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Aviation History
1942
1942 - 0452.PDF
•d FLIGHT FEBRUARY 26TH, 1942 A BRITISH MASTERPIECE angle of 60 deg., the bore and stroke being 5.4m. x 6in. respectively, giving a swept volume of 1,649 cubic inches. The com pression ratio is 6 to 1. A two-speed supercharger is now fitted, the low gear ratio being 8.15 to 1 and the high gear 9.49 to 1 and, as already indi cated, it is primarily in the modifications and improvements to the supercharger by which the increase of power available has been achieved. It is of the high-speed centrifugal type embodying an impeller which is driven from the rear end of the crankshaft through a two-speed gear, the high gear being normally engaged above 6,000ft. To relieve the gear teeth from the exceptional loads which result from impeller inertia (their respective rotational speeds are 21,597 and 25,148 r.p.m.) following sudden acceleration and deceleration of the engine, a spring drive is incorporated con sisting of a torsionally flexible shaft. The impeller inertia, in combination with the friction drive which incorporates the two-speed gear mechanism illustrated, comprising three layshaft driving spindles each embodying a centrifugally loaded clutch with Ferodo linings, serves to damp out torsional oscillation of the shaft which drives the cam shaft and auxiliaries as well as the supercharger. The clutches of the two-speed mechanism are held initially in engagement by light springs, and as the speed of rotation of the crankshaft and therefore of the centrifugal weight increases, the pressure of these springs is augmented by a centrifugal force, and consequently the torque which the clutches can transmit increases as the square of the speed. As the torque required to drive the impeller increases at a somewhat similar rate, the gear is capable of driving the impeller with a predetermined margin of torque above that causing slip. The redesigned supercharger casing has an improved form of central entry which gives a better and freer flow of air to the blower; The rotating and fixed guide vanes, too, have Showing the small frontal area of a V type liquid- cooled engine 10,000ft. in been amended. The rotating guide vanes are formed of steel and are matched to the rotors of R.R.56 Aluminium Alloy. The rotors are balanced separately and together to approxi mate to dynamic balancing on knife edges. The diffuser vane ring is an aluminium die casting bolted by means of special shaped bolts to the supercharger casing. These important changes in conjunction with a larger twin-choke up-draft S.U. carburettor, designed as a complete separate unit, have resulted in a greater weight of mixture entering the cylinders and thereby a marked increase of power. Using 100-octane ft^Us the International Power Rating of the Merlin XX is with the low gear supercharger 1,240 h.p. at 2,850 r.p.m. at 10,000ft. and +9 lb. /sq. in. boost, and 1,175 b.h.p. at 2,850 r.p.m. at 17,500ft. and +9 lb./sq. in. boost. The rated altitude, that is, the lowest height at which full throttle is permissible at the international (climbing) r.p.m. is low gear and 18, oooft. in high gear. Increase of 250 h.p. These figures represent an increase of nearly 250 h.p. over the Merlin II of identical cylinder dimensions, and illustrate in a convincing manner the technical progress achieved by years of "toil, tears and sweat," to borrow a classical phrase from our worthy Prime Minister. Nor is the limit of attainment yet in sight. A talk with the Chief Engineer gave one encouragement and enlighten- fent as to further far-reaching improvements which should surely serve to maintain the Merlin on its pinnacle of fame for years to come. It should be added that the achievement of extracting so much extra power from a given cubical capacity, in this case 27 litres, has increased the dry weight of the engine by only 75 lb., that is, to 1,450 lb., which on the power rating of 1,280 b.h.p. gives a ratio of 1.13 lb. per Longitudinal section of the Rolls-Royce Merlin XX twelve-cylinder engine with two-speed supercharger. It scales 1,450 lb. dry = 1.13 lb. per horse power.
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