Ignition system components piston engines

Question 1

What are the two main classifications of ignition systems for reciprocating engines?

Answer 1

Magneto-ignition systems and Electronic Full Authority Digital Engine Control (FADEC).

Question 2

What are the sub-classifications of Magneto Ignition systems?

Answer 2

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

Question 3

Describe the basic operating principle of a Magneto-Ignition System.

Answer 3

It operates as an engine-driven AC generator using a permanent magnet, which generates a magnetic field. This field induces a current in a coil, and when interrupted, produces high voltage necessary for a spark plug to ignite the air-fuel mixture.

Question 3

What is the difference between high-tension and low-tension magneto systems?

Answer 3

The low-tension magneto system generates low voltage distributed to a transformer coil near each spark plug, while the high-tension system directly generates high voltage to fire the spark plug.

Question 4

What three essential elements are required to generate electrical power in a magneto system?

Answer 4

A permanent magnet, a coil of wire, and relative movement of the magnetic field.

Question 4

What happens when the magneto reaches the ‘full register position’?

Answer 4

The number of magnetic lines of force through the coil core is at a maximum, producing the highest amount of flux.

Question 5

What is the significance of the timing in the magneto system?

Answer 5

The timing ensures that the spark occurs when the piston is on the correct stroke, typically a specific number of degrees before reaching the top-dead-center piston position.

Question 5

What is the purpose of the secondary electrical circuit in a magneto?

Answer 5

The secondary circuit contains the secondary coil, distributor rotor, distributor cap, ignition lead, and spark plug, directing high voltage to the spark plug to ignite the air-fuel mixture.

Question 5

What are the three circuits in a high-tension magneto system?

Answer 5

The magnetic circuit, the primary electrical circuit, and the secondary electrical circuit.

Question 6

What occurs when the magneto is in the ‘neutral position’?

Answer 6

All flux lines are short-circuited, and no flux flows through the coil core, leading to zero voltage production.

Question 7

What is the role of the magnetic circuit in a magneto?

Answer 7

It consists of a permanent magnet, soft iron core, and pole shoes. The magnet rotates, creating magnetic flux that produces voltage when it interacts with the coil core.

Question 8

What components are part of the primary electrical circuit in a magneto?

Answer 8

The primary electrical circuit includes breaker contact points, a condenser, and an insulated coil.

Question 8

Describe the role of the breaker contact points in a magneto system.

Answer 8

Breaker contact points open and close the primary circuit, controlling the timing of the magnetic field’s collapse and subsequent high voltage production in the secondary circuit.

Question 8

Explain the significance of the ‘E-gap’ position in a magneto.

Answer 8

The E-gap position is where the magnetic rotor and primary coil are set to achieve maximum efficiency by holding the magnetic field at a high value until the breaker points open, causing a high rate of flux change and inducing high voltage in the secondary coil.

Question 9

What is the function of the condenser in a magneto’s primary circuit?

Answer 9

The condenser prevents arcing at the breaker points and hastens the collapse of the magnetic field, which is essential for generating high voltage.

Question 10

How does the ignition lead in a magneto system function?

Answer 10

The ignition lead directs high-voltage current from the magneto to the spark plug and also shields against electrical interference.

Question 10

Why is shielding important in the ignition harness?

Answer 10

Shielding prevents electromagnetic interference from the high-frequency oscillations of the magneto, protecting sensitive electronic equipment.

Question 11

How does lead capacitance affect the magneto system?

Answer 11

Increased lead capacitance requires more magneto current to fire the spark plug, as it stores electrical energy that discharges across the plug gap.

Question 12

What happens when the ignition switch is in the ‘off’ position?

Answer 12

The primary circuit is completed through the switch to ground, preventing the magneto from generating high voltage and firing the spark plug.

Question 12

What is the role of the ignition switch in an aircraft ignition system?

Answer 12

The ignition switch controls the system, turning it on and off by completing or breaking the primary circuit to ground.

Question 13

What are the safety requirements for the maintenance of starting and ignition systems in reciprocating engines?

Answer 13

The safety requirements include ensuring that the ignition system is made safe before working on or around the magneto system or engine. This involves placing the ignition switch to OFF, turning the fuel switches OFF, and possibly isolating the primary lead on the distributor cap. A warning sign should also be placed over the ignition switch.

Question 14

What is a magneto, and what is its significance in aircraft engines?

Answer 14

A magneto is a device that generates electrical current for the spark plugs in an aircraft engine. It is significant because it allows the engine to operate independently of the aircraft’s electrical system. Even if one magneto fails, the engine can still operate, but it is never acceptable to fly knowing that only one magneto is functioning.

Question 15

What is a magneto drop test, and why is it important?

Answer 15

A magneto drop test involves running the engine at a designed speed (around 1700 rpm) and switching between magneto positions (left, right, and both) to ensure that the magneto disconnects when grounded. It is important because it checks if the magnetos are functioning properly, and if the rpm drop exceeds specified limits, the flight should not continue.

Question 16

What is the risk associated with the high-tension (HT) part of the ignition system?

Answer 16

The HT part of the ignition system can be lethal as it typically generates voltages in excess of 10,000 volts, sometimes exceeding 25,000 volts. Proper precautions must be taken to avoid serious injury when working near it.

Question 17

What are flange-mounted magnetos, and how do they differ from base-mounted magnetos?

Answer 17

Flange-mounted magnetos are attached to the engine via a flange around the rotating shaft. They allow for timing adjustments and are more commonly used. Base-mounted magnetos are typically found only on very old or antique aircraft engines.

Question 18

What are the key features of a low-tension magneto system?

Answer 18

In a low-tension system, low-voltage is generated in the magneto and then increased by a transformer coil near the spark plug. This design reduces electrical leakage and flashover, making it particularly useful in high-altitude operations. However, it is less commonly used today due to the development of better high-tension systems.

Question 19

What does the FADEC system control in an aircraft engine?

Answer 19

The FADEC (Full Authority Digital Electronic Control) system controls ignition timing, fuel mixture, and delivery/injection as an integrated control system. It automatically adjusts these parameters based on engine operating conditions to optimize performance, eliminating the need for magnetos or manual mixture control.

Question 20

How does the FADEC system respond to loss of power?

Answer 20

In the event of loss of primary power, FADEC continues to operate using a secondary power source (SPS). It also performs self-diagnostics and provides system status information to the pilot via indicators on the health status annunciator (HSA) panel.

Question 21

What role does the low-voltage harness play in a FADEC system?

Answer 21

The low-voltage harness connects the electronic control units (ECUs) with various sensors and power sources. It acts as a signal transfer bus, ensuring that sensor inputs are transmitted to the ECUs, which in turn control ignition and fuel injection.

Question 22

What is the purpose of the Electronic Control Unit (ECU) in the FADEC system?

Answer 22

The ECU controls the fuel mixture and spark timing for the engine’s cylinders. Each ECU is assigned to a pair of cylinders, with independent control channels for each cylinder. If one control channel fails, the other can take over, ensuring continued operation.

Question 23

What happens when battery voltage is applied to the positive (+) terminal of the booster coil through the start switch?

Answer 23

When battery voltage is applied, current flows through the closed contact points to the primary coil and ground. This current flow sets up a magnetic field around the coil, which magnetizes the coil core.

Question 24

How does the PowerLink ignition system in FADEC-equipped engines operate?

Answer 24

The PowerLink ignition system uses high-voltage coils atop the ECU and a high-voltage harness to deliver spark energy to the spark plugs. It ensures that both spark plugs in a cylinder fire on the compression stroke, with timing based on engine load conditions.

Question 25

What occurs when the movable contact point is attracted to the iron core in a booster coil?

Answer 25

The primary circuit is broken, causing the magnetic field to collapse, which leads to the coil core losing its magnetism. This allows the spring to close the contact points and complete the primary circuit again.

Question 25

How does a booster coil function during the engine start cycle?

Answer 25

A booster coil generates a series of sparks independently of the magneto during the start cycle. These sparks are routed to the distributor rotor’s trailing finger and then to the appropriate cylinder, ensuring a retarded spark to facilitate engine starting.

Question 25

What are the types of auxiliary ignition units used in reciprocating engines, and why are they necessary?

Answer 25

Auxiliary ignition units like booster coils, starting vibrators, impulse couplings, or electronic ignition systems are used to provide high ignition voltage during engine starting when the magneto output is low. These systems also retard the spark to prevent engine kickback during starting.

Question 26

How does the magnetic field behave in the booster coil, and what is the result?

Answer 26

The magnetic field expands and collapses rapidly, which induces a high voltage in the secondary coil of the booster coil. This high voltage is sufficient to ignite the engine.

Question 27

What role does the condenser play in the booster coil circuit?

Answer 27

The condenser absorbs the high self-induced voltage when the primary coil’s current flow is interrupted, preventing an arc across the contact points, which would reduce voltage output and damage the contact points.

Question 28

What is the primary function of the impulse coupling in an aircraft engine’s auxiliary starting system?

Answer 28

The impulse coupling provides a brief acceleration to one of the magnetos, usually the left, to produce an intense spark for starting the engine.

Question 29

What components make up the impulse coupling?

Answer 29

The impulse coupling consists of a cam and flyweight assembly, a spring, and a body assembly.

Question 30

How does the impulse coupling operate at low engine speeds?

Answer 30

The flyweight catches on a stop pin, and as the engine rotates, the spring winds up. When the piston reaches top dead center, the flyweight releases, causing a quick twist of the magneto, generating a spark.

Question 31

What is the significance of the sharp clicking noise heard during the engine cranking with impulse coupling?

Answer 31

The clicking noise indicates that the flyweights in the impulse coupling are engaging and disengaging, which means the impulse coupling is functioning correctly.

Question 32

What are some problems associated with impulse couplings?

Answer 32

Issues can arise from magnetized flyweights or congealed oil, preventing the flyweights from engaging stop pins. Wear and tear can also cause the impulse coupling to fail, and it only produces one spark per firing cycle, which can be a disadvantage in adverse conditions.

Question 33

What does the shower of sparks system do in an aircraft ignition system?

Answer 33

The shower of sparks system provides multiple sparks at the spark plug electrodes during the starting cycle, enhancing the engine’s ability to start.

Question 34

How does the starting vibrator in the shower of sparks system operate?

Answer 34

The starting vibrator changes DC from the battery into a pulsating DC, which flows to the primary coil of the magneto, causing it to produce a shower of sparks across the spark plug gap.

Question 35

What occurs when the engine reaches its normal advance firing position in the shower of sparks system?

Answer 35

The main breaker points open, but the current continues to flow through the retard breaker points, which eventually open to allow the magneto to produce a magnetic field that induces a high-voltage surge to fire the spark plug.

Question 35

What happens when the magneto switch is in the “both” position, and the starter switch S1 is on?

Answer 35

The starter solenoid and coils are energized, causing relay contacts to close, grounding the right magneto, and allowing the shower of sparks to ignite the left magneto.

Question 36

How does the low-tension retard breaker vibrator system differ from the high-tension system?

Answer 36

The low-tension system uses a transformer coil and is designed for light aircraft. It operates similarly but with different components like the single breaker magneto and specific relay connections.

Question 37

What is the function of a spark plug in an aircraft engine?

Answer 37

The spark plug conducts a high-voltage current through the wall of the combustion chamber, providing an air gap for the spark that ignites the fuel/air mixture.

Question 38

What are the three main components of a spark plug?

Answer 38

The three main components are the electrode, insulator, and outer shell.

Question 39

How does the heat range of a spark plug affect its performance?

Answer 39

The heat range determines the plug’s ability to transfer combustion heat to the cylinder head. Hot plugs have a long insulator nose for a longer heat transfer path, while cold plugs have a shorter insulator for rapid heat transfer.

Question 40

What is the significance of a spark plug’s reach?

Answer 40

The reach of a spark plug ensures that the electrode end is correctly positioned inside the cylinder. Incorrect reach can lead to improper combustion, engine damage, or preignition.