A multicylinder internal combustion free-piston engine (FPE) with synchronized reciprocating plungers. The invention provides a solution for the problem of the slow engine speed typical of FPE's with heavy plunger mass. Bounce chambers fitted with sleeve valves control the engine's speed and stroke length. The invention's configuration prevents piston head-strikes and operates at standard compression ratios. Piston pop-top intake valves allow uniflow scavenging and connecting rod oil channels provide lubrication with no combustion chamber contamination. Poppet combustion head valves are operated by linear cams attached to the plungers. Hydraulic valve actuators implement variable valve timing under computer control.

A multicylinder internal combustion free-piston engine (FPE) with synchronized reciprocating plungers. The invention provides a solution for the problem of the slow engine speed typical of FPE's with heavy plunger mass. Bounce chambers fitted with sleeve valves control the engine's speed and stroke length. The invention's configuration prevents piston head-strikes and operates at standard compression ratios. Piston pop-top intake valves allow uniflow scavenging and connecting rod oil channels provide lubrication with no combustion chamber contamination. Poppet combustion head valves are operated by linear cams attached to the plungers. Hydraulic valve actuators implement variable valve timing under computer control.

A hydraulic cylinder is provided. The hydraulic cylinder includes a casing with a longitudinal axis defined therethrough, a first fluid inlet defined in the casing, a second fluid inlet defined in the casing, a first fluid outlet defined in the casing, a second fluid outlet defined in the casing, a shaft extending along the longitudinal axis, a first switching valve mounted to the shaft, a second switching valve mounted to the shaft, and a piston mounted to the shaft between the first switching valve and the second switching valve, wherein the hydraulic cylinder is configured such that when a fluid is supplied to the first and second fluid inlets, the shaft, the first and second switching valves, and the piston oscillate back and forth along the longitudinal axis.