ppl_06_e2

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 6

IGNITION SYSTEMS

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Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 6 : IG NIT IO N SY ST EMS

84

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER

IGNITION SYSTEMS.

All aircraft piston engines are fitted with dual-ignition; that is, they have two electrically independent ignition systems. Each cylinder, therefore, has two spark plugs, and each spark plug is fed by a separate magneto, as shown in Figure 6.1.

Figure 6.1 The Rear of an Engine showing the Two Magnetos.

Dual-ignition systems serve two purposes. Firstly they reduce the risk of engine failure due to faulty ignition. Secondly, by igniting the mixture within the cylinder at two points, the combustion time is reduced, thus reducing the chance of detonation.

(Detonation is a phenomenon which is covered in the Chapter 7, ‘Carburation’.)

6 : IG NIT IO N SY S

Spark plugs

receive their high tension

supply

from a magneto which is independent of the aircraft electrical system.

In an aircraft

engine ignition system,

the high

tension supply to the spark plugs comes from the magneto’s primary and secondary self generation and distribution system.

MAGNETOS.

Magnetos are self-contained, engine driven electrical generators which produce high voltage sparks. Magnetos are completely independent of the aircraft electrical system.

Low voltage pulses are generated in the primary coil of the magneto (see Figure 6.2) and these are transformed into high tension pulses by a secondary coil. Within the magneto, a rotary switch called a distributor directs the sparks to the plugs in the correct firing sequence.

The magneto combines two basic principles: that of the permanent magnet generator, and that of the step-up transformer.

Magnetos are self-contained, engine driven,

electrical

generators which produce high voltage sparks.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 6 : IG NIT IO N SY ST EMS

The contact breaker in the

primary circuit of a magneto

assists in the collapse of the magnetic field created by the permanent magnet generator.

The capacitor in the primary

circuit of a magneto

assists in collapsing the magnetic field, helps prevent erosion of the contact breaker points and prevents sparking at the contact breaker points.

The distributor arm rotates at half engine speed.

When a magneto

is selected OFF, the

ignition switch is closed and the circuit is earthed.

P r i n c i p l e s o f t h e Ma g n e t o .

Figure 6.2 illustrates the first basic principle of the magneto: that of the permanent magnet generator. The only modification to the standard permanent magnet generator is the addition of a contact breaker which collapses the magnetic field in the primary circuit more quickly than would normally happen in a generator of this type.

Figure 6.2 A Diagram of a Simplified Magneto Circuit.

The second basic principle of the magneto, that of the step-up transformer, is utilised in the magneto’s secondary circuit. The secondary coil is a step-up transformer which consists of thousands of turns of very thin wire. A very high voltage is induced in the secondary windings by the rapid collapse of the magnetic lines of force in the primary circuit.

T h e Ca p a c i t o r .

The faster the magnetic lines of force collapse the greater is the voltage induced in the secondary windings.

To ensure that the primary field collapses as fast as possible, and also to prevent the contact breaker points being eroded by the sparks which would occur when the points opened, a capacitor is placed in parallel with the points. This can be seen in the diagram shown in Figure 6.2.

T h e Di s t r i b u t o r .

Once the high voltage sparks have been produced, they are passed to the spark plugs in the correct firing sequence. This is achieved by the distributor, illustrated in Figure 6.2. The distributor rotor arm rotates at half the engine speed.

Ig n i t i o n Co n t r o l .

To give the pilot control of the magneto, an ignition switch is fitted in the magneto’s primary circuit. When the ignition switch is placed to the OFF position, both ends of the primary coil are earthed, which makes it impossible for current to be generated. In Figure 6.2, the ignition switch is shown in the ON position.

86

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 6 : IG NIT IO N SY S

With the ignition switch in the OFF position, the primary circuit is earthed. The secondary circuit is not now cut by the magnetic lines of force which would otherwise enable it to generate the sparks. Therefore, the ignition system is dead.

If the magneto becomes electrically disconnected from the ignition circuit, a potentially lethal situation arises. The magneto is now permanently live, and the engine will continue running even if both magnetos are selected OFF.

After engine start, the magnetos’ operation must be checked in several ways, one of which is the Dead Cut check. This, essentially, is a check to ascertain whether or not the pilot has full control of the magnetos. The check requires that each of the magnetos be selected OFF in turn, to check that there is a drop in engine RPM, but that the engine does not stop. If, during this check, it appears that the engine may stop, the pilot must allow it to do so. To do otherwise may cause the engine to suffer damage.

St a r t i n g t h e En g i n e - Sp a r k Au g m e n t a

During start-up, the engine is cranked at about 120 rpm. At this speed, the magneto is not capable of producing sparks with sufficient energy to ignite the mixture. This makes it necessary to employ various methods of spark augmentation. There are three methods of spark augmentation. They are the high tension booster coil, the low tension booster coil and, the most common in small aircraft engines, the impulse coupling.

The high tension booster coil supplies a stream of high voltage impulses to a trailing brush on the distributor rotor. This ensures that a fat retarded spark is generated in the cylinder. The retardation of the spark is necessary only during engine start, because of the low rotational speed. Once the engine has started, the magnetos will produce sparks of sufficient strength, which allow the high tension booster coil to be switched OFF. As a consequence of this, the high tension booster Coil is switched ON only for engine start.

The low tension booster coil supplies the primary coil of the magneto with a low voltage during the engine start. Once again, as soon as the engine has started, the low tension booster coil is switched OFF.

A magneto

that is disconnected

from its

ignition switch will cause the engine to continue

running when both magnetos are selected ‘OFF’.

If, while

carrying out the Dead

Cut Check,

you suspect that the engine is going to stop running, you should allow itttoistop completelyn . .

Because

the speed of rotation of

the engine

during starting is too low for the magneto to produce sufficient energy to ignite the air-fuel mixture, aircraft

ignition systems use a means of spark augmentation.

Im p u l s e Co u p l i n g

The impulse coupling, a diagram of which is shown in Figure 6.3, is a mechanical device which uses a spring to temporarily increase the speed of rotation of the magneto. This gives a fat retarded spark during the starting cycle.

The diagram shows a cutaway view of the impulse coupling within the magneto.

The cam is driven anticlockwise by gears from the crankshaft at half the crankshaft speed, and drives the hub through the spring.

Figure 6.3. The Impulse Coupling

Mechanism.

An impulse

coupling is a mechanical

device which

uses a spring to temporarily increase the speed of rotation of the magneto.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 6 : IG NIT IO N SY ST EMS

The permanent magnet of the magneto is attached to the hub, which also carries the pawls.

Initially the cam rotates, turning the spring which carries the hub round until the pawls come into contact with the stops. The cam continues to rotate, tensioning the spring, until the leading edges of the cams contact the pawls, pushing them out of contact with the stops. The hub, under the impulse of the spring, rotates very quickly, carrying its attached magnet.

Rotating the magnet this quickly enables it to generate the high voltage spark required to ignite the mixture. The spark is also retarded, which ensures that the engine is driven in the correct direction. Once the engine has started, the pawls are rotating at such a speed that centrifugal force holds them out at an angle which prevents them contacting the stops. Thus the spring has no further effect.

88

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 6 : IG NIT IO N SY ST EMS Q U

R e p r e s e n t a t i v e P P L - t y p e q u e s t i o n s t o t e k n o w l e d g e o f Ig n i t i o n Sy s t e m s .

1.The high tension supply connected to the spark plugs comes from:

a.The magneto’s primary and secondary self-generation and distribution system

b.The battery during start up and low idle

c.The battery, and is transformed by the magneto

d.The battery and the high tension coil

2.When a magneto is selected OFF, the switch located in the primary circuit:

a.Is closed and the circuit is earthed

b.Is opened, breaking circuit continuity

c.Is opened and the circuit is earthed

d.Is closed, earthing the plug leads

3.If a magneto becomes inadvertently disconnected from its ignition switch, it will:

a.Stop the engine if the other magneto is “OFF”

b.Cause a high mag drop indication

c.Cause the engine to continue running when both magnetos are selected to “OFF”

d.Cause the failure of one plug in each cylinder

4.The high tension supply to the spark plugs is supplied from:

a.The alternator

b.The magneto and is independent of the aircraft electrical system

c.The magneto, supplied from the aircraft electrical system

d.The battery on engine start-up and low idle, and the magneto once the engine is running at fast idle

5.Magnetos are:

a.Generators, driven by the cam-shaft, used to supply electrical equipment

b.Used to generate low voltage sparks for the spark plugs

c.Fitted within the distributor, and fire in the same sequence as the spark plugs

d.Self-contained, engine-driven, electrical generators which produce high voltage sparks

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 6 : IG NIT IO N SY ST EMS Q U EST IO NS

6.The function of the contact breaker in the primary circuit of a magneto is to:

a.Allow the magneto to be switched on and off

b.Time the sparks to the spark plugs

c.Assist in the collapse of the magnetic field created by the permanent magnet generator

d.Prevent the build up of excess static charge

7.Of the following, which are functions of the capacitor in the primary circuit of

amagneto?

(i)Assists in collapsing the magnetic field

(ii)Smooths the output of the magnetic field

(iii)Helps prevent erosion of the contact breaker points

(iv)Prevents sparking at the contact breaker points

a.(i) and (ii) only.

b.(iii) and (iv) only.

c.(i) and (iv) only

d.(i), (iii) and (iv).

8.The distributor arm rotates at:

a.One quarter engine speed

b.A half engine speed

c.Engine speed

d.Twice engine speed

9.Of the following statements, which relates factually to an Impulse Coupling?

a.It supplies a stream of high voltage impulses to a trailing brush on the distributor rotor

b.It supplies the primary coil of the magneto with a low voltage during engine start

c.It is a mechanical device which uses a spring to temporarily increase the speed of rotation of the magneto

d.All of the above

10.Aircraft engine ignition systems incorporate a means of spark augmentation:

a.Because, at high engine speeds, a fat spark is needed to extract maximum power from the air-fuel mixture

b.Because the speed of rotation of the engine, during starting, is too low for the magneto to produce sufficient energy to ignite the air-fuel mixture

c.In order to overcome the problem of spark-retard during starting

d.All of the above

90

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CH AP T ER 6 : IG NIT IO N SY ST EMS Q U

11.While carrying out the Dead Cut Check, with the right magneto selected, you notice that the engine falters, and you suspect it will stop running. You should:

a.Allow the engine to stop completely

b.Quickly switch to the left magneto

c.Quickly switch to both magnetos.

d.Open the throttle to keep the engine running, then select both magnetos

Question	1	2	3	4	5	6	7	8	9	10	11
Answer

T h e a n s w e r s t o t h e s e q u e s t i o n s c a n b e f o u n d a t

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Customer: Oleg Ostapenko E-mail: [e-mail]

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