Battery ignition system
Essential elements of
a battery ignition system are battery, ignition switch and ignition coil with a
ballast resistor, distributor housing, the breaker points, cam, condenser, rotor and
the advance mechanism, spark plug and low and high tension wiring.
Working of battery ignition system
The ignition coil
consists of two coils-one primary and the other secondary. The primary winding
is connected to the battery through an ignition switch and the contact breaker.
The secondary winding is connected to spark plugs through the distributor. A typical
ignition coil has 100 to 200 number of turns in primary winding and about
20,000 turns in secondary winding.
The primary coil,
battery, ignition switch and contact breaker form the primary circuit and the
secondary winding, spark plug, and distributor for the secondary circuit.
A ballast resistor is
provided in series with the primary winding to regulate primary current. For starting
purpose this is by passed so that more current can flow in the primary circuit.
A cam rotating at
camshaft speed operates the contact breaker and causes the breaker points to
open and close. When the ignition switch is on and the contact breaker points
are closed, current flows from the batter through the primary winding and builds
up a magnetic field.
When the current flow
in the primary winding is stopped by opening the contact breaker points the
magnetic field collapses, cuts across the secondary winding and induces a
voltage, which is accompanied by a current. This magnetic field however, also
cuts the primary winding and induces a voltage in this as well as in the secondary
winding. The voltage in the primary winding always opposes the action producing
it and the effect is to slow down the build-up of the current in the primary
winding when the breaker point close. This prolongs current flow after the
points open. The slow rise in the current means a slow building up of the
magnetic field and consequently a lower voltage in the secondary circuit. The continued
flow of current after the breaker points open, results in a slow collapse of
the magnetic field and will also cause the sparking to take place across the contact
breaker points because the current tries to flow across the points as they open
and in doing so jumps across the points in the form of spark. This arcing of
points increases wear and the life of the contact points appreciably reduced. Also,
most of the energy stored in the magnetic field is consumed in spark across the
contact points instead of across the spark plugs and there would be
insufficient energy to produce the necessary high voltage surge in the
secondary circuit.
A quick collapse of
the magnetic field is required to obtain high voltage in the secondary circuit.
It is also necessary to prevent the arcing and consequential burning of the
contact points. These are achieved by providing an electrical condenser across
the contact breaker. When the contact points open, the current instead of
passing across the points in the form of an arc, flows into the condenser and
is stored by it as it becomes charged. The charge in the condenser immediately
discharges back into the primary circuit in a direction reverse to the flow of
a battery current, thus assisting in a quicker collapse of magnetic field when
the contact point opens.
Due to rapidly
collapsing magnetic field, high voltage is induced in the primary circuit and
still higher voltage of the order of 11kV to 22kV in the secondary circuit. This
high voltage in the secondary circuit passes through the distributor rotor to
one of the spark plug leads, and in to the spark plug, and if the voltage is
higher than the breakdown voltage a spark occurs across the spark plug gap
causing ignition of the combustible mixture in the combustion chamber.
