Emissions of Formaldehyde and Unburned Methanol from a Spark

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Energy Fuels 2010, 24, 863–870 Published on Web 11/18/2009

: DOI:10.1021/ef9009982

Emissions of Formaldehyde and Unburned Methanol from a Spark-Ignition Methanol Engine during Cold Start Jun Li,†,‡ Changming Gong,*,† Enyu Wang,‡ Xiumin Yu,† Zhong Wang,§ and Xunjun Liu† † State Key Laboratory of Automobile Dynamic Simulation, Jilin University, Changchun 130022, China, ‡Research and Development Center, China First Automobile Works Group Corporation, Changchun 130011, China, and §United Automotive Electronic Systems Company, Limited, Shanghai 201206, China

Received September 8, 2009. Revised Manuscript Received October 30, 2009

The effects of the methanol injection quantity per cycle, the ignition timing, the methanol injection timing, the additional liquefied petroleum gas (LPG) injected into the inlet port, and the LPG injection timing delay relative to the methanol injection timing on the formaldehyde and the unburned methanol emissions from an electronically controlled inlet port methanol injection spark-ignition (SI) engine during cold start were investigated using a single-cycle fuel injection strategy. The results showed that the methanol injection quantity per cycle, the ignition timing, the methanol injection timing, the mass ratio of injected LPG/ methanol, and the LPG injection timing delay relative to the methanol injection timing affect the formaldehyde and the unburned methanol emissions significantly. Optimal control of the methanol injection quantity per cycle, the ignition timing, the methanol injection timing, and the LPG injection timing delay relative to the methanol injection timing improves firing performances and reduces the unburned methanol emission. As the mass ratio of injected LPG/methanol increases, the formaldehyde emission increases and the unburned methanol emission falls. The variations in emitted formaldehyde and unburned methanol show opposite tendencies with the variations in the methanol injection quantity per cycle, the ignition timing, the methanol injection timing, the mass ratio of injected LPG/methanol, and the LPG injection timing delay relative to the methanol injection timing.

The environmental concern of global warming and climate change has greatly increased the interests in the application study of renewable fuels to internal combustion engines. The sharply rising petroleum price on the markets worldwide has also boosted the studies and applications of renewable fuels in the area.1 Methanol (CH3OH) is considered to be one of the favorable fuels for engines. It can be produced from the widely available fossil raw materials, including coal, natural gas, and biosubstances. The methanol derived from the biological sources represents a kind of renewable energy source.2,3 Combustion of various fossil fuels leads to emission of several pollutants, which are categorized as regulated and unregulated pollutants. The former are limited by emission standards [such as United States Environmental Protection Agency (U.S. EPA), EURO, etc.], and the latter are those without legislative limitations yet. The regulated pollutants include nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbon (HC), and particulate matter (PM), and unregulated pollutants include aldehydes (RCHO), benzene, toluene, xylene

(BTX), aldehydes, sulfur dioxide (SO2), etc.4-6 These regulated as well as unregulated pollutants contribute to several harmful effects on human health, which are further classified as short- and long-term health effects. The short-term health effects are caused by CO, NOx, PM (primarily regulated pollutants), etc., while long-term health effects are caused mainly by polyaromatic hydrocarbons (PAHs), BTX, formaldehyde (primarily unregulated pollutants), etc.7,8 The organic emissions (ozone precursors) from methanol combustion will have lower reactivity, hence lower ozone-forming potential than gasoline fuels. If pure methanol is used, then the emission of benzene and PAHs is very low.9 However, more toxic formaldehyde and unburned methanol emissions are emitted from a spark-ignition (SI) methanol-fueled engine.10,11 The use of methanol will be extremely advantageous if two of the major problems, namely, cold start and formaldehyde emissions, can be overcome.12 Formaldehyde has adverse effects on the environment because of its smell, and it may have carcinogenic effects. Furthermore, once emitted, the lifetime of formaldehyde in the atmosphere is considerable, of the order of magnitude of hours or even days. It is a very active compound in the tropospheric chemistry, participating in

*To whom correspondence should be addressed: State Key Laboratory of Automobile Dynamic Simulation, Jilin University, Changchun 130022, China. E-mail: [email protected]. (1) Huang, J. C.; Wang, Y. D.; Li, S. D.; Roskilly, A. P.; Yu, H. D.; Li, H. F. Appl. Therm. Eng. 2009, 29, 2484–2490. (2) Lin, T. C.; Chao, M. R. Sci. Total Environ. 2002, 284, 61–74. (3) Heinrich, W.; Marquardt, K. J.; Schaefer, A. J. SAE Tech. Pap. 861581, 1986. (4) Guo, H.; Wang, T.; Blake, D. R.; Simpson, I. J.; Kwok, Y. H.; Li, Y. S. Atmos. Environ. 2006, 40, 2345–59. (5) Ghose, M. K.; Paul, R.; Banerjee, S. K. Environ. Sci. Policy 2004, 7, 345–351.

(6) Ghose, M. K. Dig. Energy Environ. 2002, 2, 273–282. (7) Hosseinpoor, A. R.; Forouzanfar, M. H.; Yunesian, M.; Asghari, F.; Naieni, K. H.; Farhood, D. Environ. Res. 2005, 99, 126–131. (8) Colvile, R. N.; Hutchinson, E. J.; Mindell, J. S.; Warren, R. F. Atmos. Environ. 2001, 35, 1537–1565. (9) Agarwal, A. K. Prog. Energy Combust. Sci. 2007, 33, 233–271. (10) Zervas, E.; Montagne, X.; Lahaye, J. Environ. Sci. Technol. 2002, 36, 2414–2421. (11) Chao, H. R.; Lin, T. C.; Chao, M. R.; Chang, F. H.; Huang, C. I.; Chen, C. B. J. Hazard. Mater. 2000, 73, 39–54. (12) Bruetsch, R. I.; Hellman, K. H. SAE Tech. Pap. 920196, 1992.

1. Introduction

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chain-propagating reactions through photolysis and by the interaction with OH radicals, thereby contributing to photochemical smog.13 Formaldehyde may stimulate the eyes, throat, bronchus, etc. and cause nasopharyngeal cancer in humans. With the stringency of the HC emissions standard, a proportionally larger fraction of the total emissions from the Federal Test Procedure (FTP) is emitted during the cold-start portion of the first cycle. At the ultra-low emissions vehicle (ULEV) standard, 80-90% of the tailpipe HC emissions are emitted during the first test cycle of the FTP.14 The proportion of HC emissions emitted during the cold start is expected to increase further at the super ultra-low emissions vehicle (SULEV) standard. Tailpipe HC emissions during the cold start are high because the catalyst is not active enough to oxidize efficiently the HCs. Thus, tailpipe HC emissions during the cold start are essentially equal to engine-out HC emissions until the catalyst reaches its light-off temperature (typically around 300 °C). To shorten the time to obtain catalyst light-off, advanced engine control strategies are used, such as retarding the spark to increase the exhaust gas temperature.15 Chen et al.16 found that only 20% of gasoline evaporates under this condition, in reasonable agreement with equilibrium calculations that 10-20% of the fuel vaporizes during the first few cycles of a cold start.17,18 Therefore, 8-15 times the stoichiometric amount of gasoline is injected during the first several cycles of the cold-start and warm-up transient.19 Alkidas20 investigated the effects of injection timing, coolant temperature, and fuel volatility for steady-state operation. In agreement with Yang et al.,21 they found that the HC emissions are almost independent of injection timing for closed valve injection (CVI), unless the start of injection is ∼80° crank angle (CA) or later before intake valve opening (IVO). With opening valve injection (OVI) and, especially, with cold coolant, a portion of the liquid fuel enters the combustion chamber and impacts the in-cylinder surfaces. It also appears that, if the CVI starts at ∼80° CA or later before IVO, some liquid enters the combustion chamber. Rottenkolber et al.22 found cold-start HC emissions 25-80% higher for OVI versus CVI, depending upon injector targeting. Li et al.23 studied the characteristics of transient HC emissions of the first firing cycle during cold start on an liquefied petroleum gas (LPG) SI engine. They found that the first firing cycle is very important for cold start. Misfire of the first firing cycle can lead to significant HC emissions and affect the subsequent cycles. Gong et al.24 investigated the effect of fuel injection timing on combustion and emissions of a SI methanol and methanol/LPG engine during cold start. Their

research results showed that optimal control of the fuel injection timing and the LPG injection timing delay relative to the methanol injection timing improves firing performances and reduces the HC emissions. Wei et al.25 studied formaldehyde and methanol emission characteristics as well as the three-way catalytic converter (TWC) conversion efficiency of a SI engine when it ran on gasoline, M10, M20, and M85 (gasoline blended with 10, 20, and 85% of methanol in volume), respectively, for steady-state operation. Their experimental results showed that the HCHO emission increases with the engine speed, while the CH3OH emission from a methanol/gasoline blend-fueled engine decreases with it. HCHO emission from a gasoline-fueled engine varies in a “U” curve with the engine torque. The addition of 10% methanol in gasoline doubles the HCHO emission. The increasing methanol fraction greatly improves HCHO and CH3OH emissions; their concentrations are both approximately linear to the amount of cyclically supplied fuel methanol. All of the above works in the literature paid attention to the firing performances of the SI engine during cold start fueled with gasoline, LPG, methanol, methanol/gasoline blends, and ethanol/gasoline blends,26-30 the engine performances and formaldehyde emission for steady-state operation using methanol/gasoline blends, etc.31-34 However, investigation on formaldehyde and unburned methanol emission behavior of a SI methanol-fueled engine during cold start has not been reported by means of the cycle-by-cycle control strategy. The objective of this study was to investigated the effect of the methanol injection quantity per cycle, the ignition timing, the methanol injection timing, additional LPG injected into the inlet port, and the LPG injection timing delay relative to the methanol injection timing on the cold-start formaldehyde and unburned methanol emissions using a single-cycle fuel injection strategy. 2. Experimental Setup and Procedures 2.1. Test Engine. The experiment was conducted on a singlecylinder four-stroke electronically controlled methanol-fueled engine with inlet port fuel injection (PFI). The engine specifications are listed in Table 1. 2.2. Test Fuels. Test fuels for this study were methanol and LPG. Table 2 shows the physical and chemical properties of methanol and LPG. The components of LPG are shown in Table 3. 2.3. Experimental Setup and Procedures. The experimental system is shown in Figure 1. The in-cylinder pressure was measured using a Kistler 6125B quartz crystal pressure sensor matched with a WDF-3 charge amplifier. The instantaneous angular velocity of the crankshaft was determined by an optical shaft encoder with 0.5° CA resolution. A multi-channel data

(13) Glarborg, P.; Alzueta, M. U.; Kjaergaard, K.; Dan-Johanser, K. Combust. Flame 2003, 132, 629–638. (14) Takeda, K.; Yaegashi, T.; Tai, H. SAE Tech. Pap. 950074, 1995. (15) Gulati, S. T. SAE Tech. Pap. 1999-01-0269, 1999. (16) Chen, K. C.; Cheng, W. K.; Van Doren, J. M. SAE Tech. Pap. 961955, 1996. (17) Boyle, R. J.; Doam, D. J.; Finlay, I. C. SAE Tech. Pap. 930710, 1993. (18) Santoso, H.; Cheng, W. K. SAE Tech. Pap. 2002-01-2805, 2002. (19) Heywood, J. B. Air Pollution from Internal Combustion Engines; Academic Press: New York, 1988. (20) Alkidas, A. C. SAE Tech. Pap. 941959, 1994. (21) Yang, J.; Kaiser, E. W.; Siegl, W. O.; Anderson, R. W. SAE Tech. Pap. 930711, 1993. (22) Rottenkolber, G.; Dullenkopf, K.; Wittig, S.; Kolmel, A.; Feng, B.; Spicher, U. SAE Tech. Pap. 1999-01-3644, 1999. (23) Li, L. G.; Li, G.; Qiu, D. P.; Liu, Z. M. SAE Tech. Pap. 2006-013403, 2006. (24) Gong, C. M.; Yan, S. F.; Su, Y.; Wang, Z. W. Energy Fuels 2009, 23, 3536–3542.

(25) Wei, Y. J.; Liu, S. H.; Liu, F. J.; Liu, J.; Zhu, Z.; Li, G. L. Energy Fuels 2009, 23, 3313–3318. (26) Gong, C. M.; Deng, B. Q.; Wang, S.; Su, Y.; Gao, Q.; Liu, X. J. Energy Fuels 2008, 22, 2981–2985. (27) Hochul, K.; Cha, L. M.; Simsoo, P. Fuel 2007, 86, 1475–1482. (28) Gong, C. M.; Deng, B. Q.; Wang, S.; Su, Y.; Gao, Q.; Liu, X. J. Energy Fuels 2008, 22, 3779–3784. (29) Liao, S. Y.; Jiang, D. M.; Cheng, Q.; Huang, Z. H.; Zeng, K. Energy Fuels 2006, 20, 84–90. (30) Liao, S. Y.; Jiang, D. M.; Cheng, Q.; Huang, Z. H.; Wei, Q. Energy Fuels 2005, 19, 813–819. (31) Abu-Zaid, M.; Badran, O.; Yamin, J. Energy Fuels 2004, 18, 312– 315. (32) Bilgin, A.; Sezer, I. Energy Fuels 2008, 22, 2782–2788. (33) Huang, Z. H.; Pang, J. G.; Pan, K. Y.; Jiang, D. M.; Zhou, L. B.; Yang, Z. L. Proc. Inst. Mech. Eng., Part D 1998, 212 (5), 501–505. (34) Huang, Z. H.; Miao, H. Y.; Zhou, L. B.; Jiang, D. M. Proc. Inst. Mech. Eng., Part D 1999, 214 (3), 341–346.

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limits of this method were under 5 ppb in the solution. The sampling bag was used to collect exhaust gas. The layout of the gas sampling system is shown in Figure 2. When the ambient temperature was below 16 °C, a glow plug was used as an auxiliary start aid to heat the engine inlet manifold or additional LPG was injected into the inlet port of the methanol-fueled engine. The glow plug was fixed in the intake manifold plenum. To prevent the injected methanol from combusting on the glow plug, the surface of the glow plug was covered with a copper sleeve. After the glow plug was switched on for 3 min, the temperature of inlet manifold surface reached 42 °C. The methanol and LPG fuel injection systems were installed separately and worked independently. The methanol injection nozzle was mounted on the intake manifold. The injection pressure of methanol was 0.3 MPa. The LPG injection nozzle was mounted between the methanol injection nozzle and the intake valve. The gas-phase LPG was injected at a constant pressure of 0.14 MPa with a pressure regulator. The LPG only played the part of start aid in the methanol/LPG-fueled engine. The LPG and methanol injection quantities per cycle, the ignition timing, and the injection timing were controlled by an electronic control unit (ECU). During the cold-start test, through ECU control, an electric motor cranked the engine. The cycle in which the engine starts rotating was defined as the first cycle. The methanol and LPG were injected in the first

acquisition card PLC-8018HG was used to record the in-cylinder pressure and instantaneous angular velocity synchronously. The formaldehyde and unburned methanol were analyzed by gas chromatography (Shimadzu GC2010) and liquid chromatography (Waters 600E, Milford, MA). The gas chromatography uses a flame ionization detector (FID), and the liquid chromatography uses an ultraviolet detector. The detection Table 1. Engine Specifications bore (mm) stroke (mm) displacement (cm3) compression ratio maximum power (kW)/speed (rpm) maximum torque (N m)/speed (rpm) cooling system intake valve opening (IVO) intake valve closing (IVC) exhaust valve opening (EVO) exhaust valve closing (EVC)

52.4 57.8 125 10.55:1 6.5/7500 9/6000 air cooled 15° CA BTDC 35° CA ABDC 35° CA BBDC 15° CA ATDC

Table 2. Property Comparison of Methanol and LPG property

methanol

LPG

formula relative molecular mass composition (% m) C H O density (kg/m3) boiling point (°C) RON flammability limit (% v) latent heat of vaporization (kJ/kg) lower heating value (MJ/kg) auto-ignition temperature (°C) stoichiometric air/fuel ratio flame speed (m/s)

CH3OH 32

C3H8 þ C4H10 44 58

37.5 12.5 50.0 790 65 111 6.7-36 1110 19.6 470 6.5 0.523

82 18 0 508 -42 111 2.2-9.5 426 46.1 480 15.65 0.38

83 17 0 584 -0.5 103 1.9-8.5 385 45.5 440 15.43 0.37

Table 3. Mole Fractions of Main Components in LPG propane (%)

isobutane (%)

butane (%)

dimethyl-propylene (%)

butadiene (%)

49

21

15

8

5

Figure 2. Layout of the gas sampling system.

Figure 1. Schematic layout of the experimental system: (1) LPG tank, (2) gas valve, (3) solenoid valve, (4) LPG pressure regulator, (5) mercury manometer, (6) throttle sensor, (7) methanol pressure regulator, (8) methanol filter, (9) methanol injection nozzle, (10) LPG injection nozzle, (11) glow plug, (12) K-type thermocouple, (13) spark plug, (14) in-cylinder pressure transducer, (15) charge amplifier, (16) encoder, (17) TDC marker, (18) computer, (19) methanol pump, (20) methanol tank, and (21) ECU.

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Figure 3. Effect of Qm on the formaldehyde emission from the methanol-fueled engine during cold start using glow-plug preheating at t = 11 °C, θig = -20° CA ATDC, and θm = -35° CA ATDC.

Figure 4. Effect of Qm on the unburned methanol emission from the methanol-fueled engine during cold start using glow-plug preheating at t = 11 °C, θig = -20° CA ATDC, and θm = -35° CA ATDC.

cycle by means of a respective single-cycle fuel injection system. The engine was soaked in the room at least 8 h before each test. During the cold-start test, the throttle valve was locked at 10%, the atmospheric pressure was 100.66 kPa, the electric battery voltage was 12.05 V, the electric motor cranking speed was 770 rpm, and the ambient temperature was 11 °C. A 0° CA of injection timing of methanol and LPG corresponds to the piston position at the top dead center (TDC) of the first cycle compression stroke.

3. Results and Discussion 3.1. Effect of the Methanol Injection Quantity per Cycle on Formaldehyde and Unburned Methanol Emissions from the Methanol-Fueled Engine during Cold Start. Figures 3 and 4 give the effect of the methanol injection quantity per cycle (Qm) on the formaldehyde and unburned methanol emissions from the methanol-fueled engine during cold start using glow-plug preheating at an ambient temperature (t) of 11 °C, ignition timing (θig) of -20° CA ATDC, and injection timing of methanol (θm) of -35° CA ATDC. Figure 5 shows the corresponding instantaneous engine speed (n) histories during cold start. From those figures, it can be seen that, at Qm = 54.4 mg, fuel cannot fire and most of the injected methanol is emitted without firing; therefore, the unburned methanol emission is as high as 5486 ppm and the formaldehyde emission is as low as 202 ppm. Increasing Qm to 60.4 mg, fuel can fire to result in lower unburned methanol emission and higher formaldehyde emission. When Qm further increases to 95.1 mg, although the firing behavior improves, owing to more methanol left in the inlet port, the unburned methanol emission increases and the formaldehyde emission decreases a little. Increasing Qm to 125.3 mg, the second firing of the methanol partly left in the inlet port makes the unburned methanol emission decrease and the formaldehyde emission increase. Thus, the unburned methanol and the formaldehyde emissions are closely related

Figure 5. n histories during cold start using glow-plug preheating at different Qm and t = 11 °C, θig = -20° CA ATDC, and θm = -35° CA ATDC.

Figure 6. Effect of θig on the formaldehyde emission from the methanol-fueled engine during cold start using glow-plug preheating at t = 11 °C, Qm = 74.7 mg, and θm = -35° CA ATDC.

to the firing behavior. The unburned methanol and formaldehyde emissions show opposite tendencies with the variation of Qm. 866

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Figure 7. Effect of θig on the unburned methanol emission from the methanol-fueled engine during cold start using glow-plug preheating at t = 11 °C, Qm = 74.7 mg, and θm = -35° CA ATDC.

Figure 9. Effect of θig on pmax during cold start using glow-plug preheating at t = 11 °C, Qm = 74.7 mg, and θm = -35° CA ATDC.

Figure 10. Effect of θm on the formaldehyde emission from the methanol-fueled engine during cold start using glow-plug preheating at t = 11 °C, Qm = 74.7 mg, and θig = -20° CA ATDC.

Figure 11. Effect of θm on the unburned methanol emission from the methanol-fueled engine during cold start using glow-plug preheating at t = 11 °C, Qm = 74.7 mg, and θig = -20° CA ATDC.

3.2. Effect of the Ignition Timing on Formaldehyde and Unburned Methanol Emissions from the Methanol-Fueled Engine during Cold Start. Figures 6 and 7 show the effect of θig on the formaldehyde and unburned methanol emissions from the methanol-fueled engine during cold start using glow-plug preheating at t = 11 °C, Qm = 74.7 mg, and θm = -35° CA ATDC. Figure 8 gives corresponding n histories. Figure 9 illustrates the corresponding effect of θig on the maximum combustion pressure in the cylinder (pmax) during cold start. It can be seen that the unburned methanol emission decreases from 5243 to 5045 ppm and the formaldehyde emission increases from 444 to 660 ppm when θig is retarded from -30° to -20° CA ATDC. This is because the firing behavior improves and makes the maximum engine speed (nmax) increase from 1470 to 1601 rpm and pmax increase from 4.36 to 4.47 MPa. Further retarding θig to -10° CA ATDC, the unburned methanol emission further decreases, the formaldehyde emission still increases, but nmax and pmax decrease. The reason is that the second firing occurs at θig = -10° CA ATDC. The unburned methanol emission increases and the formaldehyde emission decreases

Figure 8. n histories during cold start using glow-plug preheating at different θig and t = 11 °C, Qm = 74.7 mg, and θm = -35° CA ATDC.

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significantly when θig is retarded from -10° to 10° CA ATDC because of the late combustion and the drop in capability of doing work with the retarding of θig. Retarding θig to 10° CA ATDC makes the concentration of the mixture not reach the lean firing limit, the methanol engine can not be started, the unburned methanol is as high as 5897 ppm, and the formaldehyde emission is as low as 66 ppm. Therefore, the unburned methanol and the formaldehyde emissions also show opposite tendencies with the variation of θig.

3.3. Effect of the Methanol Injection Timing on Formaldehyde and Unburned Methanol Emissions from the MethanolFueled Engine during Cold Start. Figures 10 and 11 give the effect of θm on the formaldehyde and unburned methanol emissions from the methanol-fueled engine during cold start using glow-plug preheating at t = 11 °C, Qm = 74.7 mg, and θig = -20° CA ATDC. Figure 12 shows the corresponding in-cylinder pressure (p) histories during cold start. Figure 13 gives the corresponding n histories. When θm is -35° CA ATDC, the methanol engine fires weakly, the unburned methanol emission is high, and the formaldehyde emission is low. Retarding θm to 471° CA ATDC, the unburned methanol emission decreases obviously and formaldehyde emission increases significantly because of the improvement of the firing performance. Further retarding θm to 747° CA ATDC, owing to the misfire of methanol, the unburned

Figure 13. n histories during cold start using glow-plug preheating at different θm and t = 11 °C, Qm = 74.7 mg, and θig = -20° CA ATDC.

Figure 12. p histories during cold start using glow-plug preheating at different θm and t=11 °C, Qm=74.7 mg, and θig=-20° CA ATDC.

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Figure 18. Effect of θ on the unburned methanol emission from the methanol-fueled engine during cold start using the LPG cold-start aid at t=11 °C, Qm=51.5 mg, R=9.2%, θig=-20° CA ATDC, and θm=471° CA ATDC.

Figure 14. Effect of R on the formaldehyde emission from the methanol-fueled engine during cold start using the LPG cold-start aid at t=11 °C, Qm=51.5 mg, θig=-20° CA ATDC, θm=-35° CA ATDC, and θL=57° CA ATDC.

Figure 15. Effect of R on the unburned methanol emission from the methanol-fueled engine during cold start using the LPG cold-start aid at t=11 °C, Qm=51.5 mg, θig=-20° CA ATDC, θm=-35° CA ATDC, and θL=57° CA ATDC.

Figure 16. Effect of R on pmax from the methanol-fueled engine during cold start using the LPG cold-start aid at t=11 °C, Qm = 51.5 mg, θig=-20° CA ATDC, θm=-35° CA ATDC, and θL=57° CA ATDC.

Figure 17. Effect of θ on the formaldehyde emission from the methanol-fueled engine during cold start using the LPG cold-start aid at t=11 °C, Qm=51.5 mg, R=9.2%, θig=-20° CA ATDC, and θm=471° CA ATDC.

Figure 19. p histories from the methanol-fueled engine during cold start using the LPG cold-start aid at different θ and t=11 °C, Qm= 51.5 mg, R=9.2%, θig=-20° CA ATDC, and θm=471° CA ATDC.

methanol emission is higher and the formaldehyde emission is lower obviously than that at θm = 471° CA ATDC. Therefore, the unburned methanol and formaldehyde emissions show opposite tendencies with the variation of θm too. 3.4. Effect of the Mass Ratio of Injected LPG/Methanol and the LPG Injection Timing Delay Relative to the Methanol

Injection Timing on Formaldehyde and Unburned Methanol Emissions from the Methanol-Fueled Engine during Cold Start. Figures 14 and 15 give the effect of the mass ratio of injected LPG/methanol (R) on the formaldehyde and unburned methanol emissions from the methanol-fueled engine 869

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figures, it can be known that, at θ = 0° CA, the methanol and the LPG were injected simultaneously in the intake stroke of the second cycle, because of LPG vaporizes fast and the methanol vaporizes slower than that of LPG, LPG enters the cylinder in the second cycle, the methanol enters the cylinder in the third cycle, and LPG cannot play the part of the cold-start aid; therefore, the methanol-fueled engine cannot fire, For all of these reasons, formaldehyde emission is low and unburned emission is high. Increasing θ to 92° CA, LPG and methanol enter the cylinder in the third cycle, LPG can play the part of the cold-start aid, and the methanol-fueled engine can fire reliably; therefore, the formaldehyde emission increases and the unburned methanol emission decreases as θ increases. At θ = 276° CA, the methanol-fueled engine firing behavior improves further, leading to a reduction of the unburned methanol emission and an increase of the formaldehyde emission; therefore, the unburned methanol emission reaches as low as 1173 ppm and the formaldehyde emission reaches as high as 1612 ppm. Further increasing θ to 460° CA, most of the LPG enters the cylinder in the fourth cycle and LPG cannot play the part of the cold-start aid too; therefore, the methanol-fueled engine fires weakly, and its firing cycle is one cycle later than that at θ = 276° CA. This leads to a reduction of the formaldehyde emission and an increase of the unburned methanol emission. It can be seen that the variations in the formaldehyde and unburned methanol emissions also show opposite tendencies with the variations in R and θ too. 4. Conclusions The conclusions from this study can be summarized as follows: (1) The methanol injection quantity per cycle, the ignition timing, the methanol injection timing, the mass ratio of injected LPG/methanol, and the LPG injection timing delay relative to the methanol injection timing affect the formaldehyde and unburned methanol emissions significantly. The unburned methanol and formaldehyde emissions are closely related to the firing and combustion. (2) Optimal control of the methanol injection quantity per cycle, the ignition timing, the fuel injection timing, and the LPG injection timing delay relative to the methanol injection timing improves firing behavior and reduces the unburned methanol emissions. (3) The pmax increases with the increase of R. The firing behavior and combustion process of the methanol-fueled engine during cold start improves as R increases, the formaldehyde emission increases, and unburned methanol emission falls. (4) The variations in formaldehyde and unburned methanol emissions show opposite tendencies with the variations in the methanol injection quantity per cycle, the ignition timing, the methanol injection timing, the mass ratio of injected LPG/methanol, and the LPG injection timing delay relative to the methanol injection timing.

Figure 20. n histories from the methanol-fueled engine during cold start using the LPG cold-start aid at different θ and t=11 °C, Qm= 51.5 mg, R=9.2%, θig=-20° CA ATDC, and θm=471° CA ATDC.

during cold start using the LPG cold-start aid at t = 11 °C, Qm = 51.5 mg, θig = -20° CA ATDC, θm = -35° CA ATDC, and LPG injection timing (θL) = 57° CA ATDC. Figure 16 illustrates the corresponding effect of R on pmax. It is apparently found that the pmax increases with the increase of R. The firing behavior and combustion process of the methanol-fueled engine during cold start improves as R increases. Therefore, it results in increasing formaldehyde emission and decreasing unburned methanol emission with the increase of R. Figures 17 and 18 show the effect of the LPG injection timing delay relative to the methanol injection timing (θ) on the formaldehyde and unburned methanol emissions from the methanol-fueled engine during cold start using the LPG cold-start aid at t = 11 °C, Qm = 51.5 mg, R = 9.2%, θig = -20° CA ATDC, and θm = 471° CA ATDC. Figures 19 and 20 give the corresponding p and n histories. From those

Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grants 50576031 and 50976045) and China First Automobile Works Group Corporation.

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