Gnome 9-N Rotary Engine  

The French-built Gnome 9-N rotary "Monosoupape" engine had one valve per cylinder. Having no intake valves its fuel mixture entered the cylinders through circular holes or "ports" cut in the cylinder walls. The propeller was bolted firmly to the engine and it, along with the cylinders, turned as a single unit around a stationary crankshaft rigidly mounted to the fuselage of the airplane. The rotary engine used castor oil for lubrication.

Although the first successful air-cooled rotary engine was developed in the U.S. in 1896 by Mr. F.O. Farwell, it was the French-built Gnome rotary that was first used extensively in airplanes during the 1909-1910 period. Numerous types of Gnome engines were subsequently designed and built, one of the most famous being the 165-hp 9-N "Monosoupape" (one valve). It was used during WWI primarily in the Nieuport 28 , of which 297 were purchased by the U.S. Air Service from France.