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Ignition Systems
The purpose of the ignition system is to provide a spark at the correct time in the
combustion chamber to ignite the air-fuel mixture. Spark must occur 8-26 ° before
TDC in order to have combustion 10 ° after TDC for maximum power and least
Ignition System Types
Magneto: Used on small engines such as lawnmowers and chainsaws (no battery
needed). The primary voltage comes from the magneto (flywheel magnets), and is
then further amplified to jump across the spark plug gap. The spark is timed by the
use of mechanical breaker points.
Capacitor Discharge: Similar to magneto ignition without the use of breaker
points. The points are replaced with solid state (non moving) components which
are much more reliable, give a hotter spark and don’t require periodic adjustment.
Battery Ignition System: Primary voltage comes from a battery and is then
further amplified. Battery type is the only type used for automotive applications.
Battery ignition systems can be either the point type or the electronic type.
Magneto Ignition
The idea behind a magneto is simple. It is basically an electrical generator that has
been tuned to create a periodic high-voltage pulse rather than continuous current.
An electrical generator (or a magneto) is the reverse of an electromagnet. In an
electromagnet there is a coil of wire around an iron bar (the armature). When you
apply current to the electromagnet's coil (e.g. with a battery), the coil creates a
magnetic field in the armature. In a generator, you reverse the process. You move a
magnet past the armature to create electric current in the coil.
A magneto consists of five parts:
 An armature. The armature is shaped like a capital "U". The two ends of the
U point toward the flywheel.
 A primary coil of perhaps 200 turns of thick wire wrapped around one leg of
the U
 A secondary coil of perhaps 20,000 turns of very thin wire wrapped around
the primary coil
 A simple electronic control unit that commonly goes by the name "electronic
ignition" (or a set of breaker points and a capacitor)
 A pair of strong permanent magnets embedded in the engine's flywheel.
When the magnets fly past the U-shaped armature, they induce a magnetic field in
the armature. This field induces some small amount of current in the primary and
secondary coil. What we need, however, is extremely high voltage. Therefore, as
the magnetic field in the armature reaches its maximum, a switch in the electronic
control unit opens. This switch breaks the flow of current through the primary coil
and causes a voltage spike (of perhaps 200 volts). The secondary coil, having 100
times more turns than the primary coil, amplifies this voltage to approximately
20,000 volts, and this voltage feeds to the spark plug.
(Info from
Battery Ignition System (point type)
Ignition System Parts
Battery: Supplies 12V to ignition primary circuit.
Ignition Switch: Ensures power only supplied when starting or running engine.
Ignition Coil: A transformer which uses electromagnetic induction principles to
increase voltage from 12V to 40,000V. The voltage is increased whenever the
primary current stops (points open)
Breaker Points: Operated by distributor cam. The points switch ignition primary
circuit on and off for each cylinder that fires. Current flows through primary circuit
when points are closed. (at 100 kph a 6 cylinder engine must produce 9000
sparks per minute)
Condenser: Absorbs and stores electrical energy each time the points open. This
reduces arcing in points and makes them last longer.
Distributor: Receives and distributes secondary voltage to the appropriate
cylinder through the firing order.
High Tension Wires: Well insulated wires which can with stand the high
secondary voltage.
Spark Plug: Provides the gap for high secondary voltage to jump across and ignite
the air fuel mixture. Available in variety of heat ranges and must be matched to
the engine. Use colder plug for hot engine (lawnmower), and a hotter plug for
cold engine (snow blower). If spark plugs get too hot, the electrodes don’t last as
long, and if too cold then they foul with carbon. Some plugs are also available in
platinum and iridium tip to make them last longer. (160,000 km)
Battery Ignition System (electronic type)Transistor: Electronic
switch used to replace breaker points. (transistors have no moving parts)
Reluctor: Reluctor is used to switch the transistor on and off at the correct time.
Advantages of electronic ignition are:
Less moving parts and less maintenance
Higher secondary voltages than with point type ignition
Much more reliable
Battery Ignition System (distributorless)
Distributorless ignition systems are the best system possible for the following
No moving parts
Simple layout with fewest number of parts
Very high secondary voltages
Any number of coils, sometimes one per cylinder
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