Energy Fuels 2009, 23, 4937–4942 Published on Web 09/03/2009
: DOI:10.1021/ef900502e
Combustion and Hydrocarbon (HC) Emissions from a Spark-Ignition Engine Fueled with Gasoline and Methanol during Cold Start Jun Li,†,‡ Changming Gong,*,† Bo Liu,† Yan Su,† Huili Dou,‡ and Xunjun Liu† †
State Key Laboratory of Automobile Dynamic Simulation, Jilin University, Changchun 130022, China, and ‡Research and Development Center, China First Automobile Works Group Corporation, Changchun 130011, China Received May 22, 2009. Revised Manuscript Received August 24, 2009
The effects of the ambient temperature and the amount of fuel injected per cycle on the cold-start firing behavior as well as the combustion and the hydrocarbon (HC) emissions of an electronically controlled inlet port injection spark-ignition (SI) engine fueled with gasoline and methanol, respectively, during the cold start were studied experimentally by means of a single-cycle fuel injection strategy. The results showed that the amount of fuel injected per cycle significantly affects the gasoline-fueled engine cold-start reliability. A proper amount of fuel injected per cycle ensures a reliable start of the gasoline-fueled engine. The ambient temperature affects most significantly the cold-start ability of the methanol-fueled engine, and the amount of methanol injected per cycle takes second place. With the ambient temperature below 16 °C, the methanol-fueled engine cannot be started reliably without auxiliary start aid even with a large amount of methanol injection under low injection pressure. Using a glow plug to heat the engine inlet manifold results in a reliable firing of the methanol-fueled engine and may realize an ideal firing in the next cycle combustion after fuel injection. The ambient temperature is closely related to the fuel vaporization and fuel-air mixture preparation, especially at cold start. With a rise in the ambient temperature, the methanol and gasoline injection amounts per cycle sufficient for the reliable firing during cold start of the engine reduce obviously and the HC emissions decrease significantly. At the same injection timing, the gasoline-fueled engine may realize an ideal firing in the next cycle combustion after fuel injection and the firing of the methanol-fueled engine occurs one cycle later than that of the gasoline-fueled engine.
Jason et al.8 studied the effect of ambient temperature on cold-start emissions for a spark-ignition (SI) engine. Their test results showed that exhaust emissions at cold ambient conditions may be drastically increased relative to 25 °C. For instance, they found that the HC emissions increase by 650% at -20 °C and carbon monoxide (CO) emissions increase by 800%. Cheng et al.9 investigated the mixture preparation and HC emissions behavior in the first cycle of SI engine cranking. They found that the amount of fuel required by cold start is significantly higher than that needed for warmed-up operation. At cold conditions, only approximately 20% of the fuel is vaporized during the first few engine cycles. This overfueling results in a large amount of liquid fuel entering the cylinder, being a major source of engine-out HC emissions. Thus, the engine must be overfueled by 5 times to provide the sufficient fuel vapor to attain ignition and initial starting. Overfueling by 10 times or more may be required to ensure rapid start and stable idle operation for low-volatility gasoline. However, because there is no volatility sensor in the fueling system, the low-volatility calibration is used for all gasoline, independent of their volatility. Typically, 8-15 times the stoichiometric amount of gasoline is injected during the first several cycles of the cold-start and warm-up transient.10 Zughyer et al.11 observed the liquid fuel distribution and flame propagation inside a PFI gasoline engine using various
1. Introduction With the stringency of the hydrocarbon (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.1 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 at its operating temperature to oxidize efficiently the HCs.2 The driving cycles of Euro III, Euro IV, and the FTP-75 all take the first 40 s of idle operation into the start-up phase. Especially, Euro III and Euro IV emission standards have included a subambient cold-start test at a temperature of -7 °C. Therefore, the control of combustion and emissions during the cold start has become the hotspots in the field of vehicle engine developments in recent years.3-7 *To whom correspondence should be addressed: State Key Laboratory of Automobile Dynamic Simulation, Jilin University, Changchun 130022, China. E-mail:
[email protected]. (1) Takeda, K.; Yaegashi, T.; Tai, H. SAE Tech. Pap. 950074, 1995. (2) Gulati, S. T. SAE Tech. Pap. 1999-01-0269, 1999. (3) Bielaczyc, P.; Merkisz, J. SAE Tech. Pap. 980401, 1998. (4) Li, G.; Li, L. G.; Liu, Z. M.; Li, Z. L.; Qiu, D. P. Energy Convers. Manage. 2007, 48, 2508–2516. (5) Hu, T. G.; Wei, Y. J.; Liu, S. H.; Zhou, L. B. Energy Fuels 2007, 21, 171–175. (6) Bielaczyc, P.; Merkisz, J. SAE Tech. Pap. 1999-01-1073, 1999. (7) Lang, K. R.; Cheng, W. K. SAE Tech. Pap. 2006-01-3400, 2006. r 2009 American Chemical Society
(8) Jason, D. H.; Checkel, M. D. SAE Tech. Pap. 2003-01-0301, 2003. (9) Cheng, W. K.; Santoso, H. SAE Tech. Pap. 2002-01-2805, 2002. (10) Heywood, J. B. Air Pollution from Internal Combustion Engines; Academic Press: New York, 1998. (11) Zughyer, J.; Zhao, F. Q.; Lai, M. C.; Lee, K. SAE Tech. Pap. 2000-01-0242, 2000.
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injectors and engine conditions. It was found that most fuel under open valve injection (OVI) conditions entered the cylinder as droplet mist. Images taken just before spark timing showed that a significant fraction of the fuel was still in the liquid phase. The combustion video showed that the fueltransport process under both close valve injection (CVI) and OVI strategies may be divided into three phases. The first phase is at early cycles when most of the injected fuel ended up as liquid film on the chamber walls, with insufficiently vaporized fuel in a mixture too lean to sustain combustion and little effect on cylinder pressure. The second phase starts when visible weak flame fronts appear because of improvement in the liquid evaporation process. Diffusion-controlled pool-fire processes become obvious at this phase, and the in-cylinder pressure rises significantly. The third phase starts when the heat of combustion evaporate most of the liquid fuel and produces an overall-rich mixture that burns rapidly and produces a high in-cylinder pressure. From the flame propagation visualization and in-cylinder pressure measurements, it was found that injectors with better dispersion and injection under open valve conditions could provide better fuel distribution and evaporation, constituting an improved engine starting strategy. However, the unburned hydrocarbon (UBHC) emissions may deteriorate. The effect of liquid fuel on HC emissions is the easiest approach to understand the HC emissions mechanisms. Liquid fuel enters the combustion chamber and impacts its cold surface, where it does not vaporize, and it can exit to the exhaust system as a source of HC emissions. The in-cylinder fuel films are very thin (on the order of 50-300 μm); therefore, the fuel is at the temperature of the surface.12 Because the higher boiling-point components in gasoline vaporize over a range of temperatures from 100 to 200 °C, much of the fuel remains unburned within the cylinder for a long time following a cold start.13 One of the difficulties to identify the effects of either fuel preparation or fuel volatility on cold-start performance is that significant differences in the amount of vaporized fuel may result from changes in either fuel injection or fuel volatility. This results in the change of the air/fuel (A/F) vapor ratio that must be taken into consideration to separate the effect of liquid fuel from the large effect of the A/F ratio on HC emissions. Kaiser et al.14 investigated the effects of mixture preparation on the coldstart engine performance and emissions. They performed simulated cold-start tests using a prevaporized, central fuel injection (CFI) system and a conventional port fuel injection (PFI) system. There were significant differences in HC levels early during the test. The HC emissions from the PFI system were nearly 80% higher than those from the CFI fueling system. After 40 s, the differences in HC emissions were reduced to roughly 15%. These experimental results suggested that a large in-cylinder fuel film is produced by the PFI fuel system. The liquid fuel layer was also the cause of the lean shift in the exhaust A/F ratio early during the test. The reduction of HC emissions after the first 40 s was a result of the liquid fuel layer slowly vaporizing as the engine warms up. Yang et al.15 studied the effects of port-injection timing and fuel droplet size
on total and speciated exhaust HC emissions. They found that the HC emissions are almost independent of injection timing for CVI unless the start of injection is ∼80 °CA (crank angle) or later before IVO. With 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. Stache et al.16 compared the results for three fuels. They found that iso-octane has a boiling point near T40 (temperature of 40% volume distilled point of the certification gasoline indolene), while iso-pentane has a boiling point near the initial boiling point of the indolene. Iso-pentane produced the lowest HC emissions, while iso-octane produced the highest for all but a small range of OVI timings with the cold coolant. The reason that iso-pentane produced the lowest HC emissions was that it vaporized more before impacting the in-cylinder wall surface. Rottenkolber et al.17 found cold-start HC emissions 25-80% higher for OVI versus CVI, depending upon injector targeting. Castaing et al.18 measured both the incylinder λ (relative air/fuel ratio) using a fast-response flame ionization detector (FFID) and the HC emissions for the first fired cycle during cranking in one cylinder of a four-valve, four-cylinder engine. They varied the injector pulse width during startup and found that, when the in-cylinder λ was near stoichiometric, engine-out HC emissions were only slightly higher for OVI versus CVI. OVI led to homogeneous charge, which, in combustion with the direct wall wetting, increased cold-start HC emissions. All of the above investigations in the literature paid attention to SI gasoline- and liquefied petroleum gas (LPG)-fueled engines.19 However, a few papers were reported on the methanol-fueled cold-start performance.20,21 They investigated the combustion and firing behavior of the SI methanol engine. Methanol is characterized by a high octane number, indicating good antiknock performance, high latent heat of vaporization allowing for a denser fuel-air charge, and excellent lean burn properties.22 However, the low vapor pressure and high latent heat of vaporization of methanol may cause cold-start difficulties for the methanol-fueled engine at low ambient temperature.23 The objective of this study is to compare the combustion behavior and HC emissions of an electronically controlled inlet port injection SI engine fueled with gasoline and methanol, respectively, during cold start by means of a single-cycle fuel injection strategy. The effects of the ambient temperature and the amount of fuel injected per cycle were studied experimentally. These results are helpful to understand the cold-start behavior of the gasoline- and methanol-fueled engines. 2. Test Engine and Experimental Setup The experiment was conducted on a single-cylinder four-stroke electronically controlled SI engine fueled with gasoline and (16) Stache, I.; Alkidas, A. C. SAE Tech. Pap. 972981, 1997. (17) Rottenkolber, G.; Dullenkopf, K.; Wittig, S.; Kolmel, A.; Feng, B.; Spicher, U. SAE Tech. Pap. 1999-01-3644, 1999. (18) Castaing, B. M.; Cowart, J. S.; Cheng, W. K. SAE Tech. Pap. 2000-01-2836, 2000. (19) Hochul, K.; Cha, L. M.; Simsoo, P. Fuel 2007, 86, 1475–1482. (20) Gong, C. M.; Deng, B. Q.; Wang, S.; Su, Y.; Gao, Q.; Liu, X. J. Energy Fuels 2008, 22, 2981–2985. (21) Gong, C. M.; Deng, B. Q.; Wang, S.; Su, Y.; Gao, Q.; Liu, X. J. Energy Fuels 2008, 22, 3779–3784. (22) Frank, B. SAE Tech. Pap. 912413, 1991. (23) Bassem, H. R.; Fakhri, J. H.; Charles, L. G.; Karl, H. H.; Harold, J. S. SAE Tech. Pap. 2002-01-2702, 2002.
(12) Shin, Y.; Cheng, W. K.; Heywood, J. B. SAE Tech. Pap. 941872, 1994. (13) Shayler, P. J.; Belton, C.; Scarisbrick, A. SAE Tech. Pap. 199901-0220, 1999. (14) Kaiser, E. W.; Siegl, W. O.; Lawson, G. P.; Connolly, F. T.; Cramer, C. F.; Dobbins, K. L.; Roth, P. W.; Smokovitz, M. SAE Tech. Pap. 961957, 1996. (15) Yang, J.; Kaiser, E. W.; Siegl, W. O.; Anderson, R. W. SAE Tech. Pap. 930711, 1993.
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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) (°CA BTDC) intake valve closing (IVC) (°CA ABDC) exhaust valve opening (EVO) (°CA BBDC) exhaust valve closing (EVC) (°CA ATDC)
Table 2. Properties of Methanol and Gasoline 52.4 57.8 125 10.55:1 6.5/7500 9/6000 air cooled 15 35 35 15
Figure 1. Schematic of the experimental system: 1, throttle sensor; 2, fuel injection nozzle; 3, glow plug; 4, K-type thermocouple; 5, fuel pressure regulator; 6, fuel filter; 7, spark plug; 8, in-cylinder pressure transducer; 9, encoder; 10, TDC marker; 11, exhaust gas analyzer; 12, charge amplifier; 13, computer; 14, fuel pump; 15, fuel tank; 16, ECU.
property
methanol
gasoline
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) autoignition temperature (°C) stoichiometric air/fuel ratio flame speed (m/s)
CH3OH 32
C5-C12 90-120
37.5 12.5 50.0 790 65 111 6.7-36 1110 19.6 470 6.5 0.523
85 15 0 720-780 40-190 90-98 1.4-7.6 310 44.0 500 14.6-14.8 0.38
Figure 2. Relationship between minimum Q and t for firing at cold start. θg=35 °CA BTDC, and θm=35 °CA BTDC.
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 ignition timing of all of the tests was fixed at 20 °CA before top dead center (BTDC). A 0 °CA of injection timing of methanol and gasoline corresponds to the piston position at compression stroke top dead center (TDC) of the second cycle.
methanol, respectively, with inlet PFI. The engine specifications are listed in Table 1. The schematic of the experimental system is shown in Figure 1. The instantaneous angular velocity of the crankshaft was determined by an optical shaft encoder with 0.5° resolution. The incylinder pressure was measured using a Kistler 6125B quartz crystal pressure sensor matched with a WDF-3 charge amplifier. A multichannel data acquisition card PLC-8018HG was used to record the in-cylinder pressure and HC emissions. The HC emissions were analyzed with a FGA4015 exhaust gas analyzer. The amount of methanol and gasoline evaporation was measured with a Sartorius 2450 electronic balance. Its detection limit is 1 10-6 g. Test fuels for this study were gasoline and methanol, and their physical and chemical properties are listed in Table 2. The purity of the methanol is 99.9%, and commercial 93 gasoline (blends fuel of 90% gasoline and 10% ethanol in volume) is used. A glow plug was used as an auxiliary start aid to heat the engine inlet manifold. To prevent the injected methanol from combustion on the glow plug, the surface of the glow plug is covered with a copper sleeve. After the glow plug was switched on for 3 min, the temperature of the inlet manifold surface reached 42 °C. The injection nozzle of gasoline (or methanol) is mounted on the intake manifold. The injection pressure of gasoline (or methanol) is 0.3 MPa. The amount of the gasoline and methanol injected per cycle, ignition timing, and 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 is defined as the first cycle. The gasoline and methanol were injected in the second cycle by means of a single-cycle fuel injection strategy.
3. Results and Discussion 3.1. Effect of the Ambient Temperature on the Minimum Amount of Fuel Injected per Cycle for Engine Cold-Start Firing. Figure 2 shows the relationship between the minimum amount of fuel injected per cycle (Qmin) to ensure the reliable firing and ambient temperature (t) during the cold start at θg = 35 °CA BTDC and θm = 35 °CA BTDC. The Qmin for the reliable firing of the engine during the cold start increases significantly with the drop of the ambient temperature. Relative to 28 °C, the Qmin,g is increased by 325% at -7 °C and the Qmin,m is increased by 110% at 16 °C. With the drop of ambient temperature, increasing the amount of gasoline injected per cycle can ensure a reliable start of the gasolinefueled engine. When the ambient temperature is below 16 °C, the methanol-fueled engine cannot be started reliably without the auxiliary start aid even at a very high amount of methanol injected per cycle.20 Therefore, a glow-plug preheating was used to heat the methanol-fueled engine inlet port when t is below 16 °C. With the drop of the ambient temperature, the Qmin,m is increased more rapidly than the Qmin,g. This is due to the effect of the ambient temperature on the methanol vaporization being more significant than that of the gasoline. 4939
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Figure 7. HC emissions of the gasoline- and methanol-fueled engines (the later with a glow-plug auxiliary start aid) at different Q and t=8 °C.
Figure 3. Methanol evaporation performance at three ambient temperatures.
Figure 4. Methanol and gasoline evaporation performance at 20 °C ambient temperature.
Figure 5. pmax of the gasoline- and methanol-fueled engines (the later with a glow-plug auxiliary start aid) at different Q and t=8 °C.
Figure 8. p histories of the methanol-fueled engine during cold start at two different ambient temperatures.
methanol evaporation is a quarter of the gasoline evaporation. The effect of the ambient temperature on the amount of the methanol evaporation is very obvious. The amount of methanol evaporation at 20 °C is 5 times at 7 °C. The methanol vaporization at the inlet port injection deteriorates because of poor volatility of methanol at low ambient temperatures. 3.2. Effect of the Amount of Fuel Injected per Cycle on Engine Cold-Start Firing. Figure 5 gives the maximum combustion pressure in the cylinder (pmax) of the gasolineand methanol-fueled engines at different amounts of fuel injected per cycle (Q) during the cold start at t = 8 °C. Figures 6 and 7 show the maximum instantaneous cranking speed (nmax) and HC emissions from the gasoline- and methanol-fueled engines at different Q during the cold start
Figure 6. nmax of the gasoline- and methanol-fueled engines (the later with a glow-plug auxiliary start aid) at different Q and t=8 °C.
Figure 3 gives the methanol evaporation performance for 1000 g of methanol at different temperatures. Figure 4 shows the methanol and gasoline evaporation performance for 1000 g of methanol and 1000 g of gasoline at 20 °C ambient temperature. It can be seen that, at 20 °C, the amount of the 4940
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Figure 10. HC emissions from the methanol-fueled engine during cold start at different conditions.
Figure 9. n histories of the methanol-fueled engine during cold start at two ambient temperatures.
Figure 11. p history of the gasoline-fueled engine during cold start at Qg=35.1 mg, t=8 °C, and θg=35 °CA BTDC.
at t = 8 °C. The gasoline-fueled engine is characterized by smaller Qg for the reliable firing. The maximum combustion pressure in the cylinder reaches the highest pmax to 4.52 MPa at Qg=35.1 mg. When Qg is smaller or greater than 35.1 mg, pmax falls rapidly. At this Qg, the fuel vapor content of the fuel-air mixture may realize the ideal firing and obtain the highest pmax. When Qg is increased further, the mixture is too rich, firing performance deteriorates rapidly, until the fuel vapor content of the mixture surpasses the rich firing limit of gasoline at Qg = 44.5 mg, and the gasoline-fueled engine cannot fire. At Qg = 33 mg, the mixture is lean, which corresponds to the lean firing limit. At both the rich and lean firing limits, the gasoline-fueled engine cannot fire, nmax is low, and HC emissions are high. When Qg is between these two limits, the gasoline-fueled engine can fire reliably, resulting in high nmax and low HC emissions. For the methanolfueled engine, there exists a Qmin,m for reliable firing. When Qm is increased further, the methanol-fueled engine can still fire, the pmax and nmax increase, and HC emissions decrease. At Qm=48.5 mg, HC emissions reach the lowest at 942 ppm. Further increasing Qm leads to a increase of the HC emissions. This is because the methanol cannot fully enter into the cylinder in one cycle. The methanol-fueled engine does not have a maximum Qm for misfiring during cold start. This is because the amount of methanol vaporization is less than that of the gasoline at low ambient temperature. Unvaporized methanol cannot fully enter into the cylinder with increasing Qm, and part of the methanol goes out of the engine as HC emissions. 3.3. Effect of the Ambient Temperature on Combustion and HC Emissions of the Methanol-Fueled Engine during Cold Start. Figures 8 and 9 give the p and n histories of the methanol-fueled engine during cold-start at different ambient
temperatures with Qm = 45.8 mg. When t = 12 °C, the methanol-fueled engine cannot fire without an auxiliary start aid. When the ambient temperature is increased to 28 °C, the methanol-fueled engine can fire reliably without any auxiliary start aid and then the pmax reaches as high as 2.7 MPa and n reaches 1438 rpm. When t = 12 °C, the methanol-fueled engine can fire weakly with a glow-plug auxiliary start aid and then the pmax reaches 1.4 MPa and n reaches 1048 rpm. Because the Qm is small, the firing is weak. Figure 10 shows the HC emissions of the methanol-fueled engine during the cold start at different ambient temperatures and at Qm =45.8 mg. It can be observed that the HC emissions from the no-firing event without an auxiliary start aid are 87% higher than those from the weak-firing event with a glow-plug auxiliary start aid at t = 12 °C and HC emissions from the reliable-fire event without any auxiliary start aid at t = 28 °C are 35% lower than those from the no-firing event without an auxiliary start aid at t=12 °C. The HC emissions from the reliable-fire event with a glow-plug auxiliary start aid at t=12 °C are 18% lower than those from the reliable-fire event without any auxiliary start aid at t=28 °C. This is because the amount of methanol vaporization with a glow-plug auxiliary start aid at t = 12 °C is larger than without any auxiliary start aid at t=28 °C. 3.4. Combustion during Cold Start. Figure 11 shows the p history of the gasoline-fueled engine during cold start at Qg= 35.1 mg, t=8 °C, and θg =35 °CA BTDC. The fuel vapor content of the gasoline-air mixture reaches the lean firing limit of gasoline in the third cycle; therefore, the gasolinefueled engine fires in the third cycle at θg=35 °CA BTDC of the compression stroke in the second cycle. Figure 12 gives the p history of the methanol-fueled engine using glow-plug preheating during cold start at Qm =51.51 mg, t=8 °C, and 4941
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cycle can ensure a reliable start of the gasoline-fueled engine. Relative to 28 °C, the Qmin,g is increased by 325% for the reliable firing at -7 °C. (2) The ambient temperature affects most significantly the cold-start ability of the methanol-fueled engine, and the amount of methanol injected per cycle takes second place. When the ambient temperature is below 16 °C, the methanol-fueled engine cannot be started reliably. Relative to 28 °C, the Qmin,m is increased by 110% for the reliable firing at 16 °C. The effect of the ambient temperature on the methanol-fueled engine cold start is more significant than that of the gasoline-fueled engine. (3) With a rise in the ambient temperature, the methanol and gasoline injection amounts per cycle sufficient for the reliable firing during the cold start of the engine reduce obviously and the HC emissions decrease significantly. (4) At the same injection timing, the gasolinefueled engine may realize an ideal firing in the next cycle combustion after fuel injection and the firing of the methanolfueled engine occurs one cycle later than that of the gasolinefueled engine. By advancing the methanol injection timing, the methanol-fueled engine can realize an ideal firing in the next cycle combustion after fuel injection and obtain a very good firing performance during cold start and the pmax reaches as high as 4.63 MPa, which is the pmax 140% higher than that of injection timing at 35 °CA BTDC.
Figure 12. p history of the methanol-fueled engine using glow-plug preheating during cold start at Qm=51.51 mg, t=8 °C, and θm=35 °CA BTDC.
Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant 50576031) and China First Automobile Works Corporation. Figure 13. p history of the methanol-fueled engine using glow-plug preheating during cold start at Qm=51.51 mg, t=8 °C, and θm=249 °CA BTDC.
Nomenclature HC=hydrocarbon SI=spark ignition FTP=Federal Test Procedure ULEV=ultra-low emissions vehicle SULEV=super ultra-low emissions vehicle CO=carbon monoxide OVI=open valve injection CVI=close valve injection UBHC=unburned hydrocarbon A/F=air/fuel CFI=central fuel injection PFI=port fuel injection CA=crank angle λ=relative air/fuel ratio FFID=fast-response flame ionization detector LPG=liquefied petroleum gas ECU=electronic control unit BTDC=before top dead center TDC=top dead center t=ambient temperature (°C) t=time (h) Qmin =minimum amount of fuel injected per cycle Q=amount of fuel injected per cycle pmax =maximum combustion pressure in the cylinder p=in-cylinder pressure nmax =maximum instantaneous cranking speed n=instantaneous cranking speed R=crank angle θg =gasoline injection timing θm =methanol injection timing
θm =35 °CA BTDC. The first firing cycle of the methanolfueled engine is the fourth cycle at θm=35 °CA BTDC of the compression stroke in the second cycle. It is one cycle later than that of the gasoline-fueled engine. This is due to the fact that methanol is a single compound fuel and its boiling point (65 °C) is higher than the initial boiling point of gasoline (about 40 °C). The low vapor pressure, high latent heat of vaporization, and small injection timing of methanol cause the mixture to be too lean in the third cycle to fire at low ambient temperature. In the fourth cycle, by increasing the vaporization time available for methanol, the fuel vapor content of the methanol-air mixture surpasses the lean firing limit of methanol. Figure 13 shows the p history of the methanol-fueled engine using glow-plug preheating during the cold start at Qm=51.51 mg, t=8 °C, and θm=249 °CA BTDC of the intake stroke in the second cycle during cold start. Then, methanol fully enters into the cylinder in the third cycle, making methanol fire in the third cycle. By advancing the methanol injection timing, the methanol-fueled engine can realize an ideal firing in the next cycle combustion after fuel injection and obtain a very good firing performance during cold start and the pmax reaches as high as 4.63 MPa, which is the pmax 140% higher than that of injection timing at 35 °CA BTDC. 4. Conclusions The transient firing behavior, combustion, and HC emissions of a SI engine fueled with gasoline and methanol, respectively, during cold start were investigated by means of a single-cycle fuel injection strategy. The main conclusions can be summarized as follows: (1) With the drop of ambient temperature, increasing the amount of gasoline injected per
Subscripts g=gasoline m=methanol 4942
