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Aviation History
1933
1933 - 1231.PDF
FLIGHT, DECEMBER 14, 1933 ot greater importance to engine development a. the moment. I believe the best solution for a high octane value fuel is by means of the addition of a reasonable quantity of tetra- ethyl lead, say, 4 c.c. per gallon. It would appear that it is not wise at the present time to exceed 4 c.c. per gallon, as available fuels show a poor lead consciousness with higher concentrations, and the actual engine problems are considerably increased! The introduction, by the Air Ministry, of an 87 octane number fuel, referred to above, will be greatly appreciated by the air-cooled aero-engine designer, and will give scope for considerable development on military engines, and it is hoped that fuels of not less than 80 octane number will, at the same time, be available for civil air lines. The use of tetra-ethyl lead introduces a number of prob lems not encountered with normal fuels, including corrosion of valves, valve seats, cylinders, etc., and recent develop ments have shown the wisdom of the Air Ministry in limit ing the lead content to 4 c.c. per gallon in the new specification. A considerable amount of experimental work has been going on to overcome, the deleterious effects of lead, and I wrill refer to this matter in more detail later on in the paper. The question of the internal corrosion of engines, using high lead content, after standing for long periods, or in stores, and the harmful effects on exhaust systems, are also problems which require attention. While the higher octane number specifications referred to are a " step in the right direction," and will show con siderable improvement in engine output, I would empha sise the necessity of immediately making preparations so that the octane number may be further raised at a future date, in order that the development of the air-cooled aero engine which is possible, within the period under review, may be realised. Considerably more attention must be paid to the import ance of fuel consumption on air-cooled engines, and, apart from the standard type approval tests, Bristol engines have recently bee* subjected to a 50 hours' cruising consumption lest, in order to establish the minimum consumptions which are allowable. Fig. 2 shows the figures attained on a recent consump tion test with the Bristol " Pegasus " M. engine. The figures are in no way the ultimate to be aimed at, but are quoted as showing that the consumptions of air cooled engines are capable of treatment, and it is believed that by development along suitable lines, a considerable further improvement can be made, from the point of view of fuel economy with the compression ratio as high, and the boost as low, as is feasible. Supercharging Supercharging briefly consists of increasing the density of the charge by means of an external compressor, and is one of the most outstanding developments of the modern aircraft engine. The principle of supercharging aircraft engines has been applied with two objects in view. Firstly, the maintenance of ground level induction-pipe pressures and powers, at high altitude, and the other—the increase of ground level power over and above that which is possible by natural aspiration ; this latter form of supercharging is commonly known as ground boosting. On the earlier supercharged engines it was considered permissible, by certain engine constructors, to compensate for rather inadequate breathing organs by ground boosting, and a number of engines have been put into production, in which the valves and porting system have been on the small side, thus easing design and production, and it has been hoped by this means to obtain good efficiency. The experimental work of the Bristol Company has conclusively proved that this is bad practice. To obtain satisfactory and efficient supercharging, the engine must start off with the freest and most efficient breathing functions possible, as restricting the valve and port areas, and then super charging, only leads to a series of difficulties, and the aim should be to obtain the maximum power at rated altitude with the minimum positive boost. It is interesting to note that the latest examples of AIR CQMPPFsyp AS y*rr. 3-;THVrt;N OffuM QNO CA3f- BEgygS MTI-HWMLV nn-UM-rn umfi . nffivrN r* nwiiM PAPAL PcmtoN I cnTxn • rn ny rcnrnna DIAGRAM OF POWrRPllE VANtr TVPT SUPfRrHAReER fXHAUST OUTLfT TO ATMCyMC^t E x nRivr rspM CNore DIAGRAM OF ROOTS TYPE SUPERCHARGER ,.«. SUm vANf TVH VIA gg UT GtAPM Alg L«HAKCT- VCUDTf CHAMttK pXD. SB DIAGRAM Of EXHAUST DRIVEN TURBO (BLOWINE THRO wei DIA.GRAM OF GEAR DRIVEN CENTRIFUGAL TYPE SUPtRCNARGCR. Fig. 3 : Representative types of Supercharger. 1263
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