PURPOSE: To reduce the cost and solve a torsional vibration problem by using a device provided on an injection pipe and controlling the fuel injection quantity and injection timing.

CONSTITUTION: When the cylinder gas pressure becomes larger than the total load of the return spring 6 load of a plunger and the opening load of an injection nozzle 8 during a compression stroke, the compression gas from a cylinder gas pressure passage 1a pushes up an operation piston 2, the plunger 4 feeds fuel by pressure, and the fuel passes through an injection pipe 9, the optimized fuel is injected into a piston combustion chamber through the injection nozzle 8 via a unit 10 controlling the fuel injection quantity and injection timing for combustion, thus the cylinder pressure is rapidly increased. The operation piston 2 is further moved upward, but is stopped by a stopper 1c. During an expansion stroke, the operation piston 2 is moved downward and hits the lower end via a decrease of the cylinder gas.