Energy & Fuels 2008, 22, 2981–2985
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Combustion of a Spark-Ignition Methanol Engine during Cold Start under Cycle-by-Cycle Control Changming Gong,*,† Baoqing Deng,† Shu Wang,‡ Yan Su,† Qing Gao,† and Xunjun Liu† College of AutomotiVe Engineering, Jilin UniVersity, Changchun 130022, China, and Chongqing AutomotiVe Research Institute, Chongqing 400039, China ReceiVed March 4, 2008. ReVised Manuscript ReceiVed May 8, 2008
The effects of ambient temperature, intake air preheating, additional liquefied petroleum gas (LPG) injected into the inlet port, and methanol injection timing on the cold-start first cycle firing behavior of an electronically controlled inlet port methanol injection spark-ignition (SI) engine were investigated under a single-cycle fuel injection strategy. The results showed that the ambient temperature significantly affects the start firing behavior of the engine. When the ambient temperature is below 16 °C, the methanol engine can not be started reliably without auxiliary start aids even at the large amount of methanol injected per cycle. Using a glow plug to heat the engine inlet manifold and additional LPG injected into the inlet port result in a reliable firing of the engine. The amount of methanol injected per cycle for the reliable firing during the cold start of the engine reduces obviously, and the cold-start performance improves significantly with the rise in ambient temperature. If the fuel injection timing is controlled reasonably at the engine cold start, it is possible to ensure the most fuel-air mixture to enter the cylinder on time, to realize the ideal concept of the next cycle combustion after fuel injection. Optimal control on the fuel injection timing improves the cold-start reliability and reduces the coldstart HC emissions from the engine.
1. Introduction Because of the shortage of petroleum and the enhancement of exhaust emissions standard, the alternative fuel engine has become more and more important. Methanol (CH3OH) is considered to be one of the favorable fuels for engines. Methanol has many desirable combustion and emission characteristics. It has a high octane number indicating antiknock performance, high latent heat of vaporization allowing a denser fuel-air charge, and excellent lean burn properties.1 These properties make methanol a good fuel for SI Otto-cycle engines. However, methanol is a relatively simple, single-compound fuel. The boiling point of methanol (65 °C) is higher than the initial boiling point of gasoline (40 °C). The low vapor pressure and high latent heat of vaporization of methanol may cause coldstart difficulties for a methanol engine at low ambient temperatures.2 The driving cycles of Euro III, Euro IV, and the U.S. 1975 federal test procedure (FTP-75) all take the first 40 s of idle into the start-up phase. Especially, Euro III and Euro IV emission standards have included subambient cold-start test at a temperature of -7 °C. Nearly 50-80% of hydrocarbon (HC) and carbon monoxide (CO) emissions of driving cycles are produced at the beginning cycles of the cold-start phase.3 The reasons for high cold-start emissions are numerous but can be traced to two main factors: low fuel volatility and inactive * To whom correspondence should be addressed: College of Automotive Engineering, Jilin University, Changchun 130022, China. E-mail:
[email protected]. † Jilin University. ‡ Chongqing Automotive Research Institute. (1) Frank, B. An overview of the technique implications of methanol and ethanol as highway motor vehicle fuels. SAE paper 912413, 1991. (2) Bassem, H. R.; Fakhri, J. H.; Charles, L. G.; Karl, H. H.; Harold, J. S. Numerical evaluation of a methanol fueled directly-injected engine. SAE paper 2002-01-2702, 2002.
catalysts.4 It is necessary to enrich the fuel air mixture under cold-start conditions because fuel vaporization at low temperature is insufficient for the formation of a combustible air-fuel mixture. Incomplete combustion with excess fuel leads to increased CO and HC emission levels.5 Therefore, the control of combustion and exhaust emissions during the cold start has become the hotspots in the field of vehicle engines in recent years. Up to now, most work on the cold start was concentrated on the gasoline and liquefied petroleum gas (LPG) fuel engine,6–8 while little work was reported on the methanol engine. Fulcher et al.9 investigated the effects of fuel atomization, vaporization, and mixing on the cold-start unburned hydrocarbon (UHC) emissions from a contemporary spark-ignition (SI) engine with intake manifold injection. Their results showed the most important overall conclusion is that cycle resolved control of the equivalence ratio may be the most effective way of reducing (3) Karwa, M. K.; Hill, F. B.; Biel, P. J.; Mark, C. Integration of engine controls, exhaust components and advanced catalytic converters for ULEV and SULEV applications. SAE paper 2001-01-3664, 2001. (4) Ashford, M.; Matthews, R.; Hall, M.; et al. An on-board distillation system to reduce cold-start hydrocarbon emissions. SAE paper 2003-013239, 2003. (5) Bielaczyc, P.; Merkisz, J. Cold start emissions investigation at different ambient temperature conditions. SAE paper 980401, 1998. (6) Lang, K. R.; Cheng, W. K. Effeces of fuel injection strategy on HC emissions in a port-fuel-injection engine during fast idle. SAE paper 200601-3400, 2006. (7) Liguang, L.; Gong, L.; Dongping, Q.; Zhimin, L. The characteristic of transient HC emissions of the first firing cycle during cold start on an LPG SI engine. SAE paper 2006-01-3403, 2006. (8) Liao, S. Y.; Jiang, D. M.; Cheng, Q.; Huang, Z. H.; Wei, Q. Investigation of the cold-start combustion characteristics of ethanol-gasoline blends in a constant-volume chamber. Energy Fuels 2005, 19, 813–819. (9) Fulcher, S. K.; Gajdeczko, B. F.; Felton, P. G.; Bracco, F. V. The effects of fuel atomization, vaporization, and mixing on the cold-start UHC of a contemporary S.I. engine with intake manifold injection. SAE paper 952482, 1995.
10.1021/ef8001636 CCC: $40.75 2008 American Chemical Society Published on Web 07/02/2008
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Table 1. Engine Specifications bore stroke displacement compression ratio maximum power/speed maximum torque/speed cooling system
52.4 mm 57.8 mm 125 cm3 10.55:1 6.5 kW/7500 rpm 9 N m/6000 rpm air cooled
cold-start UHC emissions. Lang et al.10 studied the effects of fuel properties on first cycle fuel delivery in a SI engine. Their studies showed that an empirical procedure was established to assess the change of minimum required injected mass when the fuel properties were changed. Jason et al.11 investigated the effect of ambient temperature on cold-start emissions for a SI engine. They found that exhaust emissions could be drastically increased, relative to 25 °C, at cold ambient conditions. For instance, the HC emissions were found increase by 650% at -20 °C and CO emissions by 800%. Liguang et al.,12 on the basis of cycle-by-cycle control strategy, studied how to realize the controllable ignition cycle and the single- and multicycle combustions were tested. Their test results show that the first combustion cycle has an important effect on HC emission and combustion stability of following cycles at cold start. Tiegang et al.13measured HC emission from the engine fueled with methanol/gasoline blends during cold start. The measured results show that HC is reduced about 40% at 5 °C and 30% at 15 °C compared to that of the gasoline engine when the engine is fueled with M30 (30% methanol and 70% gasoline in volume). The objective of this paper is to study the cold-start firing behavior of a methanol engine based on a cycle-by-cycle control strategy. The effects of the ambient temperature, intake air preheating, additional LPG injected into the inlet port, and methanol injection timing on the cold-start first cycle firing behavior were investigated by means of the single-cycle fuel injection, which have significant meanings to ensure methanol reliable cold start. 2. Test Engine and Experimental Setup The experiment was conducted on a single-cylinder four-stroke electronically controlled methanol engine with inlet port fuel injection (PFI). The engine specifications are listed in Table 1. Figure 1 shows the schematic of the methanol engine test bench. The instantaneous anglur velocity of the crankshaft was determined by an optical shaft encoder with 0.5° resolution. In-cylinder pressure was measured using a Kistler 6125B quartz crystal pressure sensor matched with a WDF-3 charge amplifier, mounted flush to the combustion cylinder in the cylinder head. A multichannel data acquisition card PLC-8018HG was used to record the in-cylinder pressure and instantaneous angular velocity synchronously. The HC emissions were measured with a FGA4015 emission analyzer. When the ambient temperature was below 16 °C, the auxiliary start aids of a glow plug to heat the engine inlet manifold or additional LPG injected into the inlet port were used. A glow plug was fixed in the intake manifold plenum. For preventing from firing a methanol injected on glow plug, the surface of glow plug was (10) Lang, K. R.; Cheng, W. K. Effect of fuel properties on first cycle fuel delivery in a SI engine. SAE paper 2004-01-3057, 2004. (11) Jason, D. H.;Checckel, M. D. Quantifying vehicle emission factors for various ambient conditions using an on-board, real-time emissions system. SAE paper 2003-01-0301, 2003. (12) Liguang, L.; Zhensuo, W.; Changming, G.; Baoqing, D.; Zongcheng, X.; Huiping, W. Investigation of cold-start based on cycle-by-cycle control strategy in an EFI LPG engine. SAE paper 2004-01-3059, 2004. (13) Tiegang, H.; Yanjv, W.; Shenghua, L.; Longbao, Z. Improvement of spark-ignition (SI) engine combustion and emission during cold start, fueled with methanol/gasoline blends. Energy Fuels 2007, 21, 171–175.
Figure 1. Schematic of the methanol engine test bench.
Figure 2. Minimum for firing amount of methanol injected per cycle at different ambient temperatures.
Figure 3. Instantaneous engine speed during engine start at 16 °C.
covered with 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 system were used separately. Methanol injection nozzle was mounted on intake manifold near the intake valve. The LPG injection nozzle was mounted between the methanol injection nozzle and the intake valve. The LPG only played the part of start aids. Gas-phase LPG was injected at a constant pressure of 0.14 MPa with a pressure regulator. The amount of the LPG and methanol injected, ignition timing, and injection timing were controlled by the electronic control unit (ECU). A 0° crank angle corresponding to piston position was top dead center (TDC) of the second cycle compression stroke. During the cold-start test, through process control, an electric motor cranked the engine. The methanol was injected in the second cycle before top dead center (BTDC) of compression stroke and the LPG was injected in the same cycle after top dead center (ATDC) by means of the single-cycle fuel injection system, respectively. 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 BTDC. The engine was always started from the position of the piston at BTDC of the compression stroke.
3. Results and Discussion 3.1. Effect of Ambient Temperature on the Methanol Engine Cold-Start Firing Behavior. Figure 2 gives the minimum amount of methanol injected per cycle (Qm) for ensuring reliable firing at different ambient temperatures under
Combustion of a SI Methanol Engine
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Figure 5. Instantaneous engine speed traces at several amounts of injection per cycle using glow-plug preheating. Figure 4. Cylinder pressure traces at several amounts of methanol injection per cycle using glow-plug preheating.
the injection timing of 35 °CA BTDC. The figure shows that Qm for the reliable firing during the cold start of the engine reduces obviously with a rise in ambient temperature. Relative to 28 °C, the Qm is increased by 110% at 16 °C. The methanol vaporization at the inlet port injection deteriorates because of the poor volatility of methanol at low ambient temperatures. The lower the ambient temperature, the richer the air-fuel mixture that is required for a start-up. The rich air-fuel mixture results in incomplete combustion.14 Figure 3 shows the instantaneous engine speed at 16 °C (Qm ) 102 mg), without start aids. It can be seen that the first firing cycle appeared at the fifth cycle if the methanol was injected from the second cycle, but ideal firing cycle should be at the third cycle. At the moment, the combustion was very weak. This is because the amount of methanol vaporization is little at low ambient temperature. The third and fourth cycle of the concentration of the mixture can not reach the lean firing limit. When the ambient temperature is below 16 °C, the methanol engine can not be started reliably without the auxiliary start aids even at a very high amount of methanol injected per cycle. 3.2. Effect of Glow-Plug Preheating on the Methanol Engine Cold-Start Firing Behavior. Figures 4 and 5 give the cylinder pressure and the instantaneous engine speed traces at different Qm, at the injection timing of 35 °CA BTDC and the ambient temperature of 13 °C using glow-plug preheating. From the figures, it can be seen that at Qm ) 50.1 mg, the firing is (14) Hu, L.; Gorden, E. A.; Grant, Z.; Basil, D.; Margaret, B.; James, T.; Karl, R. Impact of ambient temperatures on exhaust thermal characteristics during cold start for real world SI car urban driving tests. SAE paper 2005-01-3896, 2005.
Figure 6. HC emissions at several Qm using glow-plug preheating.
weak. The cylinder pressure increases obviously when Qm is increased to 90.3 mg, resulting in reliable firing of the fuel. When Qm further increases to 113.6 mg, the second firing can be seen from Figure 5d. This is due to Qm being too large. It cannot ensure the most fuel-air mixture to enter the cylinder on time. Figure 6 shows HC emissions from the starting engine at several Qm using glow-plug preheating. Fuel can not fire at Qm ) 45.8 mg. HC emissions are as high as 1800 ppm. Qm ) 50.1 mg results in the lowest HC emissions of 960 ppm. Further increasing Qm leads to a significant increase of HC emission. The second firing of the methanol partly left in the inlet port Qm ) 113.6 mg makes the HC emissions reduce again. 3.3. Effect of Injection Timing on the Methanol Engine Cold-Start Firing Behavior. The effect of injection timing on methanol engine cylinder pressure during cold start is shown in Figure 7 at the ambient temperature of 13 °C and Qm ) 50.1 mg. It can be observed that the effect of injection timing on methanol engine cold-start firing performance is very obvious. From Figure 7a, it can be seen that the first firing occurs at the fourth cycle if the methanol is injected from the second cycle 35 °CA BTDC, but the ideal firing cycle should be at the third cycle. This is due to the methanol being a single-compound
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Figure 10. Relationship between φ and ambient temperature at Qm ) 51.5 mg.
Figure 11. Relationship between φ and HC emission at Qm ) 51.5 mg.
Figure 7. Effect of injection timing on methanol engine cylinder pressure during cold start.
Figure 8. Effect of injection timing on HC emission.
Figure 9. Effect of the ratio of injected LPG/methanol on methanol engine cold start.
fuel. The boiling point of methanol (65 °C) is higher than the initial boiling point of gasoline (40 °C). The low vapor pressure and high latent heat of vaporization of methanol causes the mixture to be too lean in the third cycle to fire at low ambient
temperatures. In the fourth cycle, the concentration of the mixture surpasses the lean firing limit of methanol. Advancing methanol injection timing to 249 °CA BTDC, the maximum cylinder pressure is 140% higher than that of 35 °CA BTDC injected methanol. In fact, the 249 °CA BTDC injection timing means that the methanol is injected in the intake stroke of the second. It is possible to ensure the most fuel-air mixture to enter the cylinder on time in the third cycle to realize the ideal concept of next cycle combustion after fuel injection. The 341 °CA BTDC injected methanol make the second cycle mixture too lean to fire. In the third cycle, the concentration of the mixture reaches the lean firing limit of methanol, but cylinder pressure is lower than that of 249 °CA BTDC. Figure 8 gives the effect of injection timing on HC emissions. HC emissions at injection timing of 249 °CA BTDC is reduced 65% compared to that of 35 °CA BTDC injection timing. 3.4. Effects of Additional LPG Injected into the Inlet Port on the Methanol Engine Cold-Start Firing Behavior. Figure 9 shows the effect of the ratio of injected LPG to methanol (φ) on the methanol engine cold-start firing behavior at the ambient temperature of 13 °C. The methanol injection timing is 35 °CA BTDC, and the LPG injection timing is 57 °CA ATDC. The results show that φ increases with a decreasing Qm. This is because LPG evaporates more easily than that of methanol. The LPG has a lower boiling temperature and lower latent heat of vaporization than methanol; therefore, the addition of LPG changes the volatilization characteristic of LPG/ methanol blend fuel. Figure 10 illustrates the relationship between φ and the ambient temperature at Qm ) 51.5 mg. It can be seen that φ is increased with the drop of the ambient temperature. The ambient temperature affects obviously methanol vaporization. However, the effect of the ambient temperature on LPG vaporization is little. Figure 11 shows the relationship between φ and HC emission at Qm ) 51.5 mg. Fuel can not fire at φ ) 7.7. HC emissions are as high as 3100 ppm. φ ) 8.6 results in the lowest HC emissions of 2000 ppm. Further increasing φ leads to an increase of the HC emission. 4. Conclusions The conclusions from this study can be summarized as follows: (1) The ambient temperature significantly affects the
Combustion of a SI Methanol Engine
start firing behavior of the engine. When the ambient temperature is below 16 °C, the methanol engine can not be started reliably without the auxiliary start aids even at very high amounts of methanol injected per cycle. (2) Using a glow plug to heat the engine inlet manifold results in a reliable firing of the engine. If the fuel injection timing is controlled reasonably at the engine cold start, it is possible to ensure the most fuel-air mixture to enter the cylinder on time to realize the ideal concept of next cycle combustion after fuel injection. Optimal control on the fuel injection timing improves the cold-start reliability and reduces the coldstart HC emissions from the engine. (3) Using an additional LPG injected into the inlet port results in a reliable firing of the engine. The ratio of injected LPG/methanol for the reliable firing during the cold start of the engine reduces with the rise in ambient temperature.
Energy & Fuels, Vol. 22, No. 5, 2008 2985 Acknowledgment. This study was supported by the National Natural Science Foundation of China (50576031).
Nomenclature LPG ) liquefied petroleum gas SI ) spark ignition FTP ) federal test procedure HC ) hydrocarbon CO ) carbon monoxide UHC ) unburned hydrocarbon PFI ) port fuel injection ECU ) electronic control unit Qm ) amount of methanol injected per cycle φ ) ratio of injected LPG/methanol TDC ) top dead center BTDC ) before top dead center ATDC ) after top dead center EF8001636
