In answer to a request from the British
Air Ministry for a high-altitude Merlin for the pressurised
Wellington VI high-altitude 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 60-series engine ran in March 1941, and was first flown in July the same year.
[4] When only 63 examples of the otherwise-cancelled Wellington VI were produced, these engines were instead introduced on the
Spitfire IX as the Merlin 61.
This model 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.[
citation needed]
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.[
citation needed]
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) above 30,000 feet (9,100 m).[
citation needed] 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.[
citation needed]
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).[
citation needed] 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.[
citation needed] 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).[
citation needed]
