In the turbojet engine, approximately 60 percent of the energy
is extracted to rotate the compressor, while the remaining 40 percent
is used to develop thrust. In the turboshaft engine, the remaining
energy is used to drive a turbine rotor attached to a transmission or
propeller. On a freeturbine engine, the gas stream passing across
the turbines is the only link between the two turbine rotors. One
turbine drives the compressor and the other turbine propels the
aircraft. The freeturbine engine is used in Army aircraft.
The gas turbine engine differs radically in construction from
the reciprocating engine in that the turbine engine has a separate
section for each function, while in the reciprocating engine all
functions are performed in the same cylinder.
Section II. Principles of Operation
This section covers the principles of turbine engine
operation. The three classifications of turbine engines are
turbojet, turboshaft, and ramjet. The term "turbo" means "turbine."
Therefore, a turboshaft engine is one which delivers power through a
OTTO AND BRAYTON CYCLES
There is an element of similarity to both the reciprocating
and jet engines, but the thermodynamic cycle of each is different
from the other. The reciprocating engine operates on the Otto cycle,
a constant volume cycle, consisting of four distinct operations.
These operations are performed intermittently by a piston
reciprocating in an enclosed cylinder. It is important to remember
that the piston in a reciprocating engine delivers power only during
one of its four strokes.
The turbine engine operates on the Brayton cycle, a constant
pressure cycle containing the same four basic operations as the Otto
cycle, but accomplishing them simultaneously and continuously so that
an uninterrupted flow of power from the engine results. Figure 1.1
shows a graph display of the Otto and Brayton cycles.
Ambient air is drawn into the inlet section by the rotating
compressor. The compressor forces this incoming air rearward and
delivers it to the combustion chamber at a higher pressure than the