Magneto Engine Ignition Systems

The fuel air mixture in the combustion chamber needs to be ignited at the correct moment to ensure efficient combustion and power generation by the engine. This is the job of the ignition system, be that the old fashion magneto of the good old days or a modern fully electronic microprocessor controlled FADEC system. For safety reasons the ignition system does not rely on the the aircraft electrical system and is dual and each system operates one of the two spark plugs in each cylinder.

These ignition systems apply to avgas (spark ignited) engines and not to diesel, which are compression ignited engines.

Ignition

The sole purpose of the ignition is to supply a high energy spark at the right moment thereby igniting the mixture so that the engine can start its power stroke. The ignition system consists of a number of components:

Magneto's

These use a magnet rotating inside a coil. The magnetic field generates a voltage in the coil which is transformed to a higher voltage by a second coil with much more windings than the primary coil. A breaker contact in the primary coil circuit interrupts the current flowing and this interrupt causes the magnetic field to collaps thereby generating a very high peak voltage in the secondary coil. This peak voltage is then conducted to the correct spark plugs by the distributor.

Two magneto's are used so that one drives the top spark plugs and the other the bottom plugs on the engine. The magneto's generate power independantly of the aircraft electrical system.

Timing

Timing is of the essence here, the breaker points (with parallel capacitor) are driven by a small cam opening at the correct crankshaft angle. Usually 25° BTDC (before top dead center). Also parallel to the breaker contact and capacitor is the ignition switch, L or R contact.

Distributor

The distributor is also part of the magneto. Its function is to guide the high energy voltage to the correct spark plug. As each cylinder fires every two revolutions of the crankshaft, the rotor in the distributor must therefore rotate at half the crankshaft speed.

Harness and Spark plugs

The ignition harness shields the high voltage and conducts it to the spark plugs, often bound together. The wires are screened or in an metal braid or conduit to shield the radio against ignition interference.

A spark plug has a central electrode and a metal body screwed into the cylinder. Ceramic insulation is used to insulate the central electrode from the engine. Built into the spark plug is a resistor giving a short duration spark and protecting against corrosion of the electrodes.

Two spark plugs and separate ignition circuits are used per cylinder for redundancy, safety and better ignition/ combustion of the mixture.

Impulse coupling

During starting of the engine, its crankshaft rotates very slowly (around 120 RPM) and the magneto's at 60 RPM. Generated voltage is very low at that point. Ignition timing is fixed at 25° BTDC, this is too early at low RPM and if a cylinder would fire it could cause violent kickback (rotates momentarily in the wrong direction) and probably damage the starter.

A device called a impulse coupling is used to retard the ignition timing to almost at TDC and an acceleration of the magnet (with a coiled spring) in the magneto to spice up the voltage to help igniting the mixture at TDC. When the engine fires and its RPM rises the timing is set back to 25° for normal operation (between 500 and 2700 RPM). The moment the engine fires and runs idle the impulse coupling detaches and timing is reset to 25° BTDC

On some engines a vibrating system is used to create a shower of sparks with the left magneto when starting.

Starting

Except for some old historic aircraft most have an electric starter system consisting of a battery, wires and starter motor connected and with an ignition switch in the cockpit. During start the starter motor will draw a very high current and therefore needs thick cables (low resistance) to minimize the voltage drop. The starter switch is connected to a relay or solenoid in the engine compartment so that the wiring is as short as possible. Some models have a red starter warning light which indicates that power is applied to the starter motor, the light must go out the moment the ignition switch is released.

Only one magneto is equipped with the impulse coupling (usually the left) therefore the right magneto is switched off during start, this is done in the ignition switch. This switch has the following positions: OFF-RIGHT-LEFT-BOTH-START.
Some older type aircraft have a start switch separate from the magneto switches.

Oil pressure

Make sure that after start the oil pressure rises within 30 seconds, and within 60 seconds in cold weather. If not, shut the engine down to avoid damage due to improper lubrication.

Dead cut check

After the engine has started, perform a dead cut check between 800 and 1000 RPM. This is to ensure that both mags are operating before you do a run-up mag check at high power. For example: PA-28's are run-up at 2000 RPM and a GA-8 Airvan at 2100 RPM. The Airvan has a 300 bhp engine and run-up is not the time to discover that a mag has failed and the engine suddenly stops at high power settings. Especially so with turbocharged engines.

Magneto check

After the dead cut check is satisfactorily completed a high power run-up with mag check can be done. Cessna run-ups are done at a typical 1700 RPM and note the RPM drop on each mag and the difference between those readings. Typical values are max drop 125 RPM and max 50 RPM difference.

If the engine starts to run rough on one magneto the spark plugs probably fouled up because of the usually rich mixture and high lead content of avgas. Run-up again and lean the mixture for a minute or so to clean the plugs, this should resolve the problem. If not, consult an aircraft engineer to have the engine checked.

Shutdown check

Some check the magneto's prior to engine shutdown. Do this as follows on 1000 RPM or lower: BOTH-LEFT-RIGHT-OFF-BOTH. The engine should experience a sudden loss power indicating that both magneto's are really switching the ignition off. You can then be sure that the aircraft has no live magneto when the keys are out of the ignition switch. Proceed after this check to pull the mixture to ICO (idle cut off) to stop the engine as usual.
ALWAYS remove the keys from the ignition switch and place them on the dash, in the side pocket or take them with you.