Packard V-1650 Merlin

V-1650 Merlin
Packard V-1650-7 Merlin
Type Liquid-cooled V-12 piston engine
National origin United States
Manufacturer Packard
First run August 1941
Major applications Avro Lancaster
Curtiss P-40 Warhawk
de Havilland Mosquito
North American P-51 Mustang
Number built 55,523
Developed from Rolls-Royce Merlin

The Packard V-1650 Merlin was a version of the Rolls-Royce Merlin aircraft engine, produced under licence in the United States by the Packard Motor Car Company.[1] The engine was licensed in order to provide a 1,500 hp (1,100 kW; 1,500 PS)-class design at a time when U.S. engines of this rating were not considered ready for use even after years of development.

The first V-1650s, with a simple one-stage supercharger, were used in the P-40F Kittyhawk fighter. Later versions included a much more advanced two-stage supercharger for greatly improved performance at high altitudes. It found its most famous application in the North American P-51 Mustang fighter, where it vastly improved that aircraft's performance at altitude, transforming the Mustang into an outstanding fighter with the range and performance to escort heavy bombers over the European continent. By 1944, P-51B, P-51C and P-51D "Merlin" Mustangs were able to escort Allied heavy bombers in daylight all the way to Berlin and back - and yet were still capable of combating German fighters attempting to intercept the bombers.

Design and development

Packard V-1650 in Deutsches Museum München

In June 1940, Henry Ford had offered to manufacture 1,000 aircraft a day if the government would let him do it his way, and during a discussion with Secretary of the Treasury Henry Morgenthau Jr. regarding what the Ford company might produce, Ford's son Edsel tentatively agreed to make 6,000 Rolls-Royce liquid-cooled engines for Great Britain and 3,000 for the U.S.[2] However, at the beginning of July, Henry Ford stated that he would manufacture only for America's defense, not for Britain, and the entire deal was declared off. Members of the Defense Advisory Commission subsequently began negotiations with other manufacturers in an effort to place the $130,000,000 Rolls-Royce order,[2] and Packard Motor Car Company was eventually chosen because the engine's British parent company was impressed by its high-quality engineering. Agreement was reached in September 1940, and the first Packard-built engine, designated V-1650-1, ran in August 1941.[3]

The first American version of the Merlin was the Packard Merlin 28 (Mark XX), designated the V-1650-1 by the American military. This engine used a single-stage, two-speed supercharger. The two-speed gearing was - as well as for the "R-R" Merlins - from a French Farman patent license. As the Merlin 28, it was used for the Avro Lancaster bomber. The USAAF V-1650-1 version of this engine was used in the Curtiss P-40Fs. The initial Packard modifications to this engine changed the main crankshaft 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.

In answer to a request from the British Air Ministry for a high-altitude Merlin for the Wellington VI bomber, a Rolls-Royce team under the direction of Stanley Hooker developed a Merlin with two-stage supercharging, which became the Merlin 60-series, the first engine being run in March 1941, and first flown in July the same year,[4] this engine, after the Wellington VI was canceled, was instead introduced on the Spitfire IX as the Merlin 61 and was later produced by Packard as the V-1650-3 and became known as the "high altitude" Merlin destined for the P-51, the first two-stage Merlin-Mustang conversion flying with a Merlin 61[5] as the Mustang X in October 1942, the production V-1650-3 engined P-51B (Mustang III) entering service in 1943. The two-speed, two-stage supercharger section of the two-stage Merlins and V-1650-3 featured two separate impellers on the same shaft that were normally driven through a gear train at a ratio 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 high as the ratio used in the Allison, but impeller speed is not the only factor that determines engine performance, which is also a function of the size and pitch of the impeller blades. The gear-driven supercharger is a parasitic accessory; therefore, impeller gearing and blade profiles are carefully designed for maximum power at altitude without compromise of available power at the critical take off stage of flight. The double staging of the compressed fuel/air mixture provided the boost pressure through a diffuser to the intake manifolds that 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 more than 1,270 horsepower (950 kW) at altitudes beyond 30,000 feet (9,100 m). The two-stage impeller created extreme heating of the fuel/air mixture during the compression process and to prevent detonation of the compressed charge, it was necessary to cool the mixture prior to entry into the cylinders. The cooling was accomplished in an intercooler passage cast into the wheel case 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 supercharger cooling system. The glycol mixture used for 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 maximum rate of 36 U.S. gallons (136 litres, 30 Imperial gallons) per minute, depending on engine rpm. This combined system reduced the charge temperature to suitable levels.

Throttle valves in the updraft carburettor throat were controlled by an automatic boost control through the throttle linkage to maintain the selected manifold pressure with changes in altitude. The valves were only partially open during ground and low level operation to prevent overboosting of the engine. As air density decreases with increased altitude, the throttle valves were progressively opened in response to the reducing atmospheric pressure. This system provided full power within engine boost limitations up to the critical altitude of 26,000 feet (7,900 m).

Measurement of boost pressure

The British measured boost pressure as lbf/in² (psi). The normal atmospheric pressure at sea level is 14.7 psi, so a reading of +6 means that the air/fuel mix is being compressed by a supercharger blower to 20.7 psi before entering the engine; +25 means that the air/fuel mix is now being compressed to 39.7 psi.

The Americans measured their boost ratings using inches of mercury (inHg). One pound-force per square inch equals 2.036 inHg or 6.895 kPa, and a standard atmosphere is 101.325 kPa =29.92 inHg =14.70 lbf/in². In early Merlin engines the maximum manifold (boost) pressure was +18. This was increased in later models.

Inches of mercury (inHg)
absolute pressure		 Pounds per square inch of boost[6]
gauge pressure
81 inHg=+25 lbf/in² boost
66.5 inHg=+18 lbf/in² boost
60 inHg=+15 lbf/in² boost
46 inHg=+8 lbf/in² boost
42 inHg=+6 lbf/in² boost

Postwar use

In the United States many war surplus engines and airframes were sold relatively cheaply – two of the most popular items were North American 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,[7] and the Thompson Trophy. Many of these engines remain heavily used to this day in Drag Racing, Hydroplane racing, and Land Speed Racing at places like the Bonneville Salt Flats. The Mynarski Lancaster flown by the Canadian Warplane Heritage Museum in Hamilton, Ontario, Canada, one of only two Lancasters flying in the world, uses four Packard Merlin engines.[8]

Work continues on increasing the power output of the Merlin for the Unlimited Class racers at the Reno Air Races. Innovations, such as the use of Allison V-1710 connecting rods and the replacement of the intercooler with ADI (Anti-Detonate Injection) (50% Distilled Water and 50% Methanol), nearly identical in chemical composition to the Luftwaffe's wartime MW 50 system, and similar to the water injection system used on Pratt & Whitney engines during World War II, have allowed great increases in power output.[9][10] Many of the fastest Unlimited racers increase Merlin manifold pressures as high as 145 inHg (56.6 psi, 4.8 atm) to obtain up to 2,835 kW (3,800 horsepower), achieving Mustang speeds up to 490 mph.[11][12]

Variants

Applications

Specifications (V-1650)

General characteristics

Components

Performance

See also

Related development
Comparable engines
Related lists

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 Stephens 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.
  • Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN 1-85310-294-6.
  • Rubbra, AA. Rolls-Royce Piston Aero Engines: A Designer Remembers. Derby, England: Rolls-Royce Heritage Trust, 1990. ISBN 1-872922-00-7.

External links

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