Study on mixture formation for a direct injection lean-burn LPG engine

Currently, commercial gasoline direct injection engines mainly use wall-guided combustion system to prepare stratified mixture. In this system, the fuel droplets injected from one cylinder side usually tend to wall wet on the piston crown and the other side of the cylinder, which causing an increases of the HC emission. The liquid phase LPG has a lower boiling point and a higher vapor pressure than gasoline. When it is injected with the pressure more than 5MPa, flashing will occur, which speeds up the mixture formation and reduces wall wet. In this paper the mixture formation of direct injection LPG engine at part load and full load were investigated by numerical simulation. The results showed that, compared with gasoline spray using a swirl injector at part load, the LPG spray employing a single-hole injector burst fast once injected, and spray tip penetration was reduced. This resulted that rich mixture was still not guided to the spark plug area, even near the end of compression stroke. Therefore, it is necessary to advance the fuel injection timing of 5∼10°CA for LPG than gasoline at part load using single-hole injector at the same injection pressure. However, at high load, the injection advance could remain the same as that of gasoline.