Rolls Royce Merlin

Rolls Royce Merlin
Rolls Royce Merlin
Type Liquid-cooled V-12 piston engine
National origin United Kingdom
Manufacturer Rolls-Royce
First run 1935
Major applications Supermarine Spitfire
Avro Lancaster
Curtiss P-40
De Havilland Mosquito
Handley-Page Halifax
Hawker Hurricane
Number built 149,659
Variants Packard V-1650

The Rolls-Royce Merlin was a liquid cooled 27 litre (1649 in3) 60° V12 piston aircraft engine which became famous in World War II. An English Icon, several versions of the Merlin were built by Rolls-Royce (in Derby, Crewe and Glasgow) by Ford of Britain (in Trafford Park, Manchester) and in the United States as the Packard V-1650. They are widely considered to be among the most successful aircraft engines produced during World War II.

The name 'Merlin' comes from a type of small falcon (Falco columbarius), in line with the convention Rolls-Royce used in naming its other piston aero-engines after birds of prey, and has no connection to King Arthur's legendary magician.


In the early 1930s, Rolls-Royce started planning for the future of its aero engine development programmes, and eventually settled on two basic designs. The 700 horsepower (500 kW) Rolls-Royce Peregrine was an updated, supercharged development of their existing V-12, 22 L Rolls-Royce Kestrel, which had been used with great success in a number of 1930s designs. Two Peregrines bolted together on a common crankshaft into an X-24 layout would create the 1,700 hp (1,300 kW) 44 L Rolls-Royce Vulture, for use in larger aircraft such as bombers. There was also the possibility that the famous 36 L 'R' engine (itself a development of the Rolls-Royce Buzzard, a scaled-up Kestrel) from the Supermarine racing planes could be developed into a 1,500 hp (1,100 kW) class engine of its own.

This left a large gap between 700 and 1,500 hp (500 and 1,100 kW), and to fill it work was started on a new 1,100 hp (820 kW) class design known as the PV-12 – PV for "private venture" as the company received no government money for work on the project. The PV-12 first flew in a Hawker Hart (serialled K3036) biplane on 21 February 1935, using the evaporative cooling system then in vogue, but this proved unreliable and so, when supplies of ethylene glycol from the US became available, the engine was changed to the conventional liquid cooling system. The Hart was subsequently delivered to Rolls-Royce and as a Merlin "Testbed" completed over 100 hour of flying with the Merlin 'C' and 'E' engines.

Initially the new engine was plagued with problems, such as failure of the gear trains and constant failure of the coolant jackets, and several different construction methods were used before the basic design of the Merlin was set. The prototype engines were:

  • PV-12:
    The original design retained the evaporative cooling system. Passed Type Testing in June 1934, generating 740 hp (552 kW) at 12,000 ft (3,657 m) equivalent.
  • Merlin B:
    Changed to ethylene glycol coolant. "Ramp" cylinder heads (inlet valves were at a 45-degree angle to the cylinder). Passed Type Testing February 1935, generating 950 hp (708 kW) at 11,000 ft (3,353 m) equivalent.
  • Merlin C:
    Crankcase and cylinder blocks became three separate castings with bolt-on cylinder heads.
  • Merlin E:
    Similar to C; minor design changes. Passed 50 hour civil test generating a constant 955 hp (712 kW) and a maximum rating of 1,045 hp (779 kW).
  • Merlin F:
    Similar to C and E. This became the first production engine; 172 were built as the Merlin I. The Merlin continued with the 'ramp head' where the inlet valves were at a 45-degree angle to the cylinder. This was not a success and only 172 were made. The Fairey Battle was the first production aircraft to be powered by the Merlin I.
  • Merlin G:
    Replaced "ramp" cylinder heads with parallel pattern heads (valves parallel to the cylinder) scaled up from the Kestrel engine. It was first widely delivered as the 1,030 hp (770 kW) Merlin II in 1938, and production was quickly stepped up.

In 1935, the Air Ministry issued a specification, F10/35, for new fighter aircraft with a minimum airspeed of 310 mph (499 km/h). Fortunately, two designs had been developed; the Supermarine Spitfire and the Hawker Hurricane, the latter designed in response to another specification, F36/34. Both were designed around the PV-12 instead of the Kestrel, and were the only modern British fighters to have been so developed. Production contracts for both aircraft were placed in 1936. The PV-12 was given top priority and became the Merlin.

Early Merlins were rather unreliable, but Rolls-Royce soon introduced a superb reliability-improvement programme. This consisted of taking random engines from the end of the assembly line and running them continuously at full power until they failed. They were then dismantled to find out which part had failed, and that part was redesigned to be stronger. After two years of this, the Merlin had matured into one of the most reliable aero engines in the world, and could be run at full power for eight-hour bombing missions with no problems.

A Rolls Royce engine preserved in Manston, Kent, UK

As it turned out the Peregrine saw use in only two aircraft, the Westland Whirlwind and the Gloster F9/37. Although the Peregrine appeared to be a satisfactory design, it was never allowed to mature: Rolls-Royce's priority was troubleshooting the Merlin. The Vulture was fitted to the Hawker Tornado and Avro Manchester, but proved unreliable owing to failures of the crankshaft to connecting-rod bearing caused by lubrication problems. With the Merlin itself soon pushing into the 1,500 hp (1,100 kW) range, the Peregrine and Vulture were both cancelled in 1943.

By the end of its production run, over 150,000 Merlin engines had been built. By mid 1943 the Merlin was supplemented in service by the larger Rolls-Royce Griffon which incorporated several design improvements.

Engine capacity and mass flow

Although it is common practice to compare different piston engines and their performance potential by referring to the Engine displacement or Swept volume this does not give an accurate reading of an engine's capabilities. A. C Lovesey, a Rolls-Royce engineer who was a central figure in the development of the Merlin said in 1946:

Coming now to specific development items we can, for convenience, divide them into three general classes:

  1. Improvement of the supercharger.
  2. Improved fuels.
  3. Development of mechanical features to take care of the improvements afforded by (1) and (2).

Dealing with (1) it can be said that the supercharger determines the capacity, or in other words the output, of the engine. The impression still prevails that the static capacity known as the swept volume is the basis of comparison of the possible power output for different types of engine, but this is not the case because the output of the engine depends solely on the mass of air it can be made to consume efficiently, and in this respect the supercharger plays the most important role. This applies equally to any engine but the engine has to be capable of dealing with the greater mass flows with respect to cooling, freedom from detonation and capable of withstanding high gas and inertia loads. During the course of research and development on superchargers it became apparent to us that any further increase in the altitude performance of the Merlin engine necessitated the employment of a two-stage supercharger.


Most of the upgrades to the Merlin were the result of ever-increasing octane ratings in the aviation fuel available from the US, and ever more efficient supercharger designs by Stanley Hooker. In 1938

At the start of the war the engine ran on the then standard 87 octane aviation spirit and could supply just over 1,000 hp (750 kW) from its 27 L displacement. However, as early as 1938 at the 16th Paris Air Show Rolls-Royce displayed two versions of the Merlin rated to use 100 octane fuel. The Merlin R.M 2M was capable of 1,265 hp (943 Kw) at 7,870 feet (2,392 m), 1,285 hp (958 Kw) at 9,180 feet (2,798 m) and 1,320 hp (984 Kw) on take-off, while the Merlin X with a two speed blower generated 1,150 hp (857 Kw) at 15,400 feet (4,694 m) and 1,160 hp (865 Kw) at 16,730 feet (5,099 m) with the supercharger in high gear.

From late 1939 100 octane fuel became available from the U.S. and Merlin IIIs were found to be capable of running on it. Use of this fuel required that small modifications were made to the engine, which was now capable of generating 1,310 hp (877 kW)

The next major version was the XX which ran on 100 octane fuel. This allowed higher manifold pressures, which were achieved by increasing the boost from the centrifugal type supercharger. The Merlin XX also incorporated the first of the two-speed superchargers designed by Rolls-Royce. The result was 1,300 hp (970 kW) at higher altitudes than previous versions. Another improvement made to the XX and future Merlin variants was a redesign of the cooling system to use a 70/30% water/glycol mix rather than the 100% glycol of the earlier versions, allowing them to run some 70 degrees C cooler. This substantially improved engine life and reliability, removed the fire hazard of flammable pure ethylene glycol, and reduced the oil leaks which had been a flaw of the early Merlin I, II and III series.

The process of improvement continued, with later versions running on further-increased octane ratings, delivering ever higher power. Fundamental design changes were also made to all key components, again increasing the engine's life and reliability. By the end of the war the "little" engine was delivering over 1,600 hp (1,200 kW) in common versions, and as much as 2,070 hp (1,544 kW) in the Merlin 130/131 versions used on the de Havilland Hornet.

In late 1943, trials were run of a new fuel "100/150 grade". This rating was achieved by adding 2.5% mono methyl aniline (M.M.A.) to 100 octane fuel; tests determined that the mixture resulted in a buildup of tetraethyl lead deposited in the combustion chambers causing excessive fouling of the spark plugs.

The new fuel allowed the boost rating of the Merlin 66 to be raised to + 25 pounds. Starting in March 1944 the Merlin 66 powered Spitfire IXs of two squadrons were cleared to use the new fuel for operation trials, followed by other fighters flown by the ADGB, including Mustang IIIs. Continued problems with backfires were not sorted out until August. In November 1944 Spitfires of the 2 TAF began using 100/150 grade fuel, with full supplies becoming available the following February. Monty Berger, Senior Intelligence Officer of 126 (RCAF) Spitfire Wing, 2 TAF, noted in his daily operational summary on 20 April 1945 after the crashes of two Spitfires; "The incidents followed a number of engine problems that were attributed to the introduction of 150-grade fuel in early February. Pilots mistrusted it, and were no doubt relieved when the AF brass decided to revert to 130-grade. The vast majority of pilots, I'm sure, were beginning to wonder if the additional seven pounds of boost they got from 150-grade fuel were worth the price being paid."

Aircraft of the US 8th Air Force were running on 100/150 grade by the second week of June 1944; problems continued to be experienced with fouled spark-plugs and a new blend of fuel called P.E.P, using Ethylene dibromide to counter this, was introduced in early 1945. In March 1945 use of the PEP fuel was discontinued and 8th AF units reverted to the standard 100/150 grade. The 100/150 grade fuel could be recognised by its bright green colour and the "awful smell".

Carburettor developments

Rolls Royce Merlin engine on display at the en:Tillamook Air Museum. Taken July 01st 2007 by Robert Wenstrand.

The Merlin's lack of direct fuel injection meant that both Spitfires and Hurricanes were, unlike the contemporary Bf-109E, unable to nose down into a deep dive. Luftwaffe fighters could therefore 'bunt' into a high-power dive to escape attack, leaving the pursuing aircraft spluttering behind as its fuel was forced by negative 'g' out of the carburettor. RAF fighter pilots soon learned to 'half-roll' their aircraft before diving to pursue their opponents. The use of carburettors was calculated to give a higher specific power output, due to the lower temperature, and hence greater density, of the fuel/air mixture, compared to injected systems. "Miss Shilling's orifice" (invented in March 1941 by Beatrice Shilling, an engineer at the Royal Aircraft Establishment, Farnborough), a holed diaphragm fitted across the float chambers, went some way towards curing the fuel starvation in a dive. Further improvements were introduced throughout the Merlins: 1943 saw the introduction of a Bendix-Stromberg pressure carburettor which injected fuel at 5 psi through a nozzle direct into the supercharger and was fitted to Merlins 66, 70, 76, 77 and 85. The final development was an SU injection carburettor which injected fuel into the supercharger using a fuel pump driven as a function of crankshaft speed and engine pressures, which was fitted to the 100 series Merlins. Production of the Griffon-engined Spitfire Mk. XII had begun the year before.

Packard V-1650

Main article: Packard V-1650

The Merlin was considered to be so important to the war effort, negotiations soon started to establish an alternative production line outside the UK. Rolls-Royce had checked out a number of North American automobile manufacturers, in order to select one to build the Merlin in the USA or Canada, and Packard Motor Car Company's attention to high quality and engineering impressed the parent British company so much, Packard was selected to build the Merlin. Agreement was reached in September 1940, and the first Packard-built engine, designated V-1650-1, ran in August 1941.

Alternative uses

A non-supercharged version of the Merlin using more steel and iron components was produced for use in tanks. This engine, the Rolls-Royce Meteor, in turn led to the smaller Rolls-Royce Meteorite.

In 1938, Rolls Royce started work on modifying some Merlins which were later to be used in British MTBs, MGBs, and RAF Air-Sea Rescue Launches. For these the superchargers were modified single-stage units and the engine was re-engineered for use in a marine environment.

A Spanish-built version of the Messerschmitt Bf 109 G-2, the 1954 Hispano Aviacion Ha 1112-M1L Buchon, was built with the Rolls-Royce Merlin 500/45 engine of 1,600 hp (1,200 kW), with four-bladed propeller, in the Hispano Aviacion factory in Seville—a fitting powerplant for the last-produced version of the famous Messerschmitt fighter, as the Bf 109 V1 prototype aircraft had been powered by the Rolls-Royce Kestrel V-12 engine in 1935.

At the end of World War 2 new versions of the Merlin (the 600 and 700 series) were designed and produced for use in commercial airliners such as the Avro Tudor. These engines were basically military with some minor changes to suit the different operating environment.

In the United States many war surplus engines and airframes were sold relatively cheaply - two of the most popular items were P-51 Mustangs and Packard V-1650 Merlin engines, several of which were "souped up" and modified for air racing in the Bendix Trophy, the Cleveland Air Races and the Thompson Trophy.

The Merlin engine continues to be used in dozens of restored World War 2 vintage aircraft all over the world.