15 Gas Turbine Engine

15.1 Fundamentals

15.1.2 Principles and Working Cycles of Gas Turbine Engines

15.1.2.1 Introduction

During the last 40 years, the development of gas turbine engines as propulsion

systems for aircraft has been very fast. It is difficult to appreciate that before the 1950s

very few people knew about this method of aircraft propulsion. Aircraft designers had

been interested in the possibility of using a reaction turbine for a long time. But initially,

the low speeds of early aircraft and the unsuitability of a piston engine for producing the

large high-velocity airflow necessary for the ’jet’ caused many problems.

A French engineer, Rene Lorin, patented a jet propulsion engine in 1913. But this

was an athodyd and, at that time, it could not be manufactured or used since suitable

heat resisting materials had not been developed.

Note: An athodyd (or: pulse jet engine) is an open tube which is shaped to produce

thrust when fuel is ignited inside it. Fuel is added to the incoming air as the

athodyd moves through the air at high speed. This burning causes air

expansion that speeds up the air and produces thrust (Figure 1, detail a)).

Refer to Figure 1.

a) Lorin’s jet engine

Figure 1 Principle of Jet Engines

Secondly, jet propulsion would have been extremely inefficient at the low speeds

of early aircraft. However, today’s modern ram jet is very similar to Lorin’s conception.

In 1930, Frank Whittle was granted his first patent for using a gas turbine to

produce a propulsive jet (Figure 1, detail b)). But it took 11 years before his engine

completed its first flight. The Whittle engine formed the basis of the modern gas turbine

engine. The ROLLS-ROYCE, DERWENT, NENE or DART engines were derived

directly from the Whittle engine.

The DERWENT and the NENE jet engines were mainly installed in military

aircraft. The DART turboprop engine became well known as the power plant for the

VICKERS Viscount aircraft.

b) Whittle-type turbojet engine

Figure 1 Principle of Jet Engines

Although other aircraft may be fitted with later engines termed ’twin-spool’,

’triple-spool’, ’by-pass', ’ducted fan’, ’unducted fan’ or ’propfan’, they are

developments of Whittle’s early engine. Refer to Figure 2.

Turbojet engine Turboprop engine

Figure 2 Comparison of Propulsion Systems

Although the jet engine appears to be very different from a piston engine with a

propeller, it applies the same basic principle to produce propulsion. Both propel the

aircraft solely by moving a large volume of air rearwards.

Although today's jet propulsion is popularly linked with the gas turbine engine,

there are other types of jet-propelled engines, such as the ram jet, the pulse jet, the

rocket, the turbo-ram jet and the turbo-rocket.

15.1.2.2 Principles of Jet Propulsion

Jet propulsion is a practical application of Sir Isaac Newton's third law of motion

which states that

‘for every force acting on a body there is an opposite and equal reaction’.

For aircraft propulsion, the ’body' is atmospheric air that is caused to accelerate

as it passes through the engine. The force required to cause this acceleration has an

equal effect in the opposite direction, i.e. it acts on the components producing the

acceleration.

A jet engine produces thrust in a way similar to the piston engine/propeller

combination. Both propel the aircraft by moving a large volume of air backwards: one

in the form of a large air slipstream at comparatively low speed and the other in the

form of a jet of gas at very high speed.

This same principle of reaction occurs in all forms of movement and has been usefully

applied in many ways. The earliest known example of jet reaction is that of Hero's

engine (Figure 3, detail a)) produced as a toy more than 2,000 years ago. This toy

showed how the momentum of steam exiting a number of jets could impart an equal and

opposite reaction to the jets themselves, causing the engine to rotate.

Refer to Figure 3.

a) Hero’s engine (probably the earliest

form of jet reaction)

b) Rotation effect by the reaction of water

jets

Figure 3 Forms of Jet Reaction

The whirling garden sprinkler (Figure 3, detail b)) is a more practical example of

this principle: its mechanism rotates due to the reaction to the water jets. The high-

pressure jets of modern fire-fighting equipment are another example of ‘jet reaction’:

due to the reaction of the water jet, the hose cannot be held or controlled by one single

fireman. Perhaps the simplest illustration of this principle is a toy balloon which, when

the air or gas is released, rushes rapidly away in the direction opposite to the jet.

Jet reaction is definitely an internal phenomenon and does not result from the

pressure of the jet on the atmosphere. In fact, the jet propulsion engine, whether rocket,

athodyd or turbojet, is a piece of equipment designed to accelerate a stream of air or gas

and to expel it at high velocity.

There are, of course, a number of ways of doing this. But in all instances, the

resultant reaction (or: thrust) exerted on the engine is proportional to the mass or

weight of air expelled by the engine and to the velocity change imparted to it.

In other words, the same thrust can be provided

□ either by giving a large mass of air a little velocity increase

□ or by giving a small mass of air a large velocity increase.

In practice, the former is preferred, since by lowering the jet velocity relative to

the atmosphere a higher propulsive efficiency is obtained.

15.1.2.3 Types of Jet Engine according to Jet Propulsion Methods

The types of jet engine, whether ram jet, pulse jet, rocket, gas turbine, turbo-ram

jet or turbo-rocket, differ only in the way in which the 'thrust provider’ (or: engine)

supplies and converts the energy into power for flight.

The ramjet engine (Figure 4, detail a)) is an athodyd (or aero-thermodynamic

duct). It has no major rotating parts and consists of a duct with a divergent entry and a

convergent or convergent/divergent exit. Refer to Figure 4.

a) Ram jet engine

Figure 4 Basic Methods of Jet Propulsion

When forward motion is imparted to it by an external source, air is forced into

the air intake. Here, it loses velocity (or: kinetic energy) and increases its pressure (or:

potential energy) as it passes through the diverging duct. Then, the total energy is

increased by the combustion of fuel. Finally, the expanding gases are expelled to the

atmosphere through the outlet duct.

A ram jet is often used as a power plant for missiles and target vehicles. But it is

unsuitable as an aircraft power plant because it requires forward motion before any

thrust can be produced.

b) Pulse jet engine

The pulse jet engine (Figure 4, detail b)) uses the principle of intermittent

combustion. Unlike the ram jet, it can be run at a static condition. The engine is formed

by an aerodynamic duct similar to the ramjet. But, due to the higher pressures involved,

it is of more robust construction. The duct inlet has a series of inlet ‘valves’ that are

spring-loaded in the 'open' position.

Air drawn in through the open valves passes into the combustion chamber and is

heated by the burning of fuel injected into the chamber.

Figure 4 Basic Methods of Jet Propulsion

The resulting expansion causes a rise in pressure, forcing the valves to close and

the expanding gases are then ejected rearwards. A depression created by the exhausting

gases allows the valves to open again and the cycle is repeated.

The pulse jet is unsuitable as an aircraft power plant because it has the high fuel

consumption and is unable to reach the performance level of the modern gas turbine

engine.

c) Rocket engine

Although a rocket engine (Figure 4, detail c)) is a jet engine, there is one major

difference: it does not use atmospheric air as the propulsive stream. Instead, it produces

its own propelling medium by the combustion of liquid or chemically decomposed fuel

with oxygen. It is able to operate outside the earth’s atmosphere.

Figure 4 Basic Methods of Jet Propulsion

d) Gas Turbine Engine

The application of the gas turbine to jet propulsion has overcome the inherent

weakness of the rocket and the athodyd: a means of producing thrust at low speeds was

provided by the introduction of a turbine-driven compressor.

The turbojet engine draws air from the atmosphere. After compressing and

heating it (a process that occurs in all heat engines) the energy and momentum, given to

the air, forces it out of the propelling nozzle at a velocity of up to 2,000 feet per second

(approx. 610 m/s or 2,200 km/h). On its way through the engine, the air gives up some of

its energy and momentum to drive the turbine that powers the compressor.

The mechanical arrangement of the gas turbine engine is simple. It consists of only

2 main rotating parts (a compressor and a turbine) and one or a number of combustion

chambers.

Note: The mechanical arrangements of various types of gas turbine engine are

shown in Figures 5 to 7.

a) Double-entry single-stage centrifugal turbojet engine

b) Single-entry 2-stage centrifugal turboprop engine

c) Twin-spool axial flow turboprop engine

Figure 5 Arrangement of Gas Turbine Engines (I)

a) Single-spool axial flow turbojet engine

b) Twin-spoot turboshaft engine (with free-power turbine)

Figure 6 Arrangement of Gas Turbine Engines (II)

a) Twin-spool by-pass turbojet engine (low by-pass ratio)

b) Triple-spool front fan turbojet engine (high by-pass ratio)

Propfan concept

c) Contra-rotating fan concept (high by-pass ratio)

Figure 7 Arrangement of Gas Turbine Engines (III)

This simplicity, however, does not apply to all aspects of the engine: the

thermodynamic and aerodynamic aspects are quite complex. They result from:

□ the high operating temperatures of the combustion chamber and the turbine

□ the effects of varying flows across the compressor and the turbine blades

□ the design of the exhaust system through which the gases are ejected to form

the propulsive jet.

At aircraft speeds below approx. 450 knots (knots = nautical miles (nm) per

hour), the pure jet engine is less efficient than a propeller-type engine, since its

propulsive efficiency largely depends on its forward speed. The pure turbojet engine is

most suitable for high forward speeds. The propeller efficiency does, however, decrease

rapidly above 350 knots due to the disturbance of the airflow caused by the high blade-

tip speeds of the propeller.

The advantages of the turbine/propeller combination have to some extent been

offset by the introduction of the by-pass, ducted fan and propfan engines.

These engines provide larger airflows and lower jet velocities than the pure jet

engine. They give a propulsive efficiency which is comparable to that of the turboprop

engine and exceeds that of the pure jet engine.

Refer to Figure 8.

Figure 8 Comparative Efficiencies

Turboshaft Engine

A gas turbine engine that delivers power through a shaft to operate something

other than a propeller is referred to as a turboshaft engine. These are widely used in

such industrial applications as electric power generating plants and surface

transportation systems, while in aviation, turboshaft engines are used to power many

modern helicopters. Refer to Figure 9.

a) Power conversion free turbine

b) An example of a free turbine engine that has been adapted for both turboprop

and turboshaft applications

Figure 9 Free Turbine Engine

The turboshaft power take-off may be coupled to and driven directly by the

turbine that drives the compressor, but it is more likely to be driver by a turbine of its

own. Engines using a separate turbine for the power take-off are called 'free turbine

engines’ or ’free-power turbine-type turboshaft engines’.

A free turbine turboshaft engine has two major sections, the gas generator and

the free turbine section. The function of the gas generator is to produce the required

energy to drive the free turbine system and it extracts about two third of the energy

available from the combustion process leaving the other one third to drive the free-

power turbine.

The turbo-ram jet engine (Figure 10, detail a)) combines the turbojet engine

(which can be used for speeds up to Mach 3) with the ram jet engine, which shows good

performance at high Mach numbers.

The engine is surrounded by a duct that has a variable intake at the front and an

afterburner jet pipe with a variable nozzle at the rear.

During take-off and acceleration, the engine works like a conventional turbojet

with afterburner. At other flight conditions up to Mach 3, the afterburner is

inoperative. As the aircraft accelerates beyond Mach 3, the turbojet is shut down and

the intake air is diverted by guide vanes from the compressor. It is ducted straight into

the afterburning jet pipe, which now works as a ram-jet combustion chamber.

This engine is suitable for an aircraft which requires high-speed and sustained-

high-Mach-number cruise conditions.

Refer to Figure 10.

a) Turbo-ram jet engine

Turbo-rocket Engine

The turbo-rocket engine (Figure 10, detail b)) is an alternative to the turbo-ram

jet. However, there is one major difference; it carries its own oxygen to provide

combustion.

b) Turbo-rocket engine

Figure 10 Schematic Cross-section of a Turbo-ram and a Rocket Engine

The engine has a low-pressure compressor driven by a mufti-stage turbine. The

power required to drive the turbine is derived from combustion of kerosine and liquid

oxygen in a rocket-type combustion chamber. Since the gas temperature is approx.

3,500 0C, additional fuel is sprayed into the combustion chamber for cooling purposes

before the gas enters the turbine. This fuel-rich mixture (gas) is then diluted with air

from the compressor. The surplus fuel is burnt in a conventional afterburning system.

Although the engine is smaller and lighter than the turbo-ram jet, it has a higher

fuel consumption. This makes it more suitable for being used in an interceptor or space-

launcher type of aircraft that requires high speed, high altitude performance and

(normally) has a flight profile which is entirely accelerative and of short duration.