The engine was first produced in 1927 at 450hp (340kW), which soon improved in the IB version to 525hp (390kW).  This variant saw widespread use in the Hawker Hart family that was the mainstay of British air power during the early 1930s.  However it was not long before line improvements increased power dramatically and the V-12 model provided 695hp (520kW) at 3 000 rpm with no basic change to the design.

One key advance in the Kestrel was the use of a pressurised cooling system.  Water boils at 100 °C at standard atmospheric pressure, but this temperature decreases with altitude.  Since the amount of heat carried out of the engine is a function of coolant temperature and volume, if the coolant has to be kept below boiling point an increasing amount of fluid has to be used, along with an increasingly large radiator to cool it.  The solution was to pressurise the entire cooling system, thereby not only preventing the decrease in cooling performance with altitude, but in fact increasing the boiling point even on the ground.  The Kestrel was built to have sufficient pressure to maintain the boiling point at about 150 °C

A total of 414 aircraft were received by the South African Air Force during the late 1930’s and early 1940’s.  The Hartbees was built in South Africa and used as a bomber during the Campaign in East Africa 1940/41.  When the aircraft was withdrawn from operational use, it was used mainly for training purposes together with the Hawker Hart, Hind and Audax.

Engine Type:  12 Cylinder V-bank in-line supercharged and water cooled
Brake Horse Power:  4 575 to 6000 BHP
Revs Per Minute:  2 500 rpm
Weight: 1 000 lbs (454kg)
Oil Consumption:  4 to 8 pints (2.2 to 4.5) litres per hour