Packard V-1650 / Merlin
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. However, at the beginning of July, Henry Ford stated that he would manufacture only for 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, 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.
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.
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 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).
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 yet were still capable of combating German fighters attempting to intercept the bombers. By late 1944, the Allies had won air supremacy over the whole of Germany, and Germany's defeat in World War II began to appear inevitable.
A total of 55,523 were built.
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, and the Thompson Trophy.
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), essentially the same idea as the Water Injection system used on Pratt & Whitney engines during World War II, have allowed great increases in power output. Many of the fastest Unlimited racers increase Merlin manifold pressures as high as 145 inHg (56.6 psi, 4.8 atm) to obtain the 3,800 horsepower, achieving Mustang speeds up to 490 mph.
V-1650-3: 1,280 hp (950 kW); Based on Merlin 63.
V-1650-5: 1,400 hp (1,000 kW); Experimental.
V-1650-7: 1,315 hp (981 kW); Similar to Merlin 66, primary powerplant of the P-51D Mustang.
V-1650-9: 1,380 hp (1,030 kW); 2,218 hp WEP with Water methanol injection.
V-1650-9A: 1,380 hp (1,030 kW);
V-1650-11: 1,380 hp (1,030 kW); Modified fuel system.
V-1650-21: 1,380 hp (1,030 kW); Opposite rotation for P-82 Twin Mustang