BRAYTON CYCLE

Four processes occur in gas turbine engines, as illustrated above. These processes, first
described by George Brayton and called the Brayton cycle, occur in all internal combustion
engines. The Brayton steps are as follows:
> Compression occurs between the intake and the outlet of the compressor (Line A-B).
During this process, pressure and temperature of the air increases.
> Combustion occurs in the combustion chamber where fuel and air are mixed to explosive proportions and ignited. The addition of heat causes a sharp increase in volume (Line B-C)
> Expansion occurs as hot gas accelerates from the combustion chamber. The gases at constant pressure and increased volume enter the turbine and expand through it. The sharp decrease in pressure and temperature (Line C-D).
> Exhaust occurs at the engine exhaust stack with a large drop in volume and at a constant pressure (Line D-A).

The number of stages of compression and the arrangement of turbines that convert the energy of accelerating hot gas into mechanical energy are design variables. However, the basic operation of all gas turbines is the same.

BASIC PRINCIPLE OF GAS TURBINE

The balloon drawings above illustrate the basic principles upon which gas turbine engines
operate. Compressed inside a balloon, as in (A) above, exerts force upon the confines of the
balloon. Air, which has weight and occupies space, by definition, has mass. The mass of the air
is roportionalt to its density, and ensityi is roportionalt to temperature and pressure. The ir
mass confined inside the balloon, accelerates from the balloon, creating a force as it is released
(B). This force increases as mass and acceleration increase, as stated in Newton's second law;
force equals mass times acceleration (F = MA). The force created by the acceleration of the air
mass nside the balloon results n an equal and opposite force that causes the balloon to be
propelled in the opposite direction, as stated in Newton's third law.
For every action, there is an equal and opposite reaction.)
Replacing the air inside the balloon, as in (C), sustains the force and, although impractical,
allows a load to be driven by the force of the air mass accelerating across and driving a turbine,
as in (D).

In (E), a more practical means of sustaining the force of an accelerating air mass used to drive a
load is illustrated. A housing contains a fixed volume of air, which is compressed by a motor-
driven compressor. Acceleration of the compressed air from the housing drives a turbine that is
connected to the load.

In (F), fuel is injected between the compressor and the turbine to further accelerate the air mass,
thus multiplying the force used to drive the load.

In (G), the motor is removed and the compressor is powered by a portion of the combustion gas,
thus making the engine self-sufficient as long as fuel is provided.

In (H), a typical gas turbine-engine operation is represented. Intake air is compressed, mixed
with fuel and ignited. The hot gas is expanded across a turbine to provide mechanical power and