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A direct injection (DI) gasoline engine having a new stratified charge combustion system has been developed. This new combustion process (NCP) was achieved by a fan-shaped fuel spray and a combustion chamber with a shell-shaped cavity in the piston. Compared with the current Toyota D-4 engine, wider engine operating area with stratified combustion and higher output performance were obtained without a swirl control valve (SCV) and a helical port. This report presents the results of combustion analyses to optimize fuel spray characteristics and piston cavity shapes. Two factors were found to be important for achieving stable stratified combustion. The first is to create a ball-shaped uniform mixture cloud in the vicinity of the spark plug. The optimum ball-shaped mixture cloud is produced with a fuel spray having early breakup characteristics and uniform distribution, and a suitable side wall shape in the piston cavity to avoid the dispersion of the mixture.