The Rolls-Royce Peregrine was a 21-litre (1,300 cu in), 885-horsepower (660 kW) liquid-cooled V-12 aero engine designed and built by Rolls-Royce in the late 1930s. First run in 1938, the Peregrine was developed during 1939 from the Kestrel. Running on 87 octane fuel, it had an international rating of 860 h.p. at 2,850 r.p.m. at 13,500 ft, and a maximum output of 885 h.p. at 3,000 r.p.m. at 15,000 ft. A number of Merlin features were incorporated and provision was made for a wide range of auxiliaries. External differences from the Kestrel included the fitting of a downdraught carburettor; this had progressive boost control, in which the "dead" range of movement of the throttle lever was eliminated.
During the 1930s the use of superchargers to increase "effective displacement" of an aircraft engine came into common use. Charging of some form was a requirement for high-altitude flight, and as the strength of the engines improved there was no reason not to use it at all times. The introduction of just such a "ground-level" supercharger to the Kestrel along with several design changes improved the power-to-weight ratio considerably, and it was generally felt that the resulting Peregrine would be the "standard" fighter engine for the impending war. Following the company convention of naming its piston aero engines after birds of prey, Rolls-Royce named the engine the Peregrine after the Peregrine Falcon (Falco peregrinus), the world's fastest and most widespread bird of prey.
Although the Peregrine appeared to be a satisfactory design, it was never allowed to mature since Rolls-Royce's priority was refining the Merlin. As a result, the Peregrine saw use in only two aircraft: the Westland Whirlwind and the Gloster F9/37.
A design feature of the Peregrine was that it was produced in both right- and left-hand tractor variants. This was done to improve aircraft handling by providing a counter-rotating propeller facility. The handing of internal parts to achieve this was a considerable complication that was later abandoned in favour of an idler gear arrangement for the Merlin propeller reduction gear.
The two aircraft types that used the Peregrine, the Westland Whirlwind and the second prototype of the Gloster F9/37, were both twin-engine designs – the prototype F9/37 had used the Bristol Taurus radial engine. The Air Ministry requirement for the F9/37, a cannon-armed fighter (the Hurricane and Spitfire were armed with machine guns only at this point), was curtailed and there was no further progress with the design. The Whirlwind, despite having excellent low-altitude performance, proved uneconomical compared with single-engined fighters, and also suffered as a consequence of the Peregrine reliability problems. Low production rates of the Peregrine caused delays in delivery to squadron use. In August 1940 Ernest Hives, head of the Rolls-Royce aero engine division, wrote to Air Chief Marshal Wilfrid Freeman expressing his wish to stop work on the Peregrine, Vulture and another engine development project, the Rolls-Royce Exe to concentrate efforts on the Merlin and Griffon, but Freeman disagreed and stated that Peregrine production should continue.