Clerget Rotary Engine - France
In 1917 the Sopwith Camel airframe cost £874 10s. The Clerget engine that powered it cost £907 10s. The fact that the power unit usually cost more than the airframe is not always recognized.
The Clerget had neither the peculiar intake arrangements of the Gnome nor the strange connecting rods of the Le Rhone. Its valves were actuated by conventional rocker arms from two pushrods per cylinder, these constituting a recognition feature. Clergets were made in 110-hp and 130-hp models, and the 130 may have been a little overdeveloped, since it was subject to overheating. It used a special type of piston ring, called an "obdurator" ring, below the wrist pin to block heat transfer from the combustion area to the lower part of the cylinder. When this ring broke, as it was prone to do, the cylinder turned blue from the heat; a blue colour meant that the steel had been heated to 600°F (316°C). Clergets were generally very well engineered except for this problem, those made in England by Gwynne being especially excellent.