Aircraft Engines

The aircraft engines of the Second World War were almost exclusively piston engines using high-octane gasoline (petrol) fuel. Jet engines were introduced towards the end of the war in Europe, but saw no operational use in the Pacific. Rocket engines were employed by the Japanese MXY7 Ohka suicide aircraft.

Photograph of Ha-40 inline aircraft engine

An Ha-40 inline aircraft engine.

U.S. Navy. Via Francillon (1979)

Photograph of Kotobuki 1 KAI 1 radial engine

A Kotobuki 1 KAI 1 radial aircraft engine.
U.S. Navy. Via Francillon (1979)

Piston engines fell into two broad categories. Inline engines typically placed the cylinders in two rows, rather like an automobile V-8 engine. Radial engines arranged the cylinders in one or more layers around the center of the engine. Inline engines had a much smaller cross-section, which greatly reduced drag; however, the inline cylinder layout could not be adequately cooled by airflow alone, so a liquid cooling system was required. Radial engines had a large cross-section, but with appropriate cooling fins, the engine could be cooled by airflow alone. The tradeoff was greater drag for the radial engine versus greater weight and vulnerability to damage for the inline engine. In practice, the tradeoff of weight for drag meant that neither the radial or inline engine gave a decisive advantage for aircraft speed. The inline-engine Mustang was very fast, but so was the radial-engine Corsair. The inline-engine Spitfire was nimble, but so was the radial-engine Hellcat. The type of engine was a key recognition feature for single-engine monoplanes: Aircraft employing radial engines had a blunt nose due to the large cross-section of the engine, while aircraft employing inline engines had a pointed, streamlined nose to take maximum advantage of the reduced engine cross-section.

The aircraft engines of the Pacific War relied on high-octane aviation gasoline. The high octane rating meant that the fuel had very little tendency to preignite when compressed by the piston in the cylinder. This in turn meant that high compression ratios could be used, which increased engine power. The use of high-octane gasoline also posed logistical challenges, since it was expensive and hazardous to transport and store -- and because aircraft consumed fuel at a prodigious rate.

Japanese Aircraft Engines. Japanese aircraft engines were designated using a two-digit system. The first digit indicated the general engine type, while the second digit indicated cylinder bore diameter and stroke distance. Manufacturer and precise model number were also part of the designation. Thus, the Nakajima [Ha-35] 25 engine was an air-cooled 14-cylinder double-row radial engine (2) with a bore of 130mm and stroke of 150mm (5) designed by Nakajima, model number 25.

Engine class identification

Ha-1: air-cooled inline engine

Ha-2: air-cooled single-row radial engine

Ha-3: air-cooled 14-cylinder double-row radial engine

Ha-4: air-cooled 18-cylinder double-row radial engine

Ha-5: air-cooled, more than 18-cylinder, multi-row radial engine

Ha-6: liquid-cooled 12-cylinder engine

Ha-7: liquid-cooled, more than 12-cylinder, engine

Ha-8: diesel engine

Ha-9: special engine

Bore and stroke identification (bore/stroke in mm)

...1: 140/130

...2: 150/170

...3: 140/150

...4: 140/160

...5: 130/150

...0: 130/160

In practice, a bewildering variety of nonstandard designations were used for Japanese aircraft engines. Designations such as Ha-26-I should not be confused with [Ha-26] 1 which are different engines.

Aircraft Engines

Engine class identification

Bore and stroke identification (bore/stroke in mm)

See also