The impingement of the spray and top face of piston is inevitable in the wall-guided GDI engine. Parts of the spray droplets would be adhered to the wall and form a film which is bad for evaporation; the other parts would be broken up and reflected,these droplets are smaller and mixing with air better. The difference in the top contours of pistons results in the difference in the fuel evaporation and gasoline film (Fig. 7). The order of pistons with decreasing fuel evaporation is A, D, C, and B (Fig. 7a). As more fuel vaporizes, the less gasoline film on piston crown will be produced (Fig. 7b). Only a little amount of gasoline film can vaporizeand participate in combustion, which may reduce fuel efficiency and increase pollutant emission. In order to reduce gasoline film,piston A showed the best performance, followed by piston D, then is piston C and piston B is the worst.