Effects of Combustion Mode on Exhaust Particle Size Distribution

Oct 16, 2008 - Exhaust particle (10−487 nm) number size distributions (NSDs) from an engine fueled with dimethyl ether (DME) for three combustion mo...
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Energy & Fuels 2008, 22, 3838–3843

Effects of Combustion Mode on Exhaust Particle Size Distribution Produced by an Engine Fueled by Dimethyl Ether (DME) Wei Liu,*,†,‡ Xinqi Qiao,† Jiasong Wang,† Zhen Wang,† and Zhen Huang† Key Laboratory for Power Machinery and Engineering of Ministry of Education (MOE), and School of EnVironmental Science and Engineering, Shanghai Jiaotong UniVersity, 200240 Shanghai, People’s Republic of China ReceiVed March 30, 2008. ReVised Manuscript ReceiVed August 14, 2008

Exhaust particle (10-487 nm) number size distributions (NSDs) from an engine fueled with dimethyl ether (DME) for three combustion modes [compression ignition direct injection (CIDI), homogeneous charge compression ignition (HCCI), and compound charge compression ignition combustion (CCCI)] were tested and compared. The results indicated that the NSD for various combustion modes showed similar unimodal structure with a nucleation mode at all test engine conditions. The exhaust particle geometric mean diameters of CCCI and HCCI were within the same range, and they were around 4-7 nm bigger than those of CIDI. Under the same engine condition (1100 revolutions/min, 20% of full load), the total exhaust particle number concentrations for CCCI (rp ) 0.18-0.85) were slightly lower than those of HCCI but 20.0-29.3% higher than those of CIDI. For CCCI, the total number decreased with the increase of the load at the same premixing ratios but increased significantly with the increase of the premixing ratio at the same engine conditions.

1. Introduction As a new alternative fuel, dimethyl ether (DME) has attracted great attention1-4 because of the extensive energy demand and environmental pressure. The chemical molecular formula of DME is CH3-O-CH3, which is the simplest ether compound. DME only has CH and C-O bonds and lacks a C-C bond, while the molecule has 34.8% oxygen content. These characteristics contribute to better spray combustion and soot free. DME is in gaseous state at the normal atmospheric environment. Its vapor pressure is 0.51 MPa and increases with temperature. It was well-realized that DME and air can rapidly form a homogeneous mixture.5 In addition, the cetane number of DME is 55, which means DME has good compression ignition characteristics.6,7 With these characteristics, various combustion modes can be organized in the engine for DME fuel. From previous studies, there are mainly three combustion modes [compression ignition direct injection (CIDI), homogeneous charge compression ignition (HCCI), and compound charge compression ignition combustion (CCCI, combination of port * To whom correspondence should be addressed. Telephone: +86-2134206859. Fax: +86-21-34205553. E-mail: [email protected]. † Key Laboratory for Power Machinery and Engineering of MOE. ‡ School of Environmental Science and Engineering. (1) Theo, F.; Chris, M. C.; Arun, B.; Carl, U.; et al. SAE Tech. Pap. 950061, 1995. (2) Sorenson, S. C.; Mikkelsen, S. E. SAE Tech. Pap. 950064, 1995. (3) Zhang, G. D.; Huang, Z.; Qiao, X. Q.; Zhou, X. P. Trans. CSICE 2003, 20, 395–398 (in Chinese). (4) Wu, J. H.; Huang, Z.; Qiao, X. Q.; Lu, J.; et al. Int. J. Automot. Technol. 2006, 7 (6), 645–652. (5) Wu, J. H. Doctor Dissertation, Shanghai Jiao Tong University, Shanghai, China, 2006. (6) Shuichi, K.; Mitsuharu, O.; Tomoya, M. SAE Tech. Pap. 2000-012004, 2000. (7) Xu, S. D.; Yao, M. F.; Xu, J. F. SAE Tech. Pap. 2001-01-0142, 2001.

introduction and direct injection of DME)]8 for DME fuel. These combustion modes and their emissions for DME have been studied9-11 widely in the past years. However, most of the studies focused on conventional emissions (NOx, CO, HC, etc.). It was found that, in comparison to CIDI, less exhaust gas (e.g., NOx) and smoke were emitted in HCCI and CCCI cases. HCCI and CCCI were considered as two “cleaner” combustion modes. Recently, many studies12,13 have found the strongest associations between concentrations of fine and ultrafine particles in urban air and an increased risk of mortality. Furthermore, many toxicological studies found that ultrafine and nanoparticles contain much more trace amounts of toxic substances (As, Se, Pb, Cr, etc.) and organic pollutants (PAHs, PCDDPFs, etc.) than fine and coarse particles. Ultrafine and nanoparticles can penetrate deep into the lung and enter interstitial tissues, causing severe respiratory inflammation and acute pulmonary toxicity.14-16 Therefore, the ultrafine particles from vehicles have attracted extensive attention because of those health problems.17-19 The major aspect of these studies involves particle emission char(8) Zhang, J. J.; Qiao, X. Q.; et al. Collected Papers of the 7th Annual Meeting of Chinese Society for Internal Combustion Engines, Shanghai, China, 2007; pp 359-362 (in Chinese). (9) Yao, M. F.; Xu, S. D.; Luo, Y. J. Combust. Sci. Technol. 2002, 8, 252–257 (in Chinese). (10) Zhang, G. D.; Huang, Z. Diesel Engine 2003, 2, 9–12 (in Chinese). (11) Song, J.; Huang, Z.; Qiao, X. Q.; et al. Chin. Sci. Bull. 2003, 48, 1157–1160. (12) Samet, J. M.; Dominici, F.; Curriero, F. C.; Coursac, I.; Zeger, S. L. N. Engl. J. Med. 2000, 343, 1742–1749. (13) Pope, C. A.; Burnett, R. T.; Thun, M. J.; et al. J. Am. Med. Assoc. 2002, 287, 1132–1141. (14) Seaton, A.; MacNee, W.; Donaldson, K.; Godden, D. Lancet 1995, 345, 176–178. (15) Donaldson, K.; Beswick, P. H.; Gilmour, P. S. Toxicol. Lett. 1996, 88, 293–298. (16) Oberdo¨ster, G.; Utell, M. J. EnViron. Health Perspect. 2002, 110, A440–A441. (17) Bagley, S. T.; Baumgard, K. J.; Gratz, L. D.; Johnson, J. H.; Leddy, D. G. Health Effects Institute Research Report, 1996; Vol. 76. (18) Baumgard, K. J.; Johnson, J. H. SAE Tech. Pap. 960131, 1996.

10.1021/ef800224f CCC: $40.75  2008 American Chemical Society Published on Web 10/17/2008

DME Exhaust Particle Size Distribution

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Figure 1. Schematic of the DME engine and sampling system.

acteristics (including size distributions and particle composition analysis) at various engine conditions. It is widely believed that the main effect factors on engine particle emissions mainly include fuels and lube oil characteristics, sampling conditions, engine parameters, and operation conditions. At present, research about the number concentration of particles emitted from engines fueled with traditional fuels has been widely carried out.20-22 However, very few studies23 addressed the size distribution of particles emitted from DME engines. Also, the effects of these combustion modes on exhaust particle size distributions were not investigated. The aim of this study is to evaluate the effects of combustion modes for DME fuel on particulate matter from a low-load steady-state modificative DME engine. Under the same engine conditions, the exhaust particle number concentrations and size distributions of various combustion modes were tested and compared. 2. Apparatus and Procedures 2.1. Engine, Fuel, and Sampling System. A DF2135-mode, 2-cylinder, 4-stroke, naturally aspirated, direct-injection (DI) diesel engine was adopted in this study. This engine was modified to a DME HCCI and CCCI with air intake premixed homogeneouscharge and in-cylinder fuel spray. The engine test bench is shown in the left dotted line frame of Figure 1. For the DI test, the DME fuel was injected into the cylinder through course as follows: fuel injection pump f injection pipe f injector. The nozzle number × orifice, nozzle-opening pressure, and injection advance angle were modified to 5 × 0.43 mm, 17 MPa, and 26 °CA BTDC, respectively. A low-pressure pump was used to increase the fuel injection pressure in the injection system. For the HCCI test, the fuel supply system was made of a decompression evaporator and a mixer; the DME fuel and air formed a homogeneous mixture in the mixer and entered the cylinder through the intake port. In addition, the fuel flow rate was controlled by a valve. Two fuel (19) Liu, W.; Huang, Z.; Wang, J.; Qiao, X.; Hou, J. Energy Fuels 2008, 22 (4), 2307–2313. (20) Wei, Q.; Kittelson, D. B.; Watts, W. F. SAE Tech. Pap. 2001-01020, 2001. (21) Ristovski, Z. D.; Jayaratne, E. R.; Lim, M. EnViron. Sci. Technol. 2006, 40, 1314–1320. (22) Desantes, J. M.; et al. J. Aerosol Sci. 2005, 36, 1251–1276. (23) Li, X. L.; Huang, Z.; Wang, J. S. Chin. Sci. Bull. 2007, 52, 1707– 1713.

Table 1. Engine Specifications mode cylinder bore × stroke (mm) compression ratio rated power [kW/(revolutions min-1)]/ speed (revolutions/min) plunger diameter (mm) nozzle-opening pressure (MPa) injection advance angle (°CA BTDC) nozzle number × orifice

DF 2-135 135 × 140 16.5:1 29.4/1500 9 17 26 4 × 0.35

supply lines were simultaneously used in the CCCI test cases. Except for the supply line, the main engine parameters were the same in these three cases. Fuel mass consumption rates of both fuel supply lines were weighed and calculated by two electric balances. In CCCI cases, the premixed ratio rp is defined using the following equation:

rp )

mp mp + md

(1)

where mp and md represent the mass consumption rate of premixed DME fuel and directly injected DME fuel, respectively. Therefore, rp ) 1.0 is equivalent to fully HCCI, and rp ) 0 means the conventional CIDI. Additional engine details are listed in Table 1. The particle size distributions were measured using a TSI 3034 scanning mobility particle sizer (SMPS, TSI, Inc., Shoreview, MN) within a range of 10-487 nm. The SMPS-3034 houses a differential mobility analyzer (DMA)-based electrostatic classifier and a condensation particle counter (CPC) in the same cabinet. With an inlet flow of 1 L min-1, electrically charged particles pass from the neutralizer into the DMA. The DMA contains two concentric metal cylinders with an electric field in between. Here, the particles are separated depending upon their electrical mobility and counted with the CPC after passing through a butanol-saturated atmosphere, which forces the particles to grow to detectable size. Number concentrations are given for 54 channels (32 channels per decade); particle surface area and volume are estimated on the basis of number concentration and assuming a spherical shape of particles. All estimates were standardized with the geometric width of the size channel (D log Dp). Measurements were performed in 3 min intervals. The total number concentration limit of the SMPS is 102-107 particles/cm3. To dilute and cool the exhaust sample for measurement by particle instruments, a single-stage sampling and dilution system was used. It is shown in the right dotted line frame of Figure 1. The sampling gas was drawn from the exhaust manifold through an insulated sampling line, and a small fraction of the

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Figure 2. Effects of the premixed ratio (rp) on combustion characteristics.

exhaust was introduced into the dilution tunnel, where it mixed with a steady flow of clean ambient air drawn through a high-flow high-efficiency particulate air filter using an air pump. The insulated sampling line was as short as possible and was heated to maintain a surface temperature of 300 °C to prevent deposition of solid particles and condensation of volatile materials on the interior wall. A valve was used in the sampling line to control a fraction of the exhaust passing through the sampling line. The dilution ratio (DR) was determined23 by measuring the ratio of the concentration of CO2 in the undiluted exhaust gas to that in the diluted sample, i.e.,

DR )

CO2 undiluted - CO2 background CO2 diluted - CO2 background

(2)

In eq 2, CO2 undiluted is the CO2 concentration in the undiluted exhaust gas, CO2 diluted is the CO2 concentration in the diluted sample, and CO2 background is the CO2 concentration in the background. The dilution ratio is about 85 ( 5. 2.2. Engine Conditions. All tests were carried out under steady engine conditions. The engine speeds included middle speed (1100 rpm) and high speed (1500 rpm). Under the same engine conditions, the characteristics of combustion and particle emissions of these three combustion modes were compared. It is necessary to note that, because of restrictions of oil supply in CIDI cases and knocking in HCCI cases, the engine can operate stably only at low loads ( 0), the combustion characteristics of CCCI are observed (rp ) 0.15; the cool flame region of HCCI combustion phase is observed, but the high-temperature HCCI combustion region is not obvious because of the low concentration of premixed DME and endothermic action of directly injected DME). In addition, the CCCI exhibits a three-stage combustion consisting of the cool flame region, the high-temperature HCCI combustion region, and the diffusive combustion because of the directly injected diesel fuel. As the rp increases (rp ) 0.25, 0.50, and 0.85), the maximum HRR of the HCCI combustion phase is found to increase gradually, the maximum HRR of CIDI combustion phase decreases gradually, the border of the premixed combustion and diffusive combustion of the CIDI combustion phase becomes obscure, and the ignition timing of the low-temperature cool flame is slightly advanced. The engine exhibits the characteristics of pure HCCI as the rp increases to

DME Exhaust Particle Size Distribution

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Figure 3. Number weighted size distribution: (a) 1100 r/min and (b) 1500 r/min.

Figure 4. Effects of the load on the total number concentration for CCCI.

1.0, exhibiting a two-stage combustion consisting of the cool flame region and high-temperature HCCI combustion region. The ignition timing is further advanced, and the in-cylinder pressure and temperature reach the peak.8 3.2. Particle Number Size Distribution. The combustion modes were switched by adjusting the premixed ratio rp. As it is mentioned above, rp ) 0, rp ) 1.0, and 0 < rp < 1.0 represents CIDI, HCCI, and CCCI operating conditions, respectively. Figure 3shows the DME exhaust particle NSD at various rp (at middle and high speeds, 20% load). The exhaust particle NSD in all cases shows similar unimodal structure with a nucleation mode (diameter of the particle < 50 nm), but the peak number concentrations of various cases are different. The number concentrations of accumulation mode (diameter of the particle > 50 nm) are very low. It is worth noting that particles smaller than 10 nm could be present in the system. However, these cannot be detected because of the limitation of detection of the SMPS. As the rp ) 0 (CIDI), the peak concentrations (peaking at 23 nm) are the lowest at both engine conditions. In comparison to CIDI, the peak concentrations of CCCI and HCCI are significantly higher and the peak diameters (about 25-30 nm) are obviously bigger. It is also observed that the peak concentrations are the highest when the rp ) 1.0 (HCCI). In addition, the results indicate that the peak concentrations increase with speed, and the speed has little impact on the diameter. To investigate the effects of load on the DME exhaust particle NSD of CCCI, measurements of the steady premixing ratio (0.5 ( 0.04) at different loads were carried out in our study. The results are shown in the Figure 4. The results reveal that the load has little impact on the NSD, but the peak concentrations

obviously decrease with the load. With the load from 20 to 40%, the peak concentration decreases about 15.8%. The nucleation mode particles are formed by the nucleation of supersaturated elementary carbon (EC) generated during the combustion process in the engine. It suggests that the nucleations of sulfuric acid and semivolatile organic compounds in the process of dilution and cooling of the vehicle exhaust produce more nucleation mode particles.24,25 The homogeneous nucleation is significantly affected by the environmental temperature, humidity, pre-existing particles in the exhaust, and processes of the deposition and volatilization in the dilution system.26,27 In contrast, particles in the accumulation mode are mainly associated with carbonaceous agglomerates and the volatile matter adsorbed on their surface.25 At low loads, with lower temperature and worse combustion, more unburned and incomplete burning fuel and lubricating oil are emitted. It is believed that this part of unburned hydrocarbons (UHCs) can form a large number of nucleation mode volatile particles in the process of dilution and cooling.28 DME is an oxygenated fuel, and the oxygen content is 34.8%. With better spraying and “self oxygen supplying”, the DME fuel molecules have more opportunities to react with oxygen in the cylinder; therefore, the hypoxia intensity of the high fuel concentration area decreases, and the fuel can be burnt more completely. In addition, DME dose not have a C-C bond, which supresses the carbon particles formation in the cylinder. As a result, the number concentration of the accumulation mode particle is very low because of the reduction of the agglomeration of carbonaceous primary nucleation mode particles. It is worth noting that a little fuel additive was added into DME fuel to improve the lubrication of the plunger in this study. Typically, adding additives may contribute to the increase of the exhaust nucleation mode particles.29 The difference of exhaust particle NSD between various combustion modes is attributed to the difference of the in-cylinder pressure and temperature in various cases. As it is mentioned above, these combustion modes exhibit different characteristics, resulting in different emission characteristics. In addition, the emissions of particle precursor gases (SO2, SO3, (24) Kwon, S. B.; Lee, K. W.; Saito, K.; Shnozaki, O.; Seto, T. EnViron. Sci. Technol. 2003, 37, 1794–1802. (25) Schneider, J.; Hock, B. N.; Weimer, S.; Borrmann, S.; et al. EnViron. Sci. Technol. 2005, 39, 6153–6161. (26) Alleman, T. L.; Cormick, M.; Robert, L. SAE Tech. Pap. 200301-0763, 2003. (27) Shi, J. P.; Markd, D.; Harrison, R. EnViron. Sci. Technol. 2000, 34, 748–755. (28) Cheng, X. B.; Huang, R. H.; Chen, D. L. J. Combust. Sci. Technol. 2006, 12, 335–340 (in Chinese). (29) Andersson, J. D.; Preston, H.; Warrens, C. SAE Tech. Pap. 200401-3012, 2004.

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Figure 5. Number concentration of the ultrafine particle: (a) number in the accumulation mode, (b) number in the nucleation mode, and (c) total number.

H2SO4, H2O, low-volatile organic species, and semivolatile organic species) are different, which have significant impact on the particle emissions. Furthermore, the peak concentration increases with speed, mainly because of the decrease of the oxidation residence time of the exhaust and more fuel delivered into the cylinder. 3.3. Number Concentration and Geometrical Mean Diameter. Figure 5 shows the number concentrations at various premixed ratios and engine conditions, including the total number (in the range of 10-487 nm), the number in the nucleation mode (in the range of 10-50 nm), and the number in the accumulation mode (in the range of 50-487 nm). The results indicate that the number in the nucleation mode particles occupies over 98% of the total number and the number in the nucleation mode is about 2 orders of magnitude higher than the number in the accumulation mode in all cases. Figure 5a shows that the rp and speed have no evident effect on the number in the accumulation mode, which is in low level in all cases. It can be seen that, from parts b and c of Figure 5, the effects of rp and engine speed on the total number mainly reflect their effects on the number in the nucleation mode and the total number increases with the increase of speed. In comparison to CIDI, the total number for HCCI significantly rises at the same engine conditions (at 1100 and 1500 rpm, 20% load, the total number higher at 32.6 and 37.3%, respectively). The results also indicate that, for CCCI, the total number is slightly less than that of HCCI cases but is higher at 20.0-29.3% than those of CIDI cases at the same engine conditions (1100 rpm, 20% load, rp ) 0.18-0.85). In addition, the total number increases with the increase of the premixing ratio at the same loads but decreases with load in the same rp cases. It is worth noting that the increase of the total particle number concentration obviously becomes slow after the premixing ratio g 0.2 (1100 rpm, 20% load).

Figure 6. Effects of rp on the GMD.

Figure 6 depicts the effects of the premixing ratio on the geometrical mean diameter (GMD) at 20% load (1100 and 1500 rpm). The results indicate that speed has no evident effect on the GMD. However, the GMD of CCCI and HCCI are in the same range, which are obviously 4-7 nm bigger than those of CIDI cases. CCCI is a combinational combustion mode of premixed combustion with direct-injection combustion. First, the HCCI combustion stage occurs, and then the diffusive combustion stage occurs in the cylinder. With the DME introduction through the intake port, more UHC is transformed from the unburned DME in the annular clearance of the piston and cylinder wall, which is due to the premixed fuel/air mixture trapped in the crevice, and the boundary layer is hard to oxidize during the low-temperature HCCI combustion stage. For HCCI, more DME is introduced into the inlet port, causing stronger crevices and quenching effect, resulting in more UHC being transformed from the unburned DME in the annular clearance of the piston and

DME Exhaust Particle Size Distribution

cylinder wall.30 Furthermore, HCCI has no diffusive combustion, which results in more UHC emission than CCCI and CIDI. Also, Ma et al.31 found that the heat release of the third-stage diffusive combustion for CCCI was beneficial for further oxidation of UHC produced in the HCCI combustion stage. In comparison to CIDI, the increases of the total number and GMD in CCCI and HCCI cases may be related to the increase of UHC emission. Many studies (e.g., Schneider et al.25 and Vaaraslahti et al.32) found that there was a close relationship between UHC and particle emissions. Also, Zhai et al.33 reported that high UHC emission favored higher exhaust particle number concentrations and bigger exhaust particle diameters. At low loads, the carbonaceous primary nucleation mode particles of all cases are very low, but UHC emissions in CCCI and HCCI cases are much higher than those of CIDI. These UHC can be transformed into a large number of nucleation mode particles34,35 via nucleation in the process of dilution and cooling of the vehicle exhaust produces and can be adsorbed on the surface of carbonaceous nucleation mode particles to increase the particle diameter. For the same engine conditions and steady premixing ratios, the total particle number concentrations decrease with the increase of the load in CCCI cases, which may be explained as the in-cylinder temperature increases at higher load. Higher temperature favors HC oxidation and reduction of the quenching region. As a result, the HC emission significantly decreases with the increase of the load. In addition, the less nucleation mode particle forms via nucleation of UHC. For CCCI, at a lower premixing ratio, the premixed DME concentration is low. The high-temperature heat release of the HCCI combustion stage is weakened because of endothermic action of direct-injection DME fuels. In addition, the in-cylinder temperature increases slightly because of the low-temperature heat release of the HCCI stage, and then the combustion delay period of CIDI DME decreases; therefore, the peak heat release rate of diffusion combustion decreases significantly. As the peak in-cylinder temperature decreases, the UHC emission increases, and then the exhaust particle number concentration increases significantly. However, with the further increase of the premixed ratio, the increase in exhaust particle number concentration slows down. This result may be due to the increased in-cylinder temperature and enhanced UHC oxidation.8 4. Conclusions (1) The CCCI exhibits a three-stage combustion consisting of the cool flame region, the high-temperature HCCI combustion (30) Li, D. G.; Huang, Z.; Qiao, X. Q.; et al. Trans. CSICE 2005, 23 (2), 193–198 (in Chinese). (31) Ma, J. J.; Lu¨, X. C.; Ji, L. B.; Huang, Z. J. Therm. Sci., doi: 10.1016/ j.ijthermalsci.2007.10.007. (32) Vaaraslahti, K.; Virtanen, A.; et al. EnViron. Sci. Technol. 2004, 38, 4884–4890. (33) Zhai, H. B.; Li, L. G.; et al. Trans. CSICE 2007, 25, 66–72 (in Chinese). (34) Rusyniak, M.; Abdelsayed, V.; Campbell, J.; et al. J. Phys. Chem. 2001, 105, 11866–11872. (35) Kwon, S. B.; Lee, K. W.; Saito, K.; Shnozaki, O.; Seto, T. EnViron. Sci. Technol. 2003, 37, 1794–1802.

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region, and the third diffusive combustion. With the increase of rp, the maximum heat release rate of the HCCI combustion phase is found to increase gradually, the maximum heat release rate of the CIDI combustion phase decreases gradually, the boundary of the premixed combustion and diffusive combustion of the CIDI combustion phase becomes unremarkable, and the ignition timing of the low-temperature cool flame is advanced. (2) Under test engine conditions, the exhaust particle size distributions in all cases show similar unimodal structure (peaking at 20-25 nm in CIDI cases and 25-30 nm in CCCI and HCCI cases) with a nucleation mode. The number in the accumulation mode is very low. (3) In comparison to CIDI, the total number for HCCI is significantly elevated (at 1100 and 1500 rpm, 20% load, the total number higher at 32.6 and 37.3%, respectively) and the geometric mean diameters of exhaust particles increase 4-7 nm at same engine conditions (1100 and 1500 rpm, 20% of full load). The total number for CCCI is slightly less than those of HCCI cases but is higher at 20.0-29.3% than those of CIDI cases at the same engine conditions (1100 rpm, 20% load, rp ) 0.18-0.85). In addition, the GMD of CCCI is in the same range with those of HCCI cases, which are obviously bigger at 4-7 nm than those of CIDI cases. For these new combustion modes (HCCI and CCCI), their individual application cannot reduce the total number of particles; the UHC reduction aftertreatment device might be an effective complement to them for particle reduction. (4) For CCCI, the total number increases with the increase of the premixing ratio at the same loads but decreases with the load in all constant rp cases. In addition, the increase of the total number can obviously be found to be slow after the premixing ratio g 0.2 (1100 rpm and 20% load). Acknowledgment. The research is sponsored by the Research Fund for the Doctoral Program of Higher Education (20070248024 and 20050248013).

Nomenclature NSD ) size distributions of the number concentration DME ) dimethyl ether CIDI ) compression ignition direct injection HCCI ) homogeneous charge compression ignition CCCI ) compound charge compression ignition combustion rp ) premixed ratio SMPS ) scanning mobility particle sizer CPC ) condensation particle counter DR ) dilution ratio HRR ) heat release rate EC ) elementary carbon UHC ) unburned hydrocarbon GMD ) geometrical mean diameter EF800224F