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Aviation History
1939
1939 - 0814.PDF
• 2go FLIGHT. MARCH 23, 1939 CARBURETTER or INJECTION ? Relative Merits Reviewed : R.Ae.S. Lecture by Messrs. J. E. Ellor and F. M. Owner mum power. It might be possible'to use low-grade fuels for cruising. Tests with large valve overlaps had shown improvement in B.M.E.P. as great as 10 Ib./sq. in. but there were a number of difficulties. The revival of the exhaust gas turbine might alter the position, as the back pressure tended to follow varia tions in boost and to be independent of altitude. These were the conditions required for good scavenging. Manifold injection was easy to arrange, and an engine de signed for carburettor operation could' readily be converted. The Bristol '' Draco '' engine of a few years ago employed that system. Combustion Chamber Injection Although having obvious similarities, diesel practice and petrol injection remained fundamentally different. The diescl required a slow-burning fuel which was easy to ignite, while the petrol engine wanted a rapidly burning fuel, difficult to ignite. The mechanical problems of the two types were similar. In the petrol engine using injection directly into the com bustion chamber, the problem was simplified because the injec tion period occurred during the induction stroke, so that a longer period was available for mixing. The authors then discussed various control systems, and con cluded with a general summary-, which is given in full here after. '' In view of the mass of experience behind the universal adoption of the carburetter, its displacement by any form of fuel injection system can only be brought about by a convinc ing demonstration of the latter's general superiority. Although carburetter systems now operate reasonably satisfactorily, there undoubtedly exist a number of inherent undesirable features, but it is believed that the few functional troubles sometimes encounteied will definitely be eliminated, thus rendering such systems 100 per cent, reliable. This from the pilot's point of view is of primary importance. His next worry in order of importance is the complication of manual control, especially on multi-engine machines. To minimise the number of controls and to ensure minimum fuel consumptions compatible with engine requirements, automatic devices are incorporated and calibrated during flight to suit operational demands. The care ful thought and extensive development put into these devices have provided a fairly satisfactory solution in the fully auto matically controlled carburetter. At the moment, two controls per engine are arranged in the pilot's compartment, the main throttle and a two position, weak and normal mixture strength, lever, which will probably be eliminated by the latter develop ment. The absence of material facts based on extensive development rather handicaps the case for fuel injection and leaves a good deal for conjecture. (Concluded on page 299.) COMPARISON BETWEEN CARBURETTER AND PETROL INJECTION Distribution Atomisation Freezing Cooling effect of petrol on charge ... Cooling effect of petrol on charge in combustion chamber. Effect on supercharger Scavenging ... ... Fire risk (backfires) ... Use of safety fuels Aerobatics ,. Acceleration of engine Slow running... Cruising consumption Maximum power Simplicity Tuning Timing Weight Cost ... Controls! Carburetter Continuous Injection Dependent on induction svstem. Dependent on jet characteristics. Can be prevented by special precautions. Considerable. Small. Very good. Eliminated. Considerable. Small. Improves performance. Impossible without loss of fuel. Eliminated by use of flame traps. Impossible. Reasonably satisfactory. Special devices required. Sensitive. Difficult. Good. Special devices probably required. Less difficult. Dependent on distribution. Good. No continuous mechanical motions. When correctly calibrated is reliable. None. Normal. Normal. Good. Mechanical drive usually required. Probably delicate. None. Probably slightly less. Probably slightly more. Complex on modern supercharged engines. Timed Injection Induction Pile Excellent. Moderately good. Eliminated. Small. Measurable in single cylinder unit. None. Feasible. Eliminated. Possible. Good. May be satisfactory m Combustion Chamber Excellent. Moderate. Eliminated. Negligible. Possible increased breathing due to reduction in residual gas temperature. None. Feasible. Eliminated. Easy. Good. ithout special devices. Good, but special devices may be required. Reduced by good distribution and possibly by stratification. , Favoured by good distribution, but handicapped by lack 01 charge cooling. Delicate mechanically driven components. Performed by pump manufacturer, but controls will require test bed tuning. Fairly sensitive. Not sensitive. Appreciably heavier. Considerably greater. More complex. AFTER examining the subject in great detail, the authors of the paper on petrol injection or car-> buretters, J. E. Ellor and F. M. Owner, presented • to the Royal Aeronautical Society last Thursday, came to the conclusion that " Such evidence as is available in connection with multi-cylinder engines indicates no improvement in either power 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." The official title of the paper was '' A Review of the Relative Merits of Petrol Injection and Carburetters for Aero Engines." and the authors pointed out that, owing,to the meagre amount of information available on fuel injection, the paper was not as complete a review as they would have wished. In assessing the relative merits, due considerations should be given to (1) improvement in performance; (2) simplification of operation and servicing ; (3) facilities and cheapness of production; and 4) reliability. Three distinct principles, the authors stated, are involved in the possible methods of introducing the fuel into the engine cylinders- (1) continuous induced flow, as in the case of the carburetter; (2) continuous injection; and (3) timed injection. One ol the early sections of the paper dealt with the history of fuel injection, and was followed with a chapter on the problems of carburation. Then descriptions were given of modern carburetters. Continuous injection was described by the authors as the half-way line towards timed injection. It had the advantage that the density of the gas was increased, due to the cooling effect of the evaporation of the fuel. This had considerable effect on the pressure rise across a given blower and the power required to drive it was reduced. Unfortunately the gain depended largely on the volatility of the fuel so that the gain at altitude might not be so great. Timed Injection Four main advantages of timed injection were mentioned by the autnors: (1) Positive metering to each cylinder, giving perfect distribution; (2) reduction in fire risk by eliminating backfires and by using safety fuels ; (3) improved combustion chamber scavenging and cooling by appropriate valve injection timing; and (4) freedom from icing. Diesel experience had shown the problems of accurate metering to be soluble, but the pumps must necessarily be precision instruments. The possibility of using safety fuels was due to the fact that the injectors were placed close to or inside the combustion chamber. Controlled turbulence was used to get an even charge, but turbulence was a difficult factor to control. The authors thought the ideal would be the use of a stratified charge for economic operation and a uniform charge for maxi-
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