Experimental Study on Influencing Factors of iso-Octane Thermo

Mar 23, 2009 - This paper focuses on a novel combustion mode that is developed from the original concept of homogeneous charge compression ignition ...
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Energy & Fuels 2009, 23, 2405–2412

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Experimental Study on Influencing Factors of iso-Octane Thermo-atmosphere Combustion in a Dual-Fuel Stratified Charge Compression Ignition (SCCI) Engine Libin Ji, Xingcai Lu¨,* Junjun Ma, Chen Huang, Dong Han, and Zhen Huang Key Laboratory for Power Machinery and Engineering of the Ministry of Education, Shanghai Jiao Tong UniVersity, Shanghai 200030, People’s Republic of China ReceiVed NoVember 1, 2008. ReVised Manuscript ReceiVed March 2, 2009

This paper focuses on a novel combustion mode that is developed from the original concept of homogeneous charge compression ignition (HCCI) combustion. Experiments on iso-octane thermo-atmosphere combustion were carried out on a modified single-cylinder engine. n-Heptane was induced from the intake manifold to promote the ignition of iso-octane spray that was directly injected into the cylinder, through HCCI combustion of n-heptane. The combustion and emission characteristics of this novel combustion strategy were analyzed, and the effects of the iso-octane supply advance angle and n-heptane premixed ratio were investigated. The results show that the influence of the thermal effect and active radical produced by n-heptane HCCI combustion on the iso-octane ignition and emission is varied, depending upon the iso-octane supply advance angle. With the advancing of this fuel supply angle, the ignition of iso-octane is enhanced at first but then suppressed. At an iso-octane supply advance angle of 25 °CA BTDC, iso-octane is best ignited and the highest combustion efficiency and indicated thermal efficiency are obtained. Meanwhile, the level of CO in the exhaust gas is the lowest, with the penalty of the worst NOx emission.

1. Introduction Stringent regulations to reduce engine emission and eager commercial anticipations to improve fuel efficiency are prompting the development of new combustion technologies for internal combustion engines. The homogeneous charge compression ignition (HCCI) combustion has drawn extensive attention in recent years because of its potential of providing both high thermal efficiency comparable to that of conventional compression ignition (CI) engines and ultra-low NOx emission comparable to that of traditional spark ignition (SI) engines with a three-way catalyst system.1-6 Research on HCCI combustion has achieved great success with varieties of methods, such as intake air temperature * To whom correspondence should be addressed. Telephone: +86-2134206039. Fax: +86-21-34206139. E-mail: [email protected]. (1) Najt, P. M.; Foster, D. E. Compression-ignited homogeneous charge combustion. SAE 830264. (2) Lu, X. C.; Ji, L. B.; Zu, L. L.; Hou, Y. C.; Huang, C.; Huang, Z. Experimental study and chemical analysis of n-heptane homogeneous charge compression ignition combustion with port injection of reaction inhibitors. Combust. Flame 2007, 149, 261–270. (3) 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. (4) Kim, D. S.; Lee, C. S. Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR. Fuel 2006, 85, 695– 704. (5) 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. (6) Shi, L.; Deng, K. Y.; Cui, Y. Study of diesel-fuelled homogeneous charge compression ignition combustion by in-cylinder early fuel injection and negative valve overlap. Proc. Inst. Mech. Eng., Part D 2005, 219, 1193– 1201.

management,7,8 variable compression ratio (VCR),9,10 variable valve actuation (VVA),11,12 and exhaust gas recirculation (EGR).13,14 However, commercial production of a HCCI engine still faces several technical challenges, such as ignition timing control, combustion rate control, and operating range extending, attributed to the fact that the HCCI combustion process is predominantly controlled by chemical kinetics. While the basic principles of HCCI are reasonably wellunderstood as a result of recent research, the role of inhomogeneity in HCCI combustion gradually becomes a focus. For one thing, it is impossible to produce a charge that is completely homogeneous in both mixture and temperature in practical (7) Haraldsson, G.; Tunestål, P.; Johansson, B. HCCI closed-loop combustion control using fast thermal management. SAE 2004-01-0943, 2004. (8) Aroonsrisopon, T.; Foster, D.; Morikawa, T.; Lida M. Comparison of HCCI operating ranges for combinations of intake temperature, engine speed and fuel composition. SAE 2002-01-1924, 2002. (9) Christensen, M.; Hultqvist, A.; Johansson, B. Demonstrating the multi-fuel capability of a homogeneous charge compression ignition engine with variable compression ratio. SAE 1999-01-3679, 1999. (10) Ryan, T. W.; Callahan, T. J.; Mehta, D. HCCI in a variable compression ratio enginesEffect of engine variables. SAE 2004-01-1971, 2004. (11) Strandh, P.; Bengtsson, J.; Johansson, R.; Johansson, B. Variable valve actuation for timing control of a homogeneous charge compression ignition engine. SAE 2005-01-0147, 2005. (12) Agrell, F.; Ångstro¨m, H. E.; Eriksson, B.; Wikander, J.; Linderyd, J. Transient control of HCCI through combined intake and exhaust valve actuation. SAE 2003-01-3172, 2003. (13) Aleiferis, P. G.; Charalambides, A. G.; Hardalupas, Y.; Taylor, A. M. K. M.; Urata, Y. Autoignition initiation and development of n-heptane HCCI combustion assisted by inlet air heating, internal EGR or spark discharge: an optical investigation. SAE 2006-01-3273, 2006. (14) Bhave, A.; Kraft, M.; Mauss, F.; Oakley, A.; Zhao, H. Evaluating the EGR-AFR operating range of a HCCI engine. SAE 2005-01-0161, 2005.

10.1021/ef8009537 CCC: $40.75  2009 American Chemical Society Published on Web 03/23/2009

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engines. Dec et al.15 provided several sources from which naturally inhomogeneity can arise, such as incomplete fuel/air mixing, incomplete mixing of fresh charge and residuals, nonisothermal intake conditions, or heat transfer and turbulent mixing during the compression stroke. For another, inhomogeneity brings on sequential auto-ignition and distributed combustion, which are helpful to lengthen the combustion duration and smooth the heat release rate. This was illustrated experimentally for dimethyl ether (DME) fuel by Kumano et al.16 Therefore, understanding and controlling the inhomogeneity will likely play a key role in further HCCI research. However, charge inhomogeneity, which naturally arises in HCCI combustion, has a modest effect on the combustion characteristics. To obtain a higher power output and better combustion control, plenty of work has been performed to enhance this natural inhomogeneity and create a charge with an even greater degree of stratification, including temperature and/or concentration and/or composition stratification.17,18 Sjo¨berg et al. demonstrated the potential and drawbacks of enhanced natural stratification by reducing the coolant temperature and increasing the swirl ratio.19 Berntsson et al. designed a double-fuel injection strategy, which incorporated an early premixed injection with a late direct injection, to evaluate the effect of charge stratification on combustion phasing, heat release rate, and emission.20 Inagaki et al. investigated dual-fuel stratified combustion by managing different ignitability fuels and confirmed that ignitability stratification was responsible for the mild combustion rate.21 Generally speaking, the stratified charge compression ignition (SCCI) combustion mode is not only a compromise or a variant of ideal HCCI but also one of development of HCCI research. With the accomplishment of the desired stratification under all operating conditions, SCCI is capable of achieving the precise control of ignition timing and extending the operating range to higher loads, while maintaining efficiency and emission advantages of HCCI-type combustion. Nevertheless, it is not fully understood yet that how various fuel-injection strategies, introducing EGR methods, and charge mixing techniques alter HCCI combustion through partial charge stratification. Hence, research to study the fundamentals of SCCI combustion is still quite necessary. This paper presents the results of an experiment conducted with a dual-fuel HCCI engine that combined direct fuel injection with port fuel injection. n-Heptane, a surrogate fuel with high ignitability and high volatility, was supplied at an early timing by a port fuel injector from the intake manifold, and iso-octane, as representative of gasoline, was directly injected into the cylinder around top dead center (TDC). In this approach, n-heptane is well-vaporized and fully mixed with air to achieve a nearly homogeneous condition. (15) Dec, J. E.; Hwang, W.; Sjo¨berg, M. An investigation of thermal stratification in HCCI engines using chemiluminescence imaging. SAE 200601-1518, 2006. (16) Kumano, K.; Iida, N. Analysis of the effect of charge inhomogeneity on HCCI combustion by chemiluminescence measurement. SAE 2004-011902, 2004. (17) Aroonsrisopon, T.; Werner, P.; Waldman, J. O.; Sohm, V.; Foster, D. E. Expanding the HCCI operation with the charge stratification. SAE 2004-01-1756, 2004. (18) Lim, O. T.; Nakano, H.; Ilda, N. The research about the effects of thermal stratification on n-heptane/iso-octane-air mixture HCCI combustion using a rapid compression machine. SAE 2006-01-3319, 2006. (19) Sjo¨berg, M.; Dec, J. E.; Babajimopoulos, A.; Assanis, D. Comparing enhanced natural thermal stratification against retarded combustion phasing for smoothing of HCCI heat-release rates. SAE 2004-01-2994, 2004. (20) Berntsson, A W.; Denbratt I. HCCI combustion using charge stratification for combustion control. SAE 2007-01-0210, 2007. (21) Inagaki, K.; Fuyuto, T.; Nishikawa, K.; Nakakita. K.; Sakata, I. Dual-fuel PCI combustion controlled by in-cylinder stratification of ignitability. SAE 2006-01-0028, 2006.

Ji et al. Table 1. Engine Specification bore × stroke (mm) displacement (L) combustion chamber compression ratio needle open pressure (MPa) intake valve open intake valve close exhaust valve open exhaust valve close a

98 × 105 0.792 ω type 18.5 19 344 °CA ATDCa 128 °CA BTDC 114 °CA ATDC 348 °CA BTDC

TDC in this study is referred to be the combustion top dead center.

Table 2. Properties of n-Heptane and iso-Octane chemical formula molar weight (g/mol) density (g/mL at 298 K) boiling point (K) lower heat value (MJ/kg) cetane number research octane number

n-heptane

iso-octane

n-C7H16 100.16 0.688 371 44.5 56 0

i-C8H18 114.230 0.690 372 44.3 10 100

On the other hand, near the center of the combustion chamber, a concentration inhomogeneity of iso-octane remains, even as n-heptane combustion proceeds. Accordingly, ignitability inhomogeneity is established in the cylinder because of the different distributions of these two fuels that are disparate in ignitability. Thus, the distributed combustion could be achieved, and the combustion rate may be reduced. The corresponding combustion progress in this novel combustion mode can be divided into three stages. The first two stages are HCCI combustion of pilot n-heptane, including lowtemperature reaction (LTR) and high-temperature reaction (HTR). The third stage is the multipoint ignition of iso-octane induced by a thermo-atmosphere, which is provided by nheptane HCCI combustion. This dual-fuel SCCI combustion mode can also be named as iso-octane thermo-atmosphere combustion. The iso-octane supply advance angle not only influences the concentration and ignitability inhomogeneities in the cylinder but also directly determines a temporal relationship or even interactions between iso-octane spray and n-heptane HCCI combustion. Moreover, the n-heptane premixed ratio is also a key factor affecting the concentration and ignitability inhomogeneities. Thus, the objective of this study is to investigate these two key parameters impacting iso-octane thermo-atmosphere combustion. 2. Experimental Apparatus and Procedure The experiment was conducted on a single-cylinder, directinjection (DI), and four-stroke naturally aspirated diesel engine. The main engine specifications and properties of the fuels evaluated in this study are listed in Tables 1 and 2, respectively. n-Heptane and iso-octane were used in this experiment, because these two fuels are a simple attempt to mimic complex transportation fuels.22 Meanwhile, as primary reference fuels, the chemical kinetic mechanism of n-heptane and iso-octane is available,23-25 which is helpful to make a theoretical investigation in the future. (22) Zhang, H. R.; Eddings, E. G.; Sarofim, A. F.; Westbrook, C. K. Fuel dependence of benzene pathways. Proc. Combust. Inst. 2009, 32, 377– 385. (23) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. A comprehensive modeling study of n-heptane oxidation. Combust. Flame 1998, 114, 147–179. (24) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. A comprehensive modeling study of iso-octane oxidation. Combust. Flame 2002, 129, 253–280. (25) Tanaka, S.; Ayala, F.; Keck, J. C. A reduced chemical kinetic model for HCCI combustion of primary reference fuels in a rapid compression machine. Combust. Flame 2003, 133, 467–481.

iso-Octane Thermo-atmosphere Combustion

Energy & Fuels, Vol. 23, 2009 2407 Table 3. Measurement Accuracy measured parameters

measurement range

accuracy

engine speed (rpm) engine torque (N m) cylinder pressure (MPa) oil inlet temperature (°C) oil outlet temperature (°C) coolant inlet temperature (°C) coolant outlet temperature (°C) CO emission (vol %) HC emission (ppm) NOx emission (ppm) excess air coefficient fuel consumption (kg/h)

0–10000 0–200 0–25 0–150 0–150 0–150 0–150 0–10 0–20000 0–5000 0.7–32 0–20

(1a (0.4%b (1%b (1a (1a (1a (1a (5%b (5%b (5%b (1.5%b (1%b

a

Absolute accuracy. b Relative accuracy.

Figure 1. Schematic of the experimental setup. Table 4. Test Conditions engine speed n-heptane port injection timing iso-octane supply advance angle intake air temperature (°C) coolant temperature (°C) lubricant oil temperature (°C) EGR rate (%)

1800 rpm 285 °CA BTDC 15, 18, 20, 25, and 35 °CA BTDC 20 80 90 0

The EGR was not applied in the experiment, and the engine speed was kept at 1800 rpm.

3. Definitions of the Combustion Parameters

Figure 2. Descriptions of the fuel supply strategies.

Figure 1 shows the schematic of the experimental setup. An electrical port injector with an injection pressure of 5 MPa, which is usually used in a commercial gasoline engine, was mounted about 0.35 m upstream from the intake valve; therefore, the homogeneous mixture of n-heptane and air can be formed during the intake and compression stroke. In addition, the port injection (PI) timing was 285 °CA BTDC. Another injector with a cone angle of 154°, which originated from the prototype engine, was used to inject iso-octane directly into the cylinder. In addition, the DI timing could be estimated by the iso-octane supply advance angle, which ranged from 15 to 35 °CA BTDC.26 Figure 2 illustrates the fuel supply strategies and locations of the valve timing in the current study. The cylinder pressure was measured with a pressure transducer (Kistler model 6125B). The charge output from this transducer was converted to amplified voltage using an amplifier (Kistler model 5015A) and then was recorded at 0.25 °CA resolution, using a highspeed memory (Yokogawa GP-IB) with the sampling signals from the shaft encoder. According to the in-cylinder gas pressure averaged from 50 consecutive cycles for each operating point, heat release rate, bulk gas temperature, and indicated mean effective pressure were calculated from a zero-dimension combustion model.26 The exhaust gas composition CO, UHC, and NOx emissions were measured by a gas analyzer (AVL Digas 4000). Because n-heptane and iso-octane are almost smoke-free, soot emission is not measured in this study. The measured parameters and their accuracy are summarized in Table 3. The test conditions are summarized in Table 4. For all data presented, 0 °CA is defined as the TDC at the compression stroke. To ensure the repeatability and comparability of the measurements for operating conditions, the temperatures of intake air, oil, and coolant water were held accurately stable during the experiment. (26) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill Book Company: New York, 1988.

To investigate the influence of the n-heptane premixed ratio and iso-octane supply advance angle on this SCCI combustion, several basic combustion parameters are defined as follows: the start timing of n-heptane LTR is defined as the timing when the value of the heat release rate exceeds 0.5 J/°CA, as shown in Figure 3. Similarly, the start timing of n-heptane HTR is defined as the timing when the derivative of the heat release rate exceeds 0.5 J/(°CA)2. Figure 3 also shows the negative temperature coefficient (NTC) region as well as iso-octane combustion. It should be mentioned that the crank angle, which corresponds to 90% of the accumulated heat release rate, is considered as the end of combustion. For simplicity, the iso-octane supply advance angle is denoted as θfd hereon and the global fuel/air equivalence ratio, which is the sum of the n-heptane and iso-octane equivalence ratios, is denoted as Φ. In this paper, the premixed ratio rp is defined as the ratio of cycle energy Qp of premixed fuel, i.e., n-heptane, to total energy Qt, which includes premixed fuel and directly

Figure 3. Definitions of the combustion parameters.

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Figure 4. Effect of the premixed ratio on combustion and emission characteristics (θfd ) 18 °CA BTDC).

Figure 5. Effect of the premixed ratio on combustion and emission characteristics (θfd ) 25 °CA BTDC).

injected fuel. The premixed ratio can be calculated using the following formula: rp )

mphup Qp ) Qt mphup + mdhud

where mp and md represent the mass consumption rate of premixed and directly injected fuel, i.e., n-heptane and isooctane, respectively, and hup and hud are the lower heating values of n-heptane and iso-octane. 4. Results and Discussion 4.1. Effect of the Premixed Ratio on Combustion and Emission Characteristics. In this section, the global fuel/air equivalence ratio Φ is held constant at 0.37 and the proportion of n-heptane and iso-octane is changed to investigate the effect of the premixed ratio on combustion and emission characteristics at several iso-octane supply advance angles, which are representative. Figure 4 shows the influence of the premixed ratio on combustion and emission characteristics when θfd ) 18 °CA BTDC. According to the calculation, iso-octane injection delay

(in crank angle) in our experimental system is about 10 °CA.26 Thus, the actual fuel injection timing is approximate to 8 °CA BTDC, which corresponds to the duration of n-heptane HTR, as seen in Figure 4a. Under such conditions, the ignition of iso-octane spray is mainly dependent upon the thermoatmosphere formed by n-heptane HTR; therefore, we can nominate this type of combustion as exclusive thermoatmosphere combustion. Figure 4a shows that the entire combustion exhibits a three-stage combustion progress containing n-heptane LTR, HTR, and iso-octane combustion. Although the premixed ratio has little impact on the start timing of LTR, which occurs approximately at 25 °CA BTDC, the NTC region lengthens and the start timing of HTR lags distinctly as rp decreases. It can also be seen that, as rp decreases, the maximum heat release rate of n-heptane LTR and HTR decreases as well as the peak values of cylinder pressure and mass-averaged temperature and the ignition of iso-octane delays; however, the maximum heat release rate of iso-octane combustion increases. According to this, it can be concluded that a higher value of rp is favorable to promote the ignition of iso-octane, so that the combustion phase of iso-octane becomes closer to TDC. However, a higher value of rp also makes a shorter ignition

iso-Octane Thermo-atmosphere Combustion

Energy & Fuels, Vol. 23, 2009 2409

Figure 6. Effect of the premixed ratio on combustion and emission characteristics (θfd ) 35 °CA BTDC).

Figure 7. Effect of the iso-octane supply advance angle on combustion characteristics.

delay of iso-octane, which deteriorates the mixing of fuel and air and, consequently, results in a larger proportion of iso-octane diffusive combustion. As shown in Figure 4b, CO emission increases at first and then starts to decrease, as rp decreases. The reason for the CO increasing when rp decreases first is 2-fold. First, decreasing rp leads to a lower mass-averaged temperature in the cylinder as mentioned above, which decelerates the oxidation of CO into CO2, especially in cold boundary layers. Second, because of the increasing iso-octane concentration, iso-octane spray penetrates a longer distance and distributes more fuel to boundary layers, where the combustion of isooctane suffers from the lower temperature. However, further decreasing rp makes ignition of iso-octane retard; hence, the mixing of fuel and air improves. Although incomplete HCCI combustion probably happens at such relatively low equivalence ratios of n-heptane, the thermal effect and the incomplete products of this former combustion stimulate multipoint simul-

taneous ignition of a rich iso-octane/air mixture.27,28 Then, the steep temperature gradient near the wall is gradually smoothed, so that the conversion of CO to CO2 is promoted and CO emission is reduced. Decreasing rp allows less n-heptane retaining in the volume between the piston and cylinder wall above the upper piston ring, i.e., the topland, which is a major source of HC emission.19 However, further decreasing rp leads to incomplete HCCI combustion of n-heptane, which produces much HC.29 Moreover, the cetane number (CN) throughout the cylinder is lowered (27) Li, C.; Hua, Z.; Xi, J. Analysis of controlled auto-ignition/HCCI combustion in a direct injection gasoline engine with single and split fuel injections. Combust. Sci. Technol. 2008, 180, 176–205. (28) Aroonsrisopon, T.; Nitz, D. G.; Waldman, J. O.; Foster, D. E.; Lida, M. A computational analysis of direct fuel injection during the negative valve overlap period in an iso-octane fueled HCCI engine. SAE 2007-010227, 2007.

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Figure 8. CO, HC, and NOx emissions and ηit as functions of the iso-octane supply advance angle and premixed ratio.

by mixing less n-heptane with more iso-octane.30 As a result, HC emission decreases at first and then start to increase, as rp decreases. NOx emission decreases monotonically with a decreasing rp. This appears reasonable because the peak value of massaveraged temperature is reduced when rp decreases. Besides, decreasing rp mitigates the degree of inhomogeneity of the isooctane/air mixture by prolonging the iso-octane ignition delay; therefore, there is little region of locally higher equivalence ratio in the cylinder and the low-temperature combustion (LTC) can be achieved. Note that, for this fuel supply timing, the level of NOx emission is rather low. Even the maximum value of NOx emission is less than 40 ppm. The indicated thermal efficiency ηit is almost constant for all rp of this condition. The indicated thermal efficiency correlates with the fuel combustion efficiency as well as the combustion phase. To explain this trend, this range of rp should be divided into three situations. Case 1: when rp is equal to or more than 0.6, combustion efficiency of n-heptane HCCI combustion is relatively high.21 However, ηit now suffers from the overadvanced combustion phase, which causes a large amount of negative work. Case 2: when rp is equal to or less than 0.4, combustion efficiency of iso-octane thermo-atmosphere combustion is also considerable because of the homogeneous mixture of iso-octane/air and the following LTC. Nevertheless, ηit now (29) Dec, J. E. A computational study of the effects of low fuel loading and EGR on heat release rates and combustion limits in HCCI engines. SAE 2002-01-1309, 2002. (30) Kawamoto, K.; Araki, T.; Shinzawa, M.; Kimura, S.; Koide, S.; Shibuya, M. Combination of combustion concept and fuel property for ultraclean DI diesel. SAE 2004-01-1868, 2004.

suffers from the overlate combustion phase. Case 3: when rp is in the range from 0.4 to 0.6, ηit now benefits from the combustion phase, which is close to TDC. However, a weak thermo-atmosphere of n-heptane and partial mixing of isooctane/air suppress the combustion efficiency of iso-octane.31 Thus, it is evident that there is a trade-off relationship between combustion efficiency and combustion phase, and they reach a balance at the fuel supply timing of 18 °CA BTDC. Figure 5 shows the effect of the premixed ratio on combustion and emission characteristics when θfd ) 25 °CA BTDC. Different from the iso-octane supply advance angle of 18 °CA BTDC, the actual fuel injection timing is approximate to 15 °CA BTDC, which locates in the NTC region of n-heptane HCCI combustion, as seen in Figure 5a. Under such conditions, the ignition of iso-octane spray is determined not only by the thermo-atmosphere from n-heptane HCCI combustion but also a great variety of the active radical generated and accumulated during the LTR and NTC. Therefore, we can call this type of combustion active thermo-atmosphere combustion. Figure 5a illustrates that the ignition timing of iso-octane significantly advances and becomes much closer to the HTR of n-heptane, compared to 18 °CA BTDC. It can also be seen that the peak values of mass-averaged temperature are nearly the same for all rp, which are kept at about 1800 K. Although the peak value of cylinder pressure still decreases with a decreasing rp, this tendency is much weaker compared to that of 18 °CA BTDC. (31) Opat, R.; Ra, Y.; Gonzalez, D. M. A.; Krieger, R.; Reitz, R. D.; Foster, D. E. Investigation of mixing and temperature effects on HC/CO emissions for highly dilute low temperature combustion in a light duty diesel engine. SAE 2007-01-0193, 2007.

iso-Octane Thermo-atmosphere Combustion

Advancing the iso-octane supply advance angle brings on a difference in emission trends. Although the curve of CO emission in Figure 5b is similar to the one in Figure 4b, the slope hereon becomes less steep, compared to 18 °CA BTDC. CO is the dominant emission affecting combustion efficiency.31 The smaller slope of the CO emission curve indicates that the change of combustion efficiency is inconspicuous when altering rp. That is to say, in comparison to 18 °CA BTDC, the isooctane combustion is improved in the rp range of 0.4-0.6. Although the trade-off relationship between the combustion efficiency and combustion phase still exists at 25 °CA BTDC, the combustion phase may now begin to play a leading role in controlling the indicated thermal efficiency. ηit increases monotonically as rp decreases, mainly because of the less negative work of n-heptane HCCI combustion, as shown in Figure 5b. In contrast to the curve of HC emission in Figure 4b, there is a difference that HC emission gradually increases at first when decreasing rp initially. The reason for this phenomenon is under investigation. Further decreasing rp makes HC emission decrease. As the minimum value is reached, HC emission begins to increase again. This trend is similar to that observed in Figure 4b. Despite the nearly constant value of maximum mass-averaged temperature, NOx emission increases linearly with a decreasing rp. This is completely opposite to the curve of 18 °CA BTDC. The majority of NOx emission should come from iso-octane combustion, because n-heptane HCCI combustion only generates little NOx emission.32,33 As the proportion of n-heptane HCCI combustion reduces, the NOx emission gradually increases. It is worth noting that, when rp is less than 0.4, the rise rate of NOx is reduced. As mentioned above, the degree of inhomogeneity of the iso-octane/air mixture is progressively improved by stretching out the iso-octane ignition delay; therefore, there is little region of locally higher equivalence ratio as well as high combustion temperature in the cylinder. Figure 6 shows the effect of the premixed ratio on combustion and emission characteristics when θfd ) 35 °CA BTDC. At this iso-octane supply advance angle, the actual fuel injection timing is approximate to 25 °CA BTDC, which corresponds to the beginning of n-heptane LTR, as seen in Figure 6a. In this case, the ignition of iso-octane spray is also dominated by the thermoatmosphere from n-heptane HCCI combustion and the active radical generated during the LTR. However, in comparison to 25 °CA BTDC, the effect of the active radical is minor. For one thing, the concentration of the active radical in the cylinder is very low at the beginning of n-heptane LTR.28 For another, the vaporization of iso-octane spray absorbs heat from n-heptane LTR and cools down the n-heptane/air mixture around the periphery of the spray. In other words, the proceedings of n-heptane LTR may be suppressed by iso-octane injection in many places. A computational study coupling the multidimensional computational fluid dynamic (CFD) code with detailed chemical kinetics will allow us to explore these details with accuracy. Hence, this type of combustion could be named as weakened active thermo-atmosphere combustion. Figure 6a exhibits that the peak value of the cylinder pressure goes down at first and then moves up as rp decreases, as well as the peak value of the mass-averaged temperature. (32) Kim, D. S.; Lee, C. S. Effect of n-heptane premixing on combustion characteristics of diesel engine. Energy Fuels 2005, 19, 2240–2246. (33) Ma, J. J.; Lu¨, X. C.; Ji, L. B.; Huang, Z. An experimental study of HCCI-DI combustion and emissions in a diesel engine with dual fuel. Int. J. Therm. Sci. 2007, 47, 1235–1242.

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The trends of various emissions and the indicated thermal efficiency for the iso-octane supply advance angle of 35 °CA BTDC are shown in Figure 6b, which are similar to those of Figure 5b in general. However, discrepancies between the active and weakened active thermo-atmosphere combustion could be observed. First, in comparison to active thermo-atmosphere combustion, the higher level of CO emission for weakened active thermo-atmosphere combustion in the rp range of 0.4-0.6 is discerned. In addition, this difference indicates that the indicated thermal efficiency is reduced for weakened active thermo-atmosphere combustion during such a rp scale (note that the y axis scale of these two sweeps varies). It is apparent that the slight effect of active radical, which may reduce the number of ignition points and the ignition energy of iso-octane, attributes to this change. Second, the rise rate of NOx is smaller for weakened active thermo-atmosphere combustion. The reason for the second difference consists of two aspects. On one hand, the peak values of mass-averaged temperature are lower for weakened active thermo-atmosphere combustion. On the other hand, the ignition delay of iso-octane prolonged the slight effect of active radical, which results in better mixing and reduces the probability of local high combustion temperature. Third, it is manifest that the weakened active thermo-atmosphere combustion has a lower level of HC emission (note that the y axis scale is different). Possibly, this is because, for weakened active thermo-atmosphere combustion, the mixture of iso-octane/air with a higher degree of homogeneity has a tendency to smooth the temperature gradient between the hot core and cold periphery. Then, more HC and OHC in the crevices and quench layers could be oxidized. 4.2. Effect of iso-Octane Supply Advance Angle on Combustion and Emission Characteristics. In this section, the global fuel/air equivalence ratio Φ is also held constant at 0.37 and the iso-octane supply advance angle is changed to investigate the effect of θfd on combustion and emission characteristics at several n-heptane premixed ratios, which span a wide range. Figure 7 illustrates the effect of the iso-octane supply advance angle on combustion characteristics at various premixed ratios. When rp is equal to 0.3, 0.5, or 0.6, the peak value of the massaveraged temperature moves up at first and then goes down as θfd advances in Figure 7a. Note that this peak value always reaches a local maximum for the iso-octane supply advance angle of 25 °CA BTDC. For θfd of 15, 18, and 20 °CA BTDC, the iso-octane combustion belongs to exclusive thermoatmosphere combustion. In this type of combustion, the influence of thermo-atmosphere to iso-octane ignition is gradually intensified, because the combustion phase of iso-octane becomes closer to TDC when θfd is earlier. Then, the peak value of the massaveraged temperature goes up. When rp is equal to 0.7, the peak values of mass-averaged temperature almost maintain the same level (1800 K), regardless of θfd. This could be mainly ascribed to the low proportion of iso-octane. The peak value of the massaveraged temperature is mainly controlled by n-heptane HCCI combustion hereon, which is hardly affected by the type of isooctane combustion. The enhancing effect of iso-octane ignition under θfd of 25 °CA BTDC could be vividly reflected by comparing the heat release rate trace (rp ) 0.3) to that of 35 °CA BTDC. Although there is a significant gap between the two iso-octane supply advance angles (10 °CA), their ignition timing of iso-octane is nearly the same. Parts a-d of Figure 8 present the histograms for CO, HC, and NOx emissions and indicated thermal efficiency of isooctane thermo-atmosphere combustion, respectively. At any θfd,

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CO emission increases substantially at first, with the premixed ratio down to a critical value of 0.5. Because the premixed ratio is below this critical value, CO emission begins to decrease. The trends of CO emission with advancing θfd are as follows: (1) When rp is equal to 0.3, 0.5, or 0.6, CO emission decreases at first and then increases. This trend is completely opposite the change of the peak value of the mass-averaged temperature. Therefore, it is clear that CO emission is primarily dominated by the mass-averaged temperature. (2) When rp is equal to 0.7, CO emission increases with advancing θfd. This trend may be due to a slightly lower temperature during the expansion stroke when θfd is advanced, as shown in Figure 7a. At any θfd, HC emission generally experiences a similar process. Specifically speaking, with a decreasing rp, HC emission initially increases, then decreases, and finally increases again for all θfd investigated. It should be mentioned that, in Figure 8b, the initial rising trend is not obvious for three cases of exclusive thermo-atmosphere combustion (instead, a flat trend could be seen). In addition, the final declining trend cannot be seen for the active and weakened active thermo-atmosphere combustion, only because of the limitation of the rp range listed here. The trends of HC emission as a function of θfd are as follows: (1) For exclusive thermo-atmosphere combustion, HC emission reduces with advancing θfd. (2) Active thermoatmosphere combustion always has the highest level of HC emission, with the exception of rp ) 0.3. Active thermoatmosphere combustion could enhance the ignition of iso-octane; however, it also causes a worse mixing of iso-octane/air, which brings on a greater temperature gradient and hinders the oxidation of HC in crevices and quench layers. Note that the level of HC emission is about 200 ppm overall. With a retarding θfd, the trend of NOx emission as a function of the premixed ratio experiences a transition from monotonically increasing to linearly decreasing, as Figure 8c shows. The dependence of NOx emission on θfd is as follows: (1) For any rp, the NOx emission increases at first and then decreases with an advancing θfd. This trend agrees with the change of the peak value of the mass-averaged temperature. (2) Active thermoatmosphere combustion always has the highest level of NOx emission, with the exception of rp ) 0.7. With a retarding θfd, the dependence of ηit on the premixed ratio experiences a transition from monotonically increasing to simply unchanging. In other words, at any θfd, ηit becomes its maximum value when rp is equal to 0.3. The trends of ηit as a function of θfd are as follows: (1) When rp is equal to 0.3, 0.5, and 0.6, ηit of these three cases of exclusive thermo-atmosphere combustion almost maintains the same level, while active thermo-atmosphere combustion has the largest value of ηit. (2) When rp is equal to 0.7, ηit decreases with an advancing θfd.

Ji et al.

With this large premixed ratio, the influence of active radical is thoroughly overwhelmed by the strong thermal effect, which results in the rapid ignition of iso-octane for any type of combustion. Thus, ηit is reduced with an advancing θfd, because of more negative work originated from the earlier combustion phase of iso-octane. 5. Conclusions The purpose of this research is to obtain fundamental knowledge about the effects of the iso-octane supply advance angle and n-heptane premixed ratio on iso-octane thermoatmosphere combustion experimentally. The most important results presented in this paper can be summarized as follows: (1) The effect of the thermo-atmosphere and active radical on iso-octane combustion, produced by n-heptane HCCI combustion, depends upon the iso-octane supply advance angle. With the advancing of the fuel supply angle, the ignition of iso-octane is enhanced at first but then suppressed. (2) At the iso-octane supply advance angle of 15, 18, and 20 °CA BTDC, iso-octane combustion belongs exclusively to thermo-atmosphere combustion. Furthermore, in this type of combustion, the influence of thermo-atmosphere to iso-octane ignition is gradually intensified when θfd is advanced. (3) At the iso-octane supply advance angle of 25 °CA BTDC, the type of iso-octane combustion could be named as active thermo-atmosphere combustion. Under such conditions, iso-octane is best ignited and the highest combustion efficiency and indicated thermal efficiency are obtained. Meanwhile, the level of CO in the exhaust gas is lowest with the penalty of the worst NOx emission. (4) At the iso-octane supply advance angle of 35 °CA BTDC, the effect of active radical is suppressed. Hence, this type of iso-octane combustion could be called weakened active thermo-atmosphere combustion. (5) For any iso-octane supply advance angle investigated, the trends of CO and HC emissions are nearly the same. CO emission increases substantially at first and then begins to decrease, and HC emission initially increases, then decreases, and finally increases. (6) With retarding the iso-octane supply angle, the trend of NOx emission as a function of the premixed ratio experiences a transition from monotonically increasing to linearly decreasing, while the dependence of ηit on the premixed ratio experiences a transition from monotonically increasing to simply unchanging. Future research is necessary to investigate whether the results obtained in this paper would hold in general while using more complex diesel and gasoline surrogates. Acknowledgment. This work was supported by the National Basic Research Program of China (Grant 2007CB210007). EF8009537