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Aviation History
1939
1939 - 0839.PDF
MARCH 23, 1939 FLIGHT. 299 already described, but they are concerned with the machining down to the final dimensions. As has been already mentioned, the other components of the hub—and, for that matter, the governor-pump unit —call for nothing unusual in the way of production methods except the latter is subjected to a series of rigorous tests. Orthodox machining operations are employed throughout, and the finished product stands out as an example of superb workmanship and design, as is to be expected from the two firms which the name of Rotol links. (Left) Each blade is carefully balanced, in special draught-proof shops, against a master blade. of the blade root from a master on a Sundstrand Rigidmil. (Right) " Copying " both faces CARBURETTER or INJECTION? (Concluded from page 290) " Such evidence as is available in connection with multi- cylinder engines indicates no improvement in either poiver or consumption compared with the carburetter system, nor is there any good reason to expect more from the same cylinder compression ratio assuming good distribution with the existing system. "With a supercharged engine, the volumetric efficiency might conceivably be reduced unless the cooling effects of the fuel inside the cylinder during the charging stroke contracts the volume of residual gases. Claims are made of higher powers being obtained from single cylinder tests. Those who are familiar with single cylinder research with carburetter systems will appreciate the large variation in power and con sumption obtainable by ringing the changes of valve timing and induction and exhaust pipe ramming effects. Experience, however, shows that these cannot be transferred to a super charged multi-cylinder engine where common induction and ex haust manifolds are esssential. " For this reason the authors would say there is very little to be gained by the way of improvement in volumetric effi ciency due to a change in timing to suit fuel injection, if the same simplicity of pipe layout is to be maintained. As re gards consumption, the lowest theoretically possible figures have been approached on a carburetter engine having a good custributioB system; these are not likely to be improved vviih the introduction of fuel injection, and though it may be found Possible to run on ultra-weak mixtures with stratified charges, Wl th the latter system specially designed combustion cham bers would be necessary and consideration of these has been outside the scope of this paper. " The layout of a fuel injection system requires considerable care, involving as it does a number of mechanical moving parts and a complicated system of controls. It is essential also that the fuel feed pipes are all of equal length to ensure an equal discharge at each nozzle, although in practice individual ad justment of the nozzle valves may be necessary to obtain an equal mixture strength in each cylinder "if the volumetric effi ciency of the cylinders is not the same. " On a carburetter system the correct ratio of fuel and air is obtained, to a large extent, by suitable proportioning of the chokes and jets, with automatic devices to give the cor rections for altitude and special conditions outside the cruising range. The carburetter, however, requires the added compli cations of special precautions against freezing in the form of passages and piping to carry the heating medium. Cost and weight are both largely dependent on simplicity and a fuel injection system is certainly heavier and more costly than th^ corresponding carburetter system. The fine limits involved in the moving parts in an injection pump mean careful manu facture, and frequent servicing may be necessary to maintain the accurate metering claimed for this system. "Many hours' development and flying have been carried out on fuel injection, but there is not, at present, the same exper ience that' has been obtained on carburetters of all types and the undoubted possibilities of fuel injection have yet to be developed fully." NEXT WEEK: P @G0tf BRITISH AND FOREIGN MILITARY AIRCRAFT NUMBER
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