A piston steam engine is a type of engine that incorporates a cylindrical piston that moves within an enclosed cylinder. As in all piston engines, the expansion of hot gas is used to move the piston and the force of this movement is converted, through mechanical means, to perform some type of work. Unlike an internal combustion engines however, a piston steam engine relies on steam generated outside the cylinder, in a boiler, to move the piston within the cylinder.
The first engine of this type was invented in 1690 by a French inventor and engineer named Denis Papin. His simple, one-cylinder piston steam engine was a proof of concept design that showed that such a machine was possible. It worked but was not capable of repeating its action on its own and had to be taken apart and reset after each cycle. It had no means of transferring the work generated by the action of the piston but did prove that such an engine was possible.
All piston engines utilize connecting rods to connect the piston or pistons to a crankshaft. The crankshaft converts the straight-line movement of the piston to rotational movement, which is transferred to a drive mechanism, such as the wheels on a locomotive or a ship's propeller. In most internal combustion engines, the hot gas formed by the ignition and burning of fuel only pushes on one side of the piston. In a piston steam engine, the cylinder is often designed with a sliding valve, allowing the steam to work on both sides of the piston, alternating from one side to the other.
Parts for engines of this type must be manufactured with precision to ensure efficient operation. Pistons and cylinders must fit together with the minimum gap possible between them. The pistons of a piston steam engine, like other piston engines, are often fitted with rings that act as a high-temperature gasket to ensure a good seal between the piston and cylinder, preventing leakage, which reduces efficiency.
Many different types of piston steam engine have been developed since Papin's first design, but all of the various improvements and variations have not changed the basic principle of using steam to move a piston within a cylinder to perform work. Piston steam engines were used in early rail locomotives, ship engines, and even some early types of automobile. A series of vehicles produced by Stanley was known as "Stanley steamers."