Engines

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.

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.

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.

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.

Rolls-Royce Griffon

When WW II broke out, Rolls-Royce started to develop a new engine type, which should basically have the design as the Merlin, but should have a bigger output. It should also be able to be fitted into existing aircraft types in order to secure that the production of these aircraft 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. The basic design of the Griffon has some similarity the R25R engine used in the Supermarine S6B, however, it is not correct to maintain that is a scaled-up version of the Merlin.

One of the new things on the Griffon was the separate gearbox for external components like the hydraulic pump. Another is the lubrication of the crank shaft: the oil is distributed into both ends of the crank shaft. Normally the bearing caps are lubricated from outside. 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 to 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 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.

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  deliveries of wrong type of parts, two spareparts centres were set up; one for Merlin parts and one for Packard parts.

--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 and Griffon engines a longer life and higher reliability.

Some engine data further below.....

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

The intercooler system (not to be confused with the engine cooling system) - click for larger picture

A single stage Merlin engine - click for larger picture

 

An other user of the Merlin engine: The Hawker Hurricane - Click for larger picture

The difference in the front area between the  Merlin and the Griffon. Light gray = Merlin.

Griffon 65

Above and below: A rarity - a Spitfire with contra rotating prop.

 

DATA.

Merlin 61.
 
Type: 
Supercharged, geared, liquid cooled 60 degrees V-engine. 
Cylinders:
12 cylinders
Valves / cams:
2 intake- and 2 exhaust valves. 1 overhead camshaft per cylinder head.
Bore and stroke:
137,16mm x 152,4mm = 27 litres swept volume.
Compression ratio:
6,0: 1
Output:
Take-off:
1290hp at 3000 rpm.
Max.:
1565hp at 3000 rpm. at 12250 ft.
 
1390hp at 3000 rpm. at 23500 ft
Max revs.:
600 rpm during dive.
Propeller reduction gear:
Spur layshaft
Ratio:
0,42 : 1
Direction of rotation:
Propellershaft: counter clock - crankshaft: clockwise (seen from the front)
Supercharger:
Intercooled, centrifugal, 2 speed and 2 stage.
Speed controlled by the oil pressure of the scavenge system. Boost regulated automatically by a barometric servo unit connected to and controlled by the throttle.
Lubrication:
Drysump.
Induction system:
Rolls-Royce / SU updraft carburettor with automatic mixture control. Induction through the supercharger.
Ignition:
Dual ignition. 2 12-cylinder magnetos.
Cooling system:
70% water and 30% ethylene glycol. Centrifugal water pump. Max water temp. 135° Cels.A temperature sensitive device controls airflow through the radiators in the wings manually or automatically.
Starter:
Electrical (or by a hand turning gear if the electric system should fail)
Dry weight:
1,640 lbs. / 744kg.

  
 
Griffon 65.
 
Type: 
Supercharged, geared, liquid cooled 60 degrees V-engine. 
Cylinders:
12 cylinders.
Valves / camshafts:
2 intake- and 2 exhaust valves. 1 overhead camshaft per cylinder head.
Bore and stroke:
154,4mm x 167,64mm = 36,7 litres swept volume.
Output:
Take-off :
1540hp
Max.:
2035hp in 7,000 feet
 
1820hp in 21,000 feet
Propeller reduction gear:
Spur layshaft 
Reduction gear ratio:
0,51 : 1
Direction of rotation:
Propellershaft: clockwise - crankshaft: counter-clockwise (seen from the front)
Supercharger:
Intercooled, centrifugal, 2 speed and 2 stage. Speed controlled by the oil pressure of the scavenge system. Boost regulated automatically by a barometric servo unit connected to and controlled by the throttle. Same way as the Merlin 61.
Lubrication:
Dry sump. 1 pressure- and 2 scavenge pumps
Ingestion system:
Rolls-Royce Bendix updraft carburettor with automatic mixture control, FCU (Fuel Control Unit) and acceleration pump. Ingestion through the supercharger.
Ignition:
Dual ignition. 2 12-cylinder magnetos.
Cooling system:
70% water and 30% ethylene glycol. Centrifugal water pump. A temperature sensitive device controls airflow through the radiators in the wings manually or automatically.
Starter:
Cartridge starter with 5 cartridges
Dry weight:
1,980 lbs. / 898kg.