Power from Pistons to Props

by Roland H Spaulding, PhD

Aircraft performance depends upon the efficiency of the power plant. All practical, modern aviation power plants are internal combustion engines which convert thermal energy (heat) into mechanical energy. Internal combustion means that the fuel is burned within the engine itself. An engine cylinder may be compared to a cannon — closed at one end, open at the other. The piston becomes a cannon ball. The rapid expansion of burning fuel furnishes the power to push the piston toward the open end of the cylinder. Unlike the cannon ball, however, the piston is prevented from leaving the cylinder because of the connecting rod which connects it with the crank. This crank is part of the crankshaft which is connected to a gear reduction train to which is attached the propeller shaft, itself. The revolving crankshaft, by means of the connecting rod, pushes the piston back into the cylinder after it has been forced outward by the expanding gases. In order to make this possible, an exhaust valve in the closed end or cylinder head is timed to open and allow the burned gases to escape as the piston makes this inward trip.

Each trip the piston makes the length of the cylinder involves a half turn of the crankshaft (180°) and is called a stroke. The stroke just described is called the exhaust stroke. When it is completed, the piston is said to be at top dead center. At about this time, the exhaust valve is closed by means of springs and the intake valve — also in the cylinder head — is opened by means of a series of levers and push rods, operated by a cam ring which is geared to the engine crankshaft.

As the piston proceeds toward the open end of the cylinder — the intake stroke — fuel comes in through the open intake valve so that when the stroke is completed with the piston at bottom dead center, the cylinder is full of a combustible vapor consisting of about fourteen parts of air to one part of gasoline by weight. This is described as the mixture ratio and has been achieved by means of the carburetor through which both the liquid gasoline and the air have passed. The compression stroke follows as the piston travels again toward the closed end of the cylinder. In this, the volume of fuel in the cylinder, with the piston at bottom dead center, is compressed into a smaller area known as the combustion chamber, which is the amount of space left in the cylinder head after the piston has reached top dead center at the close of the compression stroke. The ratio between the first volume, with the piston at bottom dead center, and the second with the piston at top dead center, is known as the compression ratio. As the compression stroke is finished, a spark is produced in the combustion chamber by means of the ignition system and the fuel is ignited. Its rapid burning releases the inherent thermal energy of the fuel, the expanding gases force the piston toward the open end of the cylinder in what is described as a power stroke. These four strokes, power, exhaust, intake and compression, involve two complete revolutions of the crankshaft, or 720°, and make up the cycle which characterizes the four-stroke cycle internal combustion engine.

Because a one-cylinder engine has to generate momentum enough on the power stroke to carry the mechanism through the other three strokes, it is apparent that additional cylinders increase efficiency. For example, a four-cylinder engine furnishes a power stroke for each half turn of the crankshaft. There is a more constant application of power to the crankshaft and more useful work accomplished. A nine-cylinder engine would have the power stroke of one cylinder beginning before the power stroke of another had been completed. This is described as power overlap and results in a much more efficient power plant. No matter how many cylinders the engine pushes, all of them will furnish one power stroke in the same two revolutions of the crankshaft.

Aircooled Engines
Up to 300-plus horsepower

Jacobs engines were used throughout the world before before the war in single-engine commercial and private cabin planes, twin-engine light transports and military trainers; now are powering the majority of the twin-engine trainers used by the Army Air Forces and the Royal Canadian Air Force to train bomber pilots. These trainers include the AT-17 Bobcat of the AAF and the Crane of the RCAF — both built by Cessna — and the Avro Anson.

Franklin The Air-Cooled Motors Corporation produced the first of its successful series of light, horizontally-opposed air-cooled engines in 1936. The power output of its four- and six-cylinder production model Franklin engines ranges from 65 ti 130 hp. These are strictly prewar models inasmuch as today the current production is almost entirely on new-model warplane engines, developed for the Army Air Forces and the Bureau of Aeronautics, USN. No details of these engines have been released, other than the fact that they are of the air-cooled, horizontally-opposed type. The 4ACG-199-H8, now in war production, is used in the new Interstate L-6, Army liaison plane, and other lightplanes.

Kinner Two of the Kinner engines, Models B-5 and R-5, are four-cycle, air-cooled, five-cylinder radial engines. B-5 is rated normally at 125 hp at 1,925 rpm; R-5 is rated at 160 hp at 1,850 rpm. Specifications: B-5 — Bore 40675 in. Stroke 5.25 in. Displacement 141 cu in. Compression ratio 5.25 to 1. It weighs 112 lbs, uses 73-octane fuel; specific weight is 2.5 lb/hp, R-5 — Bore 5 in. Stroke 505 in. Displacement 540 cu in. Compression ratio 505 to 1. Weighs 345 lbs and uses 73-octane fuel same as B-5. Specific weight is 2.2 lbs per hp.

Heavy Air-Cooled Engines

Biggest static radial air-cooled engines are built by two manufacturers: Pratt & Whitney, builders of the famous Wasp engines, and the Wright Aeronautical, builders of the equally famous Whirlwind and Cyclone engines. Roster of fighting planes they power reads like a "Who's Who" of combat aircraft — Avenger, Flying Fortress, Liberator, Corsair, Catalina, Dauntless, Marauder, Mitchell, Thunderbolt, Wildcat and other American and British fighting planes.

The cylinder arrangement of radial engines is especially suited to air cooling because each cylinder is in the path of the cooling slip stream of the propeller. The outer surface of each cylinder consists of a great number of fins, increasing the cooling area in contact with the air. Unlike the liquid-cooled engine, there is no need for a cooling agent, although the radial-engine cylinder arrangement presents greater frontal area and usually does not lend itself to the streamlining the liquid-cooled engine allows.

Wright The Whirlwind, Cyclone, Cyclone 14, and Cyclone 18 series of engines , built by Wright Aeronautical Corporation, are some of the best known and most popular heavier horsepower engines manufactured in this country. The Whirlwind series, seven- and nine-cylinder engines, range in horsepower from 235 to 450 hp; the Cyclone series, nine-cylinder engines, cover a horsepower range of from 770 to 1,200 hp[; the Cyclone 14 series, fourteen-cylinder, double-row engines, cover the 1,600- to 1,700-hp range. The eighteen-cylinder Cyclone 18 is rated at 2,000 hp. The Boeing Flying Fortress is powered by four 1,200-hp Cyclones; the Martin Mars by four 2,000-hp Cyclone 18s; the North American BT-9 tgrainer by a 400-hp Whirlwind. These are but a few. Many Army and Navy planes of both the United States and England are powered by Wright engines.

Pratt & Whitney Engines built by Pratt & Whitney range from 450 hp to 2,000 hp, Wasp, Jr is a nine-cylinder, 450-hp, air-cooled, radial engine used largely in training planes. The others of the Wasp series, the nine-cylinder, 600-hp Wasp, the fourteen-cylinder, two-row, 1,200-hp Twin Wasp, and the eighteen-cylinder, 2,000-hp Double Wasp (opposite page), are widely used in our military and commercial aircraft.

Ranger Only production type of inline, inverted, air-cooled engines manufactured in US are by Ranger.

Liquid-Cooled Engines

Liquid-Cooled engines are engines in which the cylinders are surrounded by jackets through which a coolant, such as water, Prestone or other liquid is circulated by means of a pump. After the coolant has picked up the heat from the cylinders, it passes through a radiator where the passing air carries the heat away and the cooled water, Prestone, or other liquid returns again to the cylinder jackets. A liquid such as Prestone is used because it has a very high boiling point, permitting a higher and more efficient engine operating temperature.. Too, the freezing point of the liquid is far below any temperature likely to be encountered in operations. The higher operating temperature of the engine and the difference between the temperature of the coolant and surrounding air result in much more rapid cooling. The Allison is our most important liquid-cooled engine.

Allison The Allison is a twelve-cylinder, sixty-degree V-type, high-temperature (250° F), liquid-cooled (ethylene glycol) engine with a piston displacement of 1,710 cubic inches. Four standard models are presently in regular production: the F-20, the E-19, and the F-17-R (righthand rotation) and F-17-L (lefthand rotation) which comprise the powerplant of the Lightning.

Allison-powered combat planes have combat planes have performed superbly on all fronts in carrying out assignments for which they were specifically designed. Through variations in supercharging, Allisonos are built to be most effective at different altitudes — low, medium, and high.

The following AAF fighter planes are powered by Allisons: Lockheed Lightning, Bell Airacobra, Curtiss Tomahawk, Kittyhawk, and the North American Mustang.

Rolls-Royce The above is a cutaway sketch of the Rolls-Royce Merlin XX engine. It is a four-cycle, liquid-cooled engine; has twelve cylinders and is known as a "60-degree-V" type. Supercharged, this engine is rated at 1,240 hp at 2,850 rpm at 10,000 ft. It uses 100-octane fuel and its specific weight is given at 1.13 lb/hp. Merlin X is the latest of a series of Merlin engines

This article, heavily pictorial, was originally published in the May, 1943, issue of Skyways magazine, vol 2, no 5, pp 32-48, 51-60.
The original article includes 4 figures illustrating elements of the text, 62 photos, eight drawings and an exploded view made up of small photos.
Photos are not credited; two engine cutaway drawings courtesy of Aerosphere.

Photo captions:

Test Run
  1. In giant test cell, crew of engineers hoist a Wright 18-cylinder Cyclone of 2,000 hp up to a barrel mount where engine will be tested. These horizontal test cells are the largest in any aircraft engine factory in US
  2. Installed in its mount, Cyclone is fitted with a flight propeller. Large diameter of this prop is indicated by the size of the two men on the work platform. Engine is mounted in flight position and anchored by heavy cables
  3. After prop has been installed, oil and fuel lines checked, thermocouples and pressure gauges checked, streamlined cowling is placed over engine. Because there is no motion through air, auxiliary method of cooling engine is used
  4. From a window set high in the test cell, two engineers watch testing of engine. They record readings from hundreds of instruments and enter more than a thousand items in engine log book during course of five-or seven-hour run

Drawing captions