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.

Fig. 21-8 The effect of air temperature on a typical twin-spool engine.

35. The fuel control system (Part 10) controls the fuel flow so that the maximum fuel supply is held practically constant at low air temperature conditions, whereupon the engine speed falls but, because of the increased mass airflow as a result of the increase in air density, the thrust remains the same. For example, the combined acceleration and speed control fuel system (Part 10) schedules fuel flow to maintain a constant engine r.p.m., hence thrust increases as air temperature decreases until, at a predetermined compressor delivery pressure, the fuel flow is automatically controlled to maintain aconstant compressor delivery pressure and,therefore, thrust. Fig. 21-8 illustrates this for a twin- spool engine where the controlled engine r.p.m. is high pressure compressor speed and the compressor delivery pressure is expressed as P3. It will also be apparent from this graph that the low pressure compressor speed is always less than its limiting maximum and that the difference in the two speeds is reduced by a decrease in ambient air temperature. To prevent the L.P. compressor over- speeding, fuel flow is also controlled by an L.P.governor which, in this case, takes a passive role.
36. The pressure ratio control fuel system (Part 10) schedules fuel flow to maintain a constant engine pressure ratio and, therefore, thrust below a prede- termined ambient air temperature. Above this temperature the fuel flow is automatically controlled to prevent turbine entry temperature limitations from being exceeded, thus resulting in reduced thrust and, overall, similar curve characteristics to those shown in fig. 21-8. In the instance of a triple-spool engine the pressure ratio is expressed as P 4 /P 1 . i.e. H.P. compressor delivery pressure/engine inlet pressure.