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Aviation History
1941
1941 - 0562.PDF
THE CAS, CARB A Comparison of the Performance of Carburett THE idea that engines which are equipped with directpetrol injection systems ,are capable of better fueleconomy than equivalent carburettor engines is widely held, and seems to have been strengthened recently as a result of bench tests made on captured German aero engines using the system. Of late some prominence has been given to this theory, both in the lay and technical press, and a recent editorial article in a well-known journal stated that it was quite probable that the Germans have adopted petrol injection because it gives an advantage at altitudes above 25,000 or 30,000ft., due to the fact that no restriction is offered to the breathing of the engine. It is stated that the carburettor offers a restriction to breathing at high altitudes due to the much larger quantity of the rarefied air required. The usual claims for the direct fuel injection system, such as freedom from freezing, very good fuel economy and the possible use of slightly lower-grade fuels, are also mentioned in the article in question. Articles such as this are read by a considerable number of people, and it is surprising how many have accepted them without questioning their validity. Doubtless many o 0 n 11 f The Rolls-Royce Merlin X has cylinder capacity of 27 litres— members of the Flying Services read these articles, and it would seem a pity if they should gather the impression that the British engines are inferior in fuel economy and altitude performance to the German engines. In actual fact, the current British engines develop con- siderably greater power at high altitudes than any engines yet in service with the Luftwaffe, and it is not expected that the Germans are likely suddenly to introduce any new types with very greatly improved performance. It is hoped that the following performance comparison of a German and an English engine will show that we are in no way inferior to our adversaries in respect of aero engine design. Comparison of Fuel Economy Tests were made at the Royal Aircraft Establishment on two Junkers Jumo 21 iD. engines removed from a Heinkel 111 aircraft. These engines are fitted with the Junkers direct petrol injection equipment. Fuel consump- tion loops were taken at three different speeds. The start ing point of each loop was at the fuel consumption given by the automatic mixture regulator, and a special manual control was fitted to enable the mixture strength to be weak- ^ „ ,^ ened off from these points. The curves so ob- tained are shown on Fig. 1. It will be noticed that the con- sumptions obtainable with the flight settir-^i of the automatic mix- ture regulator are up to 14.5 percent, above the absolute best economy conditions, which are, of course, the bottom points of the consumption loops. The Rolls-Royce Merlin X engine is the nearest English equivalent to the Jumo 21 iD. engine, and has been taken for purpose of com- parison with it. Both are 12-cylinder, liquid-cooled types fitted with 2-speed Component parts ofthe Rolls-Royce Merlin carburettor, shown inthis photograph, num- ber 433> includingAmal valve and fuel pipes.
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