Pistons in cylinders first saw use in steam engines. Scotland's James Watt crafted the first good ones during the 1770s. A century later, the German inventors Nicolaus Otto and Gottlieb Daimler introduced gasoline as the fuel, burned directly within the cylinders. Such motors powered the earliest automobiles. They were lighter and more mobile than steam engines, more reliable, and easier to start.
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.
Cutaway view of a piston engine built by Germany's Gottlieb Daimler. Though dating to the 19th century, the main features of this motor appear in modern engines.
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.
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.
Numerous types of Gnome engines were designed and built, one of the most famous being the 165-hp 9-N "Monosoupape" (one valve). It was used during WWI primarily in the Nieuport 28. The engine had one valve per cylinder. Having no intake valves, its fuel mixture entered the cylinders through circular holes or "ports" cut in the cylinder walls. The propeller was bolted firmly to the engine and it, along with the cylinders, turned as a single unit around a stationary crankshaft rigidly mounted to the fuselage of the airplane. The rotary engine used castor oil for lubrication.
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).
America's greatest technological contribution during WWI was the Liberty 12-cylinder water-cooled engine. Rated at 410 hp. , it weighed only two pounds per horsepower, far surpassing similar types of engines mass-produced by England, France, Italy, and Germany at that time.
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 favor.
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.
Thus, the air-cooled "radial engine" emerged. This type of air-cooled engine arranged its cylinders to extend radially outward from its hub, like spokes of a wheel. The U.S. Navy became an early supporter of radials, which offered reliability along with light weight. This was an important feature if planes were to take off successfully from an aircraft carrier's flight deck.
With financial support from the Navy, two American firms, Wright Aeronautical and Pratt & Whitney, began building air-cooled radials. The Wright Whirlwind, in 1924, delivered 220 horsepower (164 kilowatts). A year later, the Pratt & Whitney Wasp was tested at 410 horsepower (306 kilowatts).
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.
The supercharger, spinning within a closely fitted housing (not shown), pumped additional air into aircraft piston engines.
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.
A supercharger needed power to operate. This power came from the engine itself. The supercharger, also called a centrifugal compressor, drew air through an inlet. It compressed this air and sent it into the engine. Similar compressors later found use in early jet engines.
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.
The Pratt and Whitney R-1830 Twin Wasp engine was one of the most efficient and reliable engines of the 1930s. It was a "twin-row" engine. Twin-row engines powered the warplanes of World War II.
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."
The V-1650 liquid-cooled engine was the U.S. version of the famous British Rolls-Royce "Merlin" engine which powered the "Spitfire" and "Hurricane" fighters during the Battle of Britain in 1940.
During World War II, the best piston engines used a turbocharger. This was a supercharger that drew its power from the engine' hot exhaust gases. This exhaust had plenty of power, which otherwise would have gone to waste. A turbine tapped this power and drove the supercharger. Similar turbines later appeared in jet engines.
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 kilometers per hour). Airliners, led by the Lockheed Constellation, showed that they could fly non-stop from coast to coast.
The Wasp Major engine was developed during World War II though it only saw service late in the war on some B-29 and B-50 aircraft and after the war. It represented the most technically advanced and complex reciprocating engine produced in large numbers in the United States. It was a four-row engine, meaning it had four circumferential rows of cylinders.
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.
Pratt & Whitney was the first to achieve high thrust with good fuel economy. Its J-57 engine, which did these things, first ran on a test stand in 1950. Eight such engines powered the B-52, a jet bomber with intercontinental range that entered service in 1954. Civilian versions of this engine powered the Boeing 707 and Douglas DC-8, jet airliners that began carrying passengers in 1958 and 1959, respectively. In this fashion, jet engines conquered nearly the whole of aviation.