A pre-combustion chamber (hot plug) made of the silicon nitride ceramics has been developed. The hot plug constitutes a component of the combustion chamber of swirl chamber diesel engines. And it is subjected to the severest thermal load of all the diesel engine operating components. Unlike metal hot plugs, the ceramic hot plug application requires a unique design approach to meet the ability of the ceramic materials. That is, the soft engine mounting is necessary to avoid the concentraction of mechanical stress resulting from the high Young's Modulus of the ceramic and control of temperature distribution is also required to reduce thermal stress.Due to the heat insulating construction, the ceramic hot plug permits the combustion performance to be improved at a low speed and load, resulting in improved noise, startability, and HC emission.In additional the ceramic hot plugs are already being produced for diesel passenger car.