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Rolls-Royce Merlin

The Rolls-Royce Merlin was a liquid cooled 27 litre (1649 in³) 60° V12 piston aircraft engine which became famous in World War II. Several versions of the Merlin were built by Rolls-Royce (in Derby, Crewe and Glasgow), by Ford of Britain (in Urmston, Manchester) and under licence (in the United States) by Packard.They are widely considered to be among the most successful aircraft engines produced during World War II, and perhaps the finest piston engines ever built for aviation. Merlins are highly sought-after by aviation enthusiasts even today.

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

History

In the early 1930s, Rolls 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 on the front of a Hawker Hart biplane in 1935, using the new evaporative cooling system then in vogue, but this proved unreliable and so, when supplies of ethylene glycol (Prestone) from the US became available, the engine was changed to the conventional liquid cooling system.

In 1936, the Air Ministry had a requirement for a new fighter aircraft with airspeeds that would eventually have to be over 300 mph (480 km/h). Fortunately, two designs had been developed entirely as private venture exercises: the Hawker Hurricane and Supermarine Spitfire. 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 let in 1936. The PV-12 was instantly given top priority and became the Merlin. It was first widely delivered as the 1,030 hp (770 kW) Merlin II in 1938, and production was quickly stepped up. The Merlin I had a '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 before the conventional flat head arrangement (valves parallel to the cylinder) was adopted for the Merlin II.

Early Merlins were rather unreliable, but Rolls 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.

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:

Upgrades

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. 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. From June 1940 small quantities of 100 octane fuel became available from the U.S. and Merlin IIIs were found to be capable of running on it.

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. This allowed them to run some 70 degrees C cooler, substantially improving engine life and reliability. It also removed a fire hazard from Merlin powered aircraft, as pure ethylene glycol is a flammable liquid.

The process of improvement continued, with later versions running on further-increased octane ratings, delivering higher and higher power ratings. 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, or 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 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 back to using the standard 100/150 grade. The 100/150 grade fuel could be recognised by its bright green colour and the "awful smell".

Carburettor developments

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 the 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.

Other uses for the engine

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 109G-2, the Hispano Aviacion Ha 1112M1L Buchon, was built with the Rolls-Royce Merlin 500/45 engine of , 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.

Packard's legacy

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.

The first American production of the Merlin was the Packard Merlin 28 (Mark XX). This engine was a single stage, two speed supercharger. As the Merlin 28, it was used for the Lancaster bomber. The USAAF version of this engine was used in the P-40Fs. The initial Packard modifications were done on this engine by changing the main bearings from a copper lead alloy to a silver lead combination and featured indium plating. This had been developed by General Motors' Pontiac Division to prevent corrosion which was possible with lubricating oils that were used at that time. The bearing coating also improved break-in and load carrying ability of the surface. British engineering staff assigned to Packard were astonished at the suggestion but after tear down inspections on rigidly tested engines were convinced the new design offered a decided improvement.

The real improvement Packard incorporated into the Merlin was adopting the Wright supercharger drive quill. This modification was designated the V-1650-3 and became known as the "high altitude" Merlin destined for the P-51. The two speed, two stage supercharger section of the -3 featured two separate impellers on the same shaft which were normally driven through a gear train at a speed of 6.391:1. A hydraulic gear change arrangement of oil operated clutches could be engaged by an electric solenoid to increase this ratio to 8.095:1 in high speed blower position. The high speed gear ratio of the impellers was not as great as the ratio used in the Allison but speed of the impeller alone was not the factor that increased the engine performance at altitude. The double staging of the compressed fuel/air mixture provided the boost pressure through a diffuser to the intake manifolds which increased the critical altitude of the power plant.

The ability of the supercharger to maintain a sea level atmosphere in the induction system to the cylinders allowed the Packard Merlin to develop at . The two stage impeller created extreme heating of the fuel/air mixture during the compression process and in order to prevent detonation of the compressed charge, it was necessary to cool the mixture prior to entry into the cylinders. This cooling was accomplished by the casting of an intercooler passage into the wheelcase housing between the first and second stage impellers.

Ethylene glycol coolant was circulated by a pump through this passage to carry off the excess heat generated by the impellers. Without the intercooler the temperature of the charge could be as high as 400 °F (204 °C). The intercooler in itself was not adequate to deal with the high temperature and an additional cooling fin and tube core was placed between the outlet of the blower and the induction manifold to the cylinders. This radiator was known as an aftercooler and served as a reservoir for the system. The glycol mixture used for the supercharger cooling was independent of the main engine cooling system and used a centrifugal pump driven by the engine to circulate the coolant through an aircraft radiator system at a rate of 30 gallons per minute.

This combined system reduced the charge temperature to suitable levels. The throttle valves in the updraft carburettor throat were controlled by an automatic boost control through the pilot's linkage to maintain the selected manifold pressure during changes in altitude. These valves were only partially open during ground and low level operation to prevent overboosting of the engine. As air density decreased with an increase in altitude, the throttle valves were moved to an open position by boost pressure corresponding to aircraft altitude. This system provided full power within engine boost limitations up to the critical altitude of . This was the improvement Packard brought to the Merlin.

When the first of the Packard-built Merlins arrived in Britain, the engineers at Rolls-Royce stripped it down and were amazed to find the production-line built Packard engine, far from being as bad as they expected it to be for component tolerances, was actually better. Up until then, R-R Merlins were virtual works of art, with every critical bearing surface being finished off by hand by highly-skilled and experienced craftsmen, and this time-consuming and expensive process placed great strain on the production capability of the workforce involved in the manufacture of these engines. The Packard engine changed many minds, although there were still some at R-R who remained unconvinced of the quality of the American engine, produced as it was by a largely unskilled or semi-skilled workforce (many of whom were female.) In the end, the Packard engine's performance removed any doubts about its quality and workmanship.

The Packard V-1650 so outperformed the Allison V-1710 that it supplanted the Allison in the North American P-51 Mustang, which then became one of the best fighters of the war. It was also incorporated into some models of the Curtiss P-40, specifically the P-40F and P-40L. Packard Merlins powered Canadian-built Hurricane, Lancaster, and Mosquito aircraft, as well as UK-built Spitfires in the shape of the Mark XVI, otherwise the same as the Mark IX with its British-built Merlin.

Although it is not commonly known, Packard greatly improved the engine by using interchangeable parts that required little or no hand-fitting, rather than custom-finished ones, making maintenance and repair much simpler, faster, and cheaper. Packard's engineering changes were also incorporated into subsequent British production.

However, Rolls-Royce's Stanley Hooker (then head of supercharger development) ascribes this improvement in tolerances and true mass-production standards to the involvement of Ford of Britain. After redrafting all the Merlin drawings to their higher standards, Ford were able to produce 400 Merlin engines a week at their factory in Urmston, Manchester.

A common misconception is that Packard Merlin engines were used in American PT boats; the engine used was in fact a Packard V-12 engine, a modification of the Liberty L-12, totally unrelated to the Merlin. The possibility is that these engines were also used by British MTBs and MGBs.

Civilian uses

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 surplus engines and airframes were sold relatively cheaply - two of the most popular items were P-51 Mustangs and Packard 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.

The engine was also fitted into at least two road cars in the UK. Since then, a great many Merlins have been consumed by the sport of tractor pulling.

Variants

This is an incomplete list of representative Merlin variants. Engines of the same power output were typically assigned different model numbers based on supercharger or propeller gear ratios, differences in cooling system or carburettors, engine block construction, or arrangement of engine controls. All Merlin engines were "right hand tractor", i.e. propeller rotated to the right viewed from rear, unless otherwise noted.

Merlin II or III - 1,030 hp (775 kW) at 3,000 rpm at 5,500 ft (1,680 m). With 100 Octane fuel and increased (+9lb/sq.in) Boost - 1,160 hp (865 kW) at 3,000 rpm at ; used in Spitfire Mk.I and Hurricane Mk.I fighters, the Boulton Paul Defiant, and Fairey Battle.
Merlin X - 1,130 hp (840 kW) at 3,000 rpm at 5,250 ft (1,525 m); used in Halifax Mk.I, Wellington Mk.II, and Whitley Mk.V bombers.
Merlin XII - fitted to Spitfire Mk. II.
Merlin XX - 1,480 hp (1,105 kW) at 3,000 rpm at 6,000 ft (1,830 m); used in Hurricane Mk.II and Beaufighter Mk.II fighters, Halifax Mk.II and Lancaster Mk.I bombers.
Merlin 32 - 1,645 hp (1,230 kW) at 3,000 rpm at 2,500 ft (760 m); used in Barracuda Mk.II bomber.
Merlin 45 - 1,515 hp (1,130 kW) at 3,000 rpm at 11,000 ft (3,353 m); used in Spitfire Mk.V
Merlin 46 - 1,415 hp (1,055 kW) at 3,000 rpm at 14,000 ft (4,270 m); high-altitude version used in Spitfire PR.Mk.IV and PR.Mk.VII
Merlin 50.M- 1,585 hp (1,182 kW) at 3,000 rpm at 2,750 ft (838 m); Low altitude version with supercharger impeller "cropped" to in diameter. Permitted boost was +18 lb/sq.in. instead of +16 lb/sq.in. as on a normal Merlin 50 engine. A "negative g" carburettor was fitted.
Merlin 61 - fitted with a new two-speed two-stage supercharger providing 1,565 hp (1,170 kW) at 3,000 rpm at 12,250 ft (3,740 m), and 1,390 hp (1,035 kW) at 3,000 rpm 23,500 ft (7,170 m); high-altitude version used in Spitfire Mk.VII, Mk.VIII, Mk.IX, and PR.Mk.XI
Merlin 76 & 77 - 1,233 hp (920 kW); used in the Westland Welkin high-altitude fighter and some later Spitfire and Mosquito variants. Fitted with a two-speed, two-stage supercharger and a Bendix-Stromberg carburettor. The odd-numbered mark drove a blower for pressurising the cockpit.
Merlin 130 & 131. - 2,070 hp (1,543 kW); redesigned "slimline" versions made for the de Havilland Hornet. Engine modified to decrease frontal area to minimum and was first Merlin series to use down-draught induction systems. Coolant pump moved from the bottom of the engine to the starboard side. Two -speed, two-stage supercharger and S.U Injection carburettor. Maximum Boost was 25 lb (2,053mm Hg). The Merlin 131 had an additional idler gear in the reduction gear casing allowing "reverse" (left hand tractor) rotation. Merlin 130 was fitted in starboard nacelle, Merlin 131 in port on production Hornets.
Merlin 133 & 134 - 2,030 hp (1,514 kW); Derated variants of 130/131 used in Sea Hornet F. Mk. 20, N.F. Mk. 21 and P.R. Mk. 22. Maximum Boost was lowered to 18 lb (1,691 Hg).
Merlin 266 - the prefix 2 indicates built by Packard, otherwise as Merlin 66, optimised for low-altitude operation. Fitted to the Spitfire Mk. XVI.

Applications

Specifications (Merlin II)

Specifications (Merlin 66)

References

Notes

Bibliography

Bridgman, L. (ed.) Jane's fighting aircraft of World War II. London: Crescent, 1998. ISBN 0-517-67964-7
Fozard, John W (editor).Sydney Camm and the Hurricane; Perspectives on the master fighter designer and his finest achievement. Shrewsbury, Shropshire, UK: Airlife, 1991. ISBN 1-85310-270-9
Gunston, Bill.World Encyclopaedia of Aero Engines (3rd edition). Sparkford, Somerset, UK: Patrick Stephans Limited, 1995. ISBN 1-85260-509-X
Hooker, Stanley Not Much of an Engineer London: Airlife, 1984. ISBN 1-85310-285-7
Harvey-Bailey, A. The Merlin in Perspective - the combat years. Derby, England: Rolls-Royce Heritage Trust, 1983. ISBN 1-872922-06-6
Rubbra, AA. Rolls-Royce piston aero engines: A designer remembers. Derby, England: Rolls-Royce Heritage Trust, 1990. ISBN 1-872922-00-7