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.
The second factor is to reduce the over-lean area in the piston cavity. A compact shell-shaped cavity was designed for this purpose. The resulting flame propagation improves combustion stability.
The effect of tumble motion was also investigated. The best combustion characteristics were obtained at weak tumble motion condition. Measurement results of in-cylinder flow by LDV showed that the fan-shaped fuel spray produces sufficient turbulence strength without extra intake air flow systems.
The application of the NCP to an actual 3L in-line 6 DI gasoline engine showed that in addition to satisfying Japanese regulations which start in from 2000, a fuel economy gain of more than 20% was obtained.