Engines

The Rolls-Royce Merlin

The people at Supermarine’s design department have always expressed thankfulness towards Rolls-Royce for their part in the development of the Spitfire. The development of the Spitfire is a result of the co-operation between the 2 companies, which ensured that new ideas quickly could be transferred from the one to the other.After the victories of the "R"-engine in the seaplanes in the years 1929-31, Rolls-Royce tried to maintain the position that they had gained by transferring the results to the military engines, the 490-520hp Kestrel and 820hp Buzzard, but it was hard to increase the output of those engines without loss of reliability. In 1933 87 octane fuel was available and the 100 octane in 1939-1940. This fuel could take a higher compression without causing the engine to knock (self-combust). This meant that the outputs of engines could be increased to more than 1000hp, e.g. by using superchargers, and still maintain reliability. Engine manufacturers are with few exceptions all privately owned companies who do their own design and solve their own problems. Rolls-Royce is such a private company, which at that time had Sir Henry Rolls as the general manager. He personally supervised the "PV12" project, which was the name of the new and more powerful aero-engine, because he knew what was needed. Unfortunately he did not live long enough to see the PV12 become the Merlin.

Early prototypes had weaknesses, which delayed a 100-hour test. Cracked cylinder jackets and bread-downs of the propeller reduction-gear caused these problems. When these problems were solved a 100-hour engine-run was carried out in July 1934 on an engine which had an output of 790hp at 2,500 rpm at 2 lbs. boost at an altitude equivalent to 12,000ft. The following tests resulted in modifications of the engine, which then was called "Merlin B". More problems occurred with poor crankcase castings cylinder heads and connecting rods. Finally the "Merlin E" passed a civil test. The "Merlin G" became the production model Merlin II.

X-ray view of a Merlin single-stage Merlin.

In the following years new Merlin variants were developed - such as the Merlin 61 with a 2-stage and 2-speed supercharger (or blower) and intercooler. The supercharger was driven by the oil pressure of the engine's scavenge system operating a clutch in between. The clutch should make sure that the supercharger was not damaged if the engine should momentarily run uneven, because it ran at 10 times the speed of the engine (i.e. 30,000 rpm at max. engine rpm). Because of the new supercharger the Merlin 61 had a much higher output compared to earlier Merlins.

The Merlin engine.

Since the Merlin-variants are generally alike it did not take that much work to replace one Merlin-variant with another, however, because of the new 2-stage supercharger the Merlin 60's were longer than previous versions the forward part of the Spitfire's fuselage had to be stretched. Furthermore an intercooler had to be fitted, in order to cool the pressurised fuel/air mixture before it was led to the inlet of the engine; the mixture could reach a temperature of 200° Cels. after the compression and that caused danger of combustion in the inlet, which would damage the engine. The 2-stage supercharger assisted in raising the pressure in the engine and thus increasing the output. An "inter-stage cooler" was fitted between the supercharger's stage 1 and 2.

The intercooler system (not to be confused with the engine cooling system)

A further developed supercharger increasing the manifold-pressure to 25 lbs. and the use of 150-octane fuel increased the top speed of a Spitfire Mk. IX by more than 30 mph (50 km/h). In 1944 150-octane fuel was used and water injection was fitted on a Merlin and that gave an output of 2,640 hp at 36 lbs. manifold pressure.

Merlin engines were used on a number of other aircraft types such as Avro Lancaster, DeHavilland Mosquito, Hawker Hurricane, Handley Page Halifax, Vickers Wellington II, Fairey's Barracudas and Fulmars.
During the 6 years of WW II, 88 Merlin-variants were developed and about 150,000 Merlins were built on the factories in Derby, Crewe, Glasgow and by Ford Motor Company and by Packard Motor Company in the USA.

Other users of the Merlin engine: The Avro Lancaster Mk III with Packard Merlin 28 engines. This specific aircraft is one of only two flying Lancs in the world - Maintained and operated by BBMF.

To the right: Hawker Hurricane. Below: DeHavilland Mosquito.

Rolls-Royce Griffon.

When WW II broke out, Rolls-Royce started to develop a new engine type, which basically should have the design as the Merlin, but should have a bigger output. It should also be able to be fitted into existing fighters in order to secure that the production of these fighters would not be stopped or delayed. The name of the new engine was Griffon and it brought forward the Rolls-Royce tradition for liquid cooled V-engines. All experience gained by the RAF and Fleet Air Arm, of course, was used in the new design.

One of the new things on the Griffon was the separate gearbox for external components like the hydraulic pump, instruments, brakes, flaps and generator. The early Griffons (II, III and IV) all had a single-stage and 2-speed supercharger. The Griffons III and IV were used in Spitfire Mk. XII. The Griffon II was used in Fairey's naval fighter - the Firefly. The Griffon 61 was fitted with a 2-stage and 2-speed supercharger identical to the one fitted on the Merlin 61 and it was used on Spitfire Mk. 21. The Griffon 65 was identical to Griffon 61 except for a different propeller reductiongear ratio and it was used on Spitfire Mk. XIV.

The Griffon 65 which was fitted to a Spitfire Mk. XIV.

The Griffons 72 and 74 were fitted in aircraft operated by the Royal Navy and used 150 octane fuel. The 2 engines had an output of 2,245 hp at 25 lbs. manifold pressure at 9,250 ft. (2,820 meters). The Griffon 83 were built like the 65, however, it was fitted with a propeller reduction gear with 2 counter rotating propeller shafts. The inner shaft rotated in the opposite way of the surrounding outer shaft so that a contra-rotating propeller could be used.

A rare sight: A Spitfire with counter rotation propeller. This specific engine originates from an Avro Schackelton and has an output of approx 2300 hp.

Packard.

In September 1940 the Packard Company accepted the job of building Rolls-Royce Merlin engines both for British and American aircraft. The first 2 Packard Merlins were fitted in testbeds in August 1941. In 1942 the production of Merlin 28 for Canadian built DeHavilland DH 98 Mosquito's and Avro Lancasters was running. The American Curtiss P40 was equipped with this engine, however, under the designation V-1650-1. By producing the Packard Merlin 266 (called PM 266) it also found its way to Supermarine Spitfire Mk. XVI. There were some minor differences between the Merlin 66 and the PM 266, partly in the way the tubing was done and the way the supercharger was driven. In order to avoid ordering wrong parts and erratic deliveries, two spareparts centres were set up; one for Merlin parts and one for Packard parts.

A typical user of the Packard Merlin engine is the North American P-51 Mustang

--ooOOoo--

The jet engine was on its way and by the end of the war Rolls-Royce moved their investments to this new engine type, which quickly had shown its capabilities when fitted on fighting aircraft. These old piston engines were still used in commercial aircraft for many years, so Rolls-Royce concentrated on giving the Merlin engine a longer life and reliability.

Fortunately the sound of the Merlin, Griffon and some of the other engines can still be enjoyed and the aircraft to which they are fitted can be seen.