Ignition and Starting Systems

Question 1

What is the main advantage of a magneto ignition system over a battery ignition system for an aircraft reciprocating engine?

Answer 1

A magneto has its own source of electrical energy, and it is not dependent upon the battery

Question 2

What is the function of the capacitor in an magneto?

Answer 2

• Minimizes arcing at breaker points

- Speeds up collapse of the primary current as the breaker points open

Question 3

What is compensated magneto cam, and on what kind of engine is one used?

Answer 3

Compensated Cam is a special cam used in magnetos mounted on high performance radial engines.
Cam has one lobe for each cylinder, and the lobes are ground in such a way that the breaker points open when the pistons in the different cylinders are the same linear distance from TDC

Question 4

What is the significance of the numbers on the distributor of an aircraft magneto?

Answer 4

These numbers are the sparking order of the magneto, not the firing order of the engine

Question 5

What happens in a magneto ignition system when the ignition switch is placed in the off position?

Answer 5

The primary circuit is connected to ground

Question 6

What is the reason for having a low tension ignition system on some aircraft?

Answer 6

Low tension magnetos are used on aircraft that fly at high altitudes where there is a problem with flash over in the high tension magneto distributor

Question 7

What is the basic difference between a low tension magneto and a high tension magneto?

Answer 7

High tension produces high voltage in the magneto coil, sent to correct spark plug by built in high voltage distributor.
Low tension - one coil in mag, uses carbon brush type distributor. Low voltage distributed by high tension transformers located on heads of each cylinder

Question 8

What is meant by an All-Weather spark plug?

Answer 8

A shielded spark plug that has a recess in the shielding in which a resilient grommet on the ignition lead forms a watertight seal

Question 9

What is meant by the reach of a spark plug?

Answer 9

The length of the threads on the spark plug that screw into the cylinder head

Question 10

What is the difference between a hot spark plug and a cold spark plug?

Answer 10

A hot spark plug has long path for heat to travel between nose core insulator and the spark plug shell.
A cold spark plug, the heat has a shorter distance to travel, and spark plug operates operates cooler than a hot spark plug

Question 11

What is checked when a magneto is internally timed?

Answer 11

Internally timing a magneto consists of adjusting the breaker points so they will open at the instant the rotating magnet is in its E Gap position, and the distributor rotor is in the position to direct the high voltage to cylinder number one

Question 12

What is the purpose of a vernier coupling used on some aircraft magneto drives?

Answer 12

Magnetos that are base mounted must have a vernier coupling between the magneto drive and the engine. The vernier coupling allows the magneto to engine timing to be varied in increments of less than one degree

Question 13

What is the advantage of the fine wire spark plugs over massive electrode spark plugs?

Answer 13

Fine wire spark plugs have a firing end that is more open than that of a massive electrode spark plug. The open firing end allows the gases that contain lead to be purged from the spark plug so they will not form solid lead contaminates

Question 14

Why is it important that the spark plugs be kept in numbered holes in a tray when they are removed from an engine?

Answer 14

Spark plugs tell a good deal about the internal condition of the cylinders from which they were taken. By knowing the cylinder from which each spark plug came, the mechanic can take the proper action when a spark plug indicates such condition as detonation or overheating

Question 15

What is the purpose of staggered timing between the two magnetos on an aircraft engine?

Answer 15

Engines in which the exhaust gas scavenging from the cylinders is uneven use staggered ignition timing. The spark plug nearest the exhaust valve, where the fuel-air mixture is diluted, fires before the spark plug on the intake side. By using staggered timing, the flame front caused by the two spark plugs will meet in the center of the piston.

Question 16

In what position should the ignition switch be placed when using a timing light on the magnetos?

Answer 16

In the BOTH position

Question 17

Why is it important that a torque wrench always be used when installing spark plugs in an aircraft engine?

Answer 17

If the spark plugs are not put in tight enough, there is the possibility of a poor seal, and if they are put in too tight, there is danger of cracking the insulation

Question 18

Why are pressurized magnetos used on most reciprocating engine powered aircraft that flu at high altitude?

Answer 18

Pressurized air is a better insulator than less dense air. By pressurizing the distributors, the high voltage is kept from arcing across the wrong electrode and causing vibration and loss of engine power

Question 19

What type of ignition system is used on most turbine engines?

Answer 19

High intensity, intermittent duty, capacitor discharge ignition systems

Question 20

What is meant by E Gap in magneto timing?

Answer 20

The E gap angle is the position of the rotating magnet when the primary current flowing in the magneto coil is greatest. The breaker points open when the rotating magnet is in its E gap position.

Question 21

What is the function of an impulse coupling?

Answer 21

An impulse coupling is a spring driven coupling between the magneto and the engine. When the engine is being started, the impulse coupling holds the rotating magnet until the piston passes over its top center position and starts down. The impulse coupling releases the magnet and the spring spins it fast so that it produces a hot and late spark.

Question 22

What kind of gauge should be used to measure the electrode gap in aircraft spark plugs?

Answer 22

A round wire gauge

Question 23

How many igniters are used in most turbine engines?

Answer 23

Two

Question 24

What are two types of ignition systems used in turbine engines?

Answer 24

High voltage systems and low voltage systems

Question 25

With which type of ignition system is a glow plug igniter used?

Answer 25

A low voltage system

Question 26

How is the strength of the magnet in a magneto checked?

Answer 26

The magneto is put on a test stand and rotated at a specified speed. The breaker points are held open and the primary circuit is measured. The strength of the magnet determines the amount of primary current

Question 27

In what position is the magnet in a magneto when the greatest change in flux density in the coil core takes place?

Answer 27

It is a few degrees beyond its neutral position. When it is in this position, the breaker points open and the primary current is interrupted. The flux change in the coil core is the greatest.

Question 28

In what position is the magnet in a magneto when the breaker points begin to open?

Answer 28

In its E Gap position, just a few degrees beyond its neutral position.

Question 29

What malfunction in the ignition system would cause an aircraft reciprocating engine to continue to run after the ignition switch is placed in the OFF position?

Answer 29

The ignition switch is not grounding the magneto primary circuit.

Question 30

What turns on the autoignition system in a turbo-prop engine?

Answer 30

A torque pressure switch energizes the system when the engine stops producing torque