Page 21 - The Jet Engine

2: Working cycle and airflow

Contents Page

Introduction 11
Working cycle 11

The relations between pressure,

volume and temperature 13
Changes in velocity

and pressure 14
Airflow 17

INTRODUCTION piston engine whilst they occur continuously in the
gas turbine. In the piston engine only one stroke is
1. The gas turbine engine is essentially a heat utilized in the production of power, the others being
engine using air as a working fluid to provide thrust. involved in the charging, compressing and
To achieve this, the air passing through the engine exhausting of the working fluid. In contrast, the
has to be accelerated; this means that the velocity or turbine engine eliminates the three 'idle' strokes, thus
kinetic energy of the air is increased. To obtain this enabling more fuel to be burnt in a shorter time;
increase, the pressure energy is first of all increased, hence it produces a greater power output for a given
followed by the addition of heat energy, before final size of engine.
conversion back to kinetic Energy in the form of a
high velocity jet efflux. 3. Due to the continuous action of the turbine
engine and the fact that the combustion chamber is
WORKING CYCLE not an enclosed space, the pressure of the air does
not rise, like that of the piston engine, during
2. The working cycle of the gas turbine engine is combustion but its volume does increase. This
similar to that of the four-stroke piston engine. process is known as heating at constant pressure.
However, in the gas turbine engine, combustion Under these conditions there are no peak or
occurs at a constant pressure, whereas in the piston fluctuating pressures to be withstood, as is the case
engine it occurs at a constant volume. Both engine with the piston engine with its peak pressures in
cycles (fig. 2-1) show that in each instance there is excess of 1,000 lb. per sq. in. It is these peak
induction, compression, combustion and exhaust. pressures which make it necessary for the piston
These processes are intermittent in the case of the engine to employ cylinders of heavy construction and

11