The ABC radial aero engines of the WW1 period were extremely advanced for their time, and were initially thought to be very promising indeed. Unfortunately they were all more or less plagued by problems - and although a number of types for the Royal Air Force were designed around ABC engines (especially the ill-fated Dragonfly) none of these types were to see squadron service with the RAF.
ABC also made a large number of engines for electrical generators and other purposes - mostly with a flat twin cylinder layout and unusual exhaust-over-inlet valve configuration. One of these holds the distinction of being possibly the first airborne APU when installed to power the searchlight of the Supermarine Nighthawk 'Zeppelin killer'.