magneto ignition

Magneto ignition system

a petrol engine needs a high voltage at spark plug for igniting air-fuel mixture.

the magneto ignition system produces that high voltage as follows

An ignition magneto, or high tension magneto, is a magneto that provides current for the ignition system of a spark-ignition engine, such as a petrol engine. It produces pulses of high voltage for the spark plugs. The older term tension means voltage.

The use of ignition magnetos is now confined mainly to engines where there is no other available electrical supply, for example in lawnmowers and chainsaws. It is also widely used in aviation piston engines even though an electrical supply is usually available. In this case, the magneto's self-powered operation is considered to offer increased reliability; in theory, the magneto should continue operation as long as the engine is turning.

Firing the gap of a spark plug, particularly in the combustion chamber of a high-compression engine, requires a greater voltage (or higher tension) than can be achieved by a simple magneto. The high-tension magneto combines an alternating current magneto generator and a transformer. A high current at low voltage is generated by the magneto, then transformed to a high voltage (even though this is now a far smaller current) by the transformer. In the type known as a shuttle magneto, the engine rotates a coil of wire between the poles of a magnet. In the inductor magneto, the magnet is rotated and the coil remains stationary. As the magnet moves with respect to the coil, the magnetic flux linkage of the coil changes. This induces an EMF in the coil, which in turn causes a current to flow. One or more times per revolution, just as the magnet pole moves away from the coil and the magnetic flux begins to decrease, a cam opens the contact breaker (called “the points” in reference to the two points of a circuit breaker) and interrupts the current. This causes the electromagnetic field in the primary coil to collapse rapidly. As the field collapses rapidly there is a large voltage induced (as described by Faraday's Law) across the primary coil.

As the points begin to open, point spacing is initially such that the voltage across the primary coil would arc across the points. A capacitor is placed across the points which absorbs the energy stored in the leakage inductance of the primary coil, and slows the rise time of the primary winding voltage to allow the points to open fully. The capacitor's function is similar to that of a snubber as found in a flyback converter. 

A second coil, with many more turns than the primary, is wound on the same iron core to form an electrical transformer. The ratio of turns in the secondary winding to the number of turns in the primary winding, is called the turns ratio. Voltage across the primary coil results in a proportional voltage being induced across the secondary winding of the coil. The turns ratio between the primary and secondary coil is selected so that the voltage across the secondary reaches a very high value enough to arc across the gap of the spark plug. As the voltage of the primary winding rises to several hundred volts, the voltage on the secondary winding rises to several tens of thousands of volts, since the secondary winding typically has 100 times as many turns as the primary winding.

The capacitor and the coil together form a resonant circuit which allows the energy to oscillate from the capacitor to the coil and back again. Due to the inevitable losses in the system, this oscillation decays fairly rapidly. This dissipates the energy that was stored in the condenser in time for the next closure of the points, leaving the condenser discharged and ready to repeat the cycle.

On more advanced magnetos the cam ring can be rotated by an external linkage to alter the ignition timing.

In a modern installation, the magneto only has a single low tension winding which is connected to an

external ignition coil which not only has a low tension winding, but also a secondary winding of many

thousands of turns to deliver the high voltage required for the spark plug(s). Such a system is known as an "energy transfer" ignition system. 

Initially this was done because it was easier to provide good insulation for the secondary winding of an external coil than it was in a coil buried in the construction of the magneto (early magnetos had the coil assembly externally to the rotating parts to make them easier to insulate—at the expense of efficiency). 

In more modern times, insulation materials have improved to the point where constructing self contained magnetos is relatively easy, but energy transfer systems are still used where the ultimate in reliability is required such as in aviation engines.