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Aviation History
1944
1944 - 2007.PDF
SEPTEMBER 28TH, 1944 FLIGHT 345 compression ratio, and also the effect of compression ratio on the weight of the power plant. In estimating the change In weight with compression ratio it is assumed that as the compression is raised the swept volume is increased in pro portion to the reduction in b.m.e.p. so as to maintain a constant power output- It is assumed that at 7: 1 compression ratio the weight of those parts of the power plant affected by the change in size of the engine is 3,000 lb. for a cruising power under economical low r.p.m. conditions of 1,000 b.h.p. It is also assumed that the weight of these parts will vary directly as the swept volume. Fig. 2 shows the difference between the change in power- plant weight and fuel weight, or in other words the change in pay load with compression ratio. Cost of High Compression For a rated height, under weak-mixture cruising con ditions of 15,000ft., the saving in fuel on a 10-hour flight at 1,000 h.p., resulting in a change from 7: 1 to-8: 1, will be of the order of 270 lb. On the other hand, the specific t er of the engine for a given fuel will be reduced because boost pressure will have to be lower to avoid detona- . Making due allowance for the reduced power to drive the blower, the reduction will be of the order of 14 per cent., whilst the maximum cylinder pressures will be about • the same on both engines, possibly a little higher on the high-compression, low-boost type. The increase in weight of the high-compression engine is, therefore, likely to be roughly in proportion to the reduction in specific power. Assuming an engine cruising at 50 per cent, maximum power, the increase in installed weight, including lubricat ing oil tanks, etc., would be of the order of 430 lb. On a commercial basis, therefore, it would be necessary to assess the net loss in pay load of 160 lb. against the cost of the fuel saved; about 30 shillings. Generally speaking, it does not seem that very high com pression ratios will be worth while, even in commercial operations, except for flights of over 15 hours' duration. Special high-compression engines may be made for Coastal Command aircraft for anti-submarine patrol purposes where very long endurance is required, but in general, it is believed that compression ratios will remain much as at present. On this assumption we can examine the probable •00 1 200 O 200 500 eoo cob S3 -i - LU 3 % ID -9 -5 2 I . 1 y<$> "% — Y,<• — V [JSP COM PRESS ) ^fcfo --&, ON RAID \j- 1 s. .- \ 1 \ ¥ w a \ SOO *oo si DECREAS E O F pAY- y , <;_, I 1 COMPRESSION RATIO %& \! £ ^ F'g. 1. Power-plant and fuel-weight variations with com pression ratio, assuming 2,000 h.p. engine cruising at 50 per cent, maximum power, and taking 7 :1 compression ratio as basis for comparison. Fig. 2. Pay-load variation with compression ratio, assum ing 2,000 h.p. engine cruising at 50 per cent, maximum power, and taking 7 :1 compression ratio as basis for comparison. increase in specific powers during the next seven years. Soon after the war, using existing refining equipment and technique, but at a lower rate of production, it will be possible to produce fuel capable of giving a 30 per cent, increase in b.m.e.p. without increase in lead content. Some five years after this we can confidently expect to get new fuels capable of giving at least a 50 per cent, better b.m.e.p. than the present-day 100 Octane fuel at the same induction temperature. This does not, of course, mean a 50 per cent, increase in power at any particular altitude, as higher blower compression ratios will be required to obtain the necessary boost pressures at altitude. Fig. 3 shows the power obtainable from a given fuel at constant impeller tip speed, plotted against the correspond ing power developed when the blower speed is increased to maintain a constant rated altitude of 15,000ft., both with and without after-coolers. It is assumed that the cylinder compression ratio remains constant. It will be seen that with a 50 per cent, improvement in fuel and after-cooling, one can realise an increase in power of 35 per cent, at constant height. This curve also shows the increasing value of after-cooling at a fixed rated height with improved fuels. Fig. 4 shows the relationship between b.m.e.p. and maximum cylinder pressure for normal ignition timings This can be somewhat modified by the use of very rich mixtures, such as are currently used for maximum power on air-cooled engines. The high-cylinder pressures will mean an increase in the weight of reciprocating parts, and this will mean bigger inertia loads on bearings when run ning at fight loads. At high loads the increased gas pres sures will, help to off-set these loads. 42 p.c. Power Increase in Five Years Present-day sleeve-valve engines have excellent breathing characteristics as shown in Fig. 5, and, providing airscrew and boost controls are interconnected so as to avoid the combination of light loads and high r.p.m., I think we can usefully look for a modest increase in piston speeds on existing radial engines. I think probably between five and ten per cent, in the next seven years giving, let us say, five per cent, increase in power due to higher speeds. The total increase in power that we can expect, say, five years after the end of the war is therefore about 42 per cent., so that engines at present giving 2,000 h.p. will by then be in the 3,000 h.p. class. An engine giving
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