A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, whereby each cylinder has two pistons sharing a central combustion chamber.
Benz Contramotor, the first commercial flat-engine design, ca. 1899
1954 BMW R68 flat-twin boxer engine
World War II-era Riedel starter motor
1969 Hino Motors DS140 boxer-twelve diesel engine
Internal combustion engine
An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to.
Diagram of a cylinder as found in an overhead cam 4-stroke gasoline engine: C – crankshaft E – exhaust camshaft I – inlet camshaft P – piston R – connecting rod S – spark plug V – valves. red: exhaust, blue: intake. W – cooling water jacket gray structure – engine block
Reciprocating engine of a car
Diesel generator for backup power
Bare cylinder block of a V8 engine