how jet engines produce thurst

How Jet Engines Work and Produce Thrust

Table of Contents

1.0 Introduction.........................................................................................................3

2.0 Basic Components of a Jet Engine...........................................................................4

2.2 Compressor...........................................................................................................4

2.3 Combustor............................................................................................................5

2.4 Turbine.................................................................................................................5

2.5 Exhaust Duct/Nozzle............................................................................................5

4.0 Creation of Thrust in a Turbojet Engine..................................................................7

5.0 Conclusion................................................................................................................9

6.0 References..........................................................................................................10

Jet Engine Propulsion and Thrust 2

1.0 Introduction

According to Hunecke (1997), jet engines, also known as gas turbine engines, are

the most widespread and most efficient method used for airplane propulsion currently.

The Jet engine uses basic principles and concepts of motion but applying it using a

combination of complex mechanical systems to achieve thrust. There are many types

of jet engines; however, this paper will concentrate on the Turbojet Engine to explain

the workings of the jet engine to achieve thrust and propulsion.

1.1 How the turbojet Engine Works

Turbojet Engines apply Newton’s Third Law of Motion that states, “For every

motion there is an equal and opposite reaction” (Hunecke, 1997, p. 4). Simply, when a

burnt mixture is ejected backwards from an engine, a forward force is generated on

the engine and thus on the aircraft. The bigger the backward force the bigger the

forward force (reaction force). Thrust is created when the burnt mixture pushed out

the back is ejected at higher velocity than that of the air being sucked in. (Hunecke,

1997, p. 4)

The engine’s fans suck air in at the front. A compressor, made up of fans with many

blades and attached to the shaft, elevates the pressure of the air. The compressed air is

then sprayed with fuel and an electric spark ignites the mixture. The burning gases

expand and blast out through the nozzle, at the back of the engine. As the jets of gas

shoot backward, the engine and the aircraft are thrust forward.


2.0 Basic Components of a Jet Engine

2.1 Air Intake/Inlet

The air intake acts as a fluid flow duct, which directs the airflow to ensure that

the engine functions correctly to generate thrust (Hunecke, n.d., p. 44). The intake has

to be designed to deliver the needed quantity of airflow to the engine and ensure that

airflow entering the compressor is stable and uniform. All these conditions have to be

met when the aircraft is on the ground and during flight. A good intake design is

required to ensure that engine performance is close to figures obtained during

standard testing. The Stators (stationary blades) guide the airflow of the compressed

gases.

2.2 Compressor

The function of the compressor is to increase the pressure of the airflow that

comes from the air intake. Mechanical energy is supplied to the compressor via

rotating blades that exert aerodynamic forces on the airflow (Hunecke, n.d., p. 86).

This not only adds energy to the airflow but at the same time squeezes (compresses) it

into a smaller space. The compressor is driven by the turbine. Important compressor

performance parameters include compressor efficiency, compressor total pressure

ratio and the airflow rate. These parameters influence the amount of energy required

and the quality of energy conversion that is achieved (Hunecke, n.d., p.86).

Compressor types used can be axial-flow, centrifugal-flow, axial-centrifugal-flow,

double-centrifugal-flow compressors.


2.3 Combustor

The combustor provides a stream of hot gas that provides energy to the turbine

and nozzle components of the engine. Heat is added by burning a mixture of

compressed air and vaporized fuel (Hunecke, n.d., p. 125) Minimal loss of pressure is

required in the combustion chamber during combustion. Types of combustors used

include can-type burners, annual-type burners and can-annular type burners.

2.4 Turbine

The turbine is used to drive the compressor by providing aerodynamic forces.

A high turbine power is obtained by extracting all the energy present in the hot gas.

On its own, a distinct turbine blade contributes approximately 250 hp (Hunecke, n.d.,

p. 6)

2.5 Exhaust Duct/Nozzle

The function of the exhaust nozzle is to transfer gas potential energy into

kinetic energy required to generate thrust (Hunecke, n.d., p. 155)


Figure 1: Parts of a Turbojet Engine

Hunecke, K. (1997). Jet engines: fundamentals of theory, design, and operation. WI, USA:

Motorbooks International, p.4.

3.0 The Process of Thrust and Propulsion

At the intake, air is sucked from the compressor, where there a series of blades or

airfoils. The Rotors (rotating blades), draw in the air and compress it at the same time,

while the stators (stationary blades), guide the air through the compression chamber.

As the air moves through the rows of rotors and rotors, pressure rises to as much as 40

times, consequently the temperature rises. Most modern turbojet compressors have

overall pressure ratios of 44:1. (Mattingly, 2002, p. 20)

The compressed air is then pressed toward the back into the combustion chamber,

where fuel injectors spray fuel to create a mixture. This mixture of pressurized air and

fuel is then ignited. To control burn and flow, the fuel-air mixture must be brought


almost to a stop to ensure a steady, continuous and stable flame. This occurs at the

very beginning of the combustion chamber. The aft part of this flame front is allowed

to advance toward the back. This guarantees complete combustion of the fuel as the

flame becomes hotter when it leans out, and because of the shape of the combustion

chamber the flow is accelerated rearwards. Some pressure slump is necessary, as it is

the reason why the expanding gases move out the rear of the engine rather than out

the front. Less than 25% of the air is involved in combustion, in some engines as little

as 12%, the rest acting as a reservoir to absorb the heating effects of the burning fuel.

(Hunecke, K., 1997, p.4)

The high-energy airflow coming exiting the combustion chamber goes into the

turbine, causing the turbine blades to rotate. The turbines are linked by shafts (which

have several ball-bearings in between) to turn the blades in the compressor and to spin

the intake fan at the front. This rotation reduces the energy. From the combustion

chamber, the gases produced move through the turbine and spin its blades.

After the turbine, the gases expand through the exhaust nozzle to atmospheric

pressure, generating a high velocity jet in the exhaust plume. The nozzle pressure

ratio on a turbojet is generally high enough for the expanding gases to reach Mach 1.0

and choke the throat. Usually, the flow will go supersonic in the exhaust plume

outside the engine. (Mattingly, 2002, p. 12)


4.0 Creation of Thrust in a Turbojet Engine

The creation of thrust in a jet engine is based on Newton’s third law of motion

applied to a steady flow of air (Hunecke, K., 1997, p.32)The momentum of air leaving

the engine is supposed to be higher than the momentum of the air inflowing the

engine. The outcome is a high kinetic energy for the jet. The high-energy input of the

jet comes from burning of the fuel. According to Hunecke (1997), propulsion in a jet

aircraft occurs based on the principle of reaction. This principle states that a gas jet

exhausting at elevated velocity from a nozzle generates a force in the reverse direction

that is referred to as thrust (Hunecke, 1997, p.32). The thrust force will depend on the

airflow passing through the jet engine and the exhaust velocity. The jet engine

increases the momentum of the airstream flowing through it.

4.1 Momentum change

Considering an engine on a pylon under a wing as shown below, the only

force applied is through the pylon. (See diagram 2 below)

Figure 2: A High Bypass ratio engine installed under a wing (Cumpsty, 2003, p. 25).


Assumptions made include the wing lift and drag being unaffected by the

engine and the engine being unaffected by the wing. Additionally, it is assumed that

there is uniform static pressure around the control surface (Cumpsty, 2003, p. 26). A

flow of fuel mf flows through the pylon, but with a low velocity, thus moment is

insignificant. Consequently, a mass flow of air mair enters the engine. For a bypass

engine, the velocity differences between the inner engine and the bypass streams have

to mix out. Thrust will be calculated by taking into account the flux of momentum

across the control surface around the engine. The air enters the control surface with a

velocity V. Air entering the control surface passes along the engine with only a small

portion, mair passing through the engine. According to Crumpsty (2003), the air that

passes around the engine exits the control surface with the same velocity V as the

flight speed; thus it does not contribute to thrust.

Therefore, the flux of the momentum entering the engine is given by the equation

below

In addition, the flux of momentum leaving the engine is given by the equation below

The net thrust FN is given by the difference between the two fluxes

Increase in kinetic energy causes an increase in velocity between the flow of

air entering the engine and that of the air leaving the engine (Cumpsty, 2003, p. 25).


The kinetic energy results from the effect of the work supplied by the engine to the air

via combustion.

5.0 Conclusion

This report provides an explanation of the basic components of a jet engine

and the workings of each component to produce thrust that is used for propulsion.

Airflow is directed to the compressor via the air intake. The compressor increases the

pressure of the airflow before releasing it to the combustor. The combustor increases

the energy of the airflow through burning using fuel. The turbine provides energy to

the compressor, which is used to increase pressure of the gas. The exhaust nozzle

converts the hot gas energy to kinetic energy, which generates thrust.

6.0 References

Cumpsty, N. A., 2003. Jet Propulsion: A Simple Guide to the Aerodynamics and

Thermodynamic Design and Performance of Jet Engines. Cambridge:

Cambridge University Press. p. 25-26.

Flack, R. D., 2005. Fundamentals of Jet Propulsion with Applications. New York:

Cambridge University Press.


Hunecke, K., 1997. Jet engines: fundamentals of theory, design, and operation. WI:

Motorbooks Internation. p 4-155.

Jay, S., 1972. The Jet Engine. London: Coller Nacmillan.

Mattingly, J. D., 2002. Aircraft Engine Design. New York: VA: American Institute of

Aeronautics and Astronautics. p. 12-20.

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