Page 226 - The Jet Engine

Performance

4. Whereas the thermal efficiency is often referred instance, using the symbols shown in fig. 21-1 the
to as the internal efficiency of the engine, the P 3
propulsive efficiency is referred to as the external overall compressor pressure ratio is . These
1 P
efficiency. This latter efficiency, described in para. 37,
explains why the pure jet engine is less efficient than symbols vary slightly for different types of engine; for
instance, with high by-pass ratio engines, and also
the turbo-propeller engine at lower aircraft speeds
leading to development of the by-pass principle and, when afterburning (Part 16) is incorporated,
more recently, the propfan designs. additional symbols are used.
7. To enable the performance of similar engines to
5. The thermal and the propulsive efficiency also
influence, to a large extent, the size of the be compared, it is necessary to standardize in some
compressor and turbine, thus determining the weight conventional form the variations of air temperature
and diameter of the engine for a given output. and pressure that occur with altitude and climatic
conditions. There are in use several different
6. These and other factors are presented in curves definitions of standard atmospheres, the one in most
and graphs, calculated from the basic gas laws (Part common use being the International Standard
2), and are proved in practice by bench and flight Atmosphere (I.S.A.). This is based on a temperature
testing, or by simulating flight conditions in a high lapse rate of approximately 1.98 K. degrees per
altitude test cell. To make these calculations, specific 1,000ft,, resulting in a fall from 288.15 deg.K. (15
symbols are used to denote the pressures and tem- deg.C) at sea level to 216.65 deg.K (-56.5 deg.C.) at
peratures at various locations through the engine; for 36,089 ft. (the tropopause). Above this altitude the

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

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