Chapter:Eleven

Ignition System
Pepaed By: Helen G.
IGNITION SYSTEM
üIn spark ignition engines, a device is required to ignite the
compressed air-fuel mixture at the end of compression stroke.
üIt is a part of electrical system which carries the electric
current at required voltage to the spark plug which generates
spark at correct time.
üIt consists of a battery, switch, distributor ignition coil, spark
plugs and necessary wiring.
REQUIREMENTS OF AN IGNITION SYSTEM
• The ignition system should be capable of producing high
voltage current, as high as 25000 volts, so that spark plug can
produce spark across its electrode gap.
• It should produce spark for sufficient duration so that
mixture can be ignited at all operating speeds of automobile.
• Ignition system should function satisfactory at all engine
speeds.
• Longer life of contact points and spark plug.
• Spark must generate at correct time at the end of compression
stroke in every cycle of engine operation.
Con’t……………
• The system must be easy to maintain, light in weight and
compact in size.
• There should be provision of spark advance with speed and
load.
• It should be able to function smoothly even when the spark
plug electrodes are deposited with carbon lead or oil.
TYPES OF IGNITION SYSTEMS
üThere are three types of ignition systems which are used in
petrol engines.
(a) Battery ignition system or coil ignition system.
(b) Magneto ignition system.
(c) Electronic ignition system.
üIn battery ignition system, the current in the primary winding
is supplied by a battery whereas it is supplied by a magneto in
magneto ignition system.
üBattery ignition system is used in cars and light truck.
Magneto ignition system is used in some scooters.
Con’t……………..
üB o t h t h e s y s t e m s w o r k o n t h e p r i n c i p l e o f m u t u a l
electromagnetic induction.
üElectronic ignition systems use solid state devices such as
transistors and capacitors.
• A transistor is a semiconductor device used to control the
flow of current in a circuit. It can amplify signals or act as a
switch.
Battery or Coil Ignition System
üBelow figure shows line diagram of battery ignition system
for a 4-cylinder petrol engine. It mainly consists of a 6 or 12
volt battery, ammeter, ignition switch, auto-transformer (step
up transformer), contact breaker, capacitor, distributor rotor,
distributor contact points, spark plugs, etc.
üNote that the Figure shows the ignition system for 4-cylinder
petrol engine, here there are 4-spark plugs and contact
breaker cam has 4-corners. (If it is for 6-cylinder engine it
will have 6-spark plugs and contact breaker cam will be a
perfect hexagon).
Con’t………………
üThe ignition system is divided into 2-circuits :
ØPrimary Circuit : It consists of 6 or 12 V battery, ammeter,
ignition switch, primary winding it has 200-300 turns of 20
SWG (Sharps Wire Gauge) gauge wire, contact breaker,
capacitor.
ØSecondary Circuit : It consists of secondary winding.
Secondary winding consists of about 21000 turns of 40 (S
WG) gauge wire. Bottom end of which is connected to
bottom end of primary and top end of secondary winding is
connected to centre of distributor rotor. Distributor rotors
rotate and make contacts with contact points and are
connected to spark plugs which are fitted in cylinder heads
(engine earth).
Battery – Provides initial electrical power.
Ignition Coil – Converts low voltage into high voltage for the
spark plug.
Distributor – Directs the high voltage to the correct cylinder
(not used in modern systems).
Spark Plug – Generates the spark to ignite the fuel-air
mixture.
The contact breaker regulate the ignition timing.
A capacitor is a device that stores and releases electrical energy in a
circuit.
Schematic Diagram of Coil/Battery Ignition System
Con’t……………….

Working :
ØWhen the ignition switch is closed and engine in cranked, as soon as
the contact breaker closes, a low voltage current will flow through the
primary winding. It is also to be noted that the contact beaker cam
opens and closes the circuit 4-times (for 4 cylinders) in one revolution.
ØWhen the contact breaker opens the contact, the magnetic field begins
to collapse. Because of this collapsing magnetic field, current will be
induced in the secondary winding. And because of more turns (@
21000 turns) of secondary, voltage goes unto 28000-30000 volts.
Con’t……………….
ØThis high voltage current is brought to centre of the
distributor rotor. Distributor rotor rotates and supplies this
high voltage current to proper stark plug depending upon the
engine firing order. When the high voltage current jumps the
spark plug gap, it produces the spark and the charge is
ignited-combustion starts-products of combustion expand and
produce power.
Con’t……………….
Note :
üThe Function of the capacitor is to reduce arcing at the
contact breaker (CB) points. Also when the CB opens the
magnetic field in the primary winding begins to collapse.
When the magnetic field is collapsing capacitor gets fully
charged and then it starts discharging and helps in building up
of voltage in secondary winding.
üContact breaker cam and distributor rotor are mounted on the
same shaft.