Abstract: In its simplest form an oscillating steam engine is limited to low pressure operation by a spring or set screw holding the cylinder against a stationary valve plate. This invention features trunnions not rigidly connected to the oscillating cylinder and pressure equalizing cylinder, making possible a wide and rapid variation of steam pressure and temperature with minimal valve and pressure equalizing cylinder leakage and trunnion friction and misalignment. In a compound or multiple expansion engine, high pressure steam can be fed to the low pressure cylinder for starting the engine or for increasing the power output. Automatic condensate relief is also provided for without separate manually operated valves.