Rolls-Royce Merlin
The Rolls-Royce Merlin is a British liquid-cooled V-12 piston aero engine of 27-litre 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 After the war, the Merlin was largely superseded by the Rolls-Royce Griffon for military use, although new variants were designed and built for airliners and military transport aircraft.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 Kestrel, which was being used with great success in a number of 1930s aircraft. 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 on 21 February 1935. 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.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. 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.
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 V-12 Kestrel design, while the 42-litre Rolls-Royce Vulture used four Kestrel-sized cylinder blocks fitted to a single crankcase and driving a common crankshaft, forming an X-24 layout. This was to be used in larger aircraft such as the Avro Manchester.
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. 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. 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. Early production Merlins were unreliable: common problems were cylinder head cracking, coolant leaks, and excessive wear to the camshafts and crankshaft main bearings.Early engines
The prototype, developmental, and early production engine types were the:- PV-12
- Merlin B
- Merlin C
- Merlin E
- Merlin F
- Merlin G
- Merlin III
- "Racing" Merlin
- Merlin IV
- Merlin V
- Merlin VIII
- Merlin X
- Merlin XII
- '''Merlin XX'''
Production engines
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.
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. Ultimately, during tests conducted by Rolls-Royce at Derby, an RM17SM achieved at 36 lb boost on 150-octane fuel with water injection.
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. Development of what became the "Transport Merlin" commenced with the Merlin 102 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, Avro Tudor II and IV, Tudor IVB and V, TCA Canadair North Star and BOAC Argonaut. By 1951 the time between overhauls 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.
In addition, an exhaust system to reduce noise levels to below those from ejector exhausts was devised for the North Star/Argonaut. This "cross-over" system took the exhaust flow from the inboard bank of cylinders up-and-over the engine before discharging the exhaust stream on the outboard side of the UPP nacelle. As a result, sound levels were reduced by between 5 and 8 decibels. The modified exhaust also conferred an increase in horsepower over the unmodified system of, resulting in a 5 knot improvement in true air speed. Still-air range of the aircraft was also improved by around 4 per cent. The modified engine was designated the "TMO" and the modified exhaust system was supplied as kit that could be installed on existing engines either by the operator or by Rolls-Royce.
Power ratings for the civil Merlin 600, 620, and 621-series was continuous cruising at, and for take-off. Merlins 622–626 were rated at continuous cruising at, and for take-off. Engines were available with single-stage, two-speed supercharging, two-stage, two-speed supercharging, and with full intercooling, or with half intercooling/charge heating, charge heating being employed for cold area use such as in Canada. Civil Merlin engines in airline service flew 7,818,000 air miles in 1946, 17,455,000 in 1947, and 24,850,000 miles in 1948.
Basic component overview (Merlin 61)
From Jane's:; Cylinders
; Pistons
; Connecting rods
; Crankshaft
; Crankcase
; Wheelcase
; Valve gear
Technical improvements
Most of the Merlin's technical improvements resulted from more efficient superchargers, designed by Stanley Hooker, and the introduction of aviation fuel with increased octane ratings. Numerous detail changes were made internally and externally to the engine to withstand increased power ratings and to incorporate advances in engineering practices.Ejector exhausts
The Merlin consumed an enormous volume of air at full power, and with the exhaust gases exiting at it was realised that useful thrust could be gained simply by angling the gases backwards instead of venting sideways.During tests, 70 pounds-force thrust at, or roughly was obtained, which increased the level maximum speed of the Spitfire by to. The first versions of the ejector exhausts featured round outlets, while subsequent versions of the system used "fishtail" style outlets, which marginally increased thrust and reduced exhaust glare for night flying.
In September 1937 the Spitfire prototype, K5054, was fitted with ejector type exhausts. Later marks of the Spitfire used a variation of this exhaust system fitted with forward-facing intake ducts to distribute hot air out to the wing-mounted guns to prevent freezing and stoppages at high altitudes, replacing an earlier system that used heated air from the engine coolant radiator. The latter system had become ineffective due to improvements to the Merlin itself which allowed higher operating altitudes where air temperatures are lower. Ejector exhausts were also fitted to other Merlin-powered aircraft.