Abstract: A four-cycle internal combustion engine having a transmission linkage between the engine piston and the power output enabling the power stroke of the engine piston to be substantially greater than the intake stroke. The preferred form of transmission linkage comprises a hydraulic circuit including a ram fixed to the engine piston and a pair of branch hydraulic lines including displacement pistons timed to produce the lengthened power stroke.Preferably, a Stirling-type engine is connected to the output and utilizes the waste heat from the internal combustion engine to improve the efficiency of the system.Another feature of the internal combustion engine system is the utilization of a unique, timed, sleeve valve mechanism for controlling the intake and exhaust ports.