Magneto Ignition System

The purpose of the magneto ignition system is to generate the energy to create the ignition spark. We can divide this system into magneto systems, battery and coil systems. We can use Magneto ignition System in aircraft engines and coil and battery ignition in automobile engines; although lately it is being displaced by electronic ignition.

In this sense, the basic principles for the operation of both systems are similar. The magneto is self-sufficient, requiring only the spark plugs and lead wires. While the battery and coil system requires other components.

The magneto ignition system of these engines is made up of these elements: The spark plugs and connection cables in a simplified way and the operation of the system. This is how the magnets generate an electrical current that is directed to the spark plugs through the connection cables.

This set works in a synchronized way, with the movements of the crankshaft. These movements cause the spark to jump in the corresponding cylinder at the right moment.

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A magneto is a current generator designed with sufficient voltage to cause a spark to jump from the spark plugs. In this way, it causes the ignition of compressed gases in an internal combustion engine.

A magnet is composed of a magnetized rotor and an armature with a primary winding of thick copper wire. It also has a secondary winding, with a large number of turns of fine wire, a circuit breaker, and a capacitor.

When the magnetic rotor driven by the movement of the motor rotates, it induces a current in the primary winding that charges the capacitor. The breaker interrupts the primary circuit when the induced current reaches its maximum value and the magnetic field around the primary winding collapses. The capacitor discharges the current stored in the primary winding by inducing a reverse magnetic field.

This collapse and reversal of the magnetic field produce a high voltage current in the secondary winding. This is distributed to the spark plugs for the ignition of the mixture.

In it, a hammer actuated directly by the piston or by means of a rod by the camshaft opened the circuit. Causing an opening overcurrent of more than 400V (capable of causing a spark).

La Mors

La Mors was the first company to apply the Magneto Ignition System. Also known as medium voltage magneto, due to the difficulty of keeping the moving parts in the combustion chamber efficient and isolated.

This system was immediately abandoned and replaced by the low voltage magnet. That is a magnet that produced alternating current. This voltage was then increased by an outer coil. This system, mounted on the 1909 Ford T, is still used in 1-2-cylinder motorcycle engines.

In automobiles, it was soon abandoned. Being replaced by the high voltage magnet. So named because there was no external coil or transformer to increase the voltage, but the alternator circuit was equipped with 2 coils, one of which acted as a transformer.

The further progress of the high-voltage magnet was linked to the improvement of dielectric materials and of magnets. This made it possible to greatly lighten the device and reduce its dimensions to allow its use on engines of up to 8 and more cylinders, both single and double ignition.

Ignition By Delco

The high voltage magnet was used exclusively, in all the world production of the 1920s. From 1925 it was gradually replaced by the Delco ignition. But in competition engines, given the optimal performance at high revs and not requiring the battery. It maintained the preferences of the manufacturers until the end of the 1950s.

Low Voltage Magnet

The name is improper. In the sense that the current produced by the magneto was of limited voltage (not higher than 280V). But the current sent to the spark plugs were then increased by an external transformer (coil), which took it to values higher than 10,000V.

The operation was very simple, it was about making a magnet move around a coil of copper wire or vice versa. In the first Magneto Ignition System, the magnet was fixed and the coil rotated inside. In this way, the current had to be obtained from the rotor through brushes or friction contacts.

Later the armature and the magnet were fixed, or the magnets took the form of rotors. There were also examples of a fixed magnet armature, between which a metallic core slid reciprocating, causing a flux variation.

Basic Principles To Consider in Magneto Ignition System

The basic principle on which the Magneto Ignition System (both low voltage and high voltage) worked. It is the same as that of the alternator, the variations of magnetic flux produced by the reciprocal movement created in the coil.

In an alternating current, if the magnet has 2 poles, the voltage of the current will have 2 minimums and 2 maximums (of about 10HP), which occur exactly when the magnetic flux experiences the maximum variation.

Maximum tension

The maximum voltage is that obtained when one pole of the magnet has moved away from the coil. If at that moment the circuit is interrupted with a breaker (made up of 2 points or, as in the case of motorcycles, by 2 tungsten contacts).

The voltage between the separate contacts reaches values of more than 200V (opening overcurrent). If a capacitor is mounted in parallel with the contacts, its effect will be to absorb the opening overcurrent (eliminating the spark) and increase it rapidly.

The voltage difference that is established between the contacts in circumstances, connecting the coil to an external transformer, a voltage increase is easily obtained, in which the spark can jump.

If the spark plugs are more than one, a voltage distributor (200V) must be installed. This will send the current to as many transformers as there are spark plugs. Or, several points can be fitted, which feed as many coils with the distributor solution.

In the case of low voltage. Friction contacts are essential, but on the other hand, very careful insulation is not necessary.


The coils that generate high voltage current, can be placed very close to the spark plugs to avoid the danger of stray or short circuit. As well as in the aeronautical engines or those of operation by methane.

The low voltage magnet has been preferred precisely because of the reduction in dispersion. These are due to the corona effect at high altitude (for airplanes). As well as the dangers of short circuits (for gas engines).

In single-cylinder engines, the low voltage magnet is still used because it is possible to build a magnetic flywheel. In this, all the current produced has low voltage values, but high in terms of current intensity. So that part of the coils can be used for lighting or onboard lights.

High Voltage Magnet

It does not differ much from the coils. It is simpler because it does not need an external coil, which directly produces a high voltage current to send it to the spark plugs. From the point of view of its construction, the possibility of having all the parts grouped in a single block is intended to produce current, increase the voltage and distribute it to the spark plugs.

This greatly reduces costs and largely reduces the possibility of breakdowns. The high voltage magnet is composed of:

  • A rotating magnet or 2 magnets. Equipped with suitable expansions to form a 2 or 4 pole rotor. Generally, tungsten steels are used, as well as chromium, molybdenum, or cobalt. These have a suitable temper, which maintains the magnetic characteristics for a long time.
  • An induced coil made up of a pack of silicon sheets (core), on which a primary circuit is wound. This is made up of several hundred copper coils, with a diameter of approximately 0.5 mm and coaxially. A secondary circuit of several thousand turns of very fine wire (0.05-0.08mm).


The circuits are connected in series at one end: A connection that goes to the points and then to the mass starts from the same point. And in parallel to a capacitor. The other end of the primary (thick wire) is connected to the mass. While the secondary goes to the rotating brush of a voltage distributor.


Made up of two contacts (one mobile and one fixed, connected to the mass), and a cam. The latter has 2 lobes both in two-cylinder engines with a rotating cam at half engine revolutions, and in 4-cylinder engines with a rotating cam at the same engine revolutions.

Its purpose is to interrupt the low voltage current at certain times and at regular intervals. This circulates in the primary coil of the armature. Thus determining the exact moment in which the spark jumps.

The square on which the fixed contact is welded can move angularly by a few degrees. This is to determine the most convenient distance between the points (generally 0.4mm).

The moving contact is fixed to a bracket electrically isolated from the rest of the magnet and mounted on a screw. This acts as a terminal to the hammer spring and to the low tension connection. Which links the breaker with the primary coil of the armature, with the magnet working.

When the contacts are closed, the current flows through the primary coil of the armature. Reaching the screw, it runs through the spring and reaches the mass through the fixed contact and the body of the magnet.


Its function is that the interruption of the current is fast. Eliminating the arc that would tend to occur between the contacts of the breaker.

At the moment they are separated by the effect of the cam that pushes the hammer skid. The capacitor that has the box connected to the mass, lets the alternating current pass. And therefore, it prevents it from being discharged on the secondary coil when the points are separated (after the spark has jumped).

Indeed, during the dead phases of the cycle would cause harmful sparks, with dangerous flashbacks. It should be noted that since the magnet has at least 2 poles, the minimum number of sparks that can be obtained from a magnet is 2 for each turn.

Therefore, if we mount the magneto directly on the crankshaft, it is suitable for a 4-stroke and 4-cylinder engine, or a 2-cylinder and 2-stroke engine. An excessive number of sparks would occur in a single-cylinder engine.

Automatic Advance Device

Its operation is similar to that used in magneto or other ignition distributors and is used to progressively vary the advance as the number of revolutions increases. It is based on 2 centrifugal masses that, moving away, vary the angular position of the cam that separates the points with respect to the shaft of the magnet.

It is important to note that the magneto ignition does not work with direct current, but alternating current. We can obtain the best spark when the current-voltage reaches its maximum value. This forces the points to separate. Only for a certain position of the magnet shaft. This position occurs when the edge of the poles of a magnet has moved a few millimeters away from the pole of the armature.

Magneto And Phase Ignition With Motor

When the magneto ignition must be put in phase with the motor It is necessary to fine-tune the magneto itself between the point of separation of the points and the position described. A trigger start that makes it possible to alleviate the low power of the spark, at low engine speeds, since the voltage of the current produced by the magnet.

It depends on the variation of the magnetic flux when the motor rotates very slowly. Or worse still, during start-up, the resulting spark is very weak. For this reason, almost all automobile magnets of the 1920s and 1930s had a spring device.

This device could be tensioned before starting the engine and disengaged the rotor when the cylinder was performing the compression phase. Therefore, the spark was independent of how fast the starter handle turned.

Voltage Distributor

It is responsible for sending the current in the established order to each of the spark plugs. In practice, it is an ebonite cap similar to Delco. This has a rotating brush that distributes the current by sparking.

The distributor for 4-stroke engines is connected to the magneto shaft by means of a pair of gears. These divide the number of revolutions by 2. But in a 2-stroke engine, the distributor can be mounted on the magneto shaft itself.

Final words about the Magnet Ignition System

We can use that Magneto Ignition System in cars, motorcycles, even airplanes. This system uses over the past 100 years was a perfect solution due to size restrictions for the use of an external battery. This system is very reliable as it provides a very hot spark for many racing engines, aircraft, or high-performance engines.



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