The water-cooled A-4 barrel engine (18 cylinders -- two groups of nine each horizontally opposed) was the fourth experimental barrel engine built for testing at McCook Field, Ohio, by its inventor, Mr. J.O. Almen of Seattle, Wash. The project began in 1921 and by the mid-1920s, the A-4 passed its acceptance tests. The engine never went into production, however, reportedly because of limited funds and a growing emphasis by the U.S. Army Air Corps on air-cooled radial engines.
This unique engine had a much smaller frontal area than other water-cooled engines of similar horsepower, thereby providing better streamlining and less air resistance. It was rated at 425 hp but weighed only 749 pounds (a power to weight ratio of better than one to two), a significant design achievement in the early 1920s.
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