Aircraft Gas Turbine Engine Operating Principles

Energy Transformation

A gas turbine engine is a form of heat engine that converts the chemical energy of fuel into heat energy. Heat energy causes an increase in gas pressure that is converted into kinetic energy in the form of a high velocity stream of air. The kinetic energy is transformed to mechanical energy as the gases rotate a series of turbine wheels to drive a compressor and accessories. In the case of turboprop or turboshaft engines, the expanding gases can also drive a second power turbine to drive a propeller or gearbox.

Energy Transformation Cycle

The energy transformation cycle in a gas turbine engine is known as the Brayton cycle (or constant pressure cycle) . Similar to the four-stroke Otto cycle, the Brayton cycle has intake, compression, combustion, and exhaust events. However, unlike a piston engine, all four events occur simultaneously and continuously in a gas turbine engine. A gas turbine engine is able to produce power continuously. To support the continuous production of power, a gas turbine engine must burn a great deal of fuel. [Figure 1]

Aircraft Gas Turbine Engine Operating Principles
Figure 1. In a gas turbine engine, air is drawn in through an air inlet, compressed in the compressor, mixed with fuel and ignited in the combustors, and exhausted through the turbines and exhaust nozzle . A gas turbine engine performs the same functions as a cylinder and piston in a reciprocating engine . In a turbine engine, these four events happen continuously

The continuous intake event in a gas turbine engine draws ambient air into the engine through an inlet duct to the first compressor stage. Each compressor stage increases static air pressure. In the combustor, fuel is sprayed into the incoming airflow and ignited, resulting in continuous combustion. The resulting release of heat energy increases the volume of the air while maintaining a relatively constant pressure.

When exhaust air exits the combustion chamber, it passes through the turbine where static air pressure drops and air volume continues to increase. Because the flow of expanding gases is relatively unobstructed , the velocity increases dramatically. [Figure 2]

Aircraft Gas Turbine Engine Operating Principles
Figure 2 . This chart illustrates the changes in pressure and volume during engine operation . Point A represents the condition of the air just before it enters the compressor. After it enters the compressor, its pressure increases and its volume decreases. Point B represents the pressure and volume of the air as it leaves the compressor. At point C, heat energy expands the volume of the air mass with little or no change in pressure. After it is heated, the air expands and loses pressure as it flows through the turbine section to point D

Operating Principles

The principle used by a gas turbine engine as it provides force to move an airplane is based on Newton’s law of momentum. This law states that for every action there is an equal and opposite reaction; therefore, if the engine accelerates a mass of air (action), it applies a force on the aircraft (reaction). The turbofan generates thrust by giving a relatively slower acceleration to a large quantity of air. The old pure turbojet engine achieves thrust by imparting greater acceleration to a smaller quantity of air. This was its main problem with fuel consumption and noise.

The mass of air is accelerated within the engine by the use of a continuous-flow cycle. Ambient air enters the inlet diffuser where it is subjected to changes in temperature, pressure, and velocity due to ram effect. The compressor then increases pressure and temperature of the air mechanically. The air continues at constant pressure to the burner section where its temperature is increased by combustion of fuel. The energy is taken from the hot gas by expanding through a turbine which drives the compressor, and by expanding through an exhaust nozzle designed to discharge the exhaust gas at high velocity to produce thrust.

The high velocity gases from the engine may be considered continuous, imparting this force against the aircraft in which it is installed, thereby producing thrust. The formula for thrust can be derived from Newton’s second law, which states that force is proportional to the product of mass and acceleration. This law is expressed in the formula:

F = M × A

F = force in pounds
M = mass in pounds per seconds
A = acceleration in feet per seconds

In the above formula, mass is similar to weight, but it is actually a different quantity. Mass refers to the quantity of matter, while weight refers to the pull of gravity on that quantity of matter. At sea level under standard conditions, 1 pound of mass has a weight of 1 pound. To calculate the acceleration of a given mass, the gravitational constant is used as a unit of comparison. The force of gravity is 32.2 feet per second squared (ft/Sec2). This means that a free falling 1 pound object accelerates at the rate of 32.2 feet per second each second that gravity acts on it. Since the object mass weighs 1 pound, which is also the actual force imparted to it by gravity, it can be assumed that a force of 1 pound accelerates a 1-1 object at the rate of 32.2 ft/Sec2.

Also, a force of 10 pound accelerates a mass of 10 pound at the rate of 32.2 ft/Sec2. This is assuming there is no friction or other resistance to overcome. It is now apparent that the ratio of the force (in pounds) is to the mass (in pounds) as the acceleration in ft/Sec2 is to 32.2. Using M to represent the mass in pounds, the formula may be expressed thus:

In any formula involving work, the time factor must be considered. It is convenient to have all time factors in equivalent units (i.e., seconds, minutes, or hours). In calculating jet thrust, the term “pounds of air per second” is convenient, since the second is the same unit of time used for the force of gravity.