Energy Fuels 2009, 23, 4337–4342 Published on Web 07/29/2009
: DOI:10.1021/ef900433t
Performance and Hydrocarbon (HC) Emissions from a Spark-Ignition Liquefied Petroleum Gas (LPG) Engine during Cold Start Jun Li,†,‡ Changming Gong,*,† 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 11, 2009. Revised Manuscript Received July 9, 2009
The effects of ambient temperature, amount of liquefied petroleum gas (LPG) injected per cycle, injection timing of LPG, ignition timing and the electric battery voltage on firing performance, and hydrocarbon (HC) emissions from the first cycle of an electronically controlled inlet port LPG injection spark-ignition (SI) engine were investigated during cold start by means of a cycle-by-cycle control strategy. The results indicated that the amount of LPG injected per cycle is the key factor to ensure the first firing cycle of the LPG engine during cold start; a proper amount of injected LPG makes a reliable start of the LPG engine. The effect of ambient temperature on the minimum amount of injected LPG required for firing is relatively small. If the amount of injected LPG and injection timing of LPG are controlled reasonably to ensure all of the fuel-air mixture to enter the cylinder on time, it is possible to realize the ideal firing in the same cycle with LPG injection during engine cold start. Optimal control of injection timing of LPG and ignition timing improves firing performance and reduces HC emissions during cold start. Increasing the electric battery voltage raises the maximum instantaneous cranking speed and reduces HC emissions from the LPG engine during cold start.
advance, injection timing, and misfire characteristic on HC emissions at cold start. Gordon et al.7 investigated the effects of ambient temperature on cold start urban traffic emissions for a real world spark-ignition (SI) car. They found that carbon monoxide (CO) emission for the cold start was reduced by a factor of 8 downstream of the catalyst when the ambient temperature rose from 271 to 305 K; the corresponding HC emissions were reduced by a factor of 4. Lang et al.8 illustrated in detail the effects of intake valve opening (IVO) timing on the first cycle fuel delivery during cranking. Their engine simulation results showed that the high velocity and shear rate at the valve curtain occurred only for a brief period (a few milliseconds) during which the lift is small (a fraction of a millimeter). Thus, the fuel transport to the cylinder would be limited by the upstream feed, and only a small amount of liquid fuel located at the valve rim and seat is benefited from the high shear rate atomization. In addition, in the first cycle, because there is no residual burned gas in the cylinder, the expansion of the charge from exhaust valve closing (EVC) to IVO significantly cools the charge, so that the first fuel entering the cylinder is exposed to cold air, which inhibits evaporation. Kidokoro et al.2 reported improvement of mixture preparation by reducing the Sauter mean diameter (SMD) from 85 to 75 μm. To have the droplets follow the airflow and avoid significant wall wetting, the drop size must be of the order of 20 μm or less. Swindal et al.9 noted that multicomponent fuel evaporation is complicated. They proposed that the droplet evaporation is a batch distillation process for cold operation and during the early part of the
1. Introduction At the ultra-low emissions vehicle (ULEV) standard, 8090% of the tailpipe hydrocarbon (HC) emissions are emitted during the cold start of the federal test procedure (FTP).1 The dominant effect of the initial tens of seconds of engine operation on the tailpipe HC emissions results from two factors. First, engine-out HCs are high during cold start and warm up for several reasons. The most important of these is that the engine must be overfueled to achieve rapid cold start and acceptable idle during warm up. Second, the exhaust catalyst is inefficient at oxidizing the engine-out HCs until it reaches light-off temperature (typically 533 K).2 Especially, the driving cycles of Euro III, Euro IV, and the U.S. FTP-75 all take the first 40 s of idle into the start phase. Euro III and Euro IV emission standards have included subambient coldstart test at a temperature of 266 K. Therefore, the control of combustion and emissions during the cold start has become the hotspot in the field of vehicle engine developments in recent years.3-5 The research of Henein et al.6 was based on a cycle-by-cycle analysis of combustion and HC emissions during the first 120 cycles and warm-up phase in a four-stroke V6 gasoline engine. They discussed the effects of the air/fuel (A/F) ratio, ignition *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.; Sekiguchi, K.; Saito, K.; Imatake, N. SAE Tech. Pap. 950074, 1995. (2) Kidokoro, T.; Hoshi, K.; Hiraku, K.; Satoya, K.; Watanabe, T.; Fujiwara, T.; Suzuki, H. SAE Tech. Pap. 2003-01-0817, 2003. (3) Bielaczyc, P.; Merkisz, J. SAE Tech. Pap. 1999-01-1073, 1999. (4) Santoso, H.; Cheng, W. K. SAE Tech. Pap. 2002-01-2805, 2002. (5) Lang, K. R.; Cheng, W. K. SAE Tech. Pap. 2006-01-3400, 2006. (6) Henein, N. K.; Tagomori, M. K.; Yassine, M. K.; Asmus, T. W.; Thomas, C. P.; Hartman, P. G. SAE Tech. Pap. 952402, 1995. r 2009 American Chemical Society
(7) Gordon, E. A.; Grant. Z.; Hu, L.; Alex, S.; James, A. W. SAE Tech. Pap. 2004-01-2903, 2004. (8) Lang, K.; Cheng, W. K. SAE Tech. Pap. 2004-01-1852, 2004. (9) Swindal, J. C.; Dragonetti, D. P.; Hahn, R. T.; Furman, P. A.; Acker, W. P. SAE Tech. Pap. 950106, 1995.
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: DOI:10.1021/ef900433t
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intake stroke. In the batch distillation, the lighter components evaporate first, leaving the smaller droplets that are enriched in heavier fuel components. Mcgee et al.10 compared opening valve injection (OVI) and closed valve injection (CVI), using a conventional port injector. For the throttle ramp transients with cold coolant and valve targeting, OVI produced smaller A/F ratio excursions and a film mass in the intake system that was half that with CVI. However, CVI produced lower engine-out HC emissions, allowed more retarding of the spark, and improved mixture homogeneity for all loads and temperatures. Castaing et al.11 using fast-response flame ionization detectors (FFIDs) measured both the in-cylinder λ (relative air/fuel ratio) 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. Hochul et al.12 studied the total hydrocarbon (THC) emission characteristics in liquid-phase liquefied petroleum gas (LPG) injection (LPLi) engines during cold start operation. They found that the control strategy of retarding spark timing during cold start in the LPLi vehicle was very effective in reducing the THC emissions. Li et al.13 investigated the real-time nitrogen oxide (NOx) emissions during the cold start in a LPG SI engine. They founded that real-time NOx emissions can be used to understand the combustion and misfire occurrence. Hu et al.14 measured HC emissions from the engine fueled with methanol/gasoline blends during the cold start. The measured results showed that HC emissions were reduced about 40% at 278 K and 30% at 288 K compared to those of the gasoline engine when the engine is fueled with M30 (30% methanol and 70% gasoline in volume). Gong et al.15,16 studied the firing behavior of the SI engine fueled with methanol, LPG, and methanol/LPG during the cold start. They found that, when the ambient temperature is below 289 K, the methanol engine cannot 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 resulted in a reliable firing of the engine. At the same injection timing of LPG and methanol, the LPGfueled engine may realize the ideal firing of the next cycle combustion after fuel injection; the firing of the methanol engine is one cycle later than that of the LPG engine. LPG is considered one of the most promising alternative automotive fuels worldwide because of its potential emission reduction and relatively low fuel price compared to gasoline. However, the firing behavior of the first cycle for LPG SI engine has been seldom reported. All of the above works cited from the literature paid attention to the SI gasoline engine during cold start, while little work was reported on the alternative fuels engine, such as LPG engines. The objective of this work is to study the effects of ambient temperature,
Table 1. Engine Specifications bore stroke displacement compression ratio maximum power/speed maximum torque/speed cooling system intake valve opening (IVO) intake valve closing (IVC) exhaust valve opening (EVO) exhaust valve closing (EVC)
56.5 mm 49.5 mm 125 cm3 9.2:1 7.3 kW/8000 rpm 8.7 N m/8000 rpm air cooled 15 °CA BTDC 35 °CA ABDC 35 °CA BBDC 15 °CA ATDC
Table 2. Property of LPG property
LPG
formula relative molecular mass density (kg/m3) boiling point (°C) RON flammability limit (% vol) latent heat of vaporization (kJ/kg) lower heating value (MJ/kg) auto-ignition temperature (°C) stoichiometric air/fuel ratio flame speed (m/s)
C3H8 þ C4H10 44 58 508 584 -42 -0.5 111 103 2.2-9.5 1.9-8.5 426 385 46.1 45.5 480 440 15.65 15.43 0.38 0.37
Table 3. Molar Fractions of the Main Components in LPG propane (%)
isobutane (%)
butane (%)
dimethyl-propylene (%)
butadiene (%)
49
21
15
8
5
amount of LPG injected per cycle, LPG injection timing, ignition timing and the electric battery voltage on firing performance, and hydrocarbon emissions from the first cycle of an electronically controlled inlet port LPG injection SI engine during the cold start by means of a cycle-by-cycle control strategy, which may contribute toward improving the LPG engine cold-start firing performance and reduce its HC emissions. 2. Test Engine and Experimental Setup The experiment was conducted on a single-cylinder four-stroke electronically controlled LPG engine with inlet port fuel injection (PFI). The engine specifications are listed in Table 1. The test fuel for this study was LPG. Table 2 shows the physical and chemical properties of the used LPG, and its components are shown in Table 3. 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. A multichannel data acquisition card PLC-8018HG was used to record the in-cylinder pressure, instantaneous engine speed, and HC emissions synchronously. The HC emissions were measured with a FGA4015 exhaust gas analyzer. Gas-phase LPG was injected at a constant pressure of 0.04 MPa with a pressure regulator. The amount of the LPG injected per cycle, the LPG injection timing, and the ignition timing were controlled by an electronic control unit (ECU). During the coldstart test, through ECU control, an electric motor cranked the engine. The cycle of moment in which the engine starts rotating was defined as the first cycle. The LPG was injected in the first cycle by means of the 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 kPa, and the 0° crank angle (CA) injection timing of LPG corresponds to the piston position at the compression stroke top dead center (TDC) of the first cycle. The engine was always started from the position
(10) Mcgee, J.; Curtis, E.; Russ, S.; Lavoie, G. SAE Tech. Pap. 200001-2834, 2000. (11) Castaing, B. M.; Cowart, J. S.; Cheng, W. K. SAE Tech. Pap. 2000-01-2836, 2000. (12) Hochul, K.; Cha, L. M.; Simsoo, P. Fuel 2007, 86, 1475–1482. (13) Li, G.; Li, L. G.; Liu, Z. M.; Li, Z. L.; Qiu, D. P. Energy Convers. Manage. 2007, 48, 2508–2516. (14) Hu, T. G.; Wei, Y. J.; Liu, S. H.; Zhou, L. B. Energy Fuels 2007, 21, 171–175. (15) Gong, C. M.; Deng, B. Q.; Wang, S.; Su, Y.; Gao, Q.; Liu, X. J. Energy Fuels 2008, 22, 2981–2985. (16) Gong, C. M.; Deng, B. Q.; Wang, S.; Su, Y.; Gao, Q.; Liu, X. J. Energy Fuels 2008, 22, 3779–3784.
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Figure 2. Firing border of the LPG engine in the Qmin ∼ T map at θin=360 °CA ATDC, θig=10 °CA BTDC, and V=13.2 V. Figure 1. Schematic layout of the experimental system: (1) LPG tank, (2) gas valve, (3) solenoid valve, (4) pressure regulator, (5) mercury manometer, (6) throttle sensor, (7) LPG injection nozzle, (8) spark plug, (9) in-cylinder pressure transducer, (10) charge amplifier, (11) encoder, (12) TDC marker, (13) exhaust gas analyzer, (14) computer, and (15) ECU.
of the piston before top dead center (BTDC) of the compression stroke.
3. Results and Discussion 3.1. Effect of the Ambient Temperature and Minimum Amount of LPG Injected Per Cycle on Engine Cold-Start Firing Performance and HC Emissions. Figure 2 gives the relationship between the minimum amount of LPG injected per cycle (Qmin) for firing and the ambient temperature (T) for the LPG engine during cold start at the injection timing of LPG (θin) of 360 °CA ATDC, ignition timing (θig) of 10 °CA BTDC, and the electric battery voltage (V) of 13.2 V. The minimum amount of LPG injected per cycle increases by 10% when the ambient temperature varies from 301 to 258 K. Therefore, a drop in the ambient temperature results in a slight increase in the minimum amount of LPG injected per cycle. This is due to the fact that LPG evaporates fast. The effect of ambient temperature on LPG vaporization is small. In comparison to the methanol engine, the ambient temperature had less of an effect on the minimum amount of LPG injected per cycle for firing.16 Relative to 301 K, the minimum amount of methanol injected per cycle is increased by 110% at 289 K and the minimum amount of LPG injected per cycle is increased by 10% at 258 K. Figure 3 shows the effect of the amount of LPG injected per cycle (Q) on the maximum combustion pressure in the cylinder (pmax) at different ambient temperatures and θin = 360 °CA ATDC, θig=10 °CA BTDC, and V=13.2 V. It will be seen that the maximum combustion pressure in the cylinder reaches the highest pmax at Q = 15.5 mg and T = 280 K. When the amount of LPG injected per cycle is smaller or greater than 15.5 mg, pmax falls obviously. The pmax at different ambient temperatures has a similar trend as the amount of LPG injected per cycle with T=280 K. At a given amount of LPG injected per cycle, the pmax is significantly affected by the ambient temperature. The pmax increases with the ambient temperature rising. When the amount of LPG injected per cycle increases, the fuel-air mixture to enter the cylinder is greater, the LPG engine firing performance improves, and pmax rises rapidly. At a proper amount of LPG injected per cycle, the concentration of the fuel-air mixture may realize the ideal firing and obtain the highest pmax. When the amount of LPG injected per cycle is increased further, the
Figure 3. Effect of Q on pmax at different T and θin = 360 °CA ATDC, θig=10 °CA BTDC, and V=13.2 V.
Figure 4. Effect of Q on n at different T and θin = 360 °CA ATDC, θig = 10 °CA BTDC, and V = 13.2 V.
Figure 5. Effect of Q on HC emissions at different T and θin = 360 °CA ATDC, θig=10 °CA BTDC, and V=13.2 V.
mixture becomes too rich, firing performance rapidly deteriorates, and until the concentration of the mixture surpasses the rich firing limit of LPG at Q=23.1 mg, the LPG engine cannot fire. Figures 4 and 5 give the effect of the amount of LPG injected per cycle on the maximum instantaneous cranking 4339
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Figure 7. Effect of θin on n at Q = 15.5 mg, T = 266 K, θig = 10 °CA BTDC, and V = 13.2 V.
Figure 8. Effect of θin on HC emissions at Q = 15.5 mg, T = 266 K, θig = 10 °CA BTDC, and V = 13.2 V.
to realize the next cycle firing after fuel injection. Retarding θin to 360 °CA ATDC, LPG enters the intake port in the intake stroke of the second cycle and the cylinder of the second cycle; hence, it can make LPG fire in the second cycle, to realize the same cycle firing after fuel injection. Owing to the fact that LPG is a gas-phase fuel, it can fully enter the cylinder in the second cycle; thus, θin has no effect on pmax between -35 and 360 °CA ATDC of θin. When θin is retarded to 400 °CA ATDC, LPG enters the intake port in the intake stroke of the second cycle, LPG cannot fully enter into the cylinder in the second cycle, and part of LPG enters into the cylinder in the third cycle. The concentration of the mixture surpasses the lean firing limit, and the LPG engine can fire in the second cycle. However, the combustion is weak; therefore, pmax is 35% lower than that of 360 °CA ATDC of θin. When θin is further retarded to 440 °CA ATDC, LPG can partly enter into the cylinder in the second and third cycles and the concentration of the mixture cannot reach the lean firing limit in the second and third cycles; therefore, the LPG engine cannot fire in the second and third cycles. Figures 7 and 8 give the effect of θin on n and HC emission at Q = 15.5 mg, T = 266 K, θig = 10 °CA BTDC, and V = 13.2 V. When θin. is between -35 and 360 °CA ATDC, θin has no effect on instantaneous engine speed and HC emissions, the nmax reaches as high as 1370 rpm, and HC emissions reach as low as 350 ppm. Further retarding θin to 440 °CA ATDC leads to a significant increase of HC emission and an obvious decrease of nmax. 3.3. Effect of the Ignition Timing on LPG Engine ColdStart Performance and HC Emissions. Figure 9 shows the effect of θig on p traces at Q=13 mg, T=266 K, θin=360 °CA ATDC, and V=13.2 V. When the ignition timing is retarded from -30 to 30 °CA ATDC, pmax drops significantly, the position of pmax changed from 190 to 266 °CA ATDC, and until θig = 30 °CA ATDC, the LPG engine cannot fire. Therefore, retarding ignition timing makes LPG engine combustion later and decreases pmax.
Figure 6. Effect of θin on p traces at Q = 15.5 mg, T = 266 K, θig = 10 °CA BTDC, and V = 13.2 V.
speed (nmax) during cold start and HC emissions at different ambient temperatures and θin = 360 °CA ATDC, θig = 10 °CA BTDC, and V=13.2 V. From the figures, it can be seen that, at Q = 10 mg, the mixture is lean and the concentration of the mixture cannot reach the lean firing limit and, at Q = 23.1 mg, the mixture is rich and the concentration of the mixture surpasses the rich firing limit; hence, the LPG engine cannot fire, nmax is low, and HC emissions are high. When Q is between 12 and 22 mg, the LPG engine can reliably fire, resulting in high nmax and low HC emissions. The nmax is increased significantly, and the HC emission is decreased obviously with the rise of the ambient temperature. 3.2. Effect of the Injection Timing of LPG on Engine ColdStart Performance and HC Emissions. Figure 6 shows the effect of θin on the in-cylinder pressure (p) traces at Q = 15.5 mg, T =266 K, θig = 10 °CA BTDC, and V= 13.2 V. When θin =-35 °CA ATDC, LPG enters the intake port in the intake stroke of the first cycle and the cylinder of the second cycle; hence, it can make LPG fire in the second cycle, 4340
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Figure 11. Effect of θig on HC emissions at Q=13 mg, T=266 K, θin=360 °CA ATDC, and V=13.2 V. Figure 9. Effect of θig on p traces at Q = 13 mg, T = 266 K, θin = 360 °CA ATDC, and V = 13.2 V.
Figure 10. Effect of θig on n at Q=13 mg, T=266 K, θin=360 °CA ATDC, and V=13.2 V. Figure 12. Instantaneous engine speed n at different V and Q = 13 mg, T=271 K, and θin=360 °CA ATDC.
Figures 10 and 11 give the effect of θig on nmax and HC emissions at Q=13 mg, T=266 K, θin=360 °CA ATDC, and V=13.2 V. It can be seen that nmax increases from 1175 to 1231 rpm when θig is retarded from 30 to 20 °CA BTDC, and when θig is further retarded to 5 °CA BTDC, n falls to 1195 rpm. It is due to the decrease in the compression negative work when θig is retarded from 30 to 20 °CA BTDC and late combustion and the drop in capability of doing work until retarding θig to 5 °CA BTDC. Retarding θig to 30 °CA ATDC makes the concentration of the mixture not reach the lean firing limit, and the LPG engine cannot be started. The HC emissions were almost unchanged (about 440 ppm) when θig is retarded from 30 °CA BTDC to 10 °CA ATDC and then increase rapidly to 1455 ppm, because no firing occurs at θig =30 °CA ATDC. 3.4. Effect of the Electric Battery Voltage on LPG Engine Cold-Start Performance and HC Emissions. The effect of the ambient temperature on the electric battery voltage is obvious. When the ambient temperature decreases, the battery voltage drops obviously. Figure 12 shows an instantaneous cranking speed (n) at different V and Q=13 mg, T=271 K, and θin =360 °CA ATDC. The instantaneous engine speed decreased significantly with the drop of the electric battery voltage. When the electric battery voltages were 14.7, 13.2, and 12.3 V, the corresponding n=620, 580, and 550 rpm on average. Figure 13 shows the effect of Q on pmax at different V and θin =360 °CA ATDC, θig =10 °CA BTDC, and T=271 K. For rich and lean mixtures, pmax is low; the pmax reaches the highest pmax at Q = 15.5 mg and V = 14.7 V. The pmax at different V had a similar trend as the amount of LPG injected per cycle with V=14.7 V. At a given amount of LPG injected per cycle, pmax is significantly affected by V. The pmax increases with V rising. When the amount of LPG injected per cycle increases, the fuel-air mixture to enter the cylinder is much greater, the LPG engine firing performance
Figure 13. Effect of Q on pmax at different V and θin = 360 °CA ATDC, θig=10 °CA BTDC, and T=271 K.
improves, and the pmax rises rapidly. At a proper amount of LPG injected per cycle, the concentration of the fuel-air mixture may realize the ideal firing and obtain the highest pmax. When the amount of LPG injected per cycle is increased further, the mixture is too rich, firing performance deteriorates rapidly, and until the LPG engine cannot fire at Q = 23.1 mg, the pmax falls to 1.3 MPa. Figures 14 and 15 give the effect of Q on nmax and HC emissions at different V and θin =360 °CA ATDC, θig =10 °CA BTDC, and T=271 K. It can be seen that, at a given V and at Q=10 mg, the mixture is lean and the concentration of the mixture cannot reach the lean firing limit and, then at Q= 23.1 mg, the mixture is rich and the concentration of the mixture surpasses the rich firing limit; hence, the LPG engine cannot fire. The instantaneous engine speed is low, and the HC emission is high. When Q is between 12 and 22 mg, the LPG engine can reliably fire, resulting in high nmax and low HC emissions, and nmax and HC emissions were almost 4341
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performance and reduces the HC emissions during the cold start. (4) Increasing the electric battery voltage raises the maximum instantaneous cranking speed and reduces the HC emissions from the LPG engine during the cold start. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant 50576031) and China First Automobile Works Corporation.
Nomenclature LPG=liquefied petroleum gas HC=hydrocarbon SI=spark ignition ULEV=ultra-low emissions vehicle FTP=federal test procedure A/F=air/fuel CO=carbon monoxide IVO=intake valve opening IVC=intake valve closing EVO=exhaust valve opening EVC=exhaust valve closing SMD=Sauter mean diameter OVI=opening valve injection CVI=closed valve injection FFID=fast-response flame ionization detector λ=relative air/fuel ratio THC=total hydrocarbon LPLi=liquid-phase LPG injection NOx =nitrogen oxide M30=30% methanol and 70% gasoline in volume PFI=port fuel injection ECU=electronic control unit CA=crank angle TDC=top dead center BTDC=before top dead center ATDC=after top dead center BBDC=before bottom dead center ABDC=after bottom dead center T=ambient temperature Q=amount of LPG injected per cycle θin =LPG injection timing θig =ignition timing V=electric battery voltage Qmin =minimum amount of LPG 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
Figure 14. Effect of Q on n at different V and θin = 360 °CA ATDC, θig = 10 °CA BTDC, and T = 271 K.
Figure 15. Effect of Q on HC emissions at different V and θin = 360 °CA ATDC, θig = 10 °CA BTDC, and T = 271 K.
unchanged. The nmax increases significantly, and the HC emissions decrease obviously with the rise of V. 4. Conclusions The effects of ambient temperature, amount of the LPG injected per cycle, LPG injection timing, ignition timing, and the electric battery voltage on firing performance and HC emissions from the first cycle of an inlet port LPG injection SI engine were investigated during cold start by means of a cycleby-cycle control strategy. The main conclusions can be summarized as follows: (1) The amount of LPG injected per cycle is the key factor to ensure the first firing cycle of the LPG engine during cold start, and a proper amount of LPG injection can ensure a reliable start of the LPG engine. (2) The effect of ambient temperature on the minimum amount of LPG injected per cycle for firing is relatively small. If the amount of LPG injected per cycle and LPG injection timing are controlled reasonably during cold start, it is possible to ensure that all of the fuel-air mixture enters the cylinder on time and to realize the ideal firing in the same cycle combustion after LPG injection. (3) Optimal control of injection timing of LPG and ignition timing improves the firing
Subscripts in=injection ig=ignition
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