The United States Army procured 660,950 aircraft engines from 1940 through 1945. During the same period the United States Navy (USN) procured 151,665 aircraft engines. The resulting 4.4 to 1 United States Army over USN ratio reflected the Army’s responsibility for the United States Army Air Force (USAAF) as well as acting as the procuring agency for most of the foreign Lend-Lease buys. The total procurement of 812,615 engines outfitted the nearly 300,000 planes delivered to all allied recipients and provided spares as well.
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During the interwar era advancements were made in engine technologies. By the start of the war two technologies stood out and air-cooled and liquid-cooled engines were the early war standards. Late war saw the emergence of jet engines, but US jet engines manufacturing in 1944-45 was in very small quantities. Overall air-cooled engines were produced over liquid-cooled engines by a 5.5 to 1 ratio.
Both engine types had advantages and disadvantages. Air-cooled engines advantages were simpler design, cheaper to manufacture, and battle survivability. Air-cooled engines disadvantages were not as aerodynamic as inline liquid-cooled engines, harder to place engines around the centerline. Liquid-cooled engines advantages were aerodynamics, ability to direct coolant flow evenly, more powerful on a pound for pound basis, and weapons placement. Liquid-cooled engines disadvantages were design complexity, weight, and not as survivable in battle if radiator is damaged.
Supercharging was first demonstrated during World War I, but were not produced in any appreciable quantities until the late 1920s when tougher materials became available. Supercharging allowed for appropriate fuel-air mixtures without increasing pressure as well as increased low altitude performance. Later two-speed superchargers further increased performance at higher altitudes.
Supercharger development was an ongoing process for all belligerents during the war. Rolls Royce successfully introduced an upgraded two stage supercharger into the Merlin engine. This engine meant that the Spitfire Mk IX could now substantially outperform the Focke Wulf 190 at altitude.
During the Dunkirk evacuations both British and German aircraft utilized 87 octane fuel. Prior to the Battle of Britain, the British aircraft were switching over to 100 octane fuel. The fuel switchover combined improved superchargers surprised the Luftwaffe pilots with their improved performance. A French petroleum engineer, Eugene Houdry, developed the Houdry process enabling a greater amount of high-octane fuel to be converted from crude oil and lignite coal. After moving to the US, the process was further developed and now twice as much high-octane could be converted from crude oil.
High octane fuel combined with supercharging gave the US and her allies improved engine performance in every aspect. During take off and climbing it was estimated that the British Spitfires now had a 25% edge over their German counterparts, a 10% edge in speed, and a 12% edge in altitude.
US Aircraft Engine Production by Type: 1940-45 Data
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Abstracted from R. H. Crawford and L. F. Cook, Statistics: Procurement, OCMH, Table PR-18, pp. 80. Source: Holley , Irving Brinton, Jr. “United States Army In World War II Special Studies Buying Aircraft: Materiel Procurement For The Army Air Forces.” 1964, Page 549, Center of Military History United States Army, Library of Congress Catalog Card Number: 64-60000, https://history.army.mil/html/books/011/11-2/index.html, Data accessed on July 12, 2022
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