Methanol

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Energy & Fuels 2007, 21, 812-821

Multidimensional Numerical Simulation on Dimethyl Ether/ Methanol Dual-Fuel Homogeneous Charge Compression Ignition (HCCI) Engine Combustion and Emission Processes Mingfa Yao,† Chen Huang,‡ and Zhaolei Zheng*,† State Key Laboratory of Engines, Tianjin UniVersity, Tianjin 300072, China, and Shanghai Jiao Tong UniVersity, Shanghai 200030, China ReceiVed September 24, 2006. ReVised Manuscript ReceiVed NoVember 25, 2006

A multidimensional model is adopted to investigate the combustion and emission formation of a dimethyl ether (DME) and methanol dual-fuel homogeneous charge compression ignition engine. The multidimensional model couples with a reduced chemical mechanism so that the heat transfer and in-cylinder turbulence are considered in the combustion modeling. The results show that the calculated results agree well with the experimental results. Both low- and high-temperature reactions take place in some specific locations in the cylinder and then propagate to the entire cylinder. The quasi-low-temperature reaction occurs in the core zone first, and the high-temperature reaction starts from the core zone adjacent to the combustion chamber axis. The main compositions of unburned hydrocarbon (UHC) emissions are the unburned fuels (DME and methanol) and CH2O. The unburned fuels mainly reside in the piston-ring crevice region, and CH2O is mainly from the region next to the cylinder-liner wall. The majority of CO emissions are located in the region near the top surface of the piston. With the increase of the total fuel/air equivalence ratio or the DME proportion, UHC and CO emissions decrease. However, when the total fuel/air equivalence ratio or the DME proportion is too small, CO emissions decrease too. When the maximum average temperature in the cylinder is over 1400 K, both UHC and CO emissions are very low.

1. Introduction The homogeneous charge compression ignition (HCCI) engine has the potential to meet the increasingly stringent emission regulations. The combustion process in HCCI engines does not involve flame propagation or flame diffusion as in conventional internal combustion engines. In fact, HCCI could be regarded as a type of operating mode rather than a type of engine. The main objective of HCCI combustion is to reduce soot and NOx emissions while maintaining high fuel efficiency at part-load conditions.1-4 However, there are still difficulties associated with the successful operation of HCCI engines. One of the principal challenges of HCCI combustion is the control of the combustion phasing. The energy release rate from HCCI combustion depends upon not only the unique reaction chemistry of the fuel but also the thermal conditions of the mixture concentration history during the intake and compression processes. Another obstacle of HCCI engine operations is the relatively high emissions of unburned hydrocarbon (UHC) and carbon monoxide (CO) because of the incomplete combustion of lowtemperature lean burn. Therefore, operating range extension is * To whom correspondence should be addressed: State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China. Telephone: 86-2227406842 ext. 8034. Fax: 86-22-27383362. E-mail: [email protected]. † Tianjin University. ‡ Shanghai Jiao Tong University. (1) Thring, R. H. SAE Tech. Pap. Ser. 1989, 892068. (2) Stanglmaier, R. H.; Roberts, C. E. SAE Tech. Pap. Ser. 1999, 199901-3682. (3) Christensen, M.; Johansson, B. SAE Tech. Pap. Ser. 1998, 982454. (4) Christensen, M.; Johansson, B.; AmnJus, P.; Mauss, F. SAE Tech. Pap. Ser. 1998, 980787.

as important as the auto-ignition process control in HCCI development. Many efforts have been made in the above difficulties in recent years. Approaches for controlling the combustion phasing include in-cylinder fuel injection timing, water injection, intake air temperature modulation, variable compression ratio, variable valve timing, varying the properties of the fuel or the fuel/air ratio, and exhaust gas recirculation (EGR).2,5-7 To expand the operating load range, the control of fuel injection timing, and thus the fuel distribution in the cylinder, increasing the EGR/residual level, in-cylinder water or reaction suppressor injection, fuel modification, and boosting have all been investigated and reported in the literature. In the research of Ogawa and his coauthors, various reaction suppressors, including water, methanol, ethanol, 1-propanol, hydrogen, and methane, were implemented to control ignition timing and expand the operating range in an HCCI engine with induced DME as the main fuel. The results indicated that ultralow NOx and smokeless combustion was realized over a wide operating range and methanol had the largest impact on the suppression of oxidation reaction rates among the suppressors.8 The previous research indicates that the dual-fuel operation is the preferred strategy for HCCI combustion.9,10 Adjustment (5) Takeda, Y.; Nakagome, K.; Niimura, K. SAE Tech. Pap. Ser. 1996, 961163. (6) Nakagome, K.; Shimazaki N.; Miimura K.; Kobayashi S. SAE Tech. Pap. Ser. 1997, 970898. (7) Christensen, M.; Johansson, B. SAE Tech. Pap. Ser. 1999, 1999-010182. (8) , Ogawa, H.; Miyamoto, N.; Kaneko, N.; Ando, H. SAE Tech. Pap. Ser. 2003, 2003-01-0746. (9) Furutani, M.; Ohta, Y.; Kono, M.; Hasegawa M. Proceedings of the 4th International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines (COMODIA), 1998; pp 173-177.

10.1021/ef0604745 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007

DME/Methanol Dual-Fuel HCCI Engine

of the proportion of high-cetane and high-octane number fuels has been reported as a method for controlling the ignition timing and expanding the operation range in HCCI combustion.11 HCCI experimental studies have been performed with fuel blends of DME added to liquefied petroleum gas (LPG), natural gas (NG), methanol, or methanol-reformed gas (MRG).12-16 It was found that the HCCI operating regime was widened considerably by the addition of these high-octane number fuels. The high-octane fuel inhibited the low-temperature reaction (LTR) of DME, and DME oxidation was delayed to obtain better combustion phasing. When the high-octane fuel was added, the timing of the low-temperature heat release was delayed and the peak heat release timing was retarded closer to top dead center (TDC). In the previous studies of the authors,14,15 a new combustion mode for methanol was presented, which the engine burned DME and methanol dual fuel in HCCI mode and DME could be converted from methanol. Both DME and methanol were injected from the intake manifold. A homogeneous mixture of DME and methanol with air was formed during the compression stroke. HCCI combustion can be obtained in dual-fuel mode, resulting in ultralow NOx emissions and high thermal efficiency. The major advantage of this combustion system is that the ignition timing and combustion rate can be controlled by adjusting the relative proportions of DME and methanol and both the low and high loads in the operating range of the HCCI engine can be extended through this system. The combustion characteristics, engine performance, operating region, and emission characteristics of this combustion system have been investigated.14 The controlling strategies of this combustion system have also been investigated. The results showed that the exhaust gas recirculation (EGR) rate and DME percentage were two important parameters to control the HCCI combustion process. The ignition timing and combustion duration could be regulated in a suitable range with high indicated thermal efficiency and low emissions by adjusting the DME percentage and EGR rate.15 Therefore, the DME/methanol dual-fuel mode is a good way to control HCCI combustion. Remarkably, performing these explorations solely in the laboratory would be inefficient, expensive, and impractical because there are many variables that exhibit a complex interaction. Rather, the control problem must be tackled using a suit of modeling tools to understand the process. Unlike in spark-ignition or conventional diesel engines, the start of HCCI combustion is dominated by the kinetics of the chemical reaction of the fuel with air.17,18 Detailed chemical kinetics is usually used to simulate HCCI combustion. The SENKIN utility of the CHEMKIN package was often used to simulate the HCCI engine process, assuming adiabatic compression and expansion. A lot of research has been (10) Akagawa, H.; Miyamoto, T.; Harada, A.; Sasaki, S.; Shimazaki, N.; Hashizume, T.; Tsujimura, K. SAE Tech. Pap. Ser. 1999, 1999-010183. (11) Zhao, F.; Thomas, W. A.; Dennis, N. A. Homogeneous Charge Compression Ignition (HCCI) Engines; Society of Automotive Engineers: Warrendale, PA, 2003; pp 325-348. (12) Chen, Z.; Konno, M.; Oguma, M.; Yanai, T. SAE Tech. Pap. Ser. 2000, 2000-01-0329. (13) Yao, M. F.; Zheng, Z. Q.; Qin, J. J. Eng. Gas Turbines Power 2006, 128, 414-420. (14) Zheng, Z. Q.; Yao, M. F.; Chen, Z.; Zhang, B. SAE Tech. Pap. Ser. 2004, 2004-01-2993. (15) Yao, M. F.; Chen, Z.; Zheng, Z. Q. Fuel 2006, 85, 2046-2056. (16) Shudo, T.; Ono, Y.; Takahashi, T. SAE Tech. Pap. Ser. 2002, 200201-2828. (17) Najt, P. M.; Foster, D. E. SAE Tech. Pap. Ser. 1983, 830264. (18) Aceves, S. M.; Flowers, D. L.; Westbrook, C. K.; Smith, J. R.; Pitz, W.; Dibble, R.; Christensen, M.; Johansson, B. SAE Tech. Pap. Ser. 2000, 2000-01-0327.

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implemented on the HCCI combustion process with different fuels using elementary reactions by Iida and his colleagues. They revealed the low-temperature reaction, high-temperature reaction, and emission mechanisms of hydrocarbon fuels (such as n-butane)19 and oxygenated fuels (such as DME).20 However, it has not been successful to use chemical kinetics alone to simulate the combustion by assuming a uniform temperature distribution, i.e., the so-called single-zone model. The in-cylinder temperature is actually nonuniform, and the high-temperature region in the center of the chamber is more responsible for the ignition. The work of Aceves and his colleagues focused on developing and validating multi-zone models for HCCI combustion to obtain some of the zonal resolution afforded by computational fluid dynamics (CFD) models.21,22 In their approach, a CFD code was run over part of the engine cycle, typically from bottom dead center (BDC) until a transition point before TDC, and then the fluid was binned into 10 masstemperature groups. The results have shown considerable success in predicting combustion and emissions over multiple geometries, fuels, and operating conditions. Multidimensional CFD models coupled directly with chemical kinetics can analyze the effects of the nonuniformities on auto-ignition and combustion, and they can also analyze the effects of the in-cylinder turbulence on combustion. Miyamoto et al.23 used KIVA-II along with a four-step reduced reaction mechanism, a turbulence mixing rate, and Zeldovich NOx kinetics to study combustion and emissions of a premixed lean diesel engine. Agarwal and Assanis24-26 reported on the coupling of a detailed chemical kinetic mechanism for NG ignition with the multidimensional reacting flow code KIVA-3V and explored the auto-ignition of NG injected in a quiescent chamber under diesel-like conditions. Song-Charng Kong et al. used a modified KIVA code along with CHEMKIN to study the effects of geometry-generated turbulence on HCCI combustion.27 However, detailed chemical kinetic calculations coupled with CFD simulations of chemically reacting flows are still unrealistic as the basis for a parametric simulation tool as a result of taking large amounts of central processing unit (CPU) time. Reduced mechanisms are excellent at predicting auto-ignition timing and adaptability to multidimensional models. In this study, DME/methanol dual-fuel HCCI combustion was investigated using the multidimensional CFD model (STARCD/KINETICS) coupled with a reduced chemical kinetics model. The HCCI combustion process and emissions formation were analyzed, which have significant meanings to offer ideas to control HCCI. 2. Computational Model 2.1. Reaction Mechanism. A DME/methanol reduced mechanism consisting of 27 species and 35 reactions was used to simulate the fuel chemistry.28 All species and reactions have been showed (19) Sato, S.; Iida, N. SAE Tech. Pap. Ser. 2003, 2003-01-1825. (20) Yamasaki, Y.; Iida, N. SAE Tech. Pap. Ser. 2003, 2003-01-1090. (21) Aceves, S. M.; Flowers, D. L.; Martinez-Frias, M.; Dibble, R.; Wright, J. F.; Akinyemi, W. C.; Hessel, R. P. SAE Tech. Pap. Ser. 2001, 2001-01-1027. (22) Aceves, S. M.; Martinez-Frias, J.; Flowers, D. L.; Smith, J. R.; Dibble, R. W.; Wright, J. F.; Hessel, R. P. SAE Tech. Pap. Ser. 2001, 200101-3612. (23) Miyomoto, T.; Harada, A.; Sasaki, S.; Akagawa, H.; Tujimura, K. J. SAE Tech. Pap. Ser. 1999, 1999-01-0229. (24) Agarwal, A.; Assanis, D. N. SAE Tech. Pap. Ser. 1997, 971711. (25) Agarwal, A.; Assanis, D. N. SAE Tech. Pap. Ser. 1998, 980136. (26) Agarwal, A.; Assanis, D. N. SAE Tech. Pap. Ser. 2000, 2000-011839. (27) Kong, S. C.; Reitz, R. D; Christensen, M.; Johansson, B. SAE Tech. Pap. Ser. 2003, 2003-01-1087.

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Yao et al. Table 2. Engine Specifications

Figure 1. Engine combustion chamber geometry and computational mesh at TDC. Table 1. Species and Reactions of the DME/Methanol Reduced Mechanism CH3OCH3, O2, CH3OCH2, HO2, CH3OCH2O2, CH2OCH2O2H, O2CH2OCH2O2H, HO2CH2OCHO, OH, OCH2OCHO, H2O, H2, CH2O, HCO2, H2O2, CH3OCH2O2H, CH3OCH2O, CH3O, CH2OH, H, HCO, O, CO, CO2, CH3, CH3OH, and N2 1. CH3OCH3 + O2 ) CH3OCH2 + HO2 19. CH2O + O ) HCO + OH 2. CH3OCH2 + O2 ) CH3OCH2O2 20. CH2O + HO2 ) HCO + H2O2 3. CH3OCH2O2 ) CH2OCH2O2H 21. CO + OH ) CO2 + H 4. CH2OCH2O2H + O2 ) 22. CO + HO2 ) CO2 + OH O2CH2OCH2O2H 5. O2CH2OCH2O2H ) 23. CH3 + HO2 ) CH3O + OH HO2CH2OCHO + OH 6. HO2CH2OCHO ) OCH2OCHO + OH 24. HCO2 + M ) H + CO2 + M 7. CH3OCH3 + OH ) CH3OCH2 + H2O 25. HCO + HO2 ) CH2O + O2 8. OCH2OCHO ) CH2O + HCO2 26. HCO + O2 ) CO + HO2 9. H2O2 (+M) ) OH + OH (+M) 27. H2O + M ) H + OH + M 10. HO2 + HO2 ) H2O2 + O2 28. HO2 + M ) H + O2 + M 11. HO2 + HO2 ) H2O2 + O2 29. CH3OCH2 ) CH2O + CH3 12. CH2OCH2O2H ) 30. CH3OH + OH ) CH2OH + H2O OH + CH2O + CH2O 13. CH3OCH2O2H ) 31. CH3OH + OH ) CH3O + H2O CH3OCH2O + OH 14. CH3OCH3 + HO2 ) 32. CH3OH + HO2 ) CH2OH + CH3OCH2 + H2O2 H2O2 15. CH3OCH2O ) CH3O + CH2O 33. CH3OH + O2 ) CH2OH + HO2 16. CH2OH + M ) CH2O + H + M 34. CH3O + O2 ) CH2O + HO2 17. CH2OH + O2 ) CH2O + HO2 35. H2O2 + OH ) H2O + HO2 18. CH2O + OH ) HCO + H2O

in Table 1. The mechanism has been validated by simulating the ignition delay and mole fractions of important intermediate species at various conditions, which are included in the interest range of the engine operation. 2.2. Implementation of the Mechanism in STAR-CD/KINETICS CFD Code. The mechanism has been implemented in the STAR-CD/KINETICS CFD code to simulate the combustion with a homogeneous charge. The STAR-CD code provides CHEMKIN the species and thermodynamic information of the computational cells, and the CHEMKIN code returns the new species information and energy release after solving the chemistry. The chemistry and flow solutions are then coupled. The RNGk - model was used for turbulence modeling. The PISO algorithm was used for the transient flow of the engine. PISO performs at each time (or iteration) step, a predictor, followed by a number of correctors, during which linear equation sets are solved iteratively for each main dependent variable. The decisions on the number of correctors and inner iterations are made internally on the basis of the splitting error and inner residual levels, respectively, according to prescribed tolerances and upper limits. 2.3. Initial Conditions. To reduce the computational time, twodimensional meshes were used, as shown in Figure 1. The multidimensional computations started at intake valve close (IVC) and ended at exhaust valve open (EVO) assuming a uniform distribution of mixture properties. The initial mixture temperature was adopted from the results of the engine heat release rate analysis. The computational time step was a 0.1° crank angle.

bore stroke displacement compression ratio engine speed intake valve open intake valve close exhaust valve open exhaust valve close

115 mm 115 mm 1200 cm3 17.0:1 1400 r/min 12° BTDC 45° ABDC 55° BBDC 14° ATDC

Engine specifications are given in Table 2. An electronically controlled fuel injection system was used for the fueling of DME and methanol. The electromagnetic valve was mounted close to the intake port. Electronically controlled fuel injection allowed for the adjustment of the injected amount of each fuel according to the engine-operating conditions. 3.2. Parameter Definition. Three scalars were used to show the proportional correlation of DME and methanol accurately: the total fuel/air equivalence ratio (φtotal), the percentage of energy released from DME in total energy (rDME), and the percentage of energy released from methanol in total energy (rMEOH). The total fuel/air equivalence ratio (φtotal) is defined as follows:

φtotal ) (GMEOHAFMEOHth + GDMEAFDMEth)/Gair

(1)

where GDME and GMEOH are mass flow rates of DME and methanol respectively, Gair is the total mass flow rate of air, and AFDMEth and AFMEOHth present the stoichiometric air/fuel ratios of DME and methanol, respectively. The percentage of energy released from DME in total energy (rDME) is defined as

rDME )

GDMELHVDME × 100% (2) GDMELHVDME + GMEOHLHVMEOH

The percentage of energy released from methanol in total energy (rMEOH) is defined as

rMEOH )

GMEOHLHVMEOH × 100% (3) GDMELHVDME + GMEOHLHVMEOH

3. Comparisons of Pressure Profiles of the Experiment with Calculation

where LHVDME and LHVMEOH are low heat values of DME and methanol, respectively. In present study, the calculation was investigated at φtotal of 0.238, rMEOH of 70.4%, and rMEOH of 29.6%. 3.3. Comparisons of Pressure Profiles of the Experiment with Calculation. Figure 2 shows the comparison between the measured cylinder pressure profile and the computed results using the zero-dimensional and multidimensional models, respectively. The computational initial temperature and pressure are 375.6 K and 0.1166 MPa, respectively, at the timing of IVC. The comparison indicates that the computed cylinder pressure result using the zero-dimensional model is the largest among the three profiles and the value simulated by the multidimensional model agrees better with the measurement result. Furthermore, the multidimensional model retards the ignition better than the zero-dimensional model. When the simulation uses the multidimensional model, the heat transfer and the nonuniformity of the in-cylinder temperature, pressure, and chemical species are taken into consideration; therefore, the result is better.

3.1. Engine Specifications. The experiment was conducted on a single-cylinder, water-cooled, direct-injection diesel engine.

4. Results and Discussion

(28) Liang, X. Numerical Study on HCCI Combustion of DME/MEOH Dual Fuels. Master’s Dissertation, Tianjin University, Tianjin, China, 2005.

4.1. Analysis of the DME/Methanol Dual-Fuel HCCI Combustion Process. The oxidation mechanism of DME/



Figure 2. Comparison between the measured and computed cylinder pressure profiles.

Figure 3. Heat release rate and average in-cylinder temperature for DME/methanol HCCI combustion.

methanol dual-fuel HCCI combustion was investigated in ref 29. The results indicate that the low-temperature reaction of DME is inhibited and the β-scission plays a more important role in the reaction paths of the hydrogen-atom abstraction product from DME because of the addition of methanol. Figure 3 shows the heat release rate and average in-cylinder temperature at above initial conditions. It can be seen that there is no obvious low-temperature reaction in the present calculation. The hightemperature reaction occurs at about 8° after top dead center (ATDC) and ends completely at 15° ATDC. Figure 4 shows mole fractions of fuels, major intermediate species, and the complete oxidation product. As can be seen, there is only one rapid consumption stage for DME because of the absence of the low-temperature reaction, which is remarkably different with pure DME oxidation. The decrease of DME is slightly earlier than that of CH3OH, but the rapid consumption of the two fuels begins at almost the same time. CH2O and H2O2 begin to increase gradually before the high temperature occurs, and there is not an obvious NTC region in dual-fuel oxidation. H2O2 decomposes to the OH radical as soon as the high temperature occurs, which controls the HCCI ignition. CH2O is oxidized by the OH radical at the beginning of the high-temperature reaction, and CO is formed at the same time. When CO decreases rapidly, the complete combustion product CO2 is generated promptly. Most CO is also oxidized to CO2 by the OH radical.29 Although the low-temperature reaction of DME is inhibited, it does not mean that the low-temperature reaction disappears (29) Yao, M. F.; Zheng, Z. L.; Liang, X. SAE Tech. Pap. Ser. 2006, 2006-01-1521.

Figure 4. Mole fractions of fuels, major intermediate species, and complete oxidation product.

Figure 5. In-cylinder temperature distribution for DME/methanol HCCI combustion (the color codes are linear between the lowest and highest values).

completely and DME is not consumed before the hightemperature reaction occurs. Figure 5 shows the in-cylinder temperature distributions at TDC, 8° ATDC, and 15° ATDC,


Figure 6. In-cylinder temperature distribution removing all of the chemical reactions.

respectively. Figure 5a indicates that the temperature is almost uniform in the cylinder, except the cold wall boundary layers at TDC. It is because of the heat transfer between the charge and the cylinder walls that the temperatures in the clearance and piston-ring crevice regions are slightly lower than those within the combustion chamber. Figure 6 shows the in-cylinder temperature distribution at TDC and 8° ATDC, assuming no chemical reactions. Figure 6a shows that if there is no reaction, the temperature of a small region in the very center of the combustion chamber is slightly higher than its peripheral regions. This is almost the result from the heat transfer. Figure 7 shows concentration distributions of DME, methanol, CH2O, and CO in the cylinder at different crank angles. As shown in Figure 7a, a small amount of DME is consumed between -4° ATDC and 5° ATDC. Furthermore, the fuel in the middle region of the combustion chamber is consumed more remarkably than the peripheral region and other regions. This is because the temperature there is slightly higher than other regions during compression, as mentioned in Figure 6a. The slight lowtemperature reaction of DME in the dual-fuel oxidation is defined as a quasi-low-temperature reaction in the present study. The above analysis also indicates that the quasi-low-temperature reaction occurs in the core zone of the combustion chamber first. Because of the presence of the quasi-low-temperature reaction, a very small amount of heat is released and the temperature region influenced by the heat transfer of the cold wall boundary layers becomes smaller, as shown in Figure 5a. Figure 5b shows that the temperature is strongly nonuniform in the cylinder at 8° ATDC. Furthermore, in the region near the combustion chamber axis, the temperatures are higher than other regions, and the highest temperature is about 1400 K. Therefore, the high-temperature reaction occurs in this region first. At the beginning of the high-temperature reaction (8° ATDC), the low-concentration region of DME is just the hightemperature region in the temperature distribution. Therefore, DME starts to consume remarkably in this region by the β-scission of the hydrogen-atom abstraction product forming CH2O and CH3. When the high-temperature reaction is ending (10° ATDC), DME is almost consumed completely. Nevertheless, a small amount of DME is left in the region near the bottom surface of the cylinder head and the clearance region because of the heat transfer. Furthermore, DME in the piston-ring crevice region is hardly consumed as a result of strong heat transfer here. If there is no reaction, the in-cylinder temperature is almost uniform, except for the temperatures near the cold wall boundary layers, as shown in Figure 6b. Figure 5c shows that the temperature is uniform in the cylinder after the high-temperature reaction, except in the clearance and crevice regions.

Yao et al.

The concentration distribution of CH3OH in the cylinder is shown in Figure 7b. It is similar to that of DME, and the quasilow-temperature reaction of CH3OH occurs later than that of DME. The oxidation of CH3OH starts from the core zone of the combustion chamber. Similar to the process of DME consumption, a small amount of CH3OH is not oxidized in the region near the bottom surface of the cylinder head and the clearance region. In addition, CH3OH in the piston-ring crevice region is hardly oxidized, and the unburned CH3OH emissions are mainly from this region. Zero-dimensional kinetic analysis indicates that CH2O is an important intermediate species in DME/methanol oxidation. CH2O is mainly formed by the β-scission of the hydrogen-atom abstraction product of DME and the O2 addition to the hydrogenatom abstraction product of CH3OH in the oxidation of dual fuel.29 Therefore, the concentration distribution of CH2O in the cylinder has a close relation with the distributions of DME and CH3OH. As shown in Figure 7c, more CH2O is formed first in the core zone of the combustion chamber than other regions at the quasi-low-temperature reaction (between -4° ATDC and 5° ATDC) because of preferential consumption of DME and CH3OH here. Remarkably, in the region near the combustion chamber axis as shown in the ellipse profile of Figure 7c, the concentrations of DME and CH3OH are very low and the concentration of CH2O is also very low at 8° ATDC. This indicates that CH2O disappears remarkably once the hightemperature reaction occurs, forming a large amount CO, as shown in Figure 7d. The remarkable consumption of CH2O is the sign that the high-temperature reaction appears; that is to say, the high-temperature reaction occurs in this region first. This is consistent with the foregoing statement. When the hightemperature reaction is ending (10° ATDC), all CH2O is almost oxidized, except a small amount near the region of the bottom surface of the cylinder head and the clearance region. In comparison to parts a-c of Figure 7, the concentrations of DME and CH3OH decrease gradually from the center region near the cylinder axis to the peripheral region, which indicates that DME and CH3OH are oxidized gradually from the center region near the cylinder axis to the peripheral region. However, there is a great CH2O concentration region around this center region at the beginning of the high-temperature reaction. This indicates that the combustion starts from the center region near the cylinder axis and extends to the peripheral region gradually. Figure 7d shows the concentration distribution of CO in the cylinder. As shown in the ellipse profile, CO is first produced in the center region near the cylinder axis with the consumption of CH2O at the starting of the high-temperature reaction. The great concentration region of CO extends from the center to the periphery in the combustion process, and the greatest concentration appears at 9° ATDC when the high-temperature reaction occurs strongly. CO is mainly produced in the core zone of the combustion chamber, and almost no CO is formed in the piston-ring crevice region. At the end of the reaction system, a small amount of CO cannot be oxidized in the clearance region and the region near the bottom surface of the cylinder head. According to analysis on the in-cylinder temperature and intermediate species concentration distributions for DME/ methanol HCCI combustion, it can be concluded that the quasilow-temperature reaction occurs in the core zone of the combustion chamber first and the high-temperature reaction starts from a small region near the cylinder axis. However, the results are absolutely different for pure DME HCCI combustion. Figure 8 shows the temperature distribution in the cylinder



Figure 7. Concentration distributions of DME, methanol, and some important intermediate species (φtotal ) 0.238, rDME ) 70.4%, and rMEOH ) 29.6%).

fueled with pure DME. It presents that the low-temperature reaction begins from the vicinity of the cylinder wall and the

bottom of the piston bowl, while the high-temperature reaction takes place around the center of the bowl region and is widely


Figure 8. In-cylinder temperature distribution fueled with pure DME.

distributed in space. The above analysis shows that the addition of methanol has a remarkable effect on the starting region of ignition. That is to say, the addition of methanol not only changes the reaction paths of DME but also changes the spatial regions that the combustion reaction starts. 4.2. Emission Analysis on HCCI Combustion. The experiments of the HCCI engine indicate that UHC and CO are main emissions of HCCI combustion. Figure 11a shows the carbonnumber percentages of UHC emission compositions. It can be seen that UHC emissions mainly consist of CH3OCH3, CH3OH, and CH2O. Other HCs account for a very small percent, and the unburned fuels (CH3OCH3 and CH3OH) are the most important UHC compositions. The largest amount of unburned fuels is also the crucial reason of the low combustion efficiency of dual-fuel HCCI combustion. At the current computational condition, the unburned DME (about 60%) is more than the unburned CH3OH (near 30%) because the percentage of DME is larger (70.4%).

Yao et al.

Figure 9 shows the concentration distributions of main UHC and CO in the cylinder at EVO. As shown in Figure 7a, a very small part of DME is consumed before 10° ATDC at the top of the piston-ring crevice region. With the piston moving down, some DME overflows from the piston-ring crevice region. The concentration distribution of DME at EVO is shown in Figure 9a. Figure 10 shows the local magnification of concentration distributions of UHC and CO emissions. Figure 10a indicates that the highest concentration of the unburned DME appears in the piston-ring crevice region. Therefore, the unburned DME emissions are mainly from the piston-ring crevice region. Similar to DME, the unburned methanol emissions are also mainly from the piston-ring crevice region (Figures 9b and 10b). As shown in Figure 7c, a small amount of CH2O is produced in the piston-ring crevice region before 1° ATDC because of the slight oxidation of DME here at the quasi-low temperature. Before 10° ATDC, some CH2O is formed in the piston-ring crevice region because of the consumption of a small amount of DME and methanol at the top of this region at the hightemperature reaction. Furthermore, the concentration of CH2O decreases gradually from the top to the bottom of the pistonring crevice region. With the piston moving down and the continuing oxidation of DME and methanol, the CH2O concentration distribution formed at EVO is shown in Figure 9c. From the figure of the local magnification of the CH2O concentration distribution (Figure 10c), it can be concluded that CH2O emissions are mainly located in the region next to the cylinder-liner wall and there is a little CH2O in the piston-ring crevice region at EVO. The results are different from pure DME

Figure 9. In-cylinder concentration distributions of main UHC and CO.

Figure 10. Local magnification of concentration distributions of main emissions.



Figure 11. Carbon-number percentage of compositions in UHC emissions.

Figure 12. Effects of the total fuel/air equivalence ratio and DME proportion on the percentage of fuel carbon into emissions.

HCCI combustion. DME has a high-cetane number and easily leads to ignition by compression. For pure DME HCCI combustion with the same fuel/air equivalence ratio, reactions happen more acutely in the cylinder; therefore, the temperature in the piston-ring crevice region is high enough to lead to the occurance of the low-temperature reaction in most parts of this region, but only a slight high-temperature reaction occurs in the top of the region, which makes a large amount of CH2O left in this region. Therefore, CH2O emissions are mainly from the piston-ring crevice region fueled with pure DME. Figure 9d shows the CO concentration distribution in the cylinder at EVO. It can be seen that the majority of CO emissions mainly reside in the region near the top surface of the piston. Figure 10d shows the local magnification of the CO concentration distribution, and it presents that there are no CO emissions at all in the piston-ring crevice region. The reason leading to the above results is as follows: The temperature in the piston-ring crevice region is too low to oxidize the small amount of CH2O to CO. While in the region near the top surface of the piston, the temperature is high enough to oxidize CH2O but it does not reach the temperature of oxidizing CO (about 1400 K). For pure DME oxidation, CO emissions are mainly from the top of the piston-ring crevice region, where CH2O is consumed slightly. Figure 11b shows the effect of the DME proportion on UHC emissions. The results found in the figure are as follows: The percentage of unburned DME in UHC emissions increases with the increase of the DME proportion in the fuels. The percentage of CH2O and other HCs increases, and the percentage of CH3OH decreases at the same time. According to the above analysis

on emissions, the unburned fuels are mainly located in the piston-ring crevice region. When the DME proportion in the fuels increases, the DME concentration in the piston-ring crevice region increases too, and this leads to the increase of the unburned DME percentage in UHC emissions. Therefore, diminishing the crevice volume is an important way to reduce UHC emissions. Figure 12 shows the effect of the total fuel/air equivalence ratio (φtotal) and DME proportion (rDME) on UHC and CO emissions. As shown in Figure 12a (at a fixed rDME of 71.2%), when φtotal is larger than 0.2, UHC and CO emissions are very low and the percentage of fuel carbon into CO2 emissions is very high, which indicates that combustion efficiency is high. When φtotal is a value between 0.2 and 0.15, the percentage of fuel carbon into CO2 emissions decreases remarkably and the combustion efficiency decreases accordingly. Furthermore, CO emissions go up distinctly, and UHC emissions rise to some extent. The reason for the above emission characters is as follows: When φtotal is between 0.2 and 0.15, incomplete combustion products increase in their main forming region, respectively. Moreover, the incomplete combustion is mainly represented as the incomplete conversion of a large amount of CO to CO2; therefore, CO emissions increase more remarkably than UHC emissions. When φtotal is smaller than 0.15, UHC emissions increase remarkably, while CO emissions decrease on the contrary. This is because the mixture is so lean that a large amount of fuels and CH2O (UHC) cannot be oxidized to CO. Therefore, the percentage of fuel carbon into CO2 emissions is very small, and the combustion efficiency is very low. Figure 12b shows the effect of the DME proportion on


emissions at the same φtotal of 0.2. When the DME proportion is larger than 64%, with the increase of the DME proportion, the percentage of fuel carbon into CO2 emissions increases and the unburned fuels, CH2O and CO emissions, decrease correspondingly. The reason for these results is that more DME can leads to a higher in-cylinder temperature, which contributes to better combustion. The above emission characters indicate that combustion efficiency increases with the increase of the DME proportion. Although UHC emissions go down, the percentage of unburned DME and CH2O increase in UHC emissions, as shown in Figure 11b. When the DME proportion is smaller than 64%, UHC emissions increase remarkably, while CO emissions decrease with the decrease of the DME concentration. This is because that, with the decrease of the DME proportion, the combustion reaction becomes more and more difficult to occur. The trend of the above computational results agrees well with that of the experimental results. Only when both UHC and CO emissions are low and hardly change with the increase of φtotal, it can be considered that the engine normally operates. Figure 13 shows the effect of the DME proportion on emissions at different total fuel/air equivalence ratios. In combination with Figures 12b and 13a, it can be seen that, if φtotal changes between 0.2 and 0.3, the DME proportion needed for low UHC and CO emissions becomes smaller with the increase of φtotal. At the φtotal of 0.2, the engine normally operates when the DME proportion is about 71%. While at the φtotal of 0.3, the corresponding DME proportion for normal operation of the engine is less than 61%. When φtotal changes from 0.2 to 0.3, the energy of the reaction system increases remarkably; therefore, the ignition becomes easier if the DME proportion is constant. Therefore, the DME proportion needed for normal operation should decrease. Parts b and c of Figure 13 show the effect of the DME proportion on emissions at the φtotal larger than 0.3. It can be seen that, with the increase of φtotal, the DME proportion in dual fuel increases on the contrary to ensure low UHC and CO emissions. The reason is that, when the mixture becomes too thick, the increase concentration of methanol plays a more important role in inhibiting the starting of the reaction system. Furthermore, as the mixture becomes thicker, the inhibiting role becomes more obvious. Figure 14 shows the relationship between the maximum average temperature in the cylinder and the percentage of fuel carbon into emissions. As shown in parts a and b of Figure 14, the concentrations of residual DME and methanol are very low when the maximum average temperature is larger than 1400 K. Furthermore, the maximum average temperature in the cylinder has almost no effect on the residual DME and methanol when the temperature is above 1400 K. The reason is that the residual unburned fuels are mainly resides in the piston-ring crevice region, where the temperature is always comparatively low because of the heat transfer. Parts d and e of Figure 14 show that, when the maximum average temperature in the cylinder is above 1400 K, CO emissions are very low and CO2 emissions are very high, correspondingly. If the maximum average temperature in the cylinder is below 1400 K, CO emissions go up and CO2 emissions go down with the decrease of the maximum average temperature in the cylinder. However, when the maximum average temperature is lower than 1000 K, CO emissions decrease on the contrary, because the temperature is so low that CH2O can only be oxidized partially. In comparison to Figure 14c, CH2O emissions increase at this temperature condition. If the maximum average temperature continues to decrease, CH2O emissions go down because fuels

Yao et al.

Figure 13. Effects of the DME proportion on emissions at different total fuel/air equivalence ratios.

are difficult to be oxidized to CH2O, Therefore, CO emissions are very low, and the unburned fuels continue to increase. If the maximum average temperature in the cylinder is above 1400 K, the temperature has almost no effect on CH2O emission. Therefore, it can be concluded that 1400 K is the critical temperature of HCCI combustion. 5. Conclusions The results obtained up to now can be summarized as follows: (1) The results computed by multidimensional CFD models (two-dimensional) coupled with chemical kinetics agree better with measured results than simulated by the zerodimensional model. (2) For DME/methanol HCCI combustion,



Figure 14. Relationship between the maximum of the in-cylinder temperature and the percentage of fuel carbon into emissions.

the quasi-low-temperature reaction occurs in the core zone of the combustion chamber first. The high-temperature reaction starts from a small region near the cylinder axis and then extends to the whole cylinder. (3) UHC emissions mainly consist of the unburned fuels and CH2O. Unburned fuels mainly reside in the piston-ring crevice region. Most of CH2O emissions are located next to the cylinder-liner wall. CO emissions are mainly from the regions near the top surface of the piston. (4) With the increase of the total fuel/air equivalence ratio or the DME proportion, UHC and CO emissions decrease. However, because the total fuel/air equivalence ratio or the DME proportion is too small, CO emissions decrease, while UHC emissions increase highly. (5) If the total fuel/air equivalence ratio is relatively smaller, the DME proportion needed for low UHC

and CO emissions becomes smaller with the increase of φtotal. When the mixture becomes too thick, the DME proportion increases on the contrary with the increase of φtotal, to ensure low UHC and CO emissions. (6) If the maximal average temperature in the cylinder is over 1400 K, UHC, CH2O, and CO emissions are very low. Acknowledgment. The research is supported by the National Natural Science Foundation of China through its project (50376046) and by the Ministry of Science and Technology through its 973 National Key Project on HCCI Combustion Engines and Fuels (2001CB209201). EF0604745

Methanol

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