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The Bentley BR.1 was a British rotary aircraft engine of the First World War. Designed by the motor car engine designer W. O. Bentley, the BR.1 was built in large numbers, being one of the main power-plants of the Sopwith Camel.
The 130 hp Clerget 9B was an important engine for the British Royal Naval Air Service and Royal Flying Corps, being license produced in Britain and powering a number of important British aircraft, including the Sopwith Camel. It was expensive, at £907 a copy, and prone to overheating, so the Admiralty asked Lieutenant W. O. Bentley, an established pre-war engine designer, to produce a modified version to solve these problems.
Bentley came up with an engine fitted with aluminium cylinders with cast iron liners, and aluminium pistons. Dual ignition was introduced to improve reliability, and the stroke increased to 6.7 in, which allowed power to be increased to 150 hp. The cost of the engine was also reduced, falling to £605 per engine.
The resulting engine, initially known as the A.R.1 for "Admiralty Rotary", but later called the BR.1 for Bentley Rotary was manufactured in quantity, initially against Admiralty orders. It was standardised for the Camel in RNAS squadrons, but unfortunately there were never enough to entirely replace the inferior and more expensive Clerget engine in British service, and most RFC Camel squadrons continued to use Clerget engines.