This article is about existing engine designs. For the quite different concept proposed by Frank Stelzer, see Stelzer engine.

An opposed piston engine is one in which the cylinders are double-ended, with a piston at each end and no cylinder-head.

This type of configuration was used in the Junkers Jumo 205 diesel aircraft engine, using two crankshafts, one at either end of a single bank of cylinders, and most remarkably in the Napier Deltic diesel engines, which used three crankshafts to serve three banks of double-ended cylinders arranged in an equilateral triangle with the crankshafts at the corners, and was used in railway locomotives and to power fast patrol boats. Both types are now obsolete.

It has also been used for marine auxiliary generators and for larger marine propulsion engines, notably Fairbanks-Morse diesel engines used in US submarines both conventional and nuclear. Fairbanks-Morse also used it in diesel locomotives, starting in 1944.

Opposed piston engines are essentially two-stroke cycle engines, as there is no cylinder head for the valves of a four stroke cycle.

Both the Jumo and Deltic engines used one piston per cylinder to expose an intake port, and the other to expose an exhaust port. Each piston is referred to as either an intake piston or an exhaust piston depending on its function in this regard. This layout gives superior scavenging, as gas flow through the cylinder is axial rather than radial, and simplifies design of the piston crowns. In the Jumo 205 and its variants, the upper crankshaft serves the exhaust pistons, and the lower crankshaft the intake pistons. In designs using multiple cylinder banks, such as the Junkers Jumo 223 and the Deltic, each big end bearing serves one inlet and one exhaust piston, using a forked connecting rod for the exhaust piston.