The Phoenix was an experimental version of the Bristol Aeroplane Company's Pegasus engine, adapted to run on the Diesel cycle. Only a few were built between 1928 and 1932, although samples fitted to a Westland Wapiti held the altitude record for diesel-powered aircraft at 27,453 ft (8,368 m) from 11 May 1934 until World War II.[1] The primary advantage of the Phoenix was better fuel efficiency at cruise, by up to 35%.