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Aviation History
1941
1941 - 0563.PDF
[ARCH 6TI FOFTTHE [ETTOR onomy and High-Altitude ejection and ngines ^'compared with the 35 litres capac- ity of the Junkers Jumo 211. superchargers and are normally rated on 87 octane fuel. Analysis of samples of German fuel indicate, however, that the luel used is of 92 octane when rated by the British method. i'he bore and stroke of the Jumo engine are 5.91m. and 6.50m. respectively, whereas the Merlin is 5.40m. x6.ooin. These dimensions give the German engine 29.5 per cent, greater cylinder capacity than the Merlin, but in spite of this its maximum power output in both supercharger gears is approximately n per cent, less than the Merlin X at the latter's maximum power altitudes. In addition, the German engine operates at 6.5 : 1 compression ratio against the 6.0: 1 of the Merlin. This difference in compression ratio should give the German engine an advantage of rather more than 3 per cent, in power output, and at the same time 3 per cent, lower fuel consumption. Tests at Same Piston Speed In view of the difference in dimensions of the two engines the Merlin consumption tests were not made at the same crankshaft speeds as the German engine tests, but at speeds which give the same piston speed. It was necessary to run the Merlin engine 8.25 per cent, faster on this account, and it would be reasonable to claim that the Merlin was, therefore, at a slight disadvantage, since the mechanical efficiency of an engine falls off as the speed; increases. At the r.p.m. giving the same piston speeds the Merlin im- peller tip speed is about 5 per cent, lower than the Jumo, and, therefore, the power to drive the supercharger is about 10 per cent, lower. On the other hand, the Merlin super- charger compresses fuel with the air and this increases the power required by some 7 per cent. Since, under economical cruising conditions, the supercharger power is small, the two effects annul one another, and it is legitimate directly to compare the performance of the two engines at the same piston speed and b.m.e.p. Fig. 2 shows the re- sults obtained on two Merlin X engines, and it will be seen that the maximum fuel economy is nowhere more than z\ per cent, less than the consumptions recorded with the car- burettor operating in the automatic weak cruising position. This compares very favour- ably with the maker's setting of the injection The Jumo 211 injectionequipment comprises 1,576 parts. This pic-ture shows one pump dismantled. pumps on the Jumo engines, which gave up to 14J per cent, above the ideal consumption. Figs. 3 and 4 compare the actual specific fuel consump- tions of the Merlin and Jumo engines, and it will be seen that, even without allowing for the 3 per cent, difference due to compression ratio, the ideal consumption of the Merlin engine is practically the same as that of the German engine, except at the lowest speed and power conditions, where it is 4 per cent, worse; but, what is of far more practical importance, the consumptions given by the auto- matic mixture regulator are lower for the Merlin engine, over the whole range of speeds, than the comparative con- sumptions obtained on the German engines. The advan- tage over the whole range is some 7 per cent, (without the 3 per cent, compression ratio allowance). The adverse ideal consumption of the Merlin at the very low speeds is probably due to the fact that the engine is designed for much higher operational speeds than the Jumo engine, and the high-speed valve timing adopted is prob- ably responsible for the difference. Altitude Performance The editorial article mentioned at the beginning of this report states that the carburettor offers a restriction' to the breathing of the engine, and presumably implies that this restriction reduces the engine power output at alti- tudes in excess of 25,000ft. to 30,000ft. relative to that which would be obtained if the same engine were fitted with direct petrol-injection equipment. As a result of tests made on the Merlin supercharger it has been estab- lished that introducing fuel before the supercharger eye increases the pressure rise through the blower by 12 per cent. There is, however, of necessity a small pressure loss in the carburettor, but with modern low-velocity carburettors If !£££? O0000 00000000 00000000••••• *l«Ht«»ll«UIIIH I
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