Experimental Study on the Combustion Characteristics and Emissions

Sep 18, 2007 - ... Compression Ignition Engines with Premixed Dimethoxymethane ... and densities of dimethoxymethane+aliphatic alcohols (C1–C4) at ...
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Energy & Fuels 2007, 21, 3144–3150

Experimental Study on the Combustion Characteristics and Emissions of Biodiesel Fueled Compression Ignition Engines with Premixed Dimethoxymethane Xingcai Lu,* Junjun Ma, Libin Ji, and Zhen Huang School of Mechanical & Power Engineering, Shanghai Jiaotong UniVersity, Shanghai, People’s Republic of China ReceiVed February 25, 2007. ReVised Manuscript ReceiVed July 16, 2007

In the present study, the combustion characteristics and emissions of biodiesel fueled direct injection engines with premixed dimethoxymethane (DMM) by port injection were investigated on a single-cylinder diesel engine. The experimental results show that, for a fixed equivalence ratio of biodiesel, the ignition timing and the center point of heat release advance, while the end point of heat release delays with the introduction of premixed DMM. For a constant equivalence ratio of premixed DMM, the ignition timing delays and the end point of heat release advances with the decrease of biodiesel quantity. Furthermore, the maximum gas pressure, the mean gas temperature, and the maximum pressure rise rate increase smoothly with the increase of premixed ratio. Meanwhile, when the premixed ratio reaches to a certain value, the maximum gas pressure and pressure rising rate increase remarkably. Regarding to the regulated emissions, CO and HC emissions are much higher than that of neat biodiesel fuel and increase up to the largest levels at a certain premixed ratio. After that, both the CO and HC emissions begin to decrease substantially. Smoke opacity decreases up to 35–55% at different equivalence ratios with the premixed DMM. At last, it is very interesting to find that the NOx emission decreases about 35% under overall operating conditions with up to 20% premixed DMM.

1. Introduction Biodiesel is an oxygenated, nontoxic, sulphur-free, biodegradable, and renewable alternative fuel. It can be used in diesel engines which need only minor modifications. Recently, a great deal of research has been conducted on the performance, combustion characteristics, and emissions (regulated and unregulated emissions) when it was used as an alternative fuel in direct/indirect-injection compression ignition engines.1–8 The major advantages of using biodiesel instead of diesel fuel in a conventional diesel engine are that it substantially reduces * Corresponding author. Tel.: +86-21-34206039; Fax: +86-21-34205553. E-mail address: [email protected]. (1) Rakopoulos, C. D.; Antonopoulos, K. A.; Rakopoulos, D. C.; Hountalas, D. T.; Giakoumis, E. G. Comparative performance and emissions study of a direct injection Diesel engine using blends of Diesel fuel with vegetable oils or bio-diesels of various origins. Energy ConVers. Manage. 2006, 47 (18–19), 3272–3287. (2) Labeckas, G.; Slavinskas, S. The effect of rapeseed oil methyl ester on direct injection Diesel engine performance and exhaust emissions. Energy ConVers. Manage. 2006, 47 (13–14), 1954–1967. (3) Shi, X. Y.; Pang, X. B.; Mu, Y. J.; He, H.; Shuai, S. J.; Wang, J. X.; Chen, H.; Li, R. L. Emission reduction potential of using ethanol-biodieseldiesel fuel blend on a heavy-duty diesel engine. Atmos. EnViron. 2006, 40 (14), 2567–2574. (4) Knothe, G.; Sharp, C. A.; Ryan, T. W. Exhaust emissions of biodiesel, petrodiesel, neat methyl esters, and alkanes in a new technology engine. Energy Fuels 2006, 20 (1), 403–408. (5) Lee, C. S.; Park, S. W.; Kwon, S. An experimental study on the atomization and combustion characteristics of biodiesel-blended fuels. Energy Fuels 2005, 19 (5), 2201–2208. (6) Sharp, C. A.; Howell, S. A.; Jobe, J. The Effect of Biodiesel Fuels on Transient Emissions from Modern Diesel Engines, Part I Regulated Emissions and Performance; SAE 2000-01-1967, Society of Automotive Engineers: Warrendale, PA, 2000. (7) Sharp, C. A.; Howell, S. A.; Jobe, J. The Effect of Biodiesel Fuels on Transient Emissions from Modern Diesel Engines, Part II Unregulated Emissions and Chemical Characterization; SAE 2000-01-1968,Society of Automotive Engineers: Warrendale, PA, 2000.

emissions such as carbon dioxide (CO2), volatile organic compounds (VOCs), unburned hydrocarbons (UHC), carbon monoxide (CO), sulfur oxides (SOx), polycyclic aromatic hydrocarbons, nitrated polycyclic aromatic hydrocarbons, and particulate matter (PM), while maintaining the same efficiency level as that of the conventional diesel fuel. However, neat biodiesel engines increase nitrogen oxides (NOx) emission by about 10–13%, mostly NO and NO2, which are considered as ozone hazardous compounds. To meet the severe emission standard, such as EURO 5 and Tier 2 of the USA for light- and heavy-duty diesel engines, it is necessary to reduce the NOx emissions of biodiesel fueled engines significantly. It is well-known that exhaust gas recirculation (EGR) is an effective strategy to depress the NOx emissions because it lowers the flame temperature and the oxygen concentration in the combustion chamber.9,10 A higher EGR level may be used to reduce the NOx emission of biodiesel engines due to the oxygen content in fuel. Then, the EGR method was used by Agarwal et al.11 with biodiesel blends, resulting in reduction in NOx emissions without any significant penalty in PM emissions or brake specific energy consumption. Nabi et al.12 also reported that the EGR strategy was used to control the NOx emission of biodiesel engines. Hess et al.13 (8) Armas, O.; Hernández, J. J.; Cárdenas, M. D. Reduction of diesel smoke opacity from vegetable oil methyl esters during transient operation. Fuel 2006, 85 (17–18), 2427–2438. (9) Lapuerta, M.; Hernandez, J.; Gimenez, F. Evaluation of exhaust gas recirculation as a technique for reducing Diesel engine NOX emissions. Proc. Inst. Mech. Eng., Part D, J. Automobile Eng. 2000, 214, 85–93. (10) Zheng, M.; Graham, T.; Reader, J.; Gary, H. Diesel engine exhaust gas recirculation-a review on advanced and novel concepts. Energy ConVers. Manage. 2004, 45, 883–900. (11) Agarwal, D.; Sinha, S.; Agarwal, A. K. Experimental investigation of control of NOx emissions in biodiesel-fueled compression ignition engine. Renewable Energy 2006, 31 (14), 2356–2369.

10.1021/ef070099w CCC: $37.00  2007 American Chemical Society Published on Web 09/18/2007

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Figure 1. Schematic of experimental system.

proposed an interesting method to control the NOx formation during the combustion process. Several antioxidants, which are capable of terminating some kinds of radical reactions, were added to a 20% soy biodiesel/80% diesel blends (B20) at a concentration of 1000 ppm. The results showed that the addition of butylated hydroxyanisole or butylated hydroxytoluene reduced NOx emissions at a moderate level. The National Renewable Energy Laboratory14 added cetane number improvers including di-tert-butyl peroxide (DTBP) and 2-ethylhexyl nitrate (2-EHN) to biodiesel. Due to the shorter ignition delay and decrease of the premixed burn, the maximum gas pressure and the peak temperature were depressed, which leads to a lower NOx emission. According to the above-mentioned studies, NOx emission of biodiesel engines can be slightly depressed by many complicated strategies, but it still can not meet the future emission standard. Recently, a new combustion mode, named homogenous charge compression ignition (HCCI) combustion, has been widely researched worldwide.15–17 It features a leaner and homogenous fuel/air mixture, compression auto-ignition, and low-temperature combustion. Consequently, it shows substantial reduction in both NOx and PM, while still providing diesel-like efficiency. However, some challenges include the control of ignition timing and combustion rate associated with the successful operation of HCCI engines. The conventional direct injection compressions engine, which is dominated by the diffusion burn of the (12) Nabi, Md. N.; Akhter, Md. S.; Shahadat, M. Md. Z. Improvement of engine emissions with conventional diesel fuel and diesel-biodiesel blends. Bioresour. Technol. 2006, 97 (3), 372–378. (13) Hess, M.A.; Haas, M. J.; Foglia, T. A.; Marmer, W. N. Effect of antioxidant addition on NOx emissions from biodiesel. Energy Fuels 2005, 19, 1749–1754. (14) McCormick, R. L.; Alvarez, J. R.; Graboski, M. S. NOx Solutions for Biodiesel; NREL/SR-510-31465, National Renewable Energy Laboratory: Washington, D.C., 2003. (15) Sjober, M.; Dec, J.; Cernansky, N. P. Potential of Thermal stratification and combustion retard for reducing pressure-rise rates in HCCI engines, based on multi-zone modeling and experiments; SAE 200501-0113, Society of Authomotive Engineers: Warrendale, PA, 2005. (16) Kim, M. Y.; Kim, J. W.; Lee, C. S. Effect of compression ratio and spray injection angle on HCCI combustion in a small DI diesel engine. Energy Fuels 2006, 20 (1), 69–76. (17) Lu, X. C; Ji, L. B.; Zu, L. L.; Hou, Y. C.; Huang, C.; Huang, Z. Experimental study and chemical analysis of n-heptane homogeneous charge compression ignition combustion with port injection of reaction inhibitors. Combust. Flame 2007, 149, 261–270.

inhomogeneous fuel/air mixture, is a very efficient power source but is hampered with the constraint of a trade-off between NOx emissions and PM emissions. Consequently, the direct injection combustion mode can be cause the lower PM but higher NOx emissions of biodiesel engines. Dimethoxymethane (DMM) is an oxygenated additive with a lower boiling point (42 °C), lower cetane number, and lower auto-ignition temperature. Supplemented with lower surface tension and lower dynamic viscosity, DMM can be used as a diesel fuel additive to reform the physical-chemical properties, improve the atomization and spray characteristics, and promote the fuel/air mixture formation in the cylinder. As a result, the combustion characteristics and emission levels may be significantly improved.18 In this paper, according to the idea of HCCI combustion, DMM was injected into the intake pipe by a gasoline injector. And then, a leaner homogenous DMM fuel/air mixture can be formed during the intake stroke and compression stroke. Near the top dead center (TDC), the biodiesel fuel, which was used to ignite the overall stratified fuel/air mixture, was directly injected into the cylinder. By using this dual fuel combustion system, the author hopes to improve the smoke opacity and NOx emission simultaneously for biodiesel fueled engines. In the paper, experimental results on the effect of the equivalence ratio and premixed ratio of fuel on the characteristics and exhaust emissions are provided, and the optimum premixed ratio with the lowest NOx and smoke opacity is determined. 2. Experimental System A single cylinder, four-stroke, natural aspirated diesel engine was employed in the test, and the engine speed was fixed at 1800 rpm. The engine was coupled to an electrical eddy dynamometer through which load was applied by increasing the field voltage. The detailed engine specifications are shown in Table 1, the experimental system is shown in Figure 1. DMM was injected into the intake pipe by an electronic fuel injector at the location of approximately 0.35 m upstream to the inlet port. The injection timing and fuel quantity are controlled by an electronic control unit (ECU). The soy-based biodiesel was (18) Lu, X. C.; Yang, J. G.; Zhang, W. G.; Huang, Z. Improving the combustion and emissions of direct injection compression ignition engines using oxygenated fuel additives combined with cetane number improver. Energy Fuels 2005, 18 (5), 1879–1888.

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Table 1. Specifications of a Single-Cylinder Engine bore × stroke combustion chamber inlet valve open inlet valve close injector open pressure (MPa)

98 × 105 ω 16 °CA BTDC 52 °CA ABDC 24

compression ratio advanced angle of injector open exhaust valve open exhaust valve close nozzle number × orifice diameter (mm)

Table 2. Properties of DMM and Soy-Based Biodiesel Fuel chemical formula density (g/mL) boiling point (deg C) lower heating value (MJ/kg) cetane number viscosity (mm2/s) oxygen content % a

DMM

biodiesel

CH3OCH2OCH3 0.860 42 22.4 3019 0.34 44.1

0.875 >320a 38.5 49.920 5.78 11

This is the end point in the distillation curve.20

directly injected into the combustion chamber by the original injection system. Table 2 gives the basic physical and chemical properties of DMM and biodiesel fuel. The aim of this work is to clarify the effect of premixed DMM on the combustion characteristics and emissions of the biodiesel engines. For this purpose, the experimental investigations were carried out under various premixed DMM quantities with the same direct injection (DI) biodiesel fuel and various DI biodiesel fuel quantities with the same premixed DMM from the intake port. To ensure the repeatability and comparability of the measurements for different operating conditions, the coolant-out temperature remained at 85 °C, held to within (2 °C, while the oil temperature was kept at 90–95 °C. The engine speed was 1800 rpm and was held to within (2 rpm. The cylinder pressure was measured using a Kistler model 6125A pressure transducer. The charge output from this transducer was converted to an amplified voltage using a Kistler model 5015 amplifier. The 1440 pulses per rotation (4 pulses per crank angle (CA)) from a shaft encoder on the engine crankshaft were used as the data acquisition clocking pulses to acquire the cylinder pressure data. Pressure data was recorded using a high-speed data acquisition apparatus (Yokogawa: GP-IB). CO, HC, and NOx emissions were measured by AVL Di-gas 4000 gas analyzer. Smoke opacity was measured by AVL 439 smoke meter. In each operating condition, the cylinder pressures recorded at each crank angle were averaged over 50 consecutive cycles for the experiment. For all data presented, the 0 °CA was defined as the top dead center (TDC) at the compression stroke. According to the averaged in-cylinder gas pressure, the heat release curve at each operating point could be calculated by the zero-dimension combustion model. The heat release rate, dx/dφ, could be calculated by the following formula:21 dx ⁄ dφ ) p

Cv dp dCv dQw Cp dV +V + mT + R dφ R dφ dφ dφ dQw ) hcA(T - Tw) dφ

(1) (2)

where, p is cylinder gas pressure, Cp is the constant pressure specific heat, CV is the constant volume specific heat, R is the gas constant, hc is the heat transfer coefficient, A is wall area, and Tw is wall temperature. During the experimental process, the air flow rate and fuel consumption rate of each test fuel were measured. Consequently, the overall equivalence ratio (φ), the partial equivalence ratio of premixed DMM (φdmm), and the partial equivalence ratio of biodiesel (φBD) can be obtained using the following formula: φdmm ) (Gdmm × AFdmm) ⁄ Gair

(3)

φBD ) (GBD × AFBD) ⁄ Gair

(4)

φ ) (Gdmm × AFdmm + GBD × AFBD) ⁄ Gair

(5)

18.5 9 °CA BTDC 66 °CA BBDC 12 °CA ATDC 5 × 0.24

where, Gair is the mass flow rate of intake air at each test point, Gdmm and GBD are the fuel consumption rates of DMM and biodiesel, and AFdmm and AFBD denote the stoichiometric A/F ratio of DMM and biodiesel, respectively.

3. Experimental Results and Discussion In this paper, the premixed ratio (PI) is defined as the ratio of cycle energy of premixed fuel, DMM in this paper, to total energy which includs premixed fuel and direct injected fuel. The PI can be calculated using the following formula. PI )

Gdmm × Hudmm × 100% GBD × HuBD + Gdmm × Hudmm

(6)

where, Hudmm and HuBD are the lower heating values of DMM and biodiesel. When PI is equal to 0, there is no premixed DMM and the engine is running with neat biodiesel. In the following analysis, the ignition timing θ1 is defined as the crank angle where the fired in-cylinder pressure separates from the motored pressure curve. θ2 is defined as the center position of the heat release curve, called CA50; θ3 is the crank angle of 90% of the accumulated heat release (CA90). The premixed burn duration is defined as the interval angle between θ2 and θ1, and the confusion burn duration is defined as the interval angle between θ3 and θ2. Figure 2 shows the effects of the premixed ratio (PI) of DMM on the in-cylinder gas pressure, heat release rate (HRR), and the mean gas temperature at a constant overall equivalence ratio. With the premixed DMM, some common characteristics can be found in these figures: (1) The maximum gas pressure increases, and its corresponding crankangle advances slowly with the lower premixed ratio. Meanwhile, the maximum gas pressure increases remarkably when the premixed ratio exceeds to a certain point. (2) The premixed ratio has a moderate effect on the maximum value of the HRR When the PI value is lower than the critical point, which is dependent on the overall equivalence ratio. (3) The maximum mean gas temperature increases smoothly with the increase of the PI. Moreover, it should be noted that there exhibits another trend of the ignition timing versus PI for different overall equivalence ratios. At a leaner fuel/air mixture, premixed DMM has a moderate effect on the ignition timing. For a larger overall equivalence ratio, the ignition timing advances clearly with the increase of the premixed ratio of DMM. Since the decrease of cetane number and increase of the oxygen content for DMM/diesel blends, the engine tests found that the ignition timing delays and the burn rate increases compared to the (19) Ren, Y.; Huang, Z. H.; Jiang, D. M.; Liu, L. X.; Zeng, K.; Liu, B.; Wang, X. B. Engine performance and emission characteristics of a compression ignition engine fuelled with diesel/dimethoxymethane blends. Proc. Inst. Mech. Eng., Part D, J. Automobile Eng. 2005, 219 (7), 905– 914. (20) Cheng, A. S.; Upatnieks, A.; Mueller, C. J. Investigation of the Impact of biodiesel fuelling on NOx emissions using an optical direct injection diesel engine. Int. J. Engine Res. 2006, 7 (4), 297–318. (21) Heywood, J. B. Internal combustion engine fundamentals; McGrawHill Book Company: New York, 1988.

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Figure 2. Effects of PI on combustion characteristics at a fixed overall equivalence ratio.

Figure 3. Comparison of the ignition timing, θ2, and θ3 for various premixed ratios at different values of φBD.

neat diesel fuel.22 Song et al.23 conducted an experimental test on a rapid compression machine by the visualization method. The results also confirmed that the DMM addition will delay the ignition timing of the blend fuel. Meanwhile, Fang et al.24 carried out an experimental test on a four-stroke, two-cylinder diesel engine with port injection of DMM for simultaneous reduction of smoke and NOx emissions. The results found that the ignition timing advances obviously with the increase of the premixed DMM at a constant engine load. The low-temperature reaction of the DMM/air mixture was attributed to this phenomenon. During the following section, (22) Huang, Z. H.; Ren, Y.; Jiang, D. M. Combustion and emission characteristics of a compression ignition engine fuelled with Dieseldimethoxy methane blends. Energy ConVers. Manage. 2006, 47 (11–12), 1402–1415. (23) Song, K. H.; Litzinger, T. A. Effects of dimethoxymethane blending into diesel fuel on soot in an optically accessible DI diesel engine. Combust. Sci. Technol. 2006, 178 (12), 2249–2280. (24) Fang, J.; Huang, Z.; Qiao, X.; Song, J. Study on a pre-mixed charge compression ignition engine by dimethoxymethane port injection. Proc. Inst. Mech. Eng., Part D, J. Automobile Eng. 2004, 218 (5), 549–555.

the author will discuss the effects of PI on combustion characteristics at a fixed partial equivalence ratio of DMM and partial equivalence ratio of biodiesel. Figure 3 displays the comparison of ignition timing, the center point of HRR (θ2), and the end point of HRR (θ3) for various premixed ratios at different partial equivalence ratios of biodiesel fuel. Some phenomena can be found from these figures such as the following. (1) With the increase of PI, both the ignition timing and θ2 advance with the same magnitude. This means that the premixed DMM by port injection has little effect on the premixed burn duration of the overall combustion event. (2) The end point of HRR delays and the diffusion burn duration prolongs substantially with the introduction of DMM from the intake port. (3) Both the premixed burn duration and diffusion burn duration are prolonged obviously with the increase of the equivalence ratio of biodiesel fuel.

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Figure 4. Ignition timing, θ2, and θ3 for various premixed ratios at constant φdmm.

Figure 5. Some performance parameters as a function of PI at different values of φ.

For a fixed partial equivalence ratio of biodiesel, the engine power increases with the DMM addition from the intake port. Then, the combustion phasing may be advanced due to the increase of the in-cylinder temperature. In addition, the cool flame of the DMM/air mixture may be triggered before the main combustion event occurred by the higher temperature of the in-cylinder residual gas. Consequently, the heat release and the active radicals which were produced in the cool flame reaction will promote the overall ignition and combustion. Disregarding the premixed ratio of the DMM, the overall combustion event was dominated by the diffusion burn mode. As a result, the combustion duration is prolonged with the increase of the premixed ratio and the overall equivalence ratio. Figure 4 illustrates the effects of PI on ignition timing and combustion rate at a constant partial equivalence ratio of DMM while varying the biodiesel quantity. In the figure, it should be noted that the increasing of PI means the decrease of the biodiesel quantity. It can be seen from the figure that the ignition timing and center point of HRR advance smoothly, but the end point of HRR delays substantially with the decrease of the PI for a given φdmm. According to Figures 3 and 4, both the DMM and biodiesel have the potential to promote the ignition. Meanwhile, for a fixed overall equivalence ratio, the increase of DMM quantity necesitates a decrease of the biodiesel quantity. As a result, an unobvious relationship between the PI and ignition timing is observed at a constant overall equivalence ratio. Figure 5 gives the maximum pressure rise rate, the maximum in-cylinder gas pressure and temperature, and indicated mean effective pressure (IMEP) versus PI at different overall equivalence ratios. From Figure 5a, it can be found that the maximum pressure rise rate increases with the increase of PI in different overall equivalence ratios. In particular, the maximum pressure

rise rate increases substantially once the PI exceeds a critical value, which depends on the φdmm. In general, the critical value reduces with an increase of the φ value. The relationship between the maximum in-cylinder gas pressure with the PI for different φ is show in Figure 5b. Regardless of the effect of PI on ignition timing, the increase of PI always leads to the increase of the maximum gas pressure. Especially, a larger PI will lead to “knock combustion” at richer fuel/air mixtures. In Figure 5c, the maximum mean gas temperature almost increases linearly with the increase of PI for different equivalence ratios. In Figure 5d, the IMEP increases linearly with the introduction of premixed DMM for a constant overall equivalence ratio. In general, with the increase of the PI, the overall HRR curve concentrated on the top dead center, which lead to less heat loss. As a result, the combustion efficiency and indicated thermal efficiency improve obviously with the introduction of premixed DMM. Figure 6 shows the regulated emissions versus PI in various overall equivalence ratios. It is interesting to note the trend observed in CO as a function of the premixed ratio, as shown in Figure 6a. CO increases at first with the premixed ratio amount up to a critical point. As the premixed ratio exceeds the critical value, CO emissions start to decrease obviously with further increases in the PI. Furthermore, the critical PI value which corresponds to the largest CO emission decreases with the increase of the overall equivalence ratio. For example, at an overall equivalence ratio of 0.39, the largest CO emission is observed at a PI of 37.5%. Meanwhile at an overall equivalence ratio of 0.68, the largest CO emission is observed at a PI of 19.5%. At last, it can be found that the CO increase magnitude drops remarkably with the increase of the overall equivalence ratio.

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Figure 6. Effect of a premixed ratio of DMM on regulated emissions.

Figure 7. Relationship between the NOx emissions and smoke opacity.

The HC emission increases with premixed fuel and did not exhibit the trend observed in CO previously discussed, as can be seen in Figure 6b. Also, HC emission deteriorates with the introduction of premixed DMM and reaches the largest value at a certain PI value which depends on the overall equivalence ratio. After that, HC emission improves slightly with the further increases of PI. In addition, the largest HC emission is nearly the same level for different overall equivalence ratios. For a DMM/biodiesel dual fuel combustion system, it is very likely that premixed fuel is trapped in the crevices of piston ring and dead zone of the combustion chamber during the compression stroke and that this fuel is out-gassed in the cycle, when the temperatures are too low for reaction. This is similar to the situation encountered in spark ignition engines. As a result, the CO and HC emissions are much higher than that of neat biodiesel engines. On the other hand, with the advancing of the ignition timing and the increasing of the maximum gas temperature, CO and HC emissions are further oxidized. On this basis, CO and HC emissions begin to drop with the larger PI value. It is well-known that PM or smoke opacity of biodiesel fueled engines is substantially improved compared to conventional diesel fueled engines. Compared to the neat biodiesel, it can be found from Figure 6c that the smoke opacity decreases slightly at a lower overall equivalence ratio but shows significant decrease at a larger overall equivalence ratio with the increase of PI. For example, when the overall equivalence ratio is held at 0.6, the smoke opacity improves up to 35% with a PI of 28%. Also, at a fixed overall equivalence ratio of 0.68, the smoke opacity improves up to 55% with a PI of 27%. The significant decrease of the smoke opacity of a DMM/biodiesel engine can be attributed to the following reasons. This dual

combustion system, in itself, of premixed charge should produce the lower soot emission. Moreover, the oxygen content in DMM is about 44.1%, which is much larger than that of biodiesel; this will promote the soot oxidation in the overall combustion cycle. In actuality, the mechanism of soot formation and oxidation in dual fuel combustion engines are greatly complicated and influenced by very many factors and are not completely understood. Accordingly, further research is required to investigate the detailed correlation between soot results and influential factors through the optical diagnostics.25 From Figure 6d, it can be found that the relationship between the NOx emission and PI shows a large difference in the trend observed in other emissions previously discussed. This figure shows significant NOx reductions at overall equivalence ratios with up to a 20% premixed ratio of DMM. After that, NOx emission levels begin to increase slightly compared to the lowest level. At larger overall equivalence ratios, for example 0.6 and 0.68, the NOx level decreases about 33–35%. In general, NOx emission is found to be strongly dependent upon the combustion temperature. From Figure 5c, it can be found that the maximum gas temperature increase with the premixed DMM addition. At lower PI values, a part of the homogenous fuel/air mixture will promote to the decrease of local high temperature. In addition, the maximum gas temperature and combustion duration show little difference with that of neat biodiesel. As a result, NOx emission is suppressed at lower PI values. At higher premixed ratios under the larger overall equivalence ratio, the NOx concentrations become larger than the lowest level. Increased NOx emissions for larger premixed ratios may be considered to the advanced combustion event prior to TDC and longer combustion duration. Figure 7 gives the relationship of NOx and smoke opacity for biodiesel a fueled engine with port injection of DMM. It is very interesting to find that, at PI values from 15% to 20%, both the NOx and smoke opacity decrease substantially. 4. Conclusions (1) For a fixed biodiesel quantity, the ignition phasing and the center point of HRR advance and the end point of HRR delays with the introduction of DMM by port injection. For a fixed DMM quantity, the ignition timing delays with the (25) Kim, D. S.; Lee, C. S. Improve emission characteristics of HCCI engine by various premixed fuels and cooled EGR. Fuel 2006, 85, 695– 704.

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decrease of the biodiesel quantity. As a result, when the overall equivalence ratio is kept constant, the premixed ratio of DMM has a moderate effect on ignition timing at a lower overall equivalence ratio but plays an important role in ignition timing at a larger overall equivalence ratio. (2) At a fixed overall equivalence ratio, the maximum gas pressure, the maximum pressure rise rate, and the maximum mean gas temperature increase with the increase of the premixed DMM. In particular, knock combustion may be observed when the PI exceeds a certain value. (3) With the introduction of premixed DMM by port injection, on the base of neat biodiesel, the smoke opacity further decreases by about 35–55% and NOx emission decreases 33–35%.

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HC and CO emissions increase up to the largest levels and then begin to decrease remarkably with the port injection of DMM fuel. (4) For biodiesel fueled direct injection engines, both of the engine performance and regular emissions can be improved with the introduction of DMM by port injection, while the optimum premix ratio of DMM is only about 15–20%. Acknowledgment. This work was supported by the Natural Science Foundation of Shanghai (Grant No. 06ZR14045). Also, this work was financially supported by Young Scholar Foundation of Shanghai Jiaotong University (Grant No. 05DBX003). EF070099W