The Liberty L-4 was a four-cylinder, water-cooled, inline engine

Inline engine (aviation)

In aviation, an inline engine means any reciprocating engine with banks rather than rows of cylinders, including straight engines, flat engines, V engines and H engines, but excluding radial engines and rotary engines....

 for aircraft developed in the United States during World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. The Liberty L-4 was an experimental engine of 102 hp, built by the Hudson Motor Co. and intended primarily for use in training airplanes. Only two were produced since other types of engines were available and already in production. It is unknown if the L-4 was ever test-flown.

This article incorporates text from http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=835, a public domain work of the United States Government.