CHAPTER 6

CHAPTER 6 – STARTING AND IGNITION SYSTEM

CONTENTS PAGEStarting Sequence 02

Starter Motor 04

Ignition System 06

STARTING AND IGNITION SYSTEM – Starting SequenceProviding RotationThe first step in starting a jet engine is to get the rotor shaft rotating.

This has been achieved in a variety of ways, from small piston engines to fast burning charges directed through the turbine (known as ‘Hot-shot’ method), miniature gas turbine starters, electric motors and, most commonly an air driven starter motor.

As the latter is the favoured method for modern jet engines, it is the system described here.

The SequenceOnce the start is initiated on the flight deck, most modern aircraft will have automatic sequencing of the start cycle, some will need pilot input to complete the steps in sequence.

InitiationWhen the start sequence is initiated from a button on the flight deck, an air valve opens allowing compressed air to the air driven starter motor. The air can come from a GPU (Ground Power Unit), from an on-board APU (Alternate Power Unit), another engine already started, or, in some cases by a buddy-buddy hose from another aircraft with an engine already running. The starter motor drives the engine rotor via the accessory gearbox and ‘Power Off-take Shaft’.

Turning the rotor induces an air flow through the engine.

Ignition and FuelAt the right rpm value the ignition is switched on and the fuel is then introduced into the combustion chamber, combustion occurs soon after.

During the early stage of combustion there is little power to drive the engine, so the starter motor continues to drive the rotor. At some point the combustion process will produce enough energy to drive the engine without help from the starter motor, this is called ‘Self Sustaining Speed’ which is around 40% of maximum speed.

Just after self sustaining speed rpm is reached, the air valve will close (the starter motor is disconnected) and the ignition is switched off. The engine then continues to accelerate up to idle rpm.

The gearbox and the accessories mounted on it are initially driven by the starter motor then by the engine when it is running.

Starter motor operation and the ignition system is described in the following pages.

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T h e S t a r t C y c l e S e q u e n c eT h e S t a r t C y c l e S e q u e n c e

S t a r t e r M o t o rS t a r t e r M o t o r E x t e r n a l G e a r b o xE x t e r n a l G e a r b o x

J E T E N G I N E S T A R T I N G A N D I G N I T I O NJ E T E N G I N E S T A R T I N G A N D I G N I T I O NS t a r t e r M o t o rS t a r t e r M o t o r E x t e r n a l G e a r b o xE x t e r n a l G e a r b o x

A i r d r i v e n s t a r t e r m o t o r A i r d r i v e n s t a r t e r m o t o r r o t a t e s e n g i n e s h a f tr o t a t e s e n g i n e s h a f t

R o t a t i o n i n d u c e s a i r f l o wR o t a t i o n i n d u c e s a i r f l o w

F u e l a n d i g n i t i o n s w i t c h e d F u e l a n d i g n i t i o n s w i t c h e d o n o n -- c o m b u s t i o n s t a r t sc o m b u s t i o n s t a r t s

I g n i t i o n a n d a i r s w i t c h e d o f f I g n i t i o n a n d a i r s w i t c h e d o f f --e n g i n e a c c e l e r a t e s t o i d l e , e n g i n e a c c e l e r a t e s t o i d l e , e n g i n e d r i v e s g e a r b o x . e n g i n e d r i v e s g e a r b o x .

STARTING AND IGNITION SYSTEM – Starter MotorOperationWhen the air control valve is opened, compressed air is fed to the turbine which provides the rotational force, the air then ejecting through the casing under the engine cowling.

The turbine drives an output shaft through a reduction gear and a clutch mechanism.

The TurbineThe air passes through nozzle guides vanes before driving the turbine rotor blades. Once the engine is rotating under its own power and the air valve is closed, the turbine stops rotating.

Some starters have a tungsten cutter fitted such that, should the starter turbine bearings fail, the turbine will be pushed on to the cutter which then cuts the turbine disc at the rim, preventing the blades from providing drive; this is to prevent the turbine wheel disintegrating which could have dangerous consequences.

Reduction GearThe reduction gear reduces the turbine wheel rotational speed to an rpm value more suited to the engine rotational speed. The reduction can be around 13:1, and the starter turbine wheel could be rotating at about 80,000 rpm when the air valve is switched off.

Clutch Mechanism

This takes the drive from the reduction gear to the output shaft, but only when the starter turbine speed is greater than the output shaft speed. When the output shaft speed is greater than the starter turbine speed, the clutch automatically disengages; this occurs when the start air valve is closed.

There are different clutch mechanism designs, from simple ratchet type to a ‘Sprag’ type. The latter provides smooth grip and slide properties and eliminates what is called ‘crash engagement’. This occurs when the output shaft is rotating, such as during run down, and the air valve is opened in error. This can cause the starter turbine to rapidly increase speed, causing the ratchet to crash into engagement, which often causes disintegration of the ratchet.

The Output ShaftThis is permanently connected to the external gearbox and continues to rotate whenever the engine is rotating.

During normal operation, the output shaft is disengaged from the starter turbine by the clutch mechanism, and the starter turbine does not rotate.

GeneralThe starter motor has its own oil supply and therefore needs to be serviced on a regular basis, including regular oil changes.

There is also a Magnetic Chip Detector, to monitor the condition of the starter bearings.

RollsRolls --Royce AVON Royce AVON -- (Graphic Representation)(Graphic Representation)

Starter Starter MotorMotor

Output Shaft

Clutch Mechanism

Reduction Gear

Turbine

Air Air InIn

Air Air OutOut

Air Air OutOut

Starter Starter MotorMotor

External External GearboxGearbox

JET ENGINE STARTING AND IGNITIONJET ENGINE STARTING AND IGNITION

Drive to Drive to Engine RotorEngine Rotor

STARTING AND IGNITION SYSTEM – Ignition SystemElectrical SupplyThe ignition system is fed from the aircraft power supply, usually around 28volts (the LT – Low Tension supply) and transforms this power to High Tension (HT) pulses of about 30,000volts, which occur at approximately one per second.

The box is controlled by an input signal from the flight deck direct or via electronic control systems.

Igniter BoxesUsually sealed units with no maintainable parts inside. Failure can be indicated by pulse sensing and/or lack of turbine gas temperature during start up.

Some engines have boxes with a cooling jacket around it, the cooling air supply comes from the under cowl flow.

HE LeadsHeavy duty cable which carries the HT pulse power to the igniter plugs. Some systems have air cooled leads as per the igniter boxes.

Igniter PlugsThere are two main types of igniter plug, - the Surface Discharge type and the Air Gap type; both are similar in appearance and have similar components, the main difference being at the tip, the part immersed into the combustion chamber.

Surface DischargeThis has a ‘semi-conductor’ pellet at the tip. When the igniter power builds to a certain level, the power to tracks across the pellet from the centre electrode to the outer electrode (casing). Once this tracking has completed the circuit the igniter power is discharged through he plug which causes a spark to blast off the tip into the fuel/air mixture.

Air GapWhen the power in the igniter box reaches a certain level, it jumps from the centre electrode across the air gap to the outer electrode, causing a spark to blast into the fuel/air mixture.

GeneralIgnition is usually selected on during the most critical phases of the flight, as well as start up, these are take off and landing, and can be selected on when flying through inclement weather; this is to reduce the risk of engine flame out.

Igniters deteriate with use, so testing and use is kept to a minimum to prolong life.

Some igniters won’t actually spark if the air pressure at the tip is above certain value such as 450psi. Therefore at take-off, when pressures are higher, the igniter doesn’t spark unless the flame goes out and pressure drops, even though continuous ignition is selected on; this prolongs igniter life.

JE T E N G IN E S T A R T IN G A N D IG N IT IO NJE T E N G IN E S T A R T IN G A N D IG N IT IO N

S u rfac e S u rface D isch arg eD isch arg e A ir G apA ir G ap

C o n n ec to r C o n n ec to r T h readT h read

S p an n er S p an n er F latsF lats

Ig n ite r Ig n ite r M o u n tin g M o u n tin g

T h readT h read

Ig n ite r T ipIg n ite r T ip

In su lato rIn su la to r

C en tre C en tre E lec tro d eE lec tro d e

S em i S em i C o n d u cto r C o n d u cto r P e lle tP e lle t

C o o lin g A ir C o o lin g A ir O u tle t H o lesO u tle t H o les

C o o lin g A ir C o o lin g A ir In le t H o lesIn le t H o les

Ig n ite r B o x Ig n ite r B o x –– (N o t to S c a le )(N o t to S ca le )

H ig h H ig h T en s io n T en s io n O u tp u tO u tp u t

L o w L o w T en s io n T en s io n

In p u tIn p u t

C o n tro l C o n tro l In p u tIn p u t

E n g in e C ro ss S ec tio nE n g in e C ro s s S ec tio n

Ig n ite r Ig n ite r P lu g sP lu g s

Ig n ite r B o xesIg n ite r B o xes

H T L ead sH T L ead s

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