H engine

Inline engine with four banks driving two crankshafts

An H engine is a piston engine comprising two separate flat engines, most often geared to a common output shaft. The name "H engine" is due to the engine blocks resembling a letter "H" when viewed from the front. The most successful "H" engine in this form was the Napier Dagger and its derivatives. The name was also applied to engines of the same basic layout, but rotated through 90 degrees—most famously the Napier Sabre series. A variation on the "H" theme were the Fairey Prince (H-16) & Fairey P.24 Monarch, where the two engines retained separate drives, driving Contra-rotating propellers through separate concentric shafts. Although successful, they only existed in prototype form.

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