In order to boost the torque of a 4-stroke cycle diesel engine, a crankcase-supercharged 4-stroke cycle diesel engine was made and tested in the firing mode.As a result, a remarkable reduction of smoke density and brake specific fuel consumption compared with the naturally aspirated baseline diesel engine was obtained.Additionally, a Jet Air Port (JAP), which was opened and closed by piston movement near bottom dead center, was installed. The JAP utilized air furnished by the crankcase-supercharging system. A reduction of smoke density and brake specific fuel consumption compared with the naturally-aspirated baseline diesel engine was found.The CRANKCASE-SUPERCHARGED 4-STROKE CYCLE ENGINE, which utilizes the crankcase below the piston of a 4-stroke cycle engine as a supercharging-pump, was made and tested in both the motoring and spark-ignited firing modes, and the effectiveness of the engine was confirmed and reported by the authors. (1) (2) (3) Through experiments we have performed so far, it was found that the volumetric efficiency and the brake horsepower can be improved remarkably in the lower and middle engine speed ranges by using reed valves at both the inlet and outlet sides of the crankcase. In addition, the volumetric efficiency can be improved over the entire engine speed range by using rotary disc valves at both the inlet and outlet sides of the crank-case combined with changing the disc valves opening angle and timing. As the normal speed range of diesel engines is lower than that of gasoline engines, diesel engines can be expected to secure the beneficial effects of crankcase-supercharging even with the reed valve system, of which the construction is simpler than that of the rotary disc valve system.The authors also performed tests with a 4-stroke gasoline engine which was equipped with a Jet Air Port (JAP). The JAP was opened and closed by piston movement near bottom dead center. The test results showed that the JAP was effective for reducing the brake specific fuel consumption and exhaust emissions concentration at idle and part load, and the effects of the JAP were increased further by increasing the pressure of the air supplied to the JAP. (4)Based on these reported results, the authors decided to study the effects of crankcase-super-charging and a JAP system, using crankcase-supercharging pressure on a 4-stroke cycle diesel engine.