FUEL CONSUMPTION AND POWER-TO-WEIGHT RELATIONSHIP

FUEL CONSUMPTION AND POWER-TO-WEIGHT RELATIONSHIP
46. Primary engine design considerations, particu- larly for commercial transport duty, are those of low specific fuel consumption and weight. Considerable improvement has been achieved by use of the by- pass principle, and by advanced mechanical and aerodynamic features, and the use of improved materials. With the trend towards higher by-pass ratios, in the range of 15:1, the triple-spool and contra-rotating rear fan engines allow the pressure and by-pass ratios to be achieved with short rotors, using fewer compressor stages, resulting in a lighter and more compact engine.

Fig. 21-10 International Standard Atmosphere.

47. S.f.c. is directly related to the thermal and propulsive efficiencies; that is, the overall efficiency of the engine. Theoretically, high thermal efficiency requires high pressures which in practice also means high turbine entry temperatures. In a pure turbo-jet engine this high temperature would result in a high jet velocity and consequently lower the propulsive efficiency (para. 40). However, by using the by-pass principle, high thermal and propulsive efficiencies can be effectively combined by bypassing a proportion of the L.P. compressor or fan delivery air to lower the mean jet temperature and velocity as referred to in para. 43. With advanced technology engines of high by-pass and overall pressure ratios, a further pronounced improvement in s.f.c. is obtained.
48. The turbines of pure jet engines are heavy because they deal with the total airflow, whereas the turbines of by-pass engines deal only with part of the flow; thus the H.P. compressor, combustion chambers and turbines, can be scaled down. The increased power per lb. of air at the turbines, to take advantage of their full capacity, is obtained by the increase in pressure ratio and turbine entry temperature. It is clear that the by-pass engine is lighter, because not only has the diameter of the high pressure rotating assemblies been reduced but the engine is shorter for a given power output. With a low by-pass ratio engine, the weight reduction compared with a pure jet engine is in the order of 20 per cent for the same air mass flow.

PROPULSIVE EFFICIENCY

PROPULSIVE EFFICIENCY

37. Performance of the jet engine is not only concerned with the thrust produced, but also with the efficient conversion of the heat energy of the fuel into kinetic energy, as represented by the jet velocity, and the best use of this velocity to propel the aircraft forward, i.e. the efficiency of the propulsive system.
38. The efficiency of conversion of fuel energy to kinetic energy is termed thermal or internal efficiency and, like all heat engines, is controlled by the cycle pressure ratio and combustion temperature. Unfortunately, this temperature is limited by the thermal and mechanical stresses that can be tolerated by the turbine. The development of new materials and techniques to minimize these limitations is continually being pursued.
39. The efficiency of conversion of kinetic energy to propulsive work is termed the propulsive or external efficiency and this is affected by the amount of kinetic energy wasted by the propelling mechanism. Waste energy dissipated in the jet wake, which represents a is the waste velocity. It is therefore apparent that at the aircraft lower speed range the pure jet stream wastes considerably more energy than a propeller system and consequently is less efficient over this range.
However, this factor changes as aircraft speed increases, because although the jet stream continues to issue at a high velocity from the engine its velocity relative to the surrounding atmosphere is reduced and, in consequence, the waste energy loss is reduced. reference to fig. 21-9 it can be seen that for aircraft designed to operate at sea level speeds below approximately 400 m.p.h. it is more effective to absorb the power developed in the jet engine by gearing it to a propeller instead of using it directly in the form of a pure jet stream. The disadvantage of the propeller at the higher aircraft speeds is its rapid fall off in efficiency, due to shock waves created around the propeller as the blade tip speed approaches Mach 1.0. Advanced propeller technology, however, has produced a multi-bladed, swept back design capable of turning with tip speeds in excess of Mach 1.0 without loss of propeller efficiency. By using this design of propeller in a contra-rotating configuration, thereby reducing swirl losses, a 'prop-fan' engine, with very good propulsive efficiency capable of operating efficiently at aircraft speeds in excess of 500 m.p.h. at sea level, can be produced.

Effect of temperature

Effect of temperature

Fig. 21-7 The effect of altitude on s.h.p. and

fuel consumption

33. On a cold day the density of the air increases so that the mass of air entering the compressor for a given engine speed is greater, hence the thrust or s.h.p, is higher. The denser air does, however, increase the power required to drive the compressor or compressors; thus the engine will require more fuel to maintain the same engine speed or will run at a reduced engine speed if no increase in fuel is available.
34. On a hot day the density of the air decreases, thus reducing the mass of air entering the compressor and, consequently, the thrust of the engine for a given r.p.m. Because less power will be required to drive the compressor, the fuel control system reduces the fuel flow to maintain a constant engine rotational speed or turbine entry temperature, as appropriate; however, because of the decrease in air density, the thrust will be lower. At a temperature of 45 deg.C., depending on the type of engine, athrust loss of up to 20 per cent may be experienced. This means that some sort of thrust augmentation, such as water injection (Part 17), may be required.

Effect of altitude

Effect of altitude

Fig. 21-6 The effects of altitude on thrust

and fuel consumption

32. With increasing altitude the ambient air pressure and temperature are reduced. This affects the engine in two interrelated ways: The fall of pressure reduces the air density and hence the mass airflow into the engine for a given engine speed. This causes the thrust or s.h.p. to fall. The fuel control system, as described in Part 10, adjusts the fuel pump output to match the reduced mass airflow, so maintaining a constant engine speed. The fall in air temperature increases the density of the air, so that the mass of air entering the compressor for a given engine speed is greater. This causes the mass airflow to reduce at a lower rate and so compensates to some extent for the loss of thrust due to the fall in atmospheric pressure. At altitudes above 36,089 feet and up to 65,617 feet, however, the temperature remains constant, and the thrust or s.h.p. is affected by pressure only. Graphs showing the typical effect of altitude on thrust, s.h.p, and fuel consumption are illustrated in fig. 21-6 and fig. 21-7.

ENGINE THRUST IN FLIGHT

ENGINE THRUST IN FLIGHT

Fig. 21-3  Thrust recovery with aircraft

speed.

17. Since reference will be made to gross thrust, momentum drag and net thrust, it will be helpful to define these terms: from Part 20, gross or total thrust is the product of the mass of air passing through the engine and the jet velocity at the propelling nozzle, expressed as: The net thrust or resultant force acting on the aircraft in flight is the difference between the gross thrust and the momentum drag.
18. From the definitions and formulae stated in para, 17; under flight conditions, the net thrust of the Effect of forward speed

19. Since reference will be made to 'ram ratio' and Mach number, these terms are defined as follows: Ram ratio is the ratio of the total air pressure at the engine compressor entry to the static air pressure at the air intake entry. Mach number is an additional means of measuring speed and is defined as the ratio of the speed of a body to the local speed of sound. Mach 1.0 therefore represents a speed equal to the local speed of sound.

Fig. 21-4 The effect of aircraft speed onthrust and fuel consumption.

20. From the thrust equation in para. 18, it is apparent that if the jet velocity remains constant, independent of aircraft speed, then as the aircraft speed increases the thrust would decrease in direct proportion. However, due to the 'ram ratio' effect from the aircraft forward speed, extra air is taken into the engine so that the mass airflow and also the jet velocity increase with aircraft speed. The effect of this tends to offset the extra intake momentum drag due to the forward speed so that the resultant loss of net thrust is partially recovered as the aircraft speed increases. A typical curve illustrating this point is shown in fig. 21-3. Obviously, the 'ram ratio' effect, or the return obtained in terms of pressure rise at entry to the compressor in exchange for the unavoidable intake drag, is of considerable importance to the turbo-jet engine, especially at high speeds. Above speeds of Mach 1.0, as a result of the formation ofshock waves at the air intake, this rate of pressure rise will rapidly decrease unless a suitably designed air intake is provided (Part 23); an efficient air intake is necessary to obtain maximum benefit from the ram ratio effect.

ENGINE THRUST ON THE TEST BENCH

ENGINE THRUST ON THE TEST BENCH

8. The thrust of the turbo-jet engine on the test bench differs somewhat from that during flight. Modern test facilities are available to simulate atmospheric conditions at high altitudes thus providing a means of assessing some of the performance capability of a turbo-jet engine in flight without the engine ever leaving the ground. This is important as the changes in ambient temperatur and pressure encountered at high altitudes consider-ably influence the thrust of the engine.

9. Considering the formula derived in Part 20 for engines operating under 'choked' nozzle conditions, it can be seen that the thrust can be further affected by a change in the mass flow rate of air through the engine and by a change in jet velocity. An increase in mass airflow may be obtained by using wate injection (Part 17) and increases in jet velocity by using afterburning (Part 16).
10. As previously mentioned, changes in ambien pressure and temperature considerably influence the thrust of the engine. This is because of the way they affect the air density and hence the mass of ai entering the engine for a given engine rotationa speed. To enable the performance of similar engines to be compared when operating under differen climatic conditions, or at different altitudes, correction factors must be applied to the calculations to return the observed values to those which would be found under I.S.A. conditions. For example, the thrus  correction for a turbo-jet engine is: Thrust (lb.) (corrected) =

11. The observed performance of the turbo- propeller engine is also corrected to I.S.A. conditions, but due to the rating being in s.h.p. and not in pounds of thrust the factors are different. For example, the correction for s.h.p. is: S.h.p. (corrected) = In practice there is always a certain amount of jet thrust in the total output of the turbo-propeller engine and this must be added to the s.h.p. The correction for jet thrust is the same as that in para. 10.

12. To distinguish between these two aspects of the power output, it is usual to refer to them as s.h.p. and thrust horse-power (t.h.p.). The total equivalent horse-power is denoted by t.e.h.p. (sometimes e.h.p.) and is the s.h.p. plus the s.h.p. equivalent to the net jet thrust. For estimation purposes it is taken that, under sea- level static conditions, one s.h.p. is equivalent to approximately 2.6 lb. of jet thrust. Therefore :

Temperature and pressure notation of a typical turbo-jet engine

INTRODUCTION

Fig. 21-1 Temperature and pressure notation of a typical turbo-jet engine.

1. The performance requirements of an engine are obviously dictated to a large extent by the type of operation for which the engine is designed. The power of the turbo-jet engine is measured in thrust, produced at the propelling nozzle or nozzles, and that of the turbo-propeller engine is measured in shaft horse-power (s.h.p.) produced at the propeller shaft. However, both types are in the main assessed on the amount of thrust or s.h.p. they develop for a given weight, fuel consumption and frontal area.

Rolls-Royce RB168 MK807 | Blackburn Nimbus

Rolls-Royce RB168 MK807

Blackburn Nimbus

 The Nimbus was developed from the A129 turbo-shaft which, in its turn, was a modified Turbomeca Artouste built under licence. The Nimbus developed 968 hp, but for helicopter use was flat-rated at 710 hp. The engine was used in Westland Wasp and Scout helicopters and four 700 hp units were used to power the experimental 5RN-2 hovercraft.