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Effects of Fuel Injection Timing on Combustion and Emissions of a Spark-Ignition Methanol and Methanol/Liquefied Petroleum Gas (LPG) Engine during Cold Start Changming Gong,*,† Shufang Yan,† Yan Su,† and Zhiwei Wang‡ State Key Laboratory of Automobile Dynamic Simulation, Jilin UniVersity, Changchun 130022, China, and China AutomotiVe Engineering Research Institute, Chongqing 40039, China ReceiVed NoVember 15, 2008. ReVised Manuscript ReceiVed May 8, 2009
The combustion and emission characteristics of spark-ignition (SI) methanol and methanol/ liquefied petroleum gas (LPG) engines under different fuel injection timings during cold start have been studied by means of a cycle-by-cycle control strategy. The results showed that the methanol injection timing, the LPG injection timing, and the LPG injection timing delay relative to the methanol injection timing affect cold-start combustion characteristics and emissions of hydrocarbons, formaldehyde, and unburned methanol significantly. The methanol injection timing, the LPG injection timing, and the LPG injection timing delay relative to the methanol injection timing can be optimized to ensure that most fuels enter into the cylinder in the injection cycle and the following cycle and that reliable ignition and combustion happen in the third cycle. Optimal control of the fuel injection timing and the LPG injection timing delay relative to the methanol injection timing improves firing performances, reduces the hydrocarbon and unburned methanol emissions during cold start, and increases formaldehyde emissions slightly. The variations in formaldehyde and unburned methanol emissions show opposite tendencies with the variations in methanol injection timing, the LPG injection timing, and the LPG injection timing delay relative to the methanol injection timing.
1. Introduction Because of limited reserves of fossil fuel, development of alternative fuel engines has attracted more and more concern in the engine community. The introduction of alternative fuels is beneficial to help alleviate the fuel shortage and reduce engine exhaust emissions.1 Methanol (CH3OH) is considered to be one of the favorable fuels for engines. It is a colorless liquid with a mild characteristic odor and can be produced from natural gas, biomass, coal, and also municipal solid wastes and sewage. Methanol is extremely toxic and has a damaging effect on the human nervous system, especially the optic nerves, and can even lead to death. It is not only methanol itself that causes the problems but also its oxidation product, formaldehyde.2 Methanol is characterized by a high octane number, indicating high antiknock performance and a high latent heat of vaporization allowing for a denser fuel/air charge and excellent lean-burn properties.3 However, methanol is a relatively simple, singlecompound fuel. The boiling point of methanol (65 °C) is higher than the initial boiling point of gasoline (about 40 °C).4 Therefore, the low vapor pressure and high latent heat of vaporization of methanol may cause cold start difficulties for a * To whom correspondence should be addressed: State Key Laboratory of Automobile Dynamic Simulation, Jilin University, Changchun 130022, China. E-mail:
[email protected]. † Jilin University. ‡ China Automotive Engineering Research Institute. (1) Huang, Z. H.; Zhang, Y.; Zeng, K.; Liu, B.; Wang, Q.; Jiang, D. M. Energy Fuels 2007, 21, 692–698. (2) Charalampos, I. A.; Anastasios, N. K.; Panagiotis, D. S. SAE Tech. Pap. 2003-32-0024, 2003. (3) Frank, B. SAE Tech. Pap. 912413, 1991. (4) Changming, G.; Baoqing, D.; Shu, W.; Yan, S.; Qing, G.; Xunjun, L. Energy Fuels 2008, 22, 2981–2985.
methanol engine at low ambient temperatures.5 The vaporization of methanol at the inlet port injection location is ineffective because of the poor volatility of methanol relative to gasoline. The majority of hydrocarbon (HC) emissions produced during the Federal Test Procedure (FTP) cycle occur during the first few cycles of engine firing. Excess fuel must be introduced into the combustion chamber to achieve a combustible mixture. Cold intake valve and port surfaces create a less-than-ideal combustible air/fuel mixture because of poor atomization.6 These effects become more severe if the ambient temperature drops below 0 °C. Also, conventional three-way catalytic converters used for exhaust gas aftertreatment of SI engines are ineffective in oxidizing unburned HCs until heated to their “light-off” temperature (about 220 °C) by the heat transfer from exhaust gases.7 Alkidas8 examined the effects of injection timing, coolant temperature, and fuel volatility for steady-state operation. In agreement with Yang et al.,9 they found that the HC emissions are almost independent of injection timing for closed valve injection (CVI) unless the start of injection is ∼80 °CA (crank angle) 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 (5) Bassem, H. R.; Fakhri, J. H.; Charles, L. G.; Karl, H. H.; Harold, J. S. SAE Tech. Pap. 2002-01-2702, 2002. (6) Koederitz, K. R.; Drallmeier, J. A. SAE Tech. Pap. 1999-01-0566, 1999. (7) Fischer, H. C.; Brereton, G. J. SAE Tech. Pap. 970040, 1997. (8) Alkidas, A. C. SAE Tech. Pap. 941959, 1994. (9) Yang, J.; Kaiser, E. W.; Siegl, W. O.; Anderson, R. W. SAE Tech. Pap. 930711, 1993.
10.1021/ef900190q CCC: $40.75 2009 American Chemical Society Published on Web 05/22/2009
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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
combustion chamber. Stache et al.10 compared the results for three fuels. They found that iso-octane has a boiling point that is 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 is that more of it vaporized before impacting the in-cylinder surface. Quader et al.11 spectroscopically measured the fraction of injected gasoline vaporized to achieve the first combustion at two different temperatures in a Waukesha CFR engine. They found that, at 22 °C, 57% of the injected fuel vaporized in the first cycle in which combustion took place versus only 30% at a temperature of 0 °C. Thus, the required enrichment was almost twice as great at the lower temperature. They found that the measured fuel vapor concentrations never reached a level as great as the calculated equilibrium-based fuel vapor/air equivalence ratios. They also found that, after the first cycle with combustion, dilution with residual gas could contribute to misfires in subsequent cycles, requiring that the residual-free fuel vapor/air equivalence ratio be greater than the lean flammability limit in succeeding cycles. Kim et al.12 used a color-image-capturing technique to quantify linear and piston wetting for OVI versus CVI and five different multihole injectors. They found that the large drops impacting the piston with CVI could be almost eliminated with some injector designs. Zughyer et al.13 visualized the liquid fuel distribution and flame propagation inside a port fuel injection (PFI) gasoline engine using various injectors under different engine conditions. It was found that most of the fuel under 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 fuel transport process under both CVI and OVI strategies could be categorized into three phases. The first phase was at early cycles when most of the injected fuel ended up as liquid film on the chamber walls, resulting in insufficiently vaporized fuel in a mixture too lean to sustain the combustion and little effect on the cylinder pressure. The second phase started when visible weak flame fronts appeared because of the liquid fuel evaporation process. Diffusion-controlled pool-fire processes became obvious at this phase, and the in-cylinder pressure increase was significant. The third phase started when the heat of combustion evaporated most of the liquid fuel and produced an overallrich mixture that combusted with a high speed and produced a high in-cylinder pressure. Castaing et al.14 used fast-response flame ionization detectors (FFIDs) to measure both the incylinder λ (relative air/fuel ratio) and the HC emissions for the (10) Stache, I.; Alkidas, A. C. SAE Tech. Pap. 972981, 1997. (11) Quader, A. A.; Majkowski, R. F. SAE Tech. Pap. 1999-01-1107, 1999. (12) Kim, H.; Yoon, S.; Lai, M. C.; Quelhas, S.; Boyd, R.; Kumar, N.; Yoo, J. H. SAE Tech. Pap. 2003-01-3240, 2003. (13) Zughyer, J.; Zhao, F. Q.; Lai, M. C.; Lee, K. SAE Tech. Pap. 200001-0242, 2000. (14) Castaing, B. M.; Cowart, J. S.; Cheng, W. K. SAE Tech. Pap. 200001-2836, 2000.
Table 2. Properties of Methanol and LPG property
methanol
LPG
formula relative molecular mass
CH3OH 32 composition (% m) C 37.5 H 12.5 O 50.0 3 density (kg/m ) 790 boiling point (°C) 65 RON 111 flammability limit (% v) 6.7-36 latent heat of vaporization (kJ/kg) 1110 lower heating value (MJ/kg) 19.6 auto-ignition temperature (°C) 470 stoichiometric air/fuel ratio 6.5 flame speed (m/s) 0.523
C3H8 44
C4H10 58
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 Molar Fractions of Main Components in LPG propane %
isobutene %
butane %
dimethyl-propylene %
butadiene %
49
21
15
8
5
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. Henein et al.15 recorded the cycle-cycle behavior of the engine startup process. They studied the effects of the air/fuel ratio, ignition timing, injection timing, and misfire characteristics on HC emissions at cold start. Lang et al.16 studied in detail the effects of 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). Hochul et al.17 investigated the total hydrocarbon (THC) emission characteristics in liquid-phase 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 a very effective method to reduce THC emissions. The amount of fuel required for the first cycle is significantly more than that required for later cycles for stable combustion without misfire during cold start.18 All of the above literature references were concerned with a SI gasoline or LPG engine during cold start, while little work was reported on the methanol engine.19 The objective of the work is to study the combustion and emission characteristics of a SI engine fueled with methanol and methanol/LPG under different fuel injection timings during cold start. The effects of fuel injection timing on combustion and emission characteristics of SI methanol and methanol/LPG engines during cold start are investigated by means of the single-cycle fuel injection. These results are helpful to understand the cold-start behavior of the methanol engine. 2. Experimental Section 2.1. Test Engine. The experiment was conducted on a singlecylinder four-stroke electronically controlled methanol engine with inlet PFI. The engine specifications are listed in Table 1. (15) Henein, N. K.; Tagomori, M. K.; Yassine, M. K.; Asmus, T. W.; Thomas, C. P.; Hartman, P. G. SAE Tech. Pap. 952402, 1995. (16) Lang, K.; Cheng, W. K. SAE Tech. Pap. 2004-01-1852, 2004. (17) Hochul, K.; Cha, L. M.; Simsoo, P. Fuel 2007, 86, 1475–1482. (18) Takeda, K.; Yaegashi, T.; Sekiguchi, K.; Saito, K.; Imatake, N. SAE Tech. Pap. 950074, 1995. (19) 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 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) exhaust gas analyzer, (19) computer, (20) methanol pump, (21) methanol tank, and (22) ECU.
Figure 2. Layout of the gas sampling system.
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. A multichannel data acquisition card PLC-8018HG was used to record the in-cylinder pressure and HC emissions. The HC emissions were analyzed by a nondispersive infrared analyzer (NDIR) in the FGA4015 exhaust gas analyzer. The formaldehyde and unburned methanol were analyzed by a gas chromatograph (Shimadzu GC2010) and a liquid chromatograph (Waters 600E). The gas chromatograph used a flame ionization detector (FID), and the liquid chromatograph used a ultraviolet detector. Values of HC concentrations in engine exhaust gases measured by a FID are about 2 times the equivalent values measured by a NDIR analyzer (on the same carbon number basis, e.g., C1).20 NDIR-obtained HC concentrations are multiplied by 2 to obtain an estimate of actual HC concentrations. This estimation method was used in the paper for HC concentrations obtained from NDIR measurements. The sampling bag was used to collect the 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 engine. To prevent the injected methanol from combustion 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 (20) Heywood, J. B. McGraw-Hill Book Company: New York, 1988.
temperature of the inlet manifold surface reached 42 °C. The methanol and LPG fuel injection systems were installed separately and worked independently. The injection nozzle of methanol 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. 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 methanol/ LPG engine. The amount of the LPG 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 was defined as the first cycle. The methanol and LPG were injected in the first cycle by means of respective single-cycle fuel injection systems. 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, the ignition timing of all of the tests was fixed at 20 °CA before top dead center (BTDC), and the ambient temperature was 8 °C. A 0 °CA of injection timing of methanol and LPG corresponds to the piston position at compression stroke top dead center (TDC) of the first cycle.
3. Results and Discussion 3.1. Effects of Methanol Injection Timing on Combustion and Emission Characteristics of Methanol Engine during Cold Start. Figures 3 and 4 give the effect of methanol injection timing (θm) on in-cylinder pressure (p) and instantaneous engine speed (n) traces of methanol engine at Qm (amount of methanol injected per cycle) ) 61.3 mg using glow-plug preheating. From the figures, it can be seen that, at θm ) -35 °CA after top dead center (ATDC), the firing occurs in the third cycle and is weak. When methanol injection timing is retarded to 379 °CA ATDC, the maximum in-cylinder pressure (pmax) and maximum engine speed (nmax) increase. When methanol injection timing is further retarded to 471 °CA ATDC, pmax and nmax increase obviously. The second firing can be seen from Figure 4c. The pmax reaches as high as 4.29 MPa, and nmax is 1377 rpm. The pmax under methanol injection at 471 °CA ATDC is 120% higher than that
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Figure 4. Effect of θm on n traces of the methanol engine at Qm ) 61.3 mg.
Figure 3. Effect of θm on p traces of the methanol engine at Qm ) 61.3 mg.
of -35 °CA ATDC, and nmax at 471 °CA ATDC injection is 9.8% higher than that of -35 °CA ATDC. Figure 3 shows that the first firing cycle appears at the third cycle if the methanol is injected during the first cycle (θm ) -35 °CA ATDC) and the first firing appears at the third cycle if the methanol is injected during the second cycle (θm ) 471 °CA ATDC). Methanol injection at 471 °CA ATDC results in the methanol burning in the following cycle after injection. This is due to the low vapor pressure and high latent heat of vaporization of methanol, which causes the mixture to be lean in the second cycle and to fire at θm ) -35 °CA ATDC, while in the third cycle, the concentration of the mixture surpasses the lean firing limit of methanol. Methanol injection at 471 °CA ATDC means that the methanol is injected in the intake stroke of the second
Figure 5. Effect of θm on HC emission from the methanol engine at Qm ) 61.3 mg.
cycle, and in the third cycle, the largest fuel/air mixture can enter the cylinder on time to achieve the ideal firing. When θm is retarded to 563 °CA ATDC, the concentration of the mixture reaches the lean-firing limit in the third cycle; meantime, the fourth and fifth cycle firing can be seen from Figure 4d. This is due to θm being too late, which means that not all of the mixture enters the cylinder in the third cycle. In the fourth and fifth
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Figure 6. Effect of θm on formaldehyde emission from the methanol engine at Qm ) 61.3 mg.
Figure 7. Effect of θm on unburned methanol emission from the methanol engine at Qm ) 61.3 mg.
Figure 9. Effect of θ on p traces of the methanol/LPG engine at Qm ) 54.7 mg, R ) 9.2%, and θm ) 471 °CA ATDC.
Figure 8. p traces of the methanol/LPG engine under different θm and θL at Qm ) 54.7 mg, R ) 9.2%, θ ) 92 °CA.
cycles, the concentration of the mixture reaches the lean-firing limit of methanol but pmax and nmax are lower than that at 471 °CA ATDC. Retarding θm to 747 °CA ATDC makes the concentration of the mixture not reach the lean firing limit of methanol in all cycles, and the methanol engine cannot be started reliably. Therefore, Figures 3 and 4 indicate that θm affects p and n significantly. Figure 5 gives the effect of θm on HC emission from the methanol engine. The HC emission at θm ) 563 °CA ATDC is the lowest at 418 ppm. It is reduced by 74% compared to that of θm ) -35 °CA ATDC. When the methanol injection is retarded from 563 to 747 °CA ATDC, the HC emission becomes 214% higher. Figures 6 and 7 show the effect of θm on formaldehyde and unburned methanol emissions from the methanol engine. The effects of θm on formaldehyde and unburned methanol emissions
are obvious. At θm ) -35 °CA ATDC, the methanol engine firing is weak, formaldehyde emission is low, and unburned methanol emission is high. When θm is retarded to 471 °CA ATDC, owing to the good firing behavior, formaldehyde emission increases and unburned methanol emission decreases significantly. When θm is further retarded to 747 °CA ATDC, because of no firing, formaldehyde emission is lower and unburned methanol emission is significantly higher than that of 471 °CA ATDC. The formaldehyde and unburned methanol emissions are closely related to the firing and combustion. The formaldehyde and unburned methanol emissions show opposite tendencies with a variation of θm. More formaldehyde is emitted under good firing, and the most unburned methanol is emitted at misfire. 3.2. Effects of Methanol and LPG Injection Timing on Combustion and Emission Characteristics of the Methanol/ LPG Engine during Cold Start. Figure 8 shows p traces of the methanol/LPG engine under different θm and θL at Qm ) 54.7 mg, R (mass ratio of injected LPG/methanol) ) 9.2%, and θ (LPG injection timing delay relative to the methanol injection timing) ) 92 °CA. From Figure 8a, it can be observed that the firing cycle occurs in the third cycle at θm ) -35 °CA ATDC. This is because the mixture is too lean in the second cycle to fire. Figure 8b shows that, when methanol is injected at 471 °CA ATDC, i.e., injected in the intake stroke of the second
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Figure 12. Effect of θL on formaldehyde emission from the methanol/ LPG engine at Qm ) 54.7 mg, R ) 9.2%, and θm ) 471 °CA ATDC.
Figure 13. Effect of θL on unburned methanol emission from the methanol/LPG engine at Qm ) 54.7 mg, R ) 9.2%, and θm ) 471 °CA ATDC.
Figure 10. Effect of θL on n traces of the ethanol/LPG engine at Qm ) 54.7 mg, R ) 9.2%, and θm ) 471 °CA ATDC.
Figure 11. Effect of θL on HC emission from the methanol/LPG engine at Qm ) 54.7 mg, R ) 9.2%, and θm ) 471 °CA ATDC.
cycle, the firing cycle occurs in the third cycle too. This is because methanol and LPG enter the cylinder in the third cycle, and the concentration of the mixture surpasses the lean firing limit of the methanol/LPG blend fuel. The pmax at θm ) 471 °CA ATDC and θL ) 563 °CA ATDC is 10% higher than that at θm ) -35 °CA ATDC and θL ) 57 °CA ATDC at θ ) 92 °CA. Figures 9 and 10 give the effect of θL on p and n traces of the methanol/LPG engine at Qm ) 54.7 mg, R ) 9.2%, and θm ) 471 °CA ATDC. When θ ) 0 °CA and θm ) θL ) 471 °CA ATDC, the methanol and LPG are injected in the intake stroke
of the second cycle. Because LPG vaporizes fast and the methanol vaporizes slowly, LPG enters the cylinder in the second cycle and the methanol enters the cylinder in the third cycle. In this case, LPG cannot play the part of a start aid in the methanol/LPG engine; therefore, the methanol/LPG engine cannot fire. When θL is retarded to 563 °CA ATDC, the methanol and LPG can enter the cylinder in the third cycle and the methanol/LPG engine can fire reliably. At θL ) 747 °CA ATDC, the methanol/LPG engine firing behavior improves further. The pmax reaches as high as 4.39 MPa, and nmax is 1438 rpm. The pmax of 747 °CA ATDC LPG injection timing is 206% higher than that of 471 °CA ATDC, and nmax is 63% higher. When θL is further retarded to 931 °CA ATDC, most LPG enter the cylinder in the fourth cycle; therefore, the concentration of the mixture cannot reach the lean firing limit of the methanol/ LPG blend fuel, and the methanol/LPG engine cannot fire in the third cycle but can fire in the fourth cycle. Therefore, the methanol/LPG engine produces a weak firing. The firing cycle at θL ) 931 °CA ATDC is one cycle later than that at θL ) 747 °CA ATDC. Figure 11 shows the effect of θL on HC emission from the methanol/LPG engine at Qm ) 54.7 mg, R ) 9.2%, and θm ) 471 °CA ATDC. At θL ) 471 °CA ATDC, the methanol/LPG engine cannot fire and HC emission reaches as high as 3085 ppm. θL ) 747 °CA ATDC results in the lowest HC emission of 1208 ppm. Further retarding θL leads to an increase of the HC emission. Figures 12 and 13 give the effect of θL on formaldehyde and unburned methanol emissions from the methanol/LPG engine at Qm ) 54.7 mg, R ) 9.2%, and θm ) 471 °CA ATDC. At θL ) 471 °CA ATDC, the methanol/LPG engine cannot fire, formaldehyde emission is high, and unburned methanol emission is low. Increasing θL to 747 °CA ATDC leads to a reduction of the unburned methanol emission and an increase of the formaldehyde emission. Formaldehyde emission reaches as high as 1537 ppm, and unburned methanol is as low as 1225 ppm. Further increasing θL to 931 °CA ATDC leads to a reduction of the formaldehyde emission and an increase of the unburned
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The effects of fuel injection timing on combustion and emissions from SI methanol and methanol/LPG engines during cold start were investigated by means of a cycle-by-cycle control strategy. The main conclusions can be summarized as follows: (1) The methanol injection timing, the LPG injection timing, and the LPG injection timing delay relative to the methanol injection timing affect the cold-start combustion characteristic and HC, formaldehyde, and unburned methanol emissions significantly. (2) The methanol and LPG injection timing and the LPG injection timing delay can be optimized to ensure that most fuels enter into the cylinder in the injection cycle and the following cycle. Therefore, reliable ignition and combustion happen in the third cycle. (3) Optimal control of the fuel injection timing and LPG injection timing delay improve firing performances, reduce the HC and unburned methanol emissions at the cold start, but increase formaldehyde emissions slightly. (4) The formaldehyde and unburned methanol emissions show opposite tendencies with changes in the methanol and LPG injection timings and the LPG injection timing delay relative to methanol injection timing.
CVI ) closed valve injection CA ) crank angle IVO ) intake valve opening OVI ) opening valve injection T40 ) temperature of 40% volume distilled point PFI ) port fuel injection FFID ) fast-response flame ionization detector λ ) relative air/fuel ratio THC ) total hydrocarbon LPLi ) liquid-phase LPG injection NDIR ) nondispersive infrared analyzer FID ) flame ionization detector ECU ) electronic control unit BTDC ) before top dead center ATDC ) after top dead center TDC ) top dead center Q ) amount of fuel injected per cycle R ) mass ratio of injected LPG/methanol pmax ) maximum combustion pressure in the cylinder p ) in-cylinder pressure nmax ) maximum instantaneous engine speed n ) instantaneous engine speed θ ) LPG injection timing delay relative to methanol injection timing θm ) methanol injection timing θL ) LPG injection timing R ) crank angle
Acknowledgment. This study was supported by the National Natural Science Foundation of China (Grant 50576031).
Subscripts m ) methanol L ) LPG
methanol emission. The formaldehyde and unburned methanol emissions show opposite tendencies with a variation of θL too. 4. Conclusions
Nomenclature LPG ) liquefied petroleum gas SI ) spark ignition HC ) hydrocarbon FTP ) Federal Test Procedure
Note Added after ASAP Publication. Reference 15 was modified in the version of this paper published ASAP May 22, 2009; the corrected version published ASAP June 1, 2009. EF900190Q
