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An aircraft piston engine, also commonly referred to as a reciprocating engine or "recip", is an internal combustion engine that uses one or more reciprocating pistons to convert pressure into a rotational motion. The aircraft piston engine operates on the same principles as the engines found in most automobiles. However, modifications, such as dual ignition systems, to improve redundancy and safety, and air cooling to reduce weight, have been incorporated into engines designed for aviation use. Turbochargers and, less commonly, superchargers can be added to piston engines to improve performance. Aircraft piston engines are most commonly fueled with AVGAS but diesel fueled engines are becoming more common, especially in light aircraft.
Engine design has varied tremendously in the century that has passed since the first powered flight. Most engines installed in current generation aircraft are of the horizontally opposed configuration. However, there are examples of virtually all of the following engine types still being flown in production, experimental and vintage aircraft.
The earliest aircraft engines were of the in-line or "straight" variety and had the cylinders in a line, similar to many automotive engines. The main advantage to this engine type is that it is narrow and allows the aircraft to have a narrow front fuselage. However, airflow around this engine type is inadequate to allow air cooling so liquid cooling is required thus reducing the power to weight ratio.
Rotary piston engines were developed during World War I for military aircraft. In this design, the entire engine rotates with the propeller, creating additional airflow for cooling.
A V-type engine is basically the equivalent of two in-line engines joined in a "V" configuration by a common crankshaft. The best known example of a V-type engine is the supercharged Rolls Royce Merlin that was used to power both the Supermarine Spitfire and the Avro Lancaster.
A radial piston engine consists of one or more rows of odd-numbered cylinders arranged in a circle around a central crankshaft. Due to the small size of the crankcase, this engine type had a better power to weight ratio than most other designs of their day. The cylinder arrangement allowed for good cooling airflow and smooth operation.
Horizontally Opposed Engines
Horizontally opposed engines are often referred to as boxer or flat engines. They have two banks of cylinders staggered on opposite sides of a central crankcase. The design is simple, reliable and easy to maintain.