Summary of Aircraft Engine Ignition Systems
Aircraft Engine Ignition Systems: Comprehensive Student Guide
Introduction
Aircraft ignition systems provide the timed electrical spark that ignites the air–fuel mixture in reciprocating engines and initiates combustion in turbine engines during start and relight. Reliable ignition is critical for safe engine operation, especially during starting, takeoff, and in flight relights.
Definition: An ignition system is the group of components that generate and deliver a timed electrical spark to a combustion chamber so that the fuel–air mixture can burn.
Overview: Two broad families
- Reciprocating-engine ignition systems (magnetos, low-tension systems, spark plugs)
- Turbine-engine ignition systems (high-intensity capacitor-discharge systems, glow plug igniters)
Magneto Ignition (Reciprocating Engines)
What is a magneto?
A magneto is a self-contained electrical generator that produces high-voltage pulses to fire spark plugs without relying on the aircraft battery.
Definition: A magneto is a self-powered ignition unit that uses a rotating magnet and coils to produce the high-voltage pulses for spark plug firing.
Main advantages
- Independent of aircraft battery/electrical system
- Reliable source of spark even if aircraft electrical power fails
Key components and terms
- Breaker points: mechanically open to interrupt primary current
- Capacitor (condenser): minimizes arcing at breaker points and speeds collapse of primary current
- Distributor: routes high voltage to the correct spark plug (high-tension systems)
- E-gap: the angular position of the rotating magnet when the primary current is highest; breaker points open at the E-gap
- Impulse coupling: spring-driven device that spins the magneto rapidly at start to produce a hot, late spark for easy starting
Definition: E-gap is the position of the rotating magnet when the primary current in the magneto coil is at maximum; breaker points open at this position.
Timing and testing
- Internal timing: adjust breaker points so they open at the E-gap and distributor rotor directs voltage to cylinder number one
- Vernier coupling: allows fine adjustment (less than 1° increments) of magneto-to-engine timing when base-mounted
- Magnet strength test: rotate magneto on a test stand at specified speed with points held open and measure primary current; stronger magnets produce more primary current
Common failures and symptoms
- If the ignition switch does not ground the magneto primary circuit when switched OFF, the engine may continue to run after the switch is OFF. In short: the ignition switch fails to ground the magneto primary circuit.
Practical notes on spark plugs
- Use a torque wrench when installing spark plugs: under-torqued plugs can leak (poor seal); over-torqued plugs can crack the insulation or thread/seating damage.
- Keep removed spark plugs in numbered tray positions to track cylinder conditions (detonation, overheating, lead fouling).
- Use a round-wire gauge to measure electrode gap.
Definition: Reach of a spark plug is the length of the threads that screw into the cylinder head.
Hot vs cold spark plugs
- Hot plug: longer heat path from nose core insulator to plug shell; retains more heat and runs hotter.
- Cold plug: shorter heat path; runs cooler to shed more heat.
All-weather spark plugs
Definition: An all-weather spark plug is a shielded plug with a recess in the shielding in which a resilient grommet on the ignition lead forms a watertight seal.
Fine-wire vs massive-electrode plugs
- Fine-wire plugs have a more open firing end that helps purge lead-containing combustion products, reducing solid lead buildup.
Low-Tension vs High-Tension Magnetos
| Feature | Low-tension magneto | High-tension magneto |
|---|---|---|
| Number of coils in magneto | 1 | 1 (but produces HV in-magneto) |
| Distribution method | Carbon-brush-type distributor; low-voltage sent to transformers on cylinder heads | Built |
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Aircraft Ignition Systems
Klíčové pojmy: Ignition switch must ground magneto primary; failure can let engine run after OFF, Magneto is self-powered and independent of aircraft battery, E-gap is magnet position where primary current peaks and breaker points open, Impulse coupling provides hot, late spark for starting, Use torque wrench on spark plugs to avoid leaks or insulated cracks, Pressurized magnetos or low-tension systems prevent distributor flashover at altitude, Most turbine engines use high-intensity CDI and two igniters per engine, Fine-wire plugs reduce lead deposit buildup by allowing purge, Internal magneto timing adjusts points to open at E-gap and rotor toward cylinder 1, Vernier coupling allows fine timing adjustments under 1° increments, Round wire gauge is used to measure spark plug electrode gap, Staggered timing balances combustion when scavenging is uneven