In the eighteen-cylinder model the impulses occur at each 40 degrees of angular rotation of the cylinders, securing an extremely even rotation of the air-screw. In 1913 the Gnome Monosoupape engine was introduced, a model in which the inlet valve to the cylinder was omitted, while the piston was of the ordinary cast-iron type.
The 100 horse-power Gnome Monosoupape was built with nine cylinders, each 4.33 inches bore by 5.9 inches stroke, and it developed its rated power at 1,200 revolutions per minute. An engine of the rotary type, almost as well known as the Gnome, is the Clerget, in which both cylinders and crank case are made of steel, the former having the usual radial fins for cooling.
An engine which was not developed to the same high point in this country as the Liberty motor is the rotary engine, of which the Gnome Monosoupape or Clerget are perhaps the best-known types. These were favorites with airmen flying fighting scout-planes. They weighed practically nothing, for an engine.