Effect of temperature
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.|
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.