HOW A JET ENGINE WORKS · Basic Concept 15/02/2019 Prepared by Guy E Davies CEng FIMechE 3 There...

HOW A JET ENGINE WORKS

15/02/2019 Prepared by Guy E Davies CEng FIMechE 1

Photo © Guy E Davies

How a Jet Engine Works

• Basic Concept

• Compressor

• Combustion Chamber

• Turbine

• Jet Pipe

• Fan

• Operating Conditions

• Fuel Control


Basic Concept


There are two questions to answer about how a jet engine works

1) How does it work internally?2) How does it generate thrust?

Before we go into details, let’s just look at the basic concept:


Basic Concept

The basic jet engine consists of four sections:

Air enters the intake and is compressed by a rapidly rotating compressor.

AirCompressor

Combustion

ChamberTurbine

Exhaust(Jet Pipe)

AirEnters

JetExhaust

Fuel


Basic Concept


Air enters the intake and is compressed by a rapidly rotating compressor.The highly compressed air enters the combustion chamber where fuel is injected and burned.

AirCompressor

Combustion

ChamberTurbine

Exhaust(Jet Pipe)

AirEnters

JetExhaust

Fuel


Basic Concept


Air enters the intake and is compressed by a rapidly rotating compressor.The highly compressed air enters the combustion chamber where fuel is injected and burned.The very hot, high pressure exhaust drives a turbine which is connected by a shaft to turn the compressor.

AirCompressor

Combustion

ChamberTurbine

Exhaust(Jet Pipe)

AirEnters

JetExhaust

Fuel


Basic Concept


Air enters the intake and is compressed by a rapidly rotating compressor.The highly compressed air enters the combustion chamber where fuel is injected and burned.The very hot, high pressure exhaust drives a turbine which is connected by a shaft to turn the compressor.The exhaust from the turbine accelerates down the jet pipe to exit at high velocity.

AirCompressor

Combustion

ChamberTurbine

Exhaust(Jet Pipe)

AirEnters

JetExhaust

Fuel


Basic Concept


Air enters the intake and is compressed by a rapidly rotating compressor.The highly compressed air enters the combustion chamber where fuel is injected and burned.The very hot, high pressure exhaust drives a turbine which is connected by a shaft to turn the compressor.The exhaust from the turbine accelerates down the jet pipe to exit at high velocity.Thrust is produced because the intake air has been accelerated and blown out of the exhaust at much higher velocity. (Force = Mass x Acceleration).

AirCompressor

Combustion

ChamberTurbine

Exhaust(Jet Pipe)

AirEnters

JetExhaust

Fuel


Basic Concept

The basic jet engine is shown as a cut-away diagram below


Basic Concept

Image by Sanjay Acharya - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4156813

J85-GE-17A turbojet engine from General Electric (1970)

Compressor


Rotor Blades Stator Blades

The compressor consists of multiple stages of rotating blades (rotor blades), each stage compressing the intake air a small amount. The rotating stages swirl the air so a ring of fixed blades (stator blades) is placed between each stage of rotating blades, and at the exit of the final stage to straighten the airflow.


Compressor

We measure the performance of a compressor in terms of pressure ratio rather than pressure rise.This is because when fitted to aircraft, the operating regime ranges from sea level to high altitude where the ambient pressure is much less.For example, at sea level the pressure is 1.013 Bar (14.69 psi) whereas at 40,000 feet altitude the pressure is only 0.188 Bar (2.72 psi).Note that these are absolute pressures.

We also correct the airflow rate for altitude and temperature effects by multiplying by the square root of the absolute temperature, and dividing by the intake pressure. This is the Normalised Mass Flow Shaft speed is also corrected by dividing by the square root of the absolute temperature to give the Normalised Speed

We measure the performance of a compressor in terms of pressure ratio rather than pressure rise.This is because when fitted to aircraft, the operating regime ranges from sea level to high altitude where the ambient pressure is much less.For example, at sea level the pressure is 1.013 Bar (14.69 psi) whereas at 40,000 feet altitude the pressure is only 0.188 Bar (2.72 psi).Note that these are absolute pressures.


Compressor


Compressor

The graph shows the typical operating plot of a compressor.

Increasing speed increases the mass flow as one would expect.


Compressor



Restricting the flow at constant speed causes the pressure ratio to increase and the mass flow to reduce, until flow instability occurs leading to a phenomenon known as ‘surge’.


Compressor



Restricting the flow at constant speed causes the pressure ratio to increase and the mass flow to reduce, until flow instability occurs leading to a phenomenon known as ‘surge’.

Note that this represents a basic compressor operating at modest pressure ratios. The normal operating line follows the line of maximum efficiency.


Compressor

A pressure ratio of say 13:1 results in a volume ratio of approximately 0.16:1, meaning that the exit flow needs passageways with a cross section of only 16% of the inlet area, as illustrated in the diagram. (There is also a temperature increase in the ratio of 2.1:1 in absolute terms).

However, this applies to full power conditions.


Compressor



When the engine is throttled back to low power, the compressor is turning more slowly and the pressure ratio is much reduced, falling to around 3.5:1 at idling power.The resulting volume ratio changes to 0.41:1 which means that the compressor exit flow now requires a cross sectional area of 41% of the inlet area, much larger than for the high power condition.


Compressor



When the engine is throttled back to low power, the compressor is turning more slowly and the pressure ratio is much reduced, falling to around 3.5:1 at idling power.The resulting volume ratio changes to 0.41:1 which means that the compressor exit flow now requires a cross sectional area of 41% of the inlet area, much larger than for the high power condition.

The inlet is now taking in more air than can be accepted by the exit area and ultimately flow breakdown can occur, causing surge. This results in unstable operation of the engine and in extreme cases can cause damage.


Compressor

The graph shows the operating plot of a high performance fixed geometry compressor (iea compressor working over a high pressure ratio range).

Note that the steady running line crosses the surge boundary as the speed is reduced.

17/02/2019 Prepared by Guy E Davies CEng FIMechEPrepared by Guy E Davies CEng FIMechE 21Prepared by Guy E Davies CEng FIMechEPrepared by Guy E Davies CEng FIMechEPrepared by Guy E Davies CEng FIMechE

Compressor

The simplest way to control this problem is simply to dump the excess airflow through valves known as Bleed Valves. These discharge compressor air overboard from the compressor exit or even from part way along.However, this is inefficient as it wastes the power already expended on compressing the air, so is normally only used at very low power.

Bleed Valve ClosedHigh Power

Bleed Valve OpenLow Power Bleed Air


Compressor

A more efficient, but more complex way to deal with low power conditions is to use variable geometry to reconfigure the compressor, and thus alter the flow.A ring of pivoted stator vanes is placed at the inlet to the compressor and these can be turned to swirl the air as it enters the inlet. At low power the vanes are turned to swirl the air in the direction of the compressor rotation, reducing the load on the first few rotor stages, and also effectively reducing the inlet area. These are called Variable Inlet Guide Vanes (VIGV)

Bleed Valve

VIGV

VIGV Actuating Ring

VIGV Actuator


Compressor

Each guide vane is pivoted in the compressor casing and connected by mechanical linkage to an actuating ring which can be rotated to turn the vanes. A hydraulic or electric actuator moves the actuating ring as part of the engine control system. Typically the vanes will be set at about 0° swirl for high power and progressively turned to give about 40° swirl at low power.

Bleed Valve

VIGV

VIGV Actuating Ring

VIGV Actuator


Compressor

Each guide vane is pivoted in the compressor casing and connected by mechanical linkage to an actuating ring which can be rotated to turn the vanes. A hydraulic or electric actuator moves the actuating ring as part of the engine control system. Typically the vanes will be set at about 0° swirl for high power and progressively turned to give about 40° swirl at low power.On some high performance compressors it is common to vary the swirl on several stages of stator vanes as well as the inlet guide vanes.

Bleed Valve

VIGV

VIGV Actuating Ring

VIGV Actuator Variable Stator Vanes


Compressor

Each guide vane is pivoted in the compressor casing and connected by mechanical linkage to an actuating ring which can be rotated to turn the vanes. A hydraulic or electric actuator moves the actuating ring as part of the engine control system. Typically the vanes will be set at about 0° swirl for high power and progressively turned to give about 40° swirl at low power.On some high performance compressors it is common to vary the swirl on several stages of stator vanes as well as the inlet guide vanes.

Bleed Valve

VIGV

VIGV Actuating Ring

VIGV Actuator Variable Stator Vanes

Typically, bleed valves may be used for very low power, with VIGV and possibly variable stator vanes being used for intermediate power.


Compressor


Compressor


Compressor

The graph shows the operating plot of a high performance compressor with variable inlet guide vanes and bleed valves.

Note that the variable inlet guide vanes raise the surge boundary at medium power, but bleed valves are still needed to operate at low power.

Combustion Chamber


Fuel is pumped at high pressure into the combustion chamber where it is atomised and mixed with the air from the compressor. It is ignited by spark igniters during engine start and then the flame is self sustaining.

Compressor Air


Combustion Chamber

Fuel is pumped at high pressure into the combustion chamber where it is atomised and mixed with the air from the compressor. It is ignited by spark igniters during engine start and then the flame is self sustaining.

The combustion chamber is designed to mix the optimum amount of air with the fuel for stable and efficient burn, with excess air then mixing with the hot flame downstream of the combustion zone.


Combustion Chamber

In early engines combustion took place in a ring of cylindrical chambers arranged round the engine, but modern engines use a single annular combustion chamber with multiple fuel injectors.

Early Multiple Can Burner

Modern Annular Combustion Chamber

Turbine


The turbine is used to drive the compressor and is connected to it by a shaft through the centre of the engine.

The expanding flow through the turbine is much more stable than the flow through the compressor, so more work can be done at each stage. This means that fewer stages are needed. Note that stator blades are still required between turbine stages.

However, the turbine gas temperature is very high and special measures must be taken to protect the blades.


TurbineIn modern engines the temperature of the gas entering the turbine from the combustion chamber (turbine inlet temperature or TET) can be 1500°C which is almost white hot and creates serious mechanical design problems for the first stage of the turbine. Indeed, many high temperature materials will melt at such temperatures.

The solution is to manufacture the turbine blades with integral cooling passageways and to pass compressor air through them. This also applies to the stator vanes.


Turbine

Another method of cooling is film cooling where compressor air is bled through small holes in the blade to pass a thin film of cool air over the blade surface.

A variation of this is transpiration cooling where the cooling air flows through a porous blade surface.

A summary of the development of blade cooling is shown below.

Jet Pipe


The function of the jet pipe is to accelerate the turbine exhaust gas to a high exit velocity to generate thrust, whilst not imposing a back pressure on the turbine, which would reduce efficiency.


Jet Pipe


By reducing the cross sectional area of the jet pipe from the turbine exhaust to the exit nozzle, the flow is accelerated to a high velocity.


Jet Pipe



Thrust is generated according to the simple law:Force = Mass x Accelerationwhich for a flowing gas stream can be re-written as:Force = Mass Flow x Change in Velocity


Jet Pipe




The mass flow is the mass of gas (mainly air) flowing per second. The change in velocity is the difference between the velocity at the engine intake and the velocity at the exhaust.


Jet Pipe




The mass flow is the mass of gas (mainly air) flowing per second. The change in velocity is the difference between the velocity at the engine intake and the velocity at the exhaust.

The maximum exit velocity occurs at full power and is usually at sonic speed.

Fan


The simple jet engine produces a relatively small jet of very high velocity exhaust gas (mainly air). For commercial aircraft, working at fairly high subsonic Mach number (0.75 – 0.85) this is not the most efficient approach. For these conditions a larger jet having a lower velocity would be more efficient.


Fan


At the other extreme, a propeller produces a very large jet (ie slipstream) with only a relatively small increase in velocity. Whilst this is highly efficient at low forward speed, it becomes much less efficient at moderately high subsonic Mach number because the blade tips start to exceed the speed of sound and serious energy losses occur.



At the other extreme, a propeller produces a very large jet (ie slipstream) with only a relatively small increase in velocity. Whilst this is highly efficient at low forward speed, it becomes much less efficient at moderately high subsonic Mach number because the blade tips start to exceed the speed of sound and serious energy losses occur.

An in-between solution is to use a large fan driven by the engine, acting like a small propeller inside a cowling. This produces a large exhaust jet with a velocity less than that from a simple jet engine, but higher than could be obtained from a propeller.

Fan


Fan

[Hi Bypass = Fan]


A typical fan engine schematic is shown below. In practice, to match the power

Fan

requirements of the fan and the compressor to the turbine, it is better to use separate turbines to drive the two. Note that the fan may also incorporate a low pressure compressor as well.


Fan

Further development of the concept led Rolls Royce to design and build their very successful three-shaft engines. In these there are two separate compressors and a large diameter fan, each driven by its own separate turbine with all three shafts being concentric.

HP Compressor & Turbine

IP Compressor & Turbine

LP Turbine

LP Compressor (Fan)

Rolls Royce RB211-524 Three Shaft Engine


Fan

TechnologyThe three-shaft architecture allows an engine to have fewer stages, giving a shorter, stiffer structure that allows the rotors to run at their optimum speeds.The innovative wide-chord fan blades allow a reduction in the number of total fan blades - increasing efficiency, reducing noise and giving unrivalled protection against foreign object damage.Through the Rolls-Royce concept of family designs, the RB211-524G/H-T was upgraded with 04 module HP (high pressure) technology developed on the Trent 700 engine family in 1997.In the RB211-524G/H-T, the HP turbine system is designed to operate at Trent 700 temperatures that are significantly higher than the original RB211-524.The Trent style combustor reduces NOx emissions by 40 per cent, making the RB211-524G/H-T the engine with the lowest NOx emissions on the Boeing 747-400.

The following description is from the Rolls Royce website:

The engine also utilises Variable Inlet Guide Vanes and Compressor Bleed Valves for good control at low and intermediate power settings. (The Trent is a later, more powerful engine of similar design).


Engine Operating Conditions

Fuel Control


When the engine is running at low or intermediate power and the throttle is opened further to accelerate to a higher power, the extra fuel being burned causes a rise in combustion temperature which in turn causes a transient increase in pressure. This back pressure at the compressor outlet tries to restrict the compressor outlet flow and if taken to extreme will cause flow instability in the compressor leading to complete flow breakdown known as ‘surge’.


Fuel Control


Consequently, it is necessary to limit the amount of extra fuel that can be added while the engine accelerates. As it does so, more and more fuel can then be added to bring the engine to full power.


Fuel Control



A fuel control unit solves those problems by acting as an intermediary between the pilot’s controls and the fuel valve. The pilot has a power lever which only controls the potential of the engine, not the actual fuel flow. The fuel control unit acts as a computer to determine the amount of fuel needed to deliver the power requested by the pilot.


Fuel Control



A fuel control unit solves those problems by acting as an intermediary between the pilot’s controls and the fuel valve. The pilot has a power lever which only controls the potential of the engine, not the actual fuel flow. The fuel control unit acts as a computer to determine the amount of fuel needed to deliver the power requested by the pilot.

A similar degree of control is also provided when throttling back to prevent flame-out occurring.


Fuel Control

The fuel control unit regulates the fuel flow to avoid surge during engine acceleration, and weak extinction during deceleration.


Fuel Control


Thus if the throttle is slammed fully open from low power, the fuel flow is limited by the control system to just below the surge limit, and is increased progressively as the engine accelerates. When the engine speed reaches required value, the fuel flow is reduced to the steady running value.


Fuel Control


Thus if the throttle is slammed fully open from low power, the fuel flow is limited by the control system to just below the surge limit, and is increased progressively as the engine accelerates. When the engine speed reaches required value, the fuel flow is reduced to the steady running value.

A similar degree of control prevents flame extinction when the throttle is closed.


Fuel Control

This diagram shows a schematic of the fuel flow control fitted to the Pratt & Whitney F100 engine used on the US Airforce F-15 and F-16 combat aircraft. The system uses digital control to compute the correct amount of fuel for any operating condition.

{Note that it also includes the fuel control for After-Burning ieReheat (A/B Fuel Flow) which is used only on combat aircraft to boost thrust}.

The gas turbine part of the fuel control is similar to that used in commercial fan engines.



There is, of course, a lot more to the jet engine.

Major systems and components include:Variable Inlet Guide Vane ControlBleed ValvesFuel Control SystemFuel PumpsLubrication SystemElectrical GeneratorStarterCabin Air BleedReheat (After-Burning) on military engines



What of the future?

The hunt for better economyIncreased pressure ratio – higher temperature – increased NOx

Efforts to reduce emissions (esp Nox)Combustion chamber designCooler burning

Search for new higher temperature materials

New manufacturing methods3D printing of components

Increased electronic controlNew sensorsControl closer to critical limits



Let’s finish with a few thoughts.First a quote from the National Academy of Sciences ca 1940




And now a couple of pictures

Whittle prototype 1939. Thrust 850 lb




And now a couple of picturesRolls Royce Trent 1000 - 2006. Thrust 74,100 lb

Whittle prototype 1939. Thrust 850 lb

Date post:	21-Mar-2020
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HOW A JET ENGINE WORKS · Basic Concept 15/02/2019 Prepared by Guy E Davies CEng FIMechE 3 There...

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