Powerplant V Module II Quiz - Ignition Systems

The basic requirements for reciprocating engine ignition systems are similar, regardless of the type of engine. All ignition systems must deliver...

A high-tension spark at a predetermined number of degrees ahead of the top dead center position of the piston,

A spark across the electrodes of each spark plug.

A high-tension spark in each cylinder of the engine in the correct firing order.

A high-tension spark.

A high-tension spark adequate to arc the gap in the spark plug electrodes under all operating conditions.

What are the two classifications for ignition systems used in modern reciprocating engine powered aircraft?

Magneto-ignition systems or electronic systems for reciprocating engines

Ignition systems can be subclassified as either single or dual magneto-ignition systems.

What is the difference between the two?

Single magnetos consist of two separate magnetos and the necessary wiring. Dual magnetos generally use one rotating magnet that feeds two complete magnetos in one magneto housing.

Which type of magneto-ignition system is still the most widely used in aircraft?

High-tension

The high-tension magneto system can be divided, for purposes of discussion, into three distinct circuits.

What are they?

Magnetic, primary electrical, and secondary electrical circuits.

The magnetic circuit consists of...

A permanent multi-pole rotating magnet, a soft iron core, and pole shoes.

What are the components of a primary electrical circuit?

A set of breaker contact points, a condenser, and an insulated coil.

The secondary magneto circuit contains... 

The secondary windings of the coil, distributor rotor, distributor cap, ignition lead, and spark plug.

What are the two things that the condenser (capacitor) does in the primary circuit?

The condenser prevents arcing at the points when the circuit is opened and hastens the collapse of the magnetic field about the primary coil.

What is Lenz's Law and how does it relate to the primary electrical circuit?

Lenz's Law states that an induced current always flows in such a direction that its magnetism opposes the motion or change that induced it.

What is the meaning of the term E-gap position? (E stands for efficiency).

The rotating magnet is in a position a few degrees past neutral, This position is called the E-gap position (E stands for efficiency).

What are the necessary conditions for firing a spark plug when it's piston is in the prescribed position?

The magneto must be in the E-gap position, the breaker points must be open, and the distributor must be aligned correctly.

What is the purpose of a breaker cam in a magneto?

To open and close the breaker points

What is a cam follower and what does it do?

A Micarta block or similar material that rides on the cam and allows the cam to force the breaker points to separate.

On what type of engine would you find a "Compensated Cam"?

A large radial engine

The cam lobes of a compensated cam are machine ground at unequal intervals to compensate for the elliptical path of the...

Articulated connecting rods.

What is the purpose of the distributor rotor in a secondary electrical circuit?

To direct high-voltage to the spark plugs

How Fast does the distributer rotate in relation to the crankshaft in all four-stroke cycle engines?

One-half crankshaft speed

What is the function of a spark plug in an ignition system?

Provides an air gap for the high-voltage spark to ignite the air-fuel charge

What are the three main components of a sparkplug?

Electrode, insulator, and outer shell

What is meant by the term," all-weather plug"?

The seal between the lead and the spark plug is water proof

What is the purpose of the insulator in a spark plug?

To provide a protective core around the electrode

Which material is used to make the insulator in a spark plug?

Aluminum oxide ceramic

What three positive attributes are provided by the aluminum oxide insulator?

Dielectric strength, high mechanical strength, and thermal conductivity

What are three advantages of using a spark plug with a fine wire electrode?

Fine wire electrodes have a longer service life, allow for a larger spark gap, and increased performance

What prevents cylinder gas pressure from escaping around the spark plug?

Close-tolerance screw threads and a copper gasket

What is the appropriate spark plug range for a low-compression engine?

Hot range plugs

What is the spark plug reach?

The length of the threaded portion

When is a spark plug considered to be fouled?

When the spark stops bridging the gap

What causes carbon fouling of spark plugs?

Air-fuel mixtures that are either too rich to burn or so lean they cause intermittent firing

The power setting most conducive to lead fouling is ______________________________.

Cruising with lean mixtures.

Why is ethylene dibromide used as a scavenging agent in aviation fuel?

To reduce lead deposits

The elimination of service difficulties caused by graphite is up to the______________________

Aircraft technician.

Why do manufacturers use a hermetically sealed resistor in the center electrode of some spark plugs?

To overcome the problem of gap erosion

If a plug is dropped on the floor or other hard surface, it should ____________________________________

not be installed in an engine.

What is the correct way to pull the ignition harness lead?

Straight out and in line with the centerline of the plug barrel

What is the consequence of applying a side load when removing the ignition harness lead?

Damage to the barrel insulator

What should be the first step in servicing spark plugs?

A visual inspection

It is good practice to soak the spark plugs in solvent to loosen carbon and lead deposits.

False

After drying the firing end of the plugs, remove the lead compound deposits using a _______

Vibrator cleaner.

After the abrasive blast, the firing end gets a _______________

thorough air blast to remove the abrasive material.

The shielding barrel insulators may be cleaned with __________________

a cotton cloth or felt swab saturated with solvent, wood alcohol, or other approved cleaner.

The firing end should be inspected using a light and a magnifying glass. If the plug passes the firing end visual and cleaning checks, then... 

the spark gap should be set using a round thickness gauge.

What should be done if the spark plug lead is heavily stained or damaged?

Replace the lead

What should be done after all spark plugs have been installed and torqued?

Perform a complete ignition system operational check

What is the purpose of a turbine engine ignition system?

To ignite the fuel in the combustor

In a turbine engine ignition system, what is the purpose of the high-voltage induced in the secondary of the triggering transformer?

To ionize the gap at the spark igniter

What is the role of the high-frequency triggering transformer in a turbine engine ignition system?

To hold the time duration of the discharge to a minimum

What is the function of transformers in a Capacitor Discharge Exciter Unit?

To step up the voltage across the capacitors

Why is air cooling important in a Capacitor Discharge Exciter Unit?

To prevent overheating during long use of continuous ignition

What is the difference between continuous and intermittent ignition in a turbine engine?

Continuous ignition is required for starting the engine, while intermittent ignition is used during operation

What is the voltage across the capacitors in a Capacitor Discharge Exciter Unit?

High-voltage, high energy

How does the electrode of an igniter plug differ from that of a spark plug?

It is capable of withstanding a higher energy current.

What is the main purpose of an igniter plug in a turbine engine ignition system?

To produce a high-intensity spark.

What role does heat play in minimizing electrode fouling in igniter plugs?

It helps to keep the electrode clean.

What is electrode erosion and how does it impact igniter maintenance?

It is the wearing down of the electrode due to high energy currents.

What is the recommended method for checking the operation of an igniter?

Listening for a snapping noise

What should be done before removing ignition leads from the engine?

Discharge any electrical charge

What is the proper clearance between chamber liner and igniter plug?

1/8 inch

Why is it important to disconnect the low-voltage primary lead before disconnecting the high-voltage cable?

To allow for proper dissipation of stored energy