Since the Armistice the gradual development of the various types of aero engines, together with the abandonment of windmill driven accessories, has necessitated an increase in the number of auxiliaries incorporated in the engine structure, and the engine designers are to be complimented for the ingenuity and compact arrangement of the auxiliary drives and housings on present-day aero engines.

The auxiliary drives for valve operation, dual magnetos, oil pressure and scavenge pumps, water and petrol pumps, supercharger, turning, starting and gun gears, are usually grouped in a housing or housings at the rear of the engine, except that on the radial air-cooled engine the valve gear is arranged at the front and the water pump is unnecessary.