Some single-piston gasoline engines entered service, but for use with airplanes, most such engines had a number of pistons, each shuttling back and forth within its own cylinder. Each piston also had a connecting rod, which pushed on a crank that was part of a crankshaft. This crankshaft drove the propeller.

The cylinder is closed on one end (the cylinder head), and the piston fits snugly in the cylinder. The piston wall is grooved to accommodate rings which fit tightly against the cylinder wall and help seal the cylinder's open end so that gases cannot escape from the combustion chamber. The combustion chamber is the area between the top of the piston and the head of the cylinder when the piston is at its uppermost point of travel.

The up-and-down movement of the piston is converted to rotary motion to turn the propeller by the connecting rod and the crankshaft, just as in most automobiles. Note the crankshaft, connecting rod, and piston arrangement and imagine how the movement of the piston is converted to the rotary motion of the crankshaft. Note particularly how the connecting rod is joined to the crankshaft in an offset manner.

The valves at the top of the cylinder open and close to let in a mixture of fuel and air and to let out, or exhaust, burned gases from the combustion chamber. The opening and closing of a valve are done by a cam geared to the crankshaft. This gearing arrangement ensures that the two valves open and close at the proper times.

Engines built for airplanes had to produce plenty of power while remaining light in weight. The first American planebuilders—Wilbur and Orville Wright, Glenn Curtiss—used motors that resembled those of automobiles. They were heavy and complex because they used water-filled plumbing to stay cool.

Gnome rotary

A French engine of 1908, the "Gnome," introduced air cooling as a way to eliminate the plumbing and lighten the weight. It was known as a rotary engine. The Wright and Curtiss motors had been mounted firmly in supports, with the shaft and propeller spinning. Rotary engines reversed that, with the shaft being held tightly—and the engine spinning! The propeller was mounted to the rotating engine, which stayed cool by having its cylinders whirl within the open air.

During World War I, rotaries attained tremendous popularity. They were less complex and easier to make than the water-cooled type. They powered such outstanding fighter planes as German's Fokker DR-1 and Britain's Sopwith Camel. They used castor oil for lubrication because it did not dissolve in gasoline. However, they tended to spray this oil all over, making a smelly mess. Worse, they were limited in power. The best of them reached 260 to 280 horsepower (190 to 210 kilowatts).

Thus, in 1917 a group of American engine builders returned to water cooling as they sought a 400-horsepower (300-kilowatt) engine. The engine that resulted, the Liberty was the most powerful aircraft engine of its day, with the U.S. auto industry building more than 20,000 of them. Water-cooled engines built in Europe also outperformed the air-cooled rotaries, and lasted longer. With the war continuing until late in 1918, the rotaries lost favour.

Liberty engine

In this fashion, designers returned to water-cooled motors that again were fixed in position. They stayed cool by having water or antifreeze flow in channels through the engine to carry away the heat. A radiator cooled the heated water. In addition to offering plenty of power, such motors could be completely enclosed within a streamlined housing, to reduce drag and thus produce higher speeds in flight. Rolls Royce, Great Britain's leading engine-builder, built only water-cooled motors.