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Aviation History
1942
1942 - 0040.PDF
FLIGHT JANUARY IST, 1942 TURBO-SUPERCHARCINC The object of supercharging-an engine is broadly this : The pressure produced by expanding or "firing" a charge of mixture depends upon two things, the weight of the charge and the compression to which it is sub jected before firing. Obviously a large cylinder can accommodate a greater weight than a small one at the same compression pressure. On the other hand, the higher the compression the greater will be the pressure developed on the firing or expansion stroke, but there i? a limit, according to the type of fuel employed, upon the degree to which compression can be raised, and if this limit be exceeded, detonation instead of smooth ex pansion takes place. If this were not so, higher powers from engines of given cylinder capacity could be obtained by using higher compressions, without supercharging. Effect of Increasing Compression Ratio Again broadly speaking, increased power can be obtained from cylinders of a given size by raising the compression, and thus the expansion pressures, or by increasing the number of expansion strokes which take place in a given time; that is to say, increasing the number of revolutions per minute. Or both methods can be combined, within reasonable limits. But the higher the engine speed, -the less is atmospheric pres sure able to give each cylinder a complete lung full of mixture. Because of the high velocities necessary past the various obstructions, such as pipes, ports and valves, the volumetric efficiency falls off. The function of supercharging is to provide means whereby the requisite weight of air can be forced into the cylinder at the desired engine speed. The density of air at sea level is relatively low, hence a considerable volume at atmospheric pressure has to be moved in order 1o supply the requisite weight, and thus a supercharger has either to be of large size or run at a very high speed. To drive it necessitates considerable power, approximately 15 to 20 per cent, of the theoretical maxi mum power of the engine. So long as the supercharger is delivering the goods it will require that power to drive it, irrespective of the method by which the power is applied. When a given brake-horse-power is required from an internal combustion engine there is a choice of two com promises. To use a large-size engine running slowly and obtaining its lungs full by means of atmospheric pressure alone, or to. use a smaller engine running at higher speed, and provided with a supercharger to make sure that the lungs are filled completely. Practical limi tations of design render the supercharged engine the better compromise for high-speed vehicles, or for air craft, though there is a much more cogent reason regard ing the latter, as will be seen when the story unfolds. The supercharged engine can be smaller in bulk, and more economical in weight. , Supercharging for High Altitude So far the matter of supercharging has been outlined in only one phase; that is to say, the conditions which exist when the engine is operating at normal atmo-' spheric pressure, or sea-level pressure. When the internal combustion engine is used on aircraft an entirely different set of conditions are encountered. The power developed by an unsupercharged engine becomes less and less as the aircraft mounts higher and higher, for the very simple reason that the air becomes less dense as the height above the earth's surface is increased. Atmospheric pressure at 20,000ft. altitude is half the figure at sea level, and the density is about 0.04 lb. per cubic ft. instead of 0.076 lb. Thus at this height the volumetric efficiency of the engine drops from 80 per cent, to round about 45 per cent. Each cylinder is only receiving about a half-weight charge. At 35,000ft. the air density is about one-quarter of the figure at sea level. This drop in efficiency with increased altitude applies not only to a plain engine but to a supercharged engine as well, if the supercharging has been designed only for sea-level work. But to overcome in part this considerable disadvan tage, engines expressly designed for aircraft are arranged so that they are, so to speak, over-supercharged and Turbo-supercharging is not confined to line type engines. Here is a section of equipment designed for a radial engine. The exposed turbine rotor is integral wi.th the shaft which carries the blower impeller. To avoid the transfer of heat, a layer of thermal insulating material is provided on the tur bine casing and air ia circulated between turbine and blower and around the main bearings. have too high a compression pressure for use at sea level. If they are allowed to operate at sea level with this condition unchecked they will more or less distintegrate, so they are prevented from using the surplus at sea level by controls which stop the throttle from being fully opened, and thus keep matters within the safe limit. As the aircraft gains altitude, the throttle can safely be opened wider and wider, and at the designed ceiling the engine is able to develop fair power. If it be desired to raise the maximum height still farther, the engine needs to be supercharged at a still greater rate, which can be done by fitting it with a two-speed gear drive to the supercharger, driving it at a higher speed for high alti tudes, so that it is able to push into the engine a greater number of cubic feet of the less dense air and thus supply the necessary weight of air. Excluding mechani cal losses due to the increased speed and the gearing, the supercharger will not require any more power to drive it at the higher speed in thin air than is needed at the lower speed in more dense air, because its job of work is the same at all times, namely, to move into the engine a certain weight of air in a given time. This approach to the subject by reviewing the elemen tary considerations should serve to bring out the par ticular aim of turbo-supercharging. The aircraft engine is required to maintain its power output irrespective of
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