Recently natural gas has attracted public attention as clean fuel for motor vehicles. We first developed a heavy-duty compressed natural gas (CNG) engine for city busses and manufactured many CNG-fueled engines. Both medium- and heavy-duty CNG engines achieved very low exhaust emissions. However, the fuel consumption of these engines for example the city-bus application are higher than that of a diesel engine. For this reason, these CNG engines always operate under the part-load conditions. Therefore, we developed a direct-injected CNG engine. Under a part-load condition, the engine is operated on the stratified-charged natural gas that is directly injected into the combustion chamber. It is the most important that the air/fuel ratio of the mixture stratified near the spark plug must be controlled to achieve the stable mixture condition. As a result of the development, the air/fuel ratio of the mixture near the spark plug could be optimized and controlled to be stable under each engine speed and part-load conditions. The stratified charged combustion of direct-injected gas has been investigated on a production multi-cylinder engine.