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Endurance and Fuel Consumption

Endurance and Fuel Consumption December, 1937 AIRCRAFT ENGINEERING 329 A Suggestion for the Reductio n of Engine Weight b y Lowering Compression Ratios By H . Constant, M.A. A.F.R.Ae.S. A calculation has been made to determine N E has become so accustomed to having the variation with compression ratio of th e total th e importance of high thermal efficiency and low fuel consumption specific weight S of power plant and fuel, the stressed and to having impressed on one th e details of which are summarised in the Table. consequent desirability of high compression The brake specific fuel consumption (under ratio, tha t there is some danger of one's outlook cruising conditions) and the brake power on the subject becoming rather one-sided. It outpu t (under take-off conditions) for a repre­ may be of some value, therefore, to consider sentative modern liquid-cooled engine ar e given as a percentage of their values a t 6 : 1 compres­ under what circumstances the fuel consumption sion ratio. The consumption figures include a n of an engine is likely to be relatively un­ important. allowance of 5 per cent for tankage weight. The power figures include allowances both for I t is well known that in an engine in which the increase in th e power required to drive the th e amount of power that can b e delivered is supercharger as the boost is increased at low limited by the occurrence of detonation, the compression ratio and also for the increase in power that just gives incipient detonation can the tendency to detonate due t o the higher be increased by lowering the compression ratio boost temperatures at the lower compression and raising the boost. Consequently, when ratios. The installed specific weight of the operating on a fuel of fixed anti-knock value, power plant alone, including radiator, piping, the available power of an engine can be in­ etc., has been taken a t 1·80 a t 6 : 1 compression creased by lowering its compression ratio, bu t ratio. The effective range of the aircraft has this can only be achieved by a sacrifice in been assumed to be 800 miles, and th e rati o of thermal efficiency. The conditions for high cruising power to take-off power has been taken power and low fuel consumption are thus t o be one half. On these assumptions th e tota l opposed and a compromise must be struck in specific weight 5 has been calculated for cruising such a way as to suit the particular requirements speeds of 100, 150, 200, 300 an d 400 m.p.h. tha t the engine has t o fulfil. The performance of an engine can be con­ The variation in total specific consumption veniently expressed by the ratio of the total with compression ratio is plotte d in the figure. power plant and fuel weight to the engine's I t will be seen tha t the value of S ha s a minimum take-off power. In such an expression neither value for each speed, and tha t this minimum th e effects of drag and frontal area on per­ value occurs at a lower compression ratio as formance nor the effect of fuel cost on th e the speed is increased. In other words, as th e economics of operation are considered. The cruising speeds of aircraft increase, a lower analysis that we propose to make will not, compression ratio becomes necessary in order where s = installed specific weight of engine however, involve large changes in drag or to obtain the lowest possible total fuel and based on take-off power frontal area, so tha t the first of these two omis­ power plant specific weight. With cruising r = range of aircraft in miles. sions in our expression for performance is no t speeds in the neighbourhood of 100 m.p.h., V — true cruising speed in m.p.h. important. With regard to the second, it such as characterised heavy bombing aircraft Some years ago, when the rang e of aircraft would perhaps be desirable to consider our some few year s ago, the optimu m compression was comparatively small and the specific weight analysis as referring only to military aircraft ratio would be about 5:1 . The variation of of engines high, the fuel consumption term in in which the cost of the fuel is of secondary tota l specific weight with compression ratio a t the above expression was relatively unimportant importance. these speeds is, however, so slight that it was compared with power plant weight. We have to enquire then, under what con­ probably worth while using a rather higher I n more recent years engine specilic weights ditions the tota l power plant and fuel specific ratio and saving in fuel cost and cooling drag have been reduced while ranges have increased weight becomes a minimum. This specific losses. But a t speeds of 200 m.p.h., and over, a and as a result the second term in th e expression weight is given by th e relation : lower compression ratio appears to be desirable. became more important than the first and more Thus a t 300 m.p.h. the saving in weight in a attention was paid to th e proble m of obtaining 2,000 h.p. aeroplane which would result from low consumptions. During this period there lowering the compression ratio from 7: 1 to where S = total specific weight of power occurred renewed interest in the compression 4½ : 1 would be abou t 760 lb. plan t and fuel ignition engine with its promise of high economy. W = installed weight of power plant To-day, however, a new state of affairs is I n the above calculations no account has f = brake specific fuel consumption arising, due mainly to the rapid increase in been taken of an y increases in drag and weight under cruising conditions operating speeds. The size of the world has which might result from the larger amount of H = endurance in hours not changed, and the distance of strategic hea t which has to be dissipated at the lower Pe = cruising power objectives iemains in general unaltered, engine compression ratios. In high-speed aeroplanes Pl = take-off power specif c weights show little signs of an y material fitted with ducted radiators designed to reduce If we assume that the cruising power is change, while the speed at which' aircraft the cooling drag to a minimum, the increase in 50 per cent of th e take-off power, we have accomplish their flights is steadily increasing. drag associated with a reduction in com­ As a consequence the fuel consumption term in pression ratio should not be large. Nor has * This article is published by permission of the Air Ministry, but our expression is again beginning to assume allowance been made for th e more arduous duty the author takes full responsibility for all statements made and a relatively unimportant position. required from valves and pistons when operating opinions expressed. a t low compression ratios. The mechanical reliability of these parts will probably provide TABL E I. th e lower limit beyond which the compression Compression ratio 3 5 8 9 4 6 7 .. .. .. .. ratio cannot safely be lowered. Although there .. .. Relativ e brake power per cent 102 134 114 100 91 8 5 82 is no doub t that the abov e effects would exert Brak e specific consumptio n f, lb./b.h.p./hr.. . 0·710 0·612 0·538 0·494 0·459 0·434 0·409 a modifying influence on the shape of th e curves, Installe d specific weight of power plant s .. 1·11 1·34 1·58 1·80 1·98 2·12 2·19 the corrections involved would not be suffi­ V m.p.h . 100. . 3·95 3·79 3·73 3·78 3·82 3·80 3·8 3 ciently serious to alter the general conclusion 150. . 3·01 2·9 8 3·02 3·12 3·20 3·2 8 3·29 Tota l specific weight of power plant that , under the conditions specified, a reduction 200. . 2·53 2·56 2·79 2·90 2·9 9 30 1 2·68 an d fuel S 300. . 2·06 2·16 2·30 2·46 2·59 2·70 2·74 in compression ratio is now desirable. 400. . 1·82 1·95 2·29 2·44 2·55 2·60 2·12 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aircraft Engineering and Aerospace Technology Emerald Publishing

Endurance and Fuel Consumption

Aircraft Engineering and Aerospace Technology , Volume 9 (12): 1 – Dec 1, 1937

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Publisher
Emerald Publishing
Copyright
Copyright © Emerald Group Publishing Limited
ISSN
0002-2667
DOI
10.1108/eb030253
Publisher site
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Abstract

December, 1937 AIRCRAFT ENGINEERING 329 A Suggestion for the Reductio n of Engine Weight b y Lowering Compression Ratios By H . Constant, M.A. A.F.R.Ae.S. A calculation has been made to determine N E has become so accustomed to having the variation with compression ratio of th e total th e importance of high thermal efficiency and low fuel consumption specific weight S of power plant and fuel, the stressed and to having impressed on one th e details of which are summarised in the Table. consequent desirability of high compression The brake specific fuel consumption (under ratio, tha t there is some danger of one's outlook cruising conditions) and the brake power on the subject becoming rather one-sided. It outpu t (under take-off conditions) for a repre­ may be of some value, therefore, to consider sentative modern liquid-cooled engine ar e given as a percentage of their values a t 6 : 1 compres­ under what circumstances the fuel consumption sion ratio. The consumption figures include a n of an engine is likely to be relatively un­ important. allowance of 5 per cent for tankage weight. The power figures include allowances both for I t is well known that in an engine in which the increase in th e power required to drive the th e amount of power that can b e delivered is supercharger as the boost is increased at low limited by the occurrence of detonation, the compression ratio and also for the increase in power that just gives incipient detonation can the tendency to detonate due t o the higher be increased by lowering the compression ratio boost temperatures at the lower compression and raising the boost. Consequently, when ratios. The installed specific weight of the operating on a fuel of fixed anti-knock value, power plant alone, including radiator, piping, the available power of an engine can be in­ etc., has been taken a t 1·80 a t 6 : 1 compression creased by lowering its compression ratio, bu t ratio. The effective range of the aircraft has this can only be achieved by a sacrifice in been assumed to be 800 miles, and th e rati o of thermal efficiency. The conditions for high cruising power to take-off power has been taken power and low fuel consumption are thus t o be one half. On these assumptions th e tota l opposed and a compromise must be struck in specific weight 5 has been calculated for cruising such a way as to suit the particular requirements speeds of 100, 150, 200, 300 an d 400 m.p.h. tha t the engine has t o fulfil. The performance of an engine can be con­ The variation in total specific consumption veniently expressed by the ratio of the total with compression ratio is plotte d in the figure. power plant and fuel weight to the engine's I t will be seen tha t the value of S ha s a minimum take-off power. In such an expression neither value for each speed, and tha t this minimum th e effects of drag and frontal area on per­ value occurs at a lower compression ratio as formance nor the effect of fuel cost on th e the speed is increased. In other words, as th e economics of operation are considered. The cruising speeds of aircraft increase, a lower analysis that we propose to make will not, compression ratio becomes necessary in order where s = installed specific weight of engine however, involve large changes in drag or to obtain the lowest possible total fuel and based on take-off power frontal area, so tha t the first of these two omis­ power plant specific weight. With cruising r = range of aircraft in miles. sions in our expression for performance is no t speeds in the neighbourhood of 100 m.p.h., V — true cruising speed in m.p.h. important. With regard to the second, it such as characterised heavy bombing aircraft Some years ago, when the rang e of aircraft would perhaps be desirable to consider our some few year s ago, the optimu m compression was comparatively small and the specific weight analysis as referring only to military aircraft ratio would be about 5:1 . The variation of of engines high, the fuel consumption term in in which the cost of the fuel is of secondary tota l specific weight with compression ratio a t the above expression was relatively unimportant importance. these speeds is, however, so slight that it was compared with power plant weight. We have to enquire then, under what con­ probably worth while using a rather higher I n more recent years engine specilic weights ditions the tota l power plant and fuel specific ratio and saving in fuel cost and cooling drag have been reduced while ranges have increased weight becomes a minimum. This specific losses. But a t speeds of 200 m.p.h., and over, a and as a result the second term in th e expression weight is given by th e relation : lower compression ratio appears to be desirable. became more important than the first and more Thus a t 300 m.p.h. the saving in weight in a attention was paid to th e proble m of obtaining 2,000 h.p. aeroplane which would result from low consumptions. During this period there lowering the compression ratio from 7: 1 to where S = total specific weight of power occurred renewed interest in the compression 4½ : 1 would be abou t 760 lb. plan t and fuel ignition engine with its promise of high economy. W = installed weight of power plant To-day, however, a new state of affairs is I n the above calculations no account has f = brake specific fuel consumption arising, due mainly to the rapid increase in been taken of an y increases in drag and weight under cruising conditions operating speeds. The size of the world has which might result from the larger amount of H = endurance in hours not changed, and the distance of strategic hea t which has to be dissipated at the lower Pe = cruising power objectives iemains in general unaltered, engine compression ratios. In high-speed aeroplanes Pl = take-off power specif c weights show little signs of an y material fitted with ducted radiators designed to reduce If we assume that the cruising power is change, while the speed at which' aircraft the cooling drag to a minimum, the increase in 50 per cent of th e take-off power, we have accomplish their flights is steadily increasing. drag associated with a reduction in com­ As a consequence the fuel consumption term in pression ratio should not be large. Nor has * This article is published by permission of the Air Ministry, but our expression is again beginning to assume allowance been made for th e more arduous duty the author takes full responsibility for all statements made and a relatively unimportant position. required from valves and pistons when operating opinions expressed. a t low compression ratios. The mechanical reliability of these parts will probably provide TABL E I. th e lower limit beyond which the compression Compression ratio 3 5 8 9 4 6 7 .. .. .. .. ratio cannot safely be lowered. Although there .. .. Relativ e brake power per cent 102 134 114 100 91 8 5 82 is no doub t that the abov e effects would exert Brak e specific consumptio n f, lb./b.h.p./hr.. . 0·710 0·612 0·538 0·494 0·459 0·434 0·409 a modifying influence on the shape of th e curves, Installe d specific weight of power plant s .. 1·11 1·34 1·58 1·80 1·98 2·12 2·19 the corrections involved would not be suffi­ V m.p.h . 100. . 3·95 3·79 3·73 3·78 3·82 3·80 3·8 3 ciently serious to alter the general conclusion 150. . 3·01 2·9 8 3·02 3·12 3·20 3·2 8 3·29 Tota l specific weight of power plant that , under the conditions specified, a reduction 200. . 2·53 2·56 2·79 2·90 2·9 9 30 1 2·68 an d fuel S 300. . 2·06 2·16 2·30 2·46 2·59 2·70 2·74 in compression ratio is now desirable. 400. . 1·82 1·95 2·29 2·44 2·55 2·60 2·12

Journal

Aircraft Engineering and Aerospace TechnologyEmerald Publishing

Published: Dec 1, 1937

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