Ignition SystemsChapter 3

Aim

To understand principals of operation of the ignition system

Objectives1.State the principal of operation of the magneto2.State the principal of operation of the spark plug3.Describe the layout of the ignition system on the

typical light aircraft engine4.Explain principal of operation of the engine

starting systems5.Describe operational aspects

1. The Magneto

The magneto works on the principal of electromagnetic inductance, as a conductor is moved across a magnetic field a voltage will be induced in the conductorTypically a mechanically driven magnet will rotate near a conductor which has a coil of wire wrapped around it, as the magnet rotates a voltage is induced and electrical current flows through the coilThe voltage of this initial current in the primary coil is too low to be used for ignition so a secondary coil is wound around it

Magneto

1. The Magneto

As current flows through the primary coil a magnetic field is created, this magnetic field induces a very high voltage (or high tension) through the secondary coilFrom here the current travels to the distributer where it is sent to the correct spark plug at the correct timeThe ignition harness must be able to withstand very high voltage (around 20,000 volts), any damage to the harness may result in rough running. The harness is usually insulated in a conduit or metal braiding to prevent radio interference

Magneto

Timing of the distributer is achieved using a rotor geared to half crankshaft speed

2. Spark Plugs

Spark plugEach spark plug has essentially three main components• A central electrode connected to the ignition harness• A metal body which when screwed in to the cylinder

acts as earth• Ceramic insulation to insulate the central electrode

from the metal bodyThe spark plug operates by providing a gap between the high tension wire coming off the distributor and earth, due to the large voltage difference a spark travels from the high tension electrode to earthIf the gap between the wire and earth is too large the air will offer too much resistance and poor ignition will resultIf the gap is too small the spark may not be sufficient to ignite the charge and poor ignition will resultAll aircraft spark plugs are shielded to prevent radio interference

3. Ignition system layoutMost aircraft will operate dual independent magneto systems for redundancy

Each of the magnetos will supply charge to one of the spark plugs in each of the cylindersHaving two spark plugs in each cylinder results in smooth and efficient combustion of the charge, in a horizontally opposed engine there will be one spark plug at the top of the cylinder and one at the bottomEach magneto is mechanically driven and supply's charge to the spark plugs via high tension ignition harnessesThe aircrafts electrical system is independent of the ignition system, if the electrical system fails the engine will still operate normally

4. Engine Starting

Impulse couplingDuring engine start there are two inherent design flaws in the magneto:1. When the engine is started it is typically rotating relatively slow (around 120

RPM). The magneto is directly driven by the crankshaft therefore at low RPM insufficient voltage will be induced to ignite the charge in the cylinder

2. Under normal running conditions the spark plug will spark around 25⁰ before TDC. If this occurs during the low RPM experienced during start the increased pressure in the cylinder may cause the piston to travel down the cylinder prematurely resulting in the crankshaft to rotating the wrong way, known as engine kickback. During start the spark must occur when the piston is at or past TDC

4. Engine Starting

Impulse couplingAn impulse coupling is used to overcome these limitations during start up. The rotational energy developed by the crankshaft at low RPM is used to tension or ‘wind up’ a coiled springWhen sufficient energy is stored the coupling releases accelerating the magnet in the magneto rapidlyThe rapidly spinning magnet is able to induce enough voltage to spark across the spark plug and the delay in moving the magnet rotating retards the spark sufficiently to avoid kick back

4. Engine Starting

Impulse couplingOnce the engine reaches normal operating RPM the magneto driveshaft accelerates away from the coiled spring which has no further effectTypically only one magneto is required to start the engine so typically only the left magneto will incorporate an impulse coupling

4. Engine Starting

The StarterMost modern piston aircraft will incorporate an electric starter motor that is normally powered by the batteryThe high current starter system is usually controlled by turning the ignition to start, a solenoid is used to remotely control the starter circuit to avoid running heavy duty cable near or through the cockpit

5. Operational Aspects

The Ignition SwitchThe ignition switch is used to select individual magnetos, run both magnetos and engage the starterThe engine can run on a single magneto should only one be selected or should a failure occur. Should only one magneto be operational a drop in RPM will be noted. This is due to only one flame front advancing through the charge instead of two, this increases the time for full combustion to occur and decreases efficiency

5. Operational Aspects

Magneto CheckA test of the ignition system should be done on the ground before take-off or shutdown in line with the aircraft flight manual. Should the engine stop when an individual mag is selected it indicates the magneto has failed and the aircraft should not be flownWhen an individual magneto is selected if the RPM drop is greater than published tolerances or rough running occurs it is an indication the sparkplug is fouled with lead or carbon deposits. In this case follow the procedure outlined in the flight manual, generally you will have to increase power and lean the engine to heat the cylinder and clear the fouling.On aircraft fitted with CHT gauges a low CHT will give an indication of which cylinder has the fouled or defective spark plugShould any problems with the ignition system be detected the aircraft should not be flown

5. Operational Aspects

Dead-cut checkA large number of aircraft manufacturers advocate a dead-cut check before the engine is run-up or shut-down. The check is done by briefly selecting each individual magneto and selecting the off position before turning the ignition switch back to bothWith a serviceable magneto system when the ignition switch is set to off the magnetos are grounded preventing them from supplying charge to the sparkplugs.If there is a fault with the grounding system (such as a loose wire) the magneto will still be live once the aircraft has shut down. If someone then tried to move the propeller the engine could inadvertently start, the results of which can be deadly

Questions?