Liberty L-4
The Liberty L-4B was an experimental water-cooled inline four-cylinder aircraft engine developed in the United States during World War I. The  engine was designed and manufactured by the Hudson Motor Car Company and was mainly intended for use in training airplanes. Only two examples of the L-4B were produced, since other types of engines were available and in production. While the engine was ground-tested, it is unknown if the L-4 was ever test-flown. The L-4 was created as part of a US aircraft engine development effort which also created the six-cylinder Liberty L-6, eight-cylinder Liberty L-8 and twelve-cylinder Liberty L-12. Specifications

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This article incorporates text from Liberty L-4, a public domain work of the United States Government. 1910s aircraft piston engines
Abandoned military aircraft engine projects of the United States
Hudson Motor Car Company