The results of the experiments carried out by changing the factors of the combustion system in the course of the development of the unique combustion system (Y.P.C.) which Yanmar adopt in their small diesel engines are introduced.In order to obtain better starting ability with manual operation, the Y.P.C. is so designed that a part of fuel injected into the divided chamber can directly reach the main combustion chamber. For obtaining the highest efficiency, it is necessary to examine the form in addition to the volume ratio, the throat area ratio and so on. Excellent combustion characteristics were obtained with comparatively low exhaust emission by optimizing these factors. The Y.P.C. has the flexibility to various kinds of fuel and the low combustion noise level.