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 may not rely on the aircraft electrical system and must be 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.

Before we head on to sophisticated electronic ignitions we will discuss the good ol' magneto from a bygone era. As you will still find these in the original and certified aircraft engines.

Magnetos are basically small generators and include a distributor to guide the high voltage to the spark plugs, the subject of this page.

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:

Magnetos

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 magnetos are used so that one drives the top spark plugs and the other the bottom plugs on the engine. The magnetos generate power independently 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 magnetos 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.