Abstract: A system for harnassing the combustion of a fuel and an oxidizer includes a base that has valved ports as inlets for the fuel and oxidizer and an outlet for the combustion products. The combustion occurs in a flexible container, preferably a thin walled metal bellows. The container expands in response to the combustion. This expansion is coupled to an output device to perform useful work, preferably hydraulically with a hydraulic fluid held in a rigid housing surrounding the flexible container. The housing, which can include the base, has a valved inlet and outlet for the hydraulic fluid. The system includes an arrangement to contract the container after combustion. Applications of this apparatus include modular hydraulic power generators, a marine propulsion system with water as the hydraulic fluid, an automotive-type engine with a conventional crank shaft, and a Wankel-type rotary engine.