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.

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.

Air-cooled rotaries were largely out of the picture after 1920. Even so, air-cooled engines offered tempting advantages. They dispensed with radiators that leaked, hoses that burst, cooling jackets that corroded, and water pumps that failed.

Aircraft designers wanted to build planes that could fly at high altitudes. High-flying planes could swoop down on their enemies and also were harder to shoot down. Bombers and passenger aircraft flying at high altitudes could fly faster because air is thin at high altitudes and there is less drag in the thinner air. These planes also could fly farther on a tank of fuel.

But because the air was thinner, aircraft engines produced much less power. They needed air to operate, and they couldn't produce power unless they had more air. Designers responded by fitting the engine with a "supercharger." This was a pump that took in air and compressed it. The extra air, fed into an engine, enabled it to continue to put out full power even at high altitude.

Early superchargers underwent tests before the end of World War I, but they were heavy and offered little advantage. The development of superchargers proved to be technically demanding, but by 1930, the best British and American engines installed such units routinely. In the United States, the Army funded work on superchargers at another engine-builder, General Electric. After 1935, engines fitted with GE's superchargers gave full power at heights above 30,000 feet (9,000 meters).

Fuels for aviation also demanded attention. When engine designers tried to build motors with greater power, they ran into the problem of "knock." This had to do with the way fuel burned within them. An airplane engine had a carburettor that took in fuel and air, producing a highly flammable mixture of gasoline vapour with air, which went into the cylinders. There, this mix was supposed to burn very rapidly, but in a controlled manner. Unfortunately, the mixture tended to explode, which damaged engines. The motor then was said to knock.

Poor-grade fuels avoided knock but produced little power. Soon after World War I, an American chemist, Thomas Midgely, determined that small quantities of a suitable chemical added to high-grade gasoline might help it burn without knock. He tried a number of additives and found that the best was tetraethyl lead. The U.S. Army began experiments with leaded aviation fuel as early as 1922; the Navy adopted it for its carrier-based aircraft in 1926. Leaded gasoline became standard as a high-test fuel, used widely in automobiles as well as in aircraft.

Leaded gas improved an aircraft engine's performance by enabling it to use a supercharger more effectively while using less fuel. The results were spectacular. The best engine of World War I, the Liberty, developed 400 horsepower (300 kilowatts). In World War II, Britain's Merlin engine was about the same size—and put out 2,200 horsepower (1,640 kilowatts). Samuel Heron, a long-time leader in the development of aircraft engines and fuels, writes that "it is probably true that about half the gain in power was due to fuel."

These advances in supercharging and knock-resistant fuels laid the groundwork for the engines of World War II. In 1939, the German test pilot Fritz Wendel flew a piston-powered fighter to a speed record of 469 miles per hour (755 kilometres per hour). U.S. bombers used superchargers to carry heavy bomb loads at 34,000 feet (10,000 meters). They also achieved long range, the B-29 bomber had the range to fly non-stop from Miami to Seattle. Fighters routinely topped 400 miles per hour (640 kilometres per hour). Airliners, led by the Lockheed Constellation, showed that they could fly non-stop from coast to coast.

By 1945, the jet engine was drawing both attention and excitement. Jet fighters came quickly to the forefront. However, while early jet engines gave dramatic increases in speed, they showed poor fuel economy. It took time before engine builders learned to build jets that could sip fuel rather than gulp it. Until that happened, the piston engine retained its advantage for use in bombers and airliners, which needed to be able to fly a great distance without refuelling.