The first airplane engines were liquid-cooled piston engines. Wilbur and Orville Wright, the first men to successfully build and fly a plane, modeled their plane's engines after automobile engines, which were kept from overheating by circulating water. Liquid-cooled piston engines are very complex, and also extremely heavy from the weight of the water or other fluid. Various fluids have been used in liquid-cooled engines: water, ethylene glycol or a combination are the most usual. The Liberty 12-cylinder piston engine, the result of a collaboration among various American aircraft engine manufacturers in 1917, was a successful liquid-cooled airplane engine, with more than 20,000 of these engines built.
In attempt to lighten the water-cooled piston engine, French inventors tried cooling an aircraft engine with air. The result was successful: an air-cooled engine is much lighter and less complex than a water-cooled piston engine. The only drawback is its lack of power, due to inefficient interaction between the engine and the propeller shaft. One of the earliest air-cooled engines was the engine in Glenn Curtis' 1908 June Bug.
Radial engines have one row or two rows of cylinders rotating around a stationary crankshaft. The larger and more powerful radial engines have two rows of 14 or 18 cylinders, while smaller engines have one row of seven or nine cylinders. Two of the most well-known radial engines are the Pratt & Whitney Wasp and Curtiss-Wright Whirlwind of the World War II era.
Opposed engines are so named because for each cylinder on one side of crankcase, there is another cylinder balancing it on the other side. Usually air-cooled, the opposed piston engine is light and small with little drag, making it a suitable engine for a small aircraft. Hugo Junkers, a German professor, developed the first opposed piston engine Jumo 204; this engine and improvements on it later powered Luftwaffe Junkers planes in World War II.
Inline engines have a small frontal area and and a low power-to-weight ratio. The cylinders are lined up in a row, and there is usually an even number of cylinders. The inline piston engine can be air-cooled or liquid-cooled and is usually used for lighter planes. The Fairchild L-440 engine, used in the Fairchild PT-19 trainer aircraft, is an example.
A V-type piston engine has two rows of cylinders, arranged in two rows and tilted 30 to 60 degrees from one anothe. It can be an air-cooled or water-cooled engine. The Rolls-Royce Merlin was a V-type engine used in the Spitfire planes from World War II.
