The Rolls-Royce Merlin is a British liquid-cooled V12 piston aero engine of 27-litre (1,650 cu in) capacity. Developed as a private venture by Rolls-Royce and first run in 1933, the engine became one of the most successful aircraft engines of World War II, most closely associated with the Hawker Hurricane and Supermarine Spitfire, although the majority of the production run was for the four-engined Avro Lancaster heavy bomber.
Initially known as the PV-12, it was dubbed Merlin after the company convention of naming its four-stroke piston aero engines for birds of prey. The engine benefitted from the racing experiences of precursor engines in the 1930s. After several modifications, the first production variants of the PV-12 were completed in 1936. The first operational aircraft to enter service using the Merlin were the Fairey Battle, Hurricane, and Spitfire. Experience in use led to a series of rapidly applied developments that markedly improved the engine's performance and durability. Starting at for the first production models, most late-war versions produced just under . The final version, used in the de Havilland Hornet, had over .
Some 50 versions of the Merlin were built by Rolls-Royce in Derby, Crewe, and Glasgow, as well as by Ford of Britain at their Trafford Park factory near Manchester. The Packard V-1650 was a version of the Merlin built in the United States. A de-rated version was the basis of the Rolls-Royce/Rover Meteor tank engine. After the war, the Merlin was largely superseded by the Rolls-Royce Griffon for military use, although new variants were designed and built for airliners and military transport aircraft. Production ceased in 1950 after a total of almost 150,000 engines had been delivered.
Merlin engines remain in Royal Air Force service today with the Battle of Britain Memorial Flight, and power many restored aircraft in private ownership worldwide.
Design and development Origin In the early 1930s, Rolls-Royce started planning its future aero-engine development programme and realised there was a need for an engine larger than their 21-litre (1,296 cu in) Kestrel, which was being used with great success in a number of 1930s aircraft.Rubbra 1990, p. 64. Consequently, work was started on a new -class design known as the PV-12, with PV standing for Private Venture, 12-cylinder, as the company received no government funding for work on the project. The PV-12 was first run on 15 October 1933 and first flew in a Hawker Hart biplane (serial number K3036) on 21 February 1935.Lumsden 2003, p. 203. The engine was originally designed to use the evaporative cooling system then in vogue. This proved unreliable and when ethylene glycol from the U.S. became available, the engine was adapted to use a conventional liquid-cooling system. The Hart, as a Merlin testbed, completed over 100 hours of flying with the Merlin C and E engines.Mason 1991, p. 168.
In 1935, the Air Ministry issued a specification, F10/35, for new fighter aircraft with a minimum airspeed of . Fortunately, two designs had been developed: the Supermarine Spitfire and the Hawker Hurricane; the latter designed in response to another specification, F36/34.McKinstry 2007, p. 53. Both were designed around the PV-12 instead of the Kestrel, and were the only contemporary British fighters to have been so developed. Production contracts for both aircraft were placed in 1936, and development of the PV-12 was given top priority as well as government funding. Following the company convention of naming its piston aero engines after birds of prey, Rolls-Royce named the engine the Merlin after a small, Northern Hemisphere falcon (Falco columbarius).Gunston 1989, p. 137.
Two more Rolls-Royce engines developed just prior to the war were added to the company's range. The Rolls-Royce Peregrine was an updated, supercharged development of their V12 Kestrel design, while the 42-litre (2,560 cu in) Rolls-Royce Vulture used four Kestrel-sized cylinder blocks fitted to a single crankcase and driving a common crankshaft, forming an X-24 layout.Rubbra 1990, p. 139. This was to be used in larger aircraft such as the Avro Manchester.Lumsden 2003, pp. 198–200.
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 fighter and one of the Gloster F.9/37 prototypes. The Vulture was fitted to the Avro Manchester bomber, but proved unreliable in service and the planned fighter using it – the Hawker Tornado – was cancelled as a result.Lumsden 2003, p. 200. With the Merlin itself soon pushing into the range, the Peregrine and Vulture were both cancelled in 1943, and by mid-1943 the Merlin was supplemented in service by the larger Griffon.Rubbra 1990, p. 118. The Griffon incorporated several design improvements and ultimately superseded the Merlin.
Development Initially the new engine was plagued with problems such as failure of the accessory gear trains and coolant jackets. Several different construction methods were tried before the basic design of the Merlin was set.Rubbra 1990, pp. 64–117. Early production Merlins were unreliable: common problems were cylinder head cracking, coolant leaks, and excessive wear to the camshafts and crankshaft main bearings.Rubbra 1990, pp. 82–92.
Early engines The prototype, developmental, and early production engine types were the: PV-12 The initial design using an evaporative cooling system. Two built, passed bench type testing in July 1934, generating at equivalent. First flown 21 February 1935. Merlin B Two built, ethylene glycol liquid cooling system introduced. "Ramp" cylinder heads (inlet valves were at a 45-degree angle to the cylinder). Passed Type Testing February 1935, generating at equivalent. Merlin C Development of Merlin B; crankcase and cylinder blocks became three separate castings with bolt-on cylinder heads. First flight in Hawker Horsley 21 December 1935, at .Morgan and Shacklady 2000, p. 607. Merlin E Similar to C with minor design changes. Passed 50-hour civil test in December 1935 generating a constant and a maximum rating of . Failed military 100-hour test in March 1936. Powered the Supermarine Spitfire prototype.Lumsden 2003, p. 204.
Merlin F (Merlin I) Similar to C and E. First flight in Horsley 16 July 1936.Morgan and Shacklady 2000, p. 610. This became the first production engine, and was designated as the Merlin I. The Merlin continued with the "ramp" head, but this was not a success and only 172 were made. The Fairey Battle I was the first production aircraft to be powered by the Merlin I and first flew on 10 March 1936. Merlin G (Merlin II) Replaced "ramp" cylinder heads with parallel pattern heads (valve stems parallel to the cylinder bore axis) scaled up from the Kestrel engine. 400-hour flight endurance tests carried out at RAE July 1937; acceptance test 22 September 1937. It was first widely delivered as the Merlin II in 1938, and production was quickly stepped up for Fairey Battle II. Merlin III Merlin II with standardised de Havilland/Rotol SBAC propeller shaft, and dual accessory-drive. at 3,000 rpm at at + boost. Formed basis for the Rolls-Royce/Rover Meteor tank engine "Racing" Merlin Racing engine for 1937/38 "Speed Spitfire" world speed record attempt. Merlin III with strengthened pistons, connecting rods, and gudgeon-pins, running on increased octane fuel, developed at 3,200 rpm and + boost, a power/weight ratio of . Completed 15-hour endurance run at , 3,200 rpm at + boost. Merlin IV Merlin with pressure-water cooling for Armstrong Whitworth Whitley IV. Merlin V Merlin for Fairey Battle V. Merlin VIII Medium-supercharged Merlin developed for Fairey Fulmar I, rated at 2,850 rpm at , at 3,000 rpm for take-off using 100-octane fuel. Merlin X First Merlin with two-speed supercharger, in low gear at , in high gear at . First of Rolls-Royce unitised "Power Plant" installation designs for this engine in 1937 and used in Handley Page Halifax I, Vickers Wellington II, and Armstrong Whitworth Whitley V and VII. Merlin XII Merlin fitted with 0.477:1 reduction gear installed in some Spitfire IIs with three-bladed Rotol constant-speed propeller. Rated at at 3,000 rpm at . Merlin XX Merlin X with Stanley Hooker re-designed supercharger"World Encyclopedia of Aero Engines – 5th edition" by Bill Gunston, Sutton Publishing, 2006, p. 190 incorporating re-designed inlet and improved guide vanes on impeller with revised blower gear ratios; 8:15:1 for low gear, 9:49:1 for high gear. New larger SU twin choke updraught carburettor. Engine interchangeable with Merlin X. Rated at at 2,850 rpm in low gear at and + boost; at 2,850 rpm in high gear at at +9 lb boost. Revised Rolls-Royce unitised "Power Plant" installation design. Engine used in Bristol Beaufighter II, Boulton Paul Defiant II, Handley Page Halifax II and V, Hawker Hurricane II and IV, and Avro Lancaster I and III. First Merlin produced by Packard Motor Car Company as V-1650-1 and designated by Rolls-Royce as Merlin 28.
Production engines The Merlin II and III series were the first main production versions of the engine. The Merlin III was the first version to incorporate a "universal" propeller shaft, allowing either de Havilland or Rotol manufactured propellers to be used.Fozard 1991, p. 125.
The first major version to incorporate changes brought about through experience in operational service was the XX, which was designed to run on 100-octane fuel. This fuel allowed higher manifold pressures, which were achieved by increasing the boost from the centrifugal supercharger. The Merlin XX also utilised the two-speed superchargers designed by Rolls-Royce, resulting in increased power at higher altitudes than previous versions. Another improvement, introduced with the Merlin X, was the use of a 70%–30% water-glycol coolant mix rather than the 100% glycol of the earlier versions. This substantially improved engine life and reliability, removed the fire hazard of the flammable ethylene glycol, and reduced the oil leaks that had been a problem with the early Merlin I, II and III series.Fozard 1991, pp. 127, 165.
The process of improvement continued, with later versions running on higher octane ratings, delivering more 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 in common versions, and as much as in the Merlin 130/131 versions specifically designed for the de Havilland Hornet.Flight January 1946, p. 93. Ultimately, during tests conducted by Rolls-Royce at Derby, an RM17SM (an improved version of the RM14SM 'low altitude' Merlin 100-Series with larger diameter supercharger rotors and different valve timing) achieved at 36 lb boost (103"Hg) on 150-octane fuel with water injection.Harvey-Bailey and Piggott 1993, p. 136.
With the end of the war, work on improving Merlin power output was halted and the development effort was concentrated on civil derivatives of the Merlin.Lovesey 1946, p. 224. Development of what became the "Transport Merlin" (TML) commenced with the Merlin 102 (the first Merlin to complete the new civil type-test requirements) and was aimed at improving reliability and service overhaul periods for airline operators using airliner and transport aircraft such as the Avro Lancastrian, Avro York (Merlin 500-series), Avro Tudor II and IV (Merlin 621), Tudor IVB and V (Merlin 623), TCA Canadair North Star (Merlin 724) and BOAC Argonaut (Merlin 724-IC). By 1951 the time between overhauls (TBO) was typically 650–800 hours depending on use. By then single-stage engines had accumulated 2,615,000 engine hours in civil operation, and two-stage engines 1,169,000.