| Low-carbon economy is a necessary requirement for sustainable development. LPG (Liquefied Petroleum Gas) mainly consists of alkanes of C3 and C4, which makes it an excellent alternative fuel. Compared with gasoline, it has lower carbon hydrogen ratio, higher octane number and vapor pressure, and lower CO2 and other pollutants emissions. Furthermore, LPG has almost the same heat value as gasoline, but is much cheaper in per Joule. Therefore, a liquid phase LPG port injection engine can obtain the same power as or more than a gasoline engine with the same displacement, as well as reducing more harmful emissions. Moreover, a DI (Direct Injection) LPG engine can further improve the power performance, economy and emission characteristics.The saturated vapor pressure of LPG is much higher than that of gasoline, which makes LPG engine more easily form a homogeneous mixture. Hence LPG has the potential of reducing injection pressure. Therefore, the mixture formation of direct injection LPG engine under the lower pressure than gasoline was investigated in this paper by numerical simulation.Firstly, the possibility of the homogeneous mixture formation under the condition of liquefied LPG with low injection pressure is analyzed by numerical simulation. The correctness of the simulation method is confirmed by comparing the results with those obtained by schlieren experiment. Based on this, the formation of homogeneous mixture with LPG injected in intake stroke at the pressure of 2MPa is further studied. The simulated results showed that, at high load conditions with two intake valve opened, the intake tumble would break into small-scale vortex (turbulence) near the end of compression stroke; while at part load conditions with one of the two intake valve closed, an inclined intake swirl was formed, which enhanced the turbulence in combustion chamber near the end of compression stroke. The results also showed that if the liquid phase LPG was injected at 60-80℃A ATDC in intake stroke, the homogeneous mixture would be formed under any engine load conditions.Stratified mixture at part load can further improve fuel economy. Currently, commercial gasoline direct injection engines mainly use wall-guided combustion system to prepare stratified mixture. In this system, the fuel droplets usually tend to adhere to the piston crown and the other side of the cylinder. This increases the HC emission. However, LPG can reduce more adhesion for its easy evaporation. Therefore, the stratified mixture formation of a DI LPG engine under the pressure of 5MPa was investigated in this thesis by numerical simulation. The results showed that, compared with gasoline at part load, the spray of LPG burst fast once it was injected from the injector, and the spray tip penetration was reduced. This resulted that rich mixture was still not be guided to the spark plug area a top curved piston crown, even near the end of compression stroke. Therefore, it is necessary to advance the fuel injection timing for 5-10℃A for LPG than gasoline at part load, if using the same injector and injection pressure. However, at high load, the injection advance could remain the same as that of gasoline.A new wall-guided combustion system with a cavity in piston to form lean stratified mixture for DI LPG engine was proposed and initially simulated. The simulation showed that, the mixture kept inside the piston cavity under the restriction of the intake inclined swirl and the cavity wall in early time. Then, the mixture gradually moved upward under the guide of the spray induced entrainment vortex and the cavity wall.At the same time, the rich mixture diffused to the surrounding. Finally, along of the combustion chamber wall, the mixture continued to move toward to the spark plug and diffusing.By the time to ignition, the ignitable stratified lean mixture is formed in the combustion chamber.
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