Section II. Principles of Operation

Army Aircraft Gas Turbine Engines
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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

1.7.

GENERAL

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

shaft.

1.8.

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.

1.9.

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

air had at the inlet. The compressed air is then mixed with fuel that