PURPOSE: To delay the opening of an exhaust valve to increase effective stroke and improve the thermal efficiency of an engine by making the piston shoulder or the No.1 step piston ring pass through an opening in a cylinder liner in an expansion stroke and thus causing the gas pressure to energize a sleeve valve so as to move upwards.

CONSTITUTION: When the shoulder of a piston 7 or a No.1 step piston ring 10 in an engine body passes through the opening of a cylinder liner in a operating piston chamber 3 in the expansion stroke, the gas pressure in a cylinder is applied to an operating piston 9 through a passage 11 and further to an oil chamber 5 through a hydraulic oil pipe 4 and pushes a sleeve valve 8 upwards. Before a piston 7 in the engine body opens a scavenging port 2 the scavenging port 2 is closed by the sleeve 8. By an optimum setting the gas pressure applied to the piston 7 is reduced with the finish of the high pressure exhaust and the sleeve valve 8 is pushed down to the opening position of the scavenging port 2 by a spring 6 to carry out the scavenging and exhaust (gas exchange).