Abstract: The four-stroke diesel engines with single-cylinders are widely possessed and used in China. However, there are problems of high fuel consumption and exhaust emission. A fuel-saving and supercharging device with a single-direction valve mounted on X-195 diesel engine between the intake manifold and crankcase was developed. When the piston is moving down, unburnt combustion gases that leak into the crankcase are pressed, and then are drawn into the combustion chamber together with fresh air that comes from the air cleaner. The air-fuel mixture in the crankcase can be reused so as to reduce the exhaust emission and enhance the charging efficiency. This device has characteristics of simple construction, reliability, low cost, convenient to mount and does not interfere with the mechanical driving arrangement. The experimental results show that it can effectively not only increase the engine output power, but also reduce its fuel consumption and emission.