PRF, where the numbers are the RON of the fuels. Apart from the PRF, others fuels have sensitivity (S), where S ) RON s MON. The MON is the motor octane number. The octane numbers were measured in the standardized research method29 and motor method30 tests with cooperative fuel research (CFR) engines. The inlet charge temperature is 325 K for the RON and 422 K for the MON, and the engine speed is 600 rpm for the RON and 900 rpm for the MON. Therefore, since the MON is measured at higher inlet charge temperature and higher speed, the MON test is close to the real state of the engine operation. These measurements were carried out by the North Institute of China Petrochemical Corp. Further specifications of the fuels are given in Table 3. As can be seen, in comparison to PRF and gasoline, the PRFE have higher a heat of vaporization and lower heating value due to addition of ethanol. In addition, there are fuels with comparable RON but (29) American Society for Testing Materials Designation, D 2699-01a. Standard test method for research octane number of spark-ignition engine fuel, 2001. (30) American Society for Testing Materials Designation: D 2700-01a. Standard test method for motor octane number of spark-ignition engine fuel, 2001.

different MON, e.g., fuels of PRF94, G94.1, and PRFE94.1; fuels of PRF97, G97.4, and PRFE96.2. 2.3. Experimental Procedure. This system was used to start and warm up the engine in the standard diesel configuration until the lubricating oil temperature reached 85 °C and the cooling water temperature reached 80 °C; then it was switched into HCCI mode. The fuel was injected till autoignition takes place as indicated by the pressure signal and the increase in power delivered by the engine. The fuel quantity was adjusted till the required normalized fueling rate was achieved. At each operating point, the engine was allowed to run for several minutes until the measured parameters were stable, at which 50 pressure cycles were acquired and stored for later analysis. In this work, six operating conditions were used; listed in Table 4. The difference between Qcyc is small and, therefore, can be considered as a constant for the different operating conditions.

3. Results and Discussion 3.1. Cylinder Pressure and Heat-Release Rate. The cylinder pressure and heat-release rate trace is presented in Figure 2 using fuels with comparable RON (∼94) under the six different operating conditions. It shows that the combustion process is different than the fuels with nearly identical RON, and these high-RON fuels have no low-temperature heat release. Figure 2a shows that the start of combustion (SOC) of the fuel G94.1 is the earliest among the three fuels, while the fuel PRF94 does not fire at this operating condition (Tin ) 110 °C, pin ) 1 bar). However, Figure 2b shows that the PRF94 can be fired and its SOC is the earliest at the higher intake pressure (pin ) 2 bar) in comparison to Figure 2a. Furthermore, the SOC of all three fuels is advanced at the higher pin. The reason is that more air can be drawn into the cylinder with an increase of boosting pressure, which increases the collision frequency among molecules and combustion reaction velocity. This indicates that the boosting can improve the autoignition ability of fuels, which will be helpful to autoignite at light load. Figure 2c shows the pressure and ROHR traces at the OP3. In comparison to Figure 2a, the Tin is increased from 110 to 150 °C. It can be observed that all fuels can be fired. With the increase of Tin, the SOC of the three fuels advances and the peak of cylinder pressure and ROHR increases relative to OP1. The fuel quantity is constant between the OP1 and OP3. The increasing Tin results in the decrease of intake air. Therefore, the fuel/air equivalence ratio at the OP3 is larger than that of OP1, resulting in the obvious increase of the peak of ROHR. Figure 2d shows the pressure and ROHR traces at the OP4. In comparison to Figure 2a, Tin and pin are increased from 110 °C and 1 bar to 150 °C and 2 bar. It can be observed that the SOC and peak pressure are nearly the same for all three fuels. That is, the fuel properties nearly have no effect on the SOC at the high Tin and pin. Parts b and c of Figure 2 show that an increase of pin or Tin can advance the SOC. Therefore, the SOC of the three fuels is advanced further with the simultaneous increase of pin and Tin. The effects of fuel properties on the combustion process are weakened due to the too early autoignition. Figure 2e shows the pressure and ROHR traces at the OP5. In comparison to Figure 2a, the engine speed is decreased from 1400 to 900 rpm. It can be observed that the fuel PRF94 can be fired and the SOC of all fuels advances relative to OP1. The reason is that the reactant mixture spends much time before autoignition at the lower engine speed, which results in the ignition delay recorded by the engine crank angle shortening. Therefore, the SOC is earlier than that at the higher speed. Figure 2f shows the pressure and ROHR traces at the OP6, in which the pin is increased in comparison to Figure 2e. It can be


Energy & Fuels, Vol. 23, 2009 1425 Table 3. Fuel Properties

PRF94 boiling point (°C) distillation (°C) T10 T50 T90 density (kg/m3) heat of vaporization (MJ/kg) lower heating value (MJ/kg) saturates (% v/v) olefins (% v/v) aromatics (% v/v) ethanol (% v/v) MTBE (% v/v)

G94.1

PRFE94.2

99.15

691.8 0.285 44.67 100 0 0 0

PRF97

G97.4

PRFE96.2

99.18 55 94 160 731.9 0.31-0.34 43.9-44.4 44 25 30

71 73.6 99 713.0 0.405 41.13 80 0 0 20

1

observed that the peak of cylinder pressure increases, and the SOC advances. As in Figure 2b, the SOC of the PRF94 is the earliest among the three fuels. Therefore, as Figure 2 shows, the ignition behavior of different fuels cannot be explained by the RON. On the other hand, the MON also cannot correlate the autoignition. The MON of G94.1, PRFE94.2, and PRF94 is 79, 88.9, and 94, respectively, as shown in Table 2. That is, according to the MON,

691.9 0.284 44.66 100 0 0 0

57 94 165 756.8 0.31-0.34 43.9-44.4 37.5 20 41

70.8 71.2 98.7 713.3 0.434 40.24 75 0 0 25

n-heptane

iso-octane

ethanol

98.4

99.2

78.4

688.0 0.317 44.93

692.0 0.283 44.65

789.0 0.854 26.78

1.5

autoignition of the G94.1 should be first and the PRF94 should be last. However, parts b and f of Figure 2 show that the SOC is reversed to the MON when intake pressure is boosted; the fuel G94.1 is last, while the PRF94 is first. Therefore, it indicates that the ignition behavior of different fuels cannot be explained by conventional measures of autoignition quality of the fuels such as RON and MON at all operating conditions. Finally, it can be observed that the effects of fuel properties on the

Figure 2. Cylinder pressure and rate of heat release with G94.1, PRFE94.2, and PRF94 under different operating conditions.


Liu et al. Table 4. Operating Conditions

Figure 3. Cylinder pressure and rate of heat release with G94.1, PRFE94.2, and PRF94 under the same heating value.

Figure 4. Cylinder pressure and rate of heat release with G94.1, PRFE94.2, and PRF94 under the same lambda.

combustion process depend upon the operating conditions. The fuel properties have different effects on the combustion process under different operating conditions. 3.2. Correlation between OI and CA50. The octane number is the percentage by volume of iso-octane mixed with n-heptane in the PRF, which gives rise to engine knock in either of the tests under the same conditions as the actual fuel. Generally speaking, the gasoline can contain only about 5% iso-octane and 0.3% n-heptane as in the reference,31 and engine temperatures, pressures, and mixture strengths are usually markedly different from those in the CFR tests. Because of these factors, the RON and MON values are an incomplete guide to practical autoignition performance. To provide a more realistic measure for expressing the autoignition quality of a sensitive fuel, the OI has been defined by Kalghatgi et al.28 From linear regression, the author expressed CA50 (the crank angle at which the cumulative heat released reaches 50%) as CA50 ) c + aRON + bMON CA50 ) c + (a + b)OI

(1) (2)

where OI ) [a ⁄ (a + b)]RON + [b ⁄ (a + b)]MON OI is the octane index. The author defined K as

(3)

K ) b ⁄ (a + b) Finally, the OI was expressed as

(4)

OI ) (1-K)RON + KMON (5) where K is a constant depending on the pressure and temperature evolution in the unburnt gas. K is not a primary property of the fuel. Equation 5 can also be expressed as OI ) RON -KS (6) where Sis the fuel sensitivity. The chemical origin of fuel octane sensitivity results from paraffin autoignition chemistry being dominated by negative temperature coefficient behavior.32 Fuels

operating conditions

n (rpm)

Tin (°C)

pin (bar, abs)

Qcyc (mg)

OP1 OP2 OP3 OP4 OP5 OP6

1400 1400 1400 1400 900 900

110 110 150 150 110 110

1 2 1 2 1 2

20.5 20.2 20.5 20.2 20 21

with higher sensitivities will exhibit a lower MON for the same RON. Kalghatgi et al.28 found that CA50 might show no correlation with either RON or MON but correlates very well with the OI. The higher the OI, the more the resistance to autoignition and the later is the heat release in the HCCI engine. Therefore, in this work, the OI and K value at different operating conditions have been calculated and are shown in Table 5. Due to the misfiring at the OP1, the OI is not calculated at this operating condition. The linear regression between OI and CA50 is analyzed. The coefficient of determination (R2) is shown in Table 5. R2 is the square of the correlation coefficient (R). R2 is a variable between 0 and 1. According to the formula R ) (nΣxy - ΣxΣy)/(nΣx2 - (Σx)2nΣy2 - (Σy)2), where n is the numbers of the sample, x is the value of the CA50, y is the value of OI. From this formula R2 can be calculated in different operating conditions. The higher the R2, the better the correlation will be. From Table 5 it is found that CA50 cannot show a correlation with the OI very well. In particular, at the OP5 R2 is 0.431, which indicates that the correlation between OI and CA50 is weak. From Table 5 it can be observed that the OI of fuels G94.1, PRFE94.2, and PRF94 is 84.69, 90.90, and 94.00, respectively, at the OP5. According to previous research, it indicated that the higher the OI, the more the resistance to autoignition. Therefore, the sequence of autoignition should be that the G94.1 is first and the PRF94 is last. However, from the Figure 2e it can be observed that autoignition of PRFE94.2 is last. A similar phenomenon can be observed in Figure 2b. The OI of fuels PRFE94.2 and G94.1 is 95.81 and 98.70, respectively, while their autoignition is nearly the same. These results show that autoignition of fuel PRFE94.2 is not relevant to its OI. In fact, the actual autoignition of PRFE94.2 retards that of the hypothesis according to its OI. Table 5 also shows that the CA50 of fuel PRFE96.2 has no correlation with its OI. The main reason is that ethanol has a higher heat of vaporization and lower heating value, which leads to the lower cylinder temperature and hence retarding autoignition. Therefore, due to addition of ethanol, autoignition of the mixtures of PRF and ethanol cannot correlate with the OI very well. Due to the lower heating value of PRFE94.2, its fuel quantity was increased from 20 to 21.5 mg/cycle. Then, the heating value of G94.1, PRFE94.2, and PRF94 is 0.89, 0.88, and 0.88 kJ per cycle. It is nearly identical in these three fuels. Figure 3 shows the traces of the cylinder pressure and rate of heat release under the same heating value. It can be observed that autoignition of the PRFE94.2 only advances a little in comparison to Figure 2e. In addition, the fuel quantity of PRFE96.2 was also increased from 20 to 21.9 mg/cycle. Then, the heating value of G97.4, PRFE97, and PRF96.2 is 0.89, 0.88, and 0.88 kJ per cycle. (31) Bradley, D.; Head, R. A. Engine autoignition: The relationship between octane numbers and autoignition delay times. Combust. Flame 2006, 147, 171–184. (32) Leppard, W. R. The Chemical Origin of Fuel Octane Sensitivity. SAE Tech. Pap. Ser. 1990, 902137. (33) Liu, H.; Yao, M.; Zhang, B.; Zheng, Z. Effects of Inlet Pressure and Octane Numbers on Combustion and Emissions of a Homogeneous Charge Compression Ignition (HCCI) Engine. Energy Fuels 2008, 22, 2207– 2215.



Table 5. CA50, K Value, and OI under Different Operating Conditions operating conditions

fuels code

RON

MON

CA50 (deg. ATDC)

Qcyc (mg)

K

OI

R2 (OI and CA50)

OP2, n ) 1400 rpm, Tin ) 110 °C, pin ) 2 bar

G94.1 PRFE94.2 PRF94 G97.4 PRFE96.2 PRF97 G94.1 PRFE94.2 PRF94 G97.4 PRFE96.2 PRF97 G94.1 PRFE94.2 PRF94 G97.4 PRFE96.2 PRF97 G94.1 PRFE94.2 PRF94 G97.4 PRFE96.2 PRF97 G94.1 PRFE94.2 PRF94 G97.4 PRFE96.2 PRF97

94.1 94.2 94 97.4 96.2 97 94.1 94.2 94 97.4 96.2 97 94.1 94.2 94 97.4 96.2 97 94.1 94.2 94 97.4 96.2 97 94.1 94.2 94 97.4 96.2 97

79 88.9 94 90 88.3 97 79 88.9 94 90 88.3 97 79 88.9 94 90 88.3 97 79 88.9 94 90 88.3 97 79 88.9 94 90 88.3 97

-5 -5.2 -7.5 -3.6 -3.5 -6 -5.5 -2.2 -1.7 -3.9 -1.8 -0.4 -9.8 -9.4 -9.6 -8.8 -8.4 -8.9 -1.5 0.6 0.2 -0.6 1.8 1.5 -8.7 -9.2 -10.8 -7.6 -7.9 -10

20.2

-0.305

0.779

20.5

2.424

20.2

0.009

20

0.623

21

-0.360

98.70 95.81 94.00 99.65 98.61 97.00 57.50 81.35 94.00 79.46 77.05 97.00 93.96 94.15 94.00 97.33 96.13 97.00 84.69 90.90 94.00 92.79 91.28 97.00 99.53 96.11 94.00 100.06 99.04 97.00





0.789

0.689

0.431

0.818

Table 6. CA50, K Value, and OI under Same Heating Value operating conditions

fuels code

RON

MON

CA50 (deg. ATDC)

Qcyc (mg)

K

OI

R2 (OI and CA50)


G94.1 PRFE94.2 PRF94 G97.4 PRFE96.2 PRF97

94.1 94.2 94 97.4 96.2 97

79 88.9 94 90 88.3 97

-1.5 0.1 0.2 -0.6 1.5 1.5

20 21.5 20 20 21.9 20

0.530

86.10 91.39 94.00 93.48 92.01 97.00

0.523

Table 7. CA50, K Value, and OI under the Same Lambda operating conditions

fuels code

RON

MON

CA50 (deg. ATDC)

λ

K

OI

R2 (OI and CA50)


G94.1 PRFE94.2 PRF94 G97.4 PRFE96.2 PRF97

94.1 94.2 94 97.4 96.2 97

79 88.9 94 90 88.3 97

-1.7 0.3 0.2 -0.7 1.6 1.6

3.5

0.613

84.84 90.95 94.00 92.86 91.36 97.00

0.517

Finally, the OI and R2 were calculated using fuels with the same heating value. Table 6 shows the results. R2 is 0.523 at the OP5, which is higher than that of Table 5. However, it is still very low. This result indicates that the heating value is not the main factor for autoignition retarding. Otherwise, when the heating value is the same, R2 should be increased obviously. This shows that the heat of vaporization should be the main factor.

Figure 5. Correlation between OI and CA50 without fuels PRFE.

On the other hand, the stoichiometric (A/F) ratio of these three fuels is different. Therefore, the relative air/fuel ratio (λ) was set to 3.5 at the OP5. Figure 4 shows the traces of the cylinder pressure and rate of heat release under the same lambda. It can be observed that autoignition of the G94.1 is the earliest among the three fuels. Autoignition of the PRFE94.2 and PRF94 is nearly the same. The values of K, OI, and R2 were calculated


Liu et al.

Table 8. CA50, K Value, and OI under Different Operating Conditions without Fuels PRFE operating conditions OP2, n ) 1400 rpm, Tin ) 110 °C, pin ) 2 bar





fuels code

RON

MON

G94.1 PRF94 G97.4 PRF97 G94.1 PRF94 G97.4 PRF97 G94.1 PRF94 G97.4 PRF97 G94.1 PRF94 G97.4 PRF97 G94.1 PRF94 G97.4 PRF97

94.1 94 97.4 97 94.1 94 97.4 97 94.1 94 97.4 97 94.1 94 97.4 97 94.1 94 97.4 97

79 94 90 97 79 94 90 97 79 94 90 97 79 94 90 97 79 94 90 97

and are shown in Table 7. R2 is 0.517 at the OP5, which is higher than that of Table 5. However, it is still very low. This shows that the CA50 and OI have no correlation under the same lambda. The above research shows that the OI cannot show the correlation with autoignition very well. The main reason should be that the fuel PRFE includes some ethanol. Therefore, in the next section, the correlation between OI and CA50 without PRFE was studied. The aim is to verify further the effect of the fuel PRFE on the combustion process. 3.3. Correlation between OI and CA50 without PRFE. Table 8 shows the value of K, OI, and R2 without the PRFE under different operating conditions. It can be observed that R2 increases dramatically. This indicates that the CA50 and OI have a good correlation. The higher the OI, the more the resistance to autoignition and the later is the heat release in the HCCI engine. Figure 5 shows the correlation between OI and CA50 without fuels PRFE under the OP2 and OP5. It can be observed that the CA50 shows correlation with the OI very well. R2 increases obviously in comparison to the same operating conditions in Table 5. These also indicate that autoignition of fuels PRFE cannot be guided by the OI due to addition of ethanol, whose vaporization heat is too high. In addition, it can also be observed from Table 8 that the K value is negative at the OP2 and OP6, in which pin is 2 bar and Tin is 110 °C. When increasing Tin to 150 °C, the K value will reach 0.047; then it will reach the highest value at the OP3 (Tin ) 150 °C, pin ) 1bar). This shows that the K value depends upon the operating conditions. It has been shown that the K value decreases as the compression temperature decreases in ref 28. In this work, the boosting can bring much air, which results in a lower compression temperature and a lower K value. On the contrary, it has a higher compression temperature at the lower pin, higher Tin condition, which leads to the K value being much higher. For a given sensitive fuel, S is constant. The OI is decided by the K value. Different K values will get different OI, which will make the combustion process different and finally get a different load range. Therefore, the operating conditions should be controlled suitably to get an appropriate K value. 3.4. Effect of Fuel Properties on Emissions. NOx formation is very sensitive to the temperature history during the cycle. At temperatures over 1800 K, the NOx formation rate increases rapidly with increasing temperature. With a homogeneous combustion of a premixed mixture, the temperature is expected to be the same in the entire combustion chamber, except near

CA50 (deg. ATDC) -5 -7.5 -3.6 -6 -5.5 -1.7 -3.9 -0.4 -9.8 -9.6 -8.8 -8.9 -1.5 0.2 -0.6 1.5 -8.7 -10.8 -7.6 -10

K

OI

R2 (OI and CA50)

20.2

-0.266

0.957

20.5

4.065

20.2

0.047

20

1.012

21

-0.321

98.11 94.00 99.37 97.00 32.72 94.00 67.32 97.00 93.39 94.00 97.05 97.00 78.81 94.00 89.91 97.00 98.95 94.00 99.78 97.00

Qcyc (mg)

0.927

0.994

0.855

0.905

the walls. This, in combination with very lean mixtures, gives a low maximum temperature during the cycle. Therefore, the NOx emissions are very low in these operating conditions. Figure 6 shows the NOx emissions at the OP5. Similarly, the NOx emissions at all other operating conditions are also very low. Therefore, only the emissions of the OP5 were shown in this paper. The low homogeneous combustion temperature restricts NOx formation, but the combustion temperature becomes too low to oxidize the fuel completely. The low combustion temperature leads to incomplete combustion and high emissions of HC and CO. Figure 7 shows the HC and CO emissions at the OP5. It can be observed that the HC and CO emissions decrease with increased cycle fuel quantity. Keeping the intake temperature and pressure constant, increased fuel quantity means less diluted mixture and higher combustion temperature, resulting in lower HC and CO emissions. The HC and CO emissions of different fuels show a good correlation with autoignition. The later the autoignition, the higher HC or CO emissions become. This indicates that the OI can show good correlation with HC and CO emissions except the fuel of PRFE. 3.5. Discussion. The results have shown that the fuel properties have different effects on the combustion process under different engine operating conditions. OI has better correlation with autoignition than conventional RON or MON. The higher the OI, the more the resistance to autoignition and the later is the heat release in the HCCI engine. However, autoignition of fuels containing ethanol (PRFE) cannot be guided by the OI. Ethanol has a higher heat of vaporization and lower heating value than that of gasoline and PRF, which leads to the lower cylinder temperature and hence retarding of autoignition. The

Figure 6. NOx emissions with G94.1, PRFE94.2, and PRF94 under OP5.



Figure 7. HC and CO emissions with G94.1, PRFE94.2, and PRF94 under OP5.

Figure 8. Load range under different intake temperature.

Figure 9. Load range under different intake pressure.

effect of the heating value on the combustion process is small. Therefore, the higher vaporization heat of ethanol may be the main factor, causing autoignition of fuels PRFE not being guided by the OI. In addition, ethanol might have a chemistry effect to the autoignition delay. However, the kinetic study on the autoignition of fuel blends containing ethanol was not carried out in this paper. A great deal of work is needed to study the autoignition reaction mechanisms of fuels with additive ethanol in the future. The sensitive fuel will be more likely to ignite at the low load and be more resistant to autoignition at the high load. Therefore, more sensitive fuels are likely to allow a HCCI engine to operate over a wider load range than less sensitive fuels with the same RON. Figures 8 and 9 show the load range in terms of the load at different intake temperature and pressure. The limits for the load range are defined by some chosen variables. The criterion for the maximum achievable load is the maximum pressure rise rate of 1.0 MPa/CA degree, beyond which combustion tends to become “knocky”. The limits for the minimum load are misfiring, defined as COV(IMEPgross) exceeding 10% for the engine.

Figure 8 shows the load range under intake air heating conditions. It can be observed that the G94.1 can reach the lowest load before misfire occurs. This indicates that the sensitive fuel is beneficial to extend the operating range toward the light load. However, the benefits of sensitive fuels at higher loads cannot be observed. The intake heating leads to the conclusion that the K value is positive at these operating conditions. According to the formula OI ) RON - KS, the OI of sensitive fuels cannot exceed its RON. Therefore, the PRF has higher OI and later autoignition than those of sensitive fuels. This means that the PRF can reach a higher load at high Tin conditions. However, in the previous study,33 the results have shown that the achievable maximum load of sensitive fuel (G94.1) is higher than that of nonsensitive fuel (PRF93) under boosting conditions, as Figure 9 shows. This indicates that the sensitive fuels are more beneficial to extend the operating range toward the high load under boosting conditions. These results show that the benefits of sensitive fuels depend upon the operating conditions. For a sensitive fuel, a higher Tin is needed to ensure a lower OI for extending the operating range toward light load. This means that the control strategies must focus on the thermal management at the light load to ensure autoignition of the fuel, such as hot internal EGR or other methods. On the contrary, a lower Tin and higher pin is needed to get a higher OI for extending the operating range toward high load. Obviously, the boost is an effective method to extend the operating range, especially for a sensitive fuel. Therefore, to stand out, the benefits of sensitive fuels and the operating conditions of the engine should be controlled suitably. 4. Conclusions To understand the effects of the fuel on the HCCI combustion process, load range, and exhaust emissions under different operating conditions, including different intake temperatures, intake pressures, and engine speeds, experiments were carried out in a modified four-cylinder direct-injection diesel engine using six fuels, including PRF, unleaded gasoline, and mixtures of PRF and ethanol. The RON of all fuels is over 90. Some main conclusions can be drawn in this paper. (1) The effects of fuel on the combustion process are strongly dependent upon the operating conditions. The fuel properties have different effects on the combustion process under different operating conditions. (2) OI shows good correlation with autoignition, HC and CO emissions, and the load range. However, the OI does not show good correlation with these parameters when the fuel includes some ethanol. (3) More sensitive gasoline fuels have more benefits than nonsensitive fuels for applying in a HCCI engine, but appropriate control strategies are needed in order


Liu et al.

to derive the benefits of sensitive fuels. A higher Tin is needed to ensure a lower OI for extending the operating range toward light load, while a lower Tin and higher pin is needed to get a higher OI for extending the operating range toward high load. Boosting is an effective method to extend the load range, especially for a sensitive fuel.

(50676066) and the National Natural Science Found of China (NSFC) through its key project “Some key questions in advanced combustion and control in engines” (50636040). The assistance of Associate Professor Robert Raine of the University of Auckland with the final editing of the paper is also gratefully acknowledged.

Acknowledgment. The research was supported by the National Natural Science Found of China (NSFC) through its project

EF800950C

Influence of Fuel and Operating Conditions on Combustion

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