Difference between Kestrel and Peregrine?

So how do you double the output of the same engine…?

Not so different from the Merlin, whose prototypes were producing less than 1,000 hp (note: horsepower, not b.h.p.) and today’s highly-developed racing powerplants push out over 3,500 hp for a life of a few minutes, all with the same 27-litre (1650 cubic inch) swept volume.

The additional boost would certainly add to the power, as would higher octane to allow such boost without detonation. It would require not just improved metallurgy but probably stud/bolt design and positioning to keep the head on – better metallurgy for the bolts would no doubt help! This suggests different cylinder heads. The supercharger change in itself is not minor. Was this an early sign of Hooker’s work, later seen on the Merlin 45?

Heavy Metal

i’d always heard the peregrine was the fall back if the PV12(merlin) didnt succeed……a late model; kestrel with a merlin styled super charger .

I don’t know too much about the Peregrine : it’s too modern for me! I understand it as the evolution of the proven Kestrel design accomodating the tweaks and inspirations discovered during the problematic development of the Merlin. If the Merlin didn’t end up being reliable then the Peregrine could fill the breach, but its swept volume was 20% smaller than the Merlin, so it was not really a contender. The Merlin now sits assured in history, but it would have been a nervous bunch at Rolls Royce grappling with Merlin development issues while German DB 600 engines proved their mettle over Spain and Bristol sleeve valves made radials seem like holding the future.

From Air Annual of the British Empire 1939 : ” The RR Peregrine…has a swept capacity of 21 litres, the same as that of the Kestrel engines (but) the Peregrine has a rated output of 860 b.h.p as compared with the 480 b.h.p of the original Kestrel.” So how do you double the output of the same engine, which is the essential difference ?

I think much has to do with progress in metallurgy. RR invested much effort in this and there as a large group of alloys known as the “RR alloys”. So a cylinder head casting made from an identical casting pattern could contain twice as much cylinder pressure using RR alloys in 1939 than the less developed alloy in the Kestrel of 1928. Another example is pistons, using “Y alloy” in the early thirties that expanded and scored the cylinder wall, compared to “Lo-Ex” alloys in 1939 that didn’t, allowing the engine to reliably develop more horsepower. In this context, the RR “R” racing engines used in the Schneider Trophy races developed as much bhp as the Merlins of a decade later, but the R engines only had a life of hours before they self destructed. This metallurgical progress went hand in hand with the development of fuels with higher octane ratings, allowing the development of higher cylinder pressures. So you could put high octane fuel in the Kestrel but it would not last as long as the Peregrine with its better metallurgy. Certainly there was constant development through the thirties of nickel chrome moly steels for conrods and crankshafts to cope with higher engine stresses.

The Kestrel had supercharging from its early days and later Kestrels had auxiliary drives for hydraulic pumps, CSUs and vacuum pumps, so you can’t say this is an essential difference with the Peregrine. Probably there are lots of little things. I understand a great deal of effort went in to the optimisation of supercharger designs, so the Peregrine, which was boosted to +8 may have had a different supercharger vane and passageways to the Kestrel boosted to +4. I do not know whether the Peregrine had Gilman bearings in the conrod big ends or the removable shell type bearings used in the Merlin, but this might be a typical improvement and major difference. Certainly RR had to pay America’s GM royalties for the use of Gilman bearings in Kestrels, something management would no doubt want to shake off. Gilman bearings are pictured here : http://forum.keypublishing.com/showthread.php?128767-RR-Kestrel-Gilman-bearings&highlight=

I think late model Kestrel Xs were just re-badged as Peregrines and the difference between the Kestrel of 1929 and Peregrine of 1939 is largely in the metallurgy.

I take it to mean that it was basically a development of the Kestrel with efficiencies in operation and manufacture, and that it had become sufficiently different that they thought it worth adopting a new name for it.

Think Lancaster > Lincoln

Moggy

Agreed!

Also…

Peregrine was downdraught carburettored, the Kestrel updraught.

Anon.

i’d always heard the peregrine was the fall back if the PV12(merlin) didnt succeed……a late model; kestrel with a merlin styled super charger .

In one of the RRHT books the Peregrine is described as a ‘Merlinised’ Kestrel.

I take it to mean that it was basically a development of the Kestrel with efficiencies in operation and manufacture, and that it had become sufficiently different that they thought it worth adopting a new name for it.

Think Lancaster > Lincoln

Moggy

… It was essentially a “rationalized” version of their Rolls-Royce Kestrel
…

Agree with the rest of the post, but I don’t understand this phrase. Please could you explain.

Regards

The Peregrine was an 885 hp liquid-cooled V-12 aircraft engine designed in the early 1930s. It was essentially a “rationalized” version of their Rolls-Royce Kestrel, which had seen widespread use

During the 1930s the use of superchargers to increase the 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 no reason not to use it at all times. The introduction of such “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 upcoming war. Two Peregrines bolted together on a common crankshaft would produce the Rolls-Royce Vulture, a 1,700 hp X-24 which would be used for bombers.

As it turned out, aircraft designs rapidly increased in size and power requirements to the point where the Peregrine was simply too small to be useful. Rolls’ internal project to “fill in the gap” between the Peregrine and Vulture resulted in the Rolls-Royce Merlin, which annihilated any demand for the smaller, less powerful Peregrine. The demand for the Merlin overshadowed development of the Peregrine and resources for it were cut back as attempts were made to quickly bring the Merlin into service.

http://www.gracesguide.co.uk/Main_Page

Moggy