a typical battery powered ignition uses a transformer, a...

Battery powered ignition

• A typical battery powered ignition uses a transformer, a several switching devices, and a power source.

• The power source is the battery.


• The first switch allows battery voltage to the primary coil.

• Voltage from the primary coil then goes to a second switch in the engine, then to ground.

• This switch is “timed” to open when spark is desired.


• The opening, causes the current to stop in the primary.

• This collapses the magnetic field around both coils.

• For every one primary turn there are 2,000 secondary turns in this coil pack.


• The resultant exchange causes 12 volts to become a 24,000 volt surge.

• The 2 amps becomes 1 milliamp


• This high voltage is enough to break down the dielectric air gap between the spark plug electrodes and produce a spark.


• Once the spark starts it will continue conducting current at a lower voltage value

• This will happen until the magnetic field is “drained” from the coil.


• Shortly after this the switch will need to close to give the magnetic field time to rebuild in the coil, for the next spark event.

• The points are often mounted on a moveable(rotating) plate to provide variable timing options

• This allows good engine operation at all RPM ranges.


• The secondary voltage is carried to the spark plug via a rotary switch called the distributor.

• Then it travels through “high tension” wires to the plugs.

• They are sequenced to the firing order needs of the engine.


• These extremely high surges in the coils cause rapid oscillations of voltage and current.

• This can damage the “engine switch” (points or contact points)

• A condenser is installed as a parallel path to ground.


• This drains the A/C oscillations to ground preventing high current arcing of the points.

• A condenser is a large capacitor.• It is imperative that the capacitor be

balanced with the inductor(coil) and the supply voltage.


• Summary:• The off/on switch is between the

battery and the coil.• The engine switch is between the coil

and ground.• Current is supplied from a battery,

which is charged by an engine driven generator.


• To defeat the system the circuit is broken stopping voltage to the coil.

• If the battery fails the systems fails.• But the battery can produce great

spark at any engine speed, including cold or hot starts.

Magneto powered ignition

• Magneto operation differs primarily because it does not use battery current.

• It incorporates its own rotating permanent magnet generator.


• The rest of the components are similar, but the theory of operation is somewhat different.

• Its primary advantage is that it can be completely self contained in a small engine driven package.


• Multiple units can be installed for redundancy and improved flame propagation.


• Two main disadvantages are that they need moderate engine RPM to function.

• They do not provide a means to vary the timing during operations other than start.

Theory of Mag Operation

• Internal circuits include:• Magnetic• Primary-electrical• Secondary-electrical


• Magnetic circuit includes:• Rotating four (or more) pole

magnet• Pole shoes w/extentions• Coil core


• Primary circuit includes:• Primary coil winding• Points• Condenser• Connecting wire• Grounding “P” lead and switch


• Secondary circuit includes:• Secondary coil winding• Distributor• High tension leads• Spark plugs


• The magnetic circuit allows the lines of flux in the rotor to oscillate.

• At neutral the poles are not lined up with the shoes.

• There is no flux concentrating in the poles and coil core.


• At all other times flux is varying in the coil core and poles.

• The flux reverses each cycle, or one time for each rotor pole.

• Due to hysterisis the plot of this is a flattened circular shape.

Lenz’ Law

• States that current inducted in a circuit will produce counter magnetic lines of flux that oppose the original induction flux.

Lenz’ Law

• This means if the flux in the pole shoes and coil core is allowed to pass through a complete circuit, the resultant current will create flux that opposes the rotor flux.

Lenz’ Law

• No coil the flux will concentrate on the core.

Lenz’ Law

• As the core moves close the lines bend to fit it.

Lenz’ Law

• They finally concentrate in it.

Lenz’ Law

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• With a coil as the core moves close counter current will cause

Lenz’ Law

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• opposite polarity magnetism, resisting a build up of flux.

Lenz’ Law

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Lenz’ Law

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• As the core moves away the current and resultant field switches

Lenz’ Law

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• The magnetic polarity is the same and the fields

Lenz’ Law

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• resist any decreases

Lenz’ Law

• Thus the addition of a coil circuit to the core will create an inductive lag in the magnetic flux.

Lenz’ Law

• This lag allows us to maximize the field buildup and minimize the collapse time when the current is cut off.


• So as a result of Lenz’ law the flux lags and stays “built” around the coil until something after the rotor gets back to neutral.


• At this time the points open, the current disappears

• the time between when the rotor passes neutral and when the points open is called E-gap, or efficiency gap.


• At this point if there had been no coil the magnetic circuit would already have been reversed.

• So rate of field collapse when the points open is enhanced some by the reversed field.


• Enhancing too much (E-Gap) then interferes with the buildup of the next field cycle.

• In a sense the primary circuit and the magnetic circuit are constantly working against each other, and the secondary is auxiliary to them.


• But what we desire from this system is the field collapse around the secondary.

• The secondary is identical to a battery operated system.

• A distributor switches each spark to the next cylinder inline for firing.


• The magnetos is disabled by adding another circuit parallel to the primary points.

• This causes the primary circuit to stay active causing constant magnetic flux lag, that never rapidly collapses around the secondary.


• These devices have no means to alter when the spark fires for each cylinder like other timing devices do.

• Primarily because the most of the power of a propellered engine is produced between 300-400 RPM.

a typical battery powered ignition uses a transformer, a...

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