Numerical Study on Optimal Operating Conditions of Homogeneous

Energy & Fuels 2009, 23, 3909–3918

3909

Numerical Study on Optimal Operating Conditions of Homogeneous Charge Compression Ignition Engines Fueled with Dimethyl Ether and n-Heptane Sung Wook Park* Department of Mechanical Engineering, Hanyang UniVersity, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea ReceiVed March 31, 2009. ReVised Manuscript ReceiVed May 7, 2009

This paper describes a numerical study for analyzing combustion and emission characteristics and for suggesting optimal operating conditions of homogeneous charge compression ignition (HCCI) engines fueled with dimethyl ether (DME). Numerous calculations for DME HCCI engines were conducted for wide ranges of intake temperatures, equivalence ratios, and boost pressures. On the basis of the results for HCCI engines, engine performance and emissions including CO, HC, NOx, and soot are shown on a peak cycle temperature and equivalence ratio map. In addition, combustion and emission characteristics of DME HCCI engines were compared to those of n-heptane HCCI engines for the same supplied lower heating values. Results of the present study showed that soot, HC, and NOx emissions are decreased but CO emission is increased by using DME instead of n-heptane for HCCI engines. Furthermore, the DME HCCI engine has a wider optimal operating range than the n-heptane HCCI engine.

1. Introduction DME (Dimethyl ether) is a promising alternative fuel for diesel engines because its cetane number is high enough for compression ignition engines and it can be produced from numerous sources including natural gas, coal, coal-bed methane, and biomass.1-5 Furthermore, DME contains around 35 wt % oxygen and does not have any carbon-carbon bonds in its chemical structure.6 Thus, internal combustion engines fueled with DME are capable of soot-free combustion and have comparable thermal efficiency to diesel engines. The other advantage of DME fuel is its high vapor pressure, which reduces wall-wetting and allows PCCI (premixed charge compression ignition).7 There exist several issues that must be solved before DME can be successfully used to fuel compression-ignition engines. DME has a higher vapor pressure than conventional diesel fuel. As the rapid vapor pressure of DME is very high, DME causes vapor lock in the fuel injection system. To solve this problem, DME should be pressurized to liquefy for use in a high-pressure injection system. Moreover, DME fuel shows significantly different physical properties from conventional diesel, including a lower critical point, lower viscosity, and lower lubricity. Lower * Phone: +82-2-2220-0430; fax: +82-2-2220-4588; e-mail: parks@ hanyang.ac.kr. (1) Yu, Y.; Kang, S.; Kim, Y.; Lee, K.-S. Energy Fuels 2008, 22, 3649– 3660. (2) Bhide, S.; Morris, D.; Leroux, J.; Wain, K. S.; Perez, J. M.; Boehman, A. L. Energy Fuels 2003, 17, 1126–1132. (3) Sorenson, S. C.; Glensvig, M.; Abata, D. SAE Tech. Pap. Ser. 981195, 1998. (4) Teng, H.; McCandless, J. C.; Schneyer, J. B. SAE Tech. Pap. Ser. 2001-01-0154, 2001. (5) Yeom, K.; Bae, C. Energy Fuels 2009, 23, 1956–1964. (6) Teng, H.; McCandless, J. C.; Schneyer, J. B. SAE Tech. Pap. Ser. 2004-01-0093, 2004. (7) Kim, M. Y.; Bang, S. H.; Lee, C. S. Energy Fuels 2007, 21, 793– 800.

lubricity demands additive to maintain acceptable lubricity for mechanical parts of engines such as high-pressure pumps and fuel injectors. The lower heating value of DME is around 64% that of conventional diesel, which means a larger mass of fuel per cycle is required to keep input energy comparable.8 In addition, the global equivalence ratio of a DME engine is different from that of a diesel engine, even for the same input energy, because of the oxygen in DME fuel. For these reasons, it is believed that the optimal operating conditions of DME engines are considerably different from those of conventional diesel engines. Much experimental and numerical research has been performed to find an optimal strategy for the use of DME in compression ignition engines. Some researchers have investigated the addition of DME to other alternative fuels such as methane and LPG (liquefied petroleum gas) to enhance ignitibility. Morsy9 attempted to control ignition by adding DME to LPG in a compression ignition engine. On the basis of numerical results using single-zone zero-dimensional code, he found that the ratio of DME fuel to methane required to achieve a near-TDC ignition increases linearly with engine speed and decreases as global equivalence ratio increases. Yao and Qin10 also simulated the homogeneous charge compression ignition processes using a detailed kinetic model for a DME-methane fuel mixture and reported that low-temperature reaction is retarded but high-temperature reaction is facilitated by adding methane to DME fuel. Experimental studies have also been conducted on DME fuel sprays because it is believed that the spray characteristics of DME fuel are significantly different from those of diesel fuel. Macroscopic spray characteristics of DME fuel such as penetra(8) Longbao, Z.; Hewu, W.; Deming, J.; Zuohua, H. SAE Tech. Pap. Ser. 1999-01-3669, 1999. (9) Morsy, M. H. Fuel 2007, 86, 533–540. (10) Yao, M.; Qin, J. SAE Tech. Pap. Ser. 2004-01-2951, 2004.

10.1021/ef900276v CCC: $40.75  2009 American Chemical Society Published on Web 06/01/2009

3910

Energy & Fuels, Vol. 23, 2009

tion and spray angle were investigated and compared to those of diesel by Yu et al.11 in a common-rail fuel injection system. Using a phase Doppler particle analyzer, Suh et al.12 found that the Sauter mean diameter of DME spray is lower than that of diesel spray because of lower surface tension and quicker evaporation of DME droplets. Sidu et al.13 measured the spray penetration of DME at high ambient pressure and reported that DME spray has a shorter penetration and wider spray cone angle compared to diesel fuel due to increased atomization and evaporation. Combustion and emission characteristics of DME fuel engines have been studied by many researchers both experimentally and numerically. Kim et al.7 built a common-rail single-cylinder engine fueled with DME and studied emission characteristics. In their study, a high-pressure pump operated with high-pressure air was used to pressurize DME. They concluded that the NOx emission of DME fuel is higher than that of diesel fuel at the same indicated mean effective pressure. On the other hand, Longbao et al.8 asserted that NOx emission is decreased by switching to DME from diesel because the greater latent heat of evaporation of DME increases ignition delay. As can be seen from the previous studies on NOx emissions from DME engines,7,8 there exist some uncertainties on the combustion and emission characteristics of DME-fueled engines. To compare DME engine performance to that of conventional diesel engines, emission maps can be utilized. It is known that CO, NOx, and soot emissions are highly dependent on the temperature and equivalence ratio. Thus, important information has been obtained from emission maps. Akihama et al.14 found that simultaneously soot-free and NOx-free combustion can be achieved at a specific temperature and equivalence ratio. However, the wall heat transfer and the effect of crevice on emissions were not considered because zero-dimensional HCCI conditions were assumed in their research. Park and Reitz15 summarized CO (carbon monoxide), HC (hydrocarbon), NO, and soot emissions on peak cycle temperature-equivalence ratio maps based on two-dimensional calculations using KIVA-Chemkin code. They calculated more than 500 HCCI cases for each map, taking wall heat into consideration. Their results reveal that a low emission window of operation in which emissions are lower than 10 g/kg · f CO, 10 g/kg · f HC, 0.5 g/kg · f NO and 0.5 g/kg · f soot, is possible in HCCI combustion. However, the effect of the crevice region on CO and HC emission was not considered in this study. As shown above, DME emission maps on CO, HC, NOx, and soot are interesting because they allows analysis of fuel effects on emissions. Furthermore, optimal operating ranges can be found for DME engines and can be compared to those for n-heptane, a surrogate for diesel fuel. In the present study, numerical investigations of the effects of equivalence ratio, initial temperature, and boost pressure were conducted under assumed HCCI combustion. Twodimensional calculation mesh with crevice area was used for the calculations in order to consider the effect of wall heat transfer and crevice area on CO and HC emissions. Engine performance (e.g., IMEP (indicated mean effective pressure) (11) Yu, J.; Lee, J.; Bae, C. SAE Tech. Pap. Ser. 2002-01-2898, 2002. (12) Suh, H. K.; Park, S. W.; Lee, C. S. Energy Fuels 2006, 20, 1471– 1481. (13) Sidu, X.; Mingfa, Y.; Junfeng, X. SAE Tech. Pap. Ser. 2001-010142, 2001. (14) Akihama, K.; Takatori, Y.; Inagaki, Z.; Sasaki, Z.; Dean, A. M. SAE Tech. Pap. Ser. 2001-01-0655, 2001. (15) Park, S. W.; Reitz, R. D. Combust. Sci. Technol. 2007, 179, 2279– 2307.

Wook Park

and ISFC (indicated specific fuel consumption)) and emission maps were made for DME and n-heptane based on results from around 1000 two-dimensional KIVA-Chemkin calculations for each fuels. In addition, optimal operating conditions for each fuel are suggested. 2. Model Formulation A modified version of KIVA-3 V code16 was used to simulate HCCI combustion for DME and n-heptane. The code contains improvements in numerous physical and chemistry models developed at the Engine Research Center, University of Wisconsin-Madison.17-20 2.1. Fuel Oxidation Chemistries for DME and n-Heptane. Combustion and emission characteristics are highly dependent on the reaction chemistry of the fuel. Due to computational cost, skeletal mechanisms are generally used for n-heptane. In contrast, the relatively simple chemical structure of DME (CH3OCH3) allows the use of its full chemistry in calculations. To calculate HCCI reactions for n-heptane, a reduced reaction mechanism for n-heptane suggested by Patel et al.21 was used. Additional species and reactions are added to simulated acetylene and NOx formations.15 Thus, the mechanism consists of 36 species and 74 reactions selected from detailed mechanisms such as the Lawrence Livermore National Laboratory mechanism containing 179 species and 1642 reactions22 and the Chalmers University mechanism.23 This reduced n-heptane mechanism has been used in previous studies on diesel combustion and has shown good agreement with experimental results.24-26 On the other hand, the detailed kinetic mechanism for DME contains 351 reactions among 79 species,27-29 which is small compared to the detailed mechanism for n-heptane because the chemical structure of DME is simpler. Therefore, the detailed mechanism for DME was used for the present study without alterations. This mechanism has been validated under jet-stirred reactor conditions for pressure and shock tube conditions.29 Since chemical mechanisms were used in this study instead of a combustion model, the Chemkin chemistry solver30 was integrated into KIVA-3 V in order to solve the chemistry. The KIVA code calculated species and thermodynamic information (16) Amsden, A. A. KIVA-3 V Release 2, ImproVement to KIVA 3-V; Los Alamos National Laboratory: 1999; LA-UR-99-915. (17) Kong, S.-C.; Sun, Y.; Reitz, R. D. J. Eng. Gas Turbines Power 2007, 129, 245–251. (18) Kong, S.-C.; Kim, H.; Reitz, R. D.; Kim, Y. J. Eng. Gas Turbines Power 2007, 129, 252–260. (19) Ra, Y.; Reitz, R. D.; Jarrett, M. W.; Shyu, T. P. SAE Tech. Pap. Ser. 2006-01-1149, 2006. (20) Kim, H.; Reitz, R. D.; Kong, S.-C. SAE Tech. Pap. Ser. 200601-1150, 2006. (21) Patel, A.; Kong, S.-C.; Reitz, R. D. SAE Tech. Pap. Ser. 200401-0558, 2004. (22) Curran, H. J.; Gaffuri, J. P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 1998, 114, 149–177. (23) Golovitchev, V. I. http://www.tfd.chalmers.se/∼valeri/MECH.html; Chalmers University of Technology: Gothenburg, Sweden, 2000. (24) Sun, Y.; Reitz, R. D. SAE Tech. Pap. Ser. 2006-01-0027, 2006. (25) Park, S. W.; Reitz, R. D. J. Eng. Gas Turbines Power 2008, 130, 032805. (26) Park, S. W.; Reitz, R. D. Fuel 2009, 88, 843–852. (27) Fischer, S. L.; Dryer, F. L.; Curran, H. J. Int. J. Chem. Kinet. 2000, 32, 713–740. (28) Curran, H. J.; Fischer, S. L.; Dryer, F. L. Int. J. Chem. Kinet. 2000, 32, 741–759. (29) Kaiser, E. W.; Wallington, T. J.; Hurley, M. D.; Platz, J.; Curran, H. J.; Pitz, W. J.; Westbrook, C. K. J. Phys. Chem. A 2000, 104, 8194– 8206.

Dimethyl Ether and n-Heptane HCCI Engines

Energy & Fuels, Vol. 23, 2009 3911

for each cell and provided them to Chemkin. Chemkin subsequently solves the chemistry and returns the new species information. 2.2. Emission Models. For predicting CO and HC emissions, no additional emission model is needed because they are species represented in the mechanism. In the present study, HC emission is defined as the total mass of species that contain both hydrogen and carbon regardless of combination with oxygen. A two-step phenomenological model was used to simulate soot emission. The present soot model considers both soot formation and oxidation. Soot formation was calculated by using a modified Hiroyasu model that considers acetylene (C2H2) as a soot precursor.17 On the other hand, the Nagle-Strickland-Constable model was employed to simu˙ S within late soot oxidation.31 The rate of soot mass change M a computational cell is calculated from the soot formation ˙ SO.17 ˙ Sf and soot oxidation rate M rate M dMSf dMSO dMS ) dt dt dt

(1)

An Arrhenius expression is used for the soot oxidation rate based on a carbon oxidation model.

( )

ESf dMSf ) AsfMC2H2Pn exp dt RT

(2-1)

6MWc dMSO ) MR dt FsDs s Total

(2-2)

where, ASf ) 200, ESf ) 12 500 (cal/mol), MC2H2 is the mass of C2H2, MWc is the carbon molecular weight, Fs is the soot density (2.0 g/cm3), Ds is the soot diameter (2.5 × 10-6cm), and RTotal is the net reaction rate. The present two-step soot model has been validated for n-heptane mechanism by many researches.17,18,20,24-26 Although, the present soot has not been validated for DME engines, it is believed that reasonable soot simulations are able to be done by the present soot model because acetylene (C2H2) is also used as a soot precursor. A NOx mechanism that is reduced from the Gas Research Institute (GRI) NO mechanism32 was used for the present study. The reduced NOx mechanism consists of nine reactions and four species (i.e., N, NO, NO2, and N2O) to describe the formation of nitric oxides, as shown below. In the present simulations, NOx was defined as sum of NO and NO2. N + NO S N2 + O

(3-1)

(30) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin- II: a Fortran chemical kinetics package for the analyses of gas phase chemical kinetics; Sandia Report: 1989; SAND 89-8009. (31) Han, Z.; Uludogan, A.; Hampson, G. J.; Reitz, R. D. SAE Tech. Pap. Ser. 960633, 1996. (32) Dougan, C. L., Kong, S.-C., Reitz, R. D. Proceedings of ICES2005; ICES2005-1020; 2005.

N + O2 S NO + O

(3-2)

N2O + O S 2NO

(3-3)

N2O + OH S N2+HO2

(3-4)

N 2O + m S N 2 + O + m

(3-5)

HO2 + NO S NO2 + OH

(3-6)

NO + O + m S NO2 + m

(3-7)

NO2 + O S NO + O2

(3-8)

NO2 + H S NO + OH

(3-9)

2.3. Calculation Conditions. The computational mesh of the present study was generated based on a single-cylinder diesel engine with 75 mm bore size and 84.5 mm stroke as shown in Figure 1. Calculations were performed using a two-dimensional mesh from intake valve closure to exhaust valve opening to save calculation time. At intake valve closure, it is assumed that the fuel is completely evaporated and mixed with air homogeneously as done in the previous studies on HCCI combustion.15,32 The wall temperature was assumed to be fixed through intake valve closure to exhaust valve opening as listed in Table 1. Therefore, wall heat transfer was considered in the present calculations. Many calculations were done to complete equivalence ratio-peak cycle temperature maps for engine performance (e.g., IMEP and ISFC) and emissions, covering a wide range of operating conditions. The equivalence ratio was varied from 0.2 to 2.0, and intake temperature ranged from 300 to 460 K. In addition, boost pressures were set to cover 0.1-3.0 MPa. The fuel amounts per cycle for DME and n-heptane were set to 30 and 19 mg, respectively, since the lower heating value of DME is around 64% that of diesel fuel.8 Thus, 990 calculations using a 2D mesh for HCCI combustion were conducted for each fuel. The fuel amounts of the present study correspond to medium load. For each calculation, the initial mole fraction of the mixture at intake valve closure is required. Since fuel mass, intake

Figure 1. Two-dimensional computational mesh at top dead center. Table 1. Engine Specifications and Calculation Conditions fuel bore × stroke compression ratio fuel amount displacement engine speed equivalence ratio intake temperature boost pressure intake valve closure exhaust valve opening swirl ratio @ IVC wall temperature

DME (CH3OCH3), n-Heptane (C7H16) 75.0 × 84.5 mm 17.8 30 mg/cyc (DME), 19 mg/cyc (C7H16) 373.3 cm3 1500 rpm 0.2-2.0 by 0.2 300-460 K by 20 K 1.0-3.0 bar by 0.2 bar -128° ATDC 172°ATDC 1.63 553 K (piston) 523 K (head) 433 K (cylinder)

3912


Wook Park

Figure 2. Combustion characteristics of HCCI engines fueled with DME and n-heptane for 0.6 and 1.6 equivalence ratios (Tini ) 360 K, Pboost ) 0.2 MPa).

temperature, boost pressure, and in-cylinder volume at intake valve closure are fixed for each calculation, the oxygen mole fraction can be calculated to make a given equivalence ratio. Then it is assumed that the remaining portion of the mixture is N2 and CO2,which are the main species of the EGR (exhaust gas recirculation). The proportions of N2 and CO2 were calculated based on the specific heat ratio of the products of complete combustion at 600 K as suggested by Kim et al.33 Therefore, the initial mixture is composed of fuel, O2, N2, and CO2 in the present calculations. The detailed calculation conditions are listed in Table 1. The global equivalence ratio (φ) is calculated based on moles of hydrogen ([H]), carbon ([C]), and oxygen ([O]) at intake valve closure, φ)

2[H] + 0.5[C] [O]

(4)

In the present study, IMEP is calculated using work done from -180° ATDC to +180° ATDC. In-cylinder pressures before IVC and after EVO are assumed to be identical to those at IVC and EVO, respectively. 3. Results and Discussion Combustion and emission characteristics of DME fuel are analyzed and compared to n-heptane for the conditions representing low (φ ) 0.6) and high (φ ) 1.6) equivalence ratios. Engine performance and emissions are plotted on a peak cycle temperature and global equivalence ratio maps. In addition, optimal operating ranges for each fuel are suggested based on a merit function that considers both engine performance and emissions. 3.1. Combustion and Emission Characteristics of DME and Comparisons to n-Heptane. Figure 2 shows combustion characteristics of a DME HCCI engine and comparisons to those

of an n-heptane-fueled engine, in which n-heptane is used as a surrogate for diesel fuel. In the figure, initial temperature and boost pressure are fixed at 360 K and 0.2 MPa, respectively. Global equivalence ratios (φ) are 0.6 and 1.6, which represent lean and rich combustions. The required mole fraction of oxygen for a specific equivalence ratio for DME fuel is higher than that of n-heptane at φ ) 0.6, but lower at φ ) 1.6 (see Figure 2a) because the fuel amounts are different and DME contains a significant amount of oxygen. In Figure 2a, pressure histories indicate that the ignition delay of DME is slightly shorter than that of n-heptane. As indicated by Hoffman and Abraham,34 DME has a lower NTC (negative temperature coefficient behavior) than n-heptane. Thus, DME has a shorter dwell time between low-temperature and high-temperature reactions, which can also be seen in Figure 2b. As can be seen in Figure 2b, total heat release is quite similar for DME and n-heptane because the fuel amounts were chosen to yield similar lower heating values. Furthermore, indicated mean effective pressures are close for the given equivalence ratios. There is no significant difference between DME and n-heptane in mean cylinder temperature of Figure 2c. In the present study, the peak cycle temperature is defined as the peak value of mean cylinder temperature as shown in Figure 2c, and the values are used to plot peak cycle temperature-equivalence ratio maps. Emissions of the DME HCCI engine and comparisons to those of the n-heptane HCCI engine are shown in Figure 3. The unit used for emissions are [g/kW · hr] rather than [g/kg · f] because the lower heating values for DME and n-heptane are different. In Figure 3a, the CO emissions of DME are greater than those of n-heptane for rich combustion (φ ) 1.6). It is known that CH3OCH2 is first combined with O2 and then OH is abstracted, that is, CO cannot be abstracted directly from



Figure 3. Emission characteristics of HCCI engines fueled with DME and n-heptane for 0.6 and 1.6 equivalence ratios (Tini ) 360 K, Pboost ) 0.2 MPa).

Figure 4. Correlation between peak cycle temperature and ISFC for DME HCCI engine.

DME.27,28,35 In the present study, the mole fraction of oxygen is reduced for DME compared to n-heptane to keep the equivalence ratio constant (see Figure 2a). Therefore, it is believed that because less oxygen is used when DME is the fuel, less OH is produced, which makes the CO oxidation rate lower than if n-heptane were used. However, when the amount of oxygen provided is identical, DME is still observed to produce less CO.36 In Figure 3b, the HC emissions of DME are greater than those of n-heptane before combustion begins because fuel vapor is regarded as hydrocarbon. However HC for DME is decreased dramatically after combustion begins and becomes slightly lower than that of n-heptane at a 1.6 equivalence ratio. NOx emissions of DME are higher than those of n-heptane in Figure 3c because the ignition delay of DME is shorter, which leads to a higher temperature as shown in Figure 2a. In addition, the DME HCCI

engine shows slightly lower soot emissions than the n-heptane engine, as shown in Figure 3d. 3.2. Correlation between Peak Cycle Temperature and Performance for DME HCCI Engines. Previous study has shown that there is a close correlation between peak cycle temperature and emissions for n-heptane HCCI engines, and a low emission window can be suggested based on peak cycle temperature and equivalence ratio maps.15 The correlation between peak cycle temperature and combustion characteristics for DME HCCI engines is investigated in this section. In Figures 4 and 5, each symbol indicates a result for each HCCI case. Figure 4 shows the correlation between peak cycle temperature and ISFC for equivalence ratios of 0.6 and 1.6. In the figure, it can be seen that incomplete combustion leads to a deterioration in fuel consumption at an equivalence ratio of 1.6. ISFC numbers are higher for DME than for n-heptane20,24-26 at every peak cycle temperature due to the different lower heating value of DME. In addition, deterioration of ISFC for higher peak cycle temperatures is observed for both equivalence ratios. It is evident that higher initial temperature increases peak cycle temperature and advances ignition timing simultaneously. Therefore, it is believed that ISFC deteriorates at higher peak cycle temperatures due to excessively advanced ignition timing. The correlation between peak cycle temperature and pollutant emissions is shown in Figure 5. In Figure 5a, a higher concentration of CO is found at peak cycle temperatures below 1600 K for an equivalence ratio of 0.6. As indicated by Park and Reitz,15 two factors contribute to incomplete oxidation of CO; low temperature (e.g., below 1600 K) and an equivalence ratio greater than 1. At an equivalence ratio of 1.6, CO emission increases with peak cycle temperature. Park and Reitz15 demonstrated that there

3914


Wook Park

Figure 5. Correlation between peak cycle temperature and pollutant emissions.

Figure 6. CO emissions in [g/kW · hr] on peak cycle temperature-equivalence ratio map.

is a uniform CO distribution in [g/kg · f] for a fixed equivalence ratio of 1.6. Therefore, it is believed that higher ISFC as peak cycle temperature increases (see Figure 4) causes an increase in CO emissions in [g/kW · hr]. Hydrocarbon emissions increase as equivalence ratio increases and as peak cycle temperature decreases in Figure 5b. Figure 5c shows NOx emissions, which are strongly dependent on temperature. NOx starts to be formed when the peak cycle temperature is above 2000 K for the 0.6 equivalence ratio case. Soot emissions shows that soot is formed at a band of peak cycle temperature in Figure 5d. In Figure 6, a strong relation is shown between peak cycle temperature and pollutant emissions, including CO, HC, NOx, and soot emission. Thus, it is believed that emission maps for

the DME HCCI engine can be completed on peak cycle temperature-equivalence ratio. 3.3. Emission Maps for DME and n-Heptanes-fueled HCCI Engines. Figure 6 illustrates CO emissions for DME and n-heptane on a peak cycle temperature-equivalence ratio map. For both DME and n-heptane maps, the highest concentration of CO is found at high peak cycle temperature and equivalence ratio. It is believed that an increase in ISFC due to high intake temperature and insufficient oxygen to oxidize CO (33) Kim, D.; Ekoto, I.; Colban, W. F.; Miles, P. C. SAE Tech. Pap. Ser. 2008-01-1602, 2008. (34) Hoffman, S. R.; Abraham, J. Fuel 2009, 88, 1099–1108. (35) Kim, H.; Pae, A.; Min, K. Combust. Sci. Technol. 2004, 174, 221– 238. (36) Egnell, R. SAE Tech. Pap. Ser. 2001-01-0651, 2001.



Figure 7. HC emissions in [g/kW · hr] on peak cycle temperature-equivalence ratio map.

Figure 8. Spatial distributions of CO and HC at 30 degree ATDC for 1.6 equivalence ratio, 360 K initial temperature, and 0.2 MPa boost pressure (DME cases).

Figure 9. NOx emissions in [g/kW · hr] on peak cycle temperature-equivalence ratio map. NOx is defined as sum of NO and NO2.

increases CO emission at an equivalence ratio above 1.5 and a peak cycle temperature above 1800K. It can be seen that CO emissions for DME are greater than those for n-heptane because of a lower initial mole fraction of oxygen (also see Figure 3a). Hydrocarbon emission maps for DME and n-heptane are shown in Figure 7. When the equivalence ratio is below one, HC emissions for DME and n-heptane are quite similar to each other. However, HC emissions of DME are lower than those of n-heptane at a peak cycle temperature below 1600 K and equivalence ratios greater than 1.5. Thus, HC emissions can be reduced by substituting DME for diesel fuel for high

equivalence ratio and low-temperature combustion. In Figures 6 and 7 it can be seen that CO and HC emissions are greater compared to those in previous research by Park and Reitz.15 In the present study, crevice area, which was not considered in Park and Reitz’s study,15 was included in the calculation mesh as shown in Figure 1. As shown in Figure 8, a significant amount of CO and HC emissions are observed in the crevice area. Figure 9 shows NOx emissions for DME and n-heptane. For both fuels, NOx starts to be produced at a peak cycle temperature of around 1900 K and at an equivalence ratio of less than one.

3916


Wook Park

Figure 10. Soot emissions in [g/kW · hr] on peak cycle temperature-equivalence ratio map. C2H2 is regarded as a precursor of soot formation.

Figure 11. Indicated specific fuel consumption in [g/kW · hr] on peak cycle temperature-equivalence ratio map.

Figure 12. Comparisons of indicated mean effective pressure between DME and n-heptane in [MPa] on peak cycle temperature-equivalence ratio map.

It can be seen that the 0.5 g/kW · hr NOx line of DME is close to that of n-heptane. However NOx emissions for DME increase more rapidly as peak cycle temperature increases compared to those for n-heptane. In the soot emission map for DME of Figure 10a, soot emission occurs between a peak cycle temperature of 1700 and 1800 K at high equivalence ratio. However, the high

soot concentration area is moved toward higher peak cycle temperature (i.e., between 1800 and 2000 K peak cycle temperature), and soot emission is slightly increased for n-heptane compared to DME. Therefore, it can be seen that soot is produced at lower temperatures and quantity of soot emission is decreased for the DME HCCI engine compared to the n-heptane engine.



Figure 13. Effect of peak cycle temperature and equivalence ratio on total heat releases [J] for DME and n-heptane.

Figure 14. CA 30 [deg., ATDC] distributions. CA 30 indicates a crank angle when the accumulated heat release reaches 30% of total heat release, which represents start of combustion.

Figure 15. Merit of DME HCCI engines on a peak cycle temperature and equivalence ratio map. Optimal operating conditions are seen between 1700 and 1850 K peak cycle temperature and below 0.5 equivalence ratio. The dashed white line indicates a merit function value of 500.

3.4. Combustion Characteristics and Merit Analysis of DME HCCI Engine. Indicated specific fuel consumption of DME is expected to be worse than that of n-heptane due to its smaller, lower heating value. In Figure 11a it is seen that ISFC for DME fuel is greater than 275 g/kW · hr, whereas ISFC is lower than 200 g/kW · hr at wide operating range (i.e., below

Figure 16. Merit of n-heptane HCCI engine on peak cycle temperature and equivalence ratio map. Dashed white line indicates 500 merit line.

1800 K peak cycle temperature and stoichiometric fuel to oxidizer ratio) for n-heptane, as shown in Figure 11b. However, Figure 12 shows that the indicated mean effective pressure for DME is comparable to that for n-heptane. In Figures 11 and

3918


12, optimal operating range for the best ISFC or IMEP can be observed at low peak cycle temperature (e.g., below 1600 K for DME) and below stoichiometric fuel to oxidizer ratio. Considering that total heat release is nearly constant below stoichiometric for both fuels, as shown in Figure 14, it is believed that the timing of combustion has an important effect on IMEP. Figure 14 shows the start of combustion, which is represented by the time when the accumulated heat release reaches 30% of total heat release. As shown in this figure, the start of combustion approaches top dead center for low peak cycle temperature and lower than stoichiometric fuel to oxidizer ratio. To find optimal operating conditions for DME and n-heptane considering IMEP, CO, NOx, and soot emissions, a merit function is defined in the present study as, fmerit ) 1000 (1.0 MPa/IMEP)5 + (CO/(10 g/kW · h))2 + (HC/(10 g/kW · h))2 + (NOx /(0.5 g/kW · h))2 + (Soot/(0.2 g/kW · h))2 (5)

In the above equation, IMEP is weighted to consider both engine performance and emissions reasonably. A greater function value indicates better engine operating condition in Figures 15 and 16. In Figure 15, the optimal operating range can be found between 1700 and 1850 K peak cycle temperature and below 0.5 equivalence ratio for the DME HCCI engine (see dashed white line in Figure 15). It can be seen that the higher equivalence ratio region is limited by excessive CO, HC, soot, and poor IMEP. The high peak cycle temperature region (i.e., above 2000 peak cycle temperature) is limited by excessive NOx and poor IMEP. In addition, operating conditions for peak cycle temperatures below 1400 K are made suboptimal by CO and HC oxidation. The merit function for n-heptane in Figure 16 shows that n-heptane has a narrower optimal operating range than DME. 5. Conclusions In the present study, combustion and emission characteristics for DME and n-heptane HCCI engines were investigated over wide ranges to produce contour plots for IMEP and pollutant emissions such as CO, HC, NOx, and soot on

Wook Park

peak cycle temperature-equivalence ratio maps. The amount of fuel provided for n-heptane was adjusted to yield a similar total heat release to that for DME. The differences between DME and n-heptane in combustion and emission characteristics were studied. On the basis of maps for engine performance and emissions, optimal operating conditions for DME and n-heptane HCCI engines were suggested. The following conclusions can be drawn from the results of this study. (1) Strong correlations are found between peak cycle temperature and emissions including CO, NOx, HC, and soot for DME HCCI engines. Thus, emissions can be analyzed on peak cycle temperature and equivalence ratio maps for the DME HCCI engine. (2) The highest concentration of CO is found at high peak cycle temperature and equivalence ratio for DME and n-heptane fuels because of poor ISFC due to high intake temperature and insufficient oxygen to oxidize CO increases CO emission. CO emissions for DME are greater than those for n-heptane because of a lower initial mole fraction of oxygen. (3) HC emissions for DME and n-heptane are quite similar below stoichiometric. However HC emissions for DME are lower than those for n-heptane below a peak cycle temperature of 1600 K and over an equivalence ratio of 1.5. Thus, HC emissions can be reduced by substituting DME for diesel fuel for high equivalence ratio and low-temperature combustion. (4) In DME HCCI engines, soot is produced at lower temperatures than in n-heptane HCCI engines. Soot emission can be reduced by substituting DME for n-heptane fuel. (5) The optimal operating range for the best ISFC or IMEP is at low peak cycle temperature and below stoichiometric for both fuels because ignition is near TDC for that area. Total heat release of DME is close to that of n-heptane in the present study. (6) On the basis of the merit function, which considers both engine performance and emissions, the optimal operating range is between 1700 and 1850 K peak cycle temperature and below 0.5 equivalence ratio for DME HCCI engines. Results of the present study are only based on numerical calculation. Although only validated models are used, experiments are suggested to validate the present numerical results at the optimal operating conditions of the present study. EF900276V

Numerical Study on Optimal Operating Conditions of Homogeneous

Recommend Documents