2. Operation of Starting Circuit
• The starting circuit converts electrical energy
from the battery into mechanical energy for
starting the engine
3. Basic starting circuit has 4 parts:
a) Battery - supply energy to the circuit
b) Starter switch - activate the circuit
c) Motor switch - engages the motor drive
with the engine flywheel
d) Starting motor
– Drives the flywheel to start the engine. As soon as
the engine starts, the starting motor disengages. It
is designed to handle heavy loads for short periods
of time only
4. Battery charging
The amount of electrical current a battery can produced is
limited by the amount of chemical reaction which can take
place within it.
When chemical reaction in a battery has ended – battery is
discharged and can no longer produce a flow of electrical
current
During discharged current flow from +ve terminal to –ve
terminal
The battery can be recharged by causing direct current from
outside source to flow from –ve to +ve terminals
5. Coil Ignition
Ignition Circuit (spark ignition engines)
The ignition circuit creates the spark which ignites fuel-
air mixture to power petrol engines. To do this, it must :
a. Step up low voltage to high voltage surges
b. Time these surges to the engine cycle
6. Ignition circuit has these parts:
a. Ignition coil
b. Condenser
c. Distributor
d. Spark plugs
e. Ignition switch
7. a) Ignition Coil
The coil transforms low voltage from the battery to
the high voltage for producing a spark.
Operation of Ignition Coil
The ignition circuit must take low voltage from the
battery and create high voltage to fire spark plugs
It must do this very accurately and very rapidly
100 or more times per second
This is why maintenance is so important
8. i) Primary Circuit
• Path for low voltage current from source (diagram)
• It includes these parts:
– Ignition switch
– Coil primary winding
– Distributor contact points
– Condenser
9. ii) Secondary Circuit
High voltage path for current stepped up by the coil
It includes these parts:
Coil secondary winding
Distributor rotor
Distributor cap
Spark plug wires
Spark plug
10. Operation of the Circuit
Divided into 2 parts
1) Before distributor points open
– Before engine start, distributor points closed
– When ignition is turned on, current flows from battery into
primary windings of the coil
– This current creates a magnetic field around the winding
– From primary winding, current at low voltage travels
through the closed distributor points and back to ground
– This is why the battery can be completely discharged if the
switch is left on when the engine is not running
11. 2) After distributor points open
– As the engine rotates in starting, it drives the
distributor shaft and the breaker arm
– When the breaker cam opens the distributor points, the
second phase of ignition begins
– As the points open, they open the primary circuit and
divert the flow of current to condenser, which stops
flow in the primary circuit
– Stopping this flow causes the magnetic field around
the primary winding to collapse very quickly
– This collapse induces a voltage in the primary winding
which causes current to flow. The current in the
primary circuit is absorbed by the condenser
12. • The collapsing field further collapses across the
secondary winding and induce voltage in it also
• The secondary has more turns compares to the primary
winding (stronger winding). Therefore higher voltage is
induced – 4,000 to 20,000 volts
• High voltage pushes current through second winding and
high tension terminal into the distributor cap (Diagram)
• The rotor inside the distributor cap turns to a spark plug
terminal, directing the voltage surge to correct spark plug
through insulated cables
• At the spark plug, current flows down the center
electrode, jumps the gap, and creates the spark
• The whole cycle happens from 50 – 150 second
depending on the speed of the engine
13. Maintenance of Electrical System
i) Servicing and maintenance of battery
ii) Servicing of spark plugs
iii)Servicing the distributor
iv) Replacing breaker point
v) Servicing alternator and starting motor
