Page 233 - The Jet Engine

Performance

termined ambient air temperature. Above this energy wasted by the propelling mechanism. Waste
temperature the fuel flow is automatically controlled energy dissipated in the jet wake, which represents a
to prevent turbine entry temperature limitations from W v ( J − V ) 2
being exceeded, thus resulting in reduced thrust and, loss, can be expressed as where (v J -V)
g
2
overall, similar curve characteristics to those shown is the waste velocity. It is therefore apparent that at
in fig. 21-8. In the instance of a triple-spool engine the aircraft lower speed range the pure jet stream
the pressure ratio is expressed as P /P . i.e. H.P. wastes considerably more energy than a propeller
4
1
compressor delivery pressure/engine inlet pressure. system and consequently is less efficient over this
range. However, this factor changes as aircraft
speed increases, because although the jet stream
PROPULSIVE EFFICIENCY continues to issue at a high velocity from the engine
its velocity relative to the surrounding atmosphere is
37. Performance of the jet engine is not only reduced and, in consequence, the waste energy loss
concerned with the thrust produced, but also with the is reduced.
efficient conversion of the heat energy of the fuel into
kinetic energy, as represented by the jet velocity, and 40. Briefly, propulsive efficiency may be expressed
the best use of this velocity to propel the aircraft as: Work done on the aircraft
forward, i.e. the efficiency of the propulsive system. or simply
Energy imparted to engine airflow
38. The efficiency of conversion of fuel energy to Work done
kinetic energy is termed thermal or internal efficiency Work done + work wasted in exhaust
and, like all heat engines, is controlled by the cycle
pressure ratio and combustion temperature. Work done is the net thrust multiplied by the aircraft
Unfortunately, this temperature is limited by the speed. Therefore, progressing from the net thrust
thermal and mechanical stresses that can be equation given in para. 18, the following equation is
tolerated by the turbine. The development of new arrived at:
materials and techniques to minimize these Propulsive efficiency =
limitations is continually being pursued.  W(v − V )
V  (P - P 0 )A + J 
39. The efficiency of conversion of kinetic energy to  g 
propulsive work is termed the propulsive or external V  (P - P )A + W(v − V ) + W(v − V ) 2
J
J
efficiency and this is affected by the amount of kinetic   0 g   2 g

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

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