Abstract: A rotary power device of modular construction is disclosed in which one or more pistons are reciprocably received in a cylinder sleeve, and the sleeve is rotatably supported in a bore within a housing. A cam and follower mechanism acting between the housing and piston governs reciprocating motion of the piston(s) within the sleeve. Rotary power can be extracted from the sleeve, or reciprocating power can be extracted from the piston(s). The rotating cylinder functions as a sleeve valve and permits, depending on the porting, use of the device as a gas-expansion engine (e.g. steam or compressed air) or pump, or as a two-stroke or four stroke internal combustion engines using spark or compression ignition. Engines built according to the invention are simples, compact, and can be perfectly balanced, and multiple engine modules may be coupled together in various configurations to form power plants of various sizes.