INTRODUCTION
Automobile
electrical system includes starting system, charging system, ignition system
and lighting system and some accessories. The accessories include cigarette
lighter horn and mobile charging system, etc.
IGNITION SYSTEM
(a) Spark plugs (for petrol vehicle)
(b) Distributor
(c) Ignition
coil
(d) Ignition
switch, etc.
CHARGING SYSTEM
(a) Alternator
(b) Regulator,
etc.
STARTING SYSTEM
(a) Battery
(b) Starting
motor
(c) Wiring,
(d) Switches,
etc.
IGNITION SYSTEM
In spark ignition engines, a device is required to ignite the compressed air-fuel mixture at the end of compression stroke.
Ignition system
fulfils this requirement. It is a part of electrical system which carries the
electric current at required voltage to the spark plug which generates spark at
correct time.
It consists of
a battery, switch, distributor ignition coil, spark plugs and necessary wiring.
A compression
ignition engine, i.e. a diesel engine does not require any ignition system.
Because, self-ignition of fuel air mixture takes place when diesel is injected
in the
REQUIREMENTS OF AN IGNITION SYSTEM
(a) 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.
(b) It should
produce spark for sufficient duration so that mixture can be ignited at all
operating speeds of automobile.
(c) Ignition system
should function satisfactory at all engine speeds.
(d) Longer life
of contact points and spark plug.
Automobile
Electrical Systems
(e) Spark must generate at correct time at the end of compression stroke in every cycle of engine operation.
(f) The system
must be easy to maintain, light in weight and compact in size.
(g) There
should be provision of spark advance with speed and load.
(h) It should
be able to function smoothly even when the spark plug electrodes are deposited
with carbon lead or oil.
TYPES OF IGNITION SYSTEMS
(a) Battery
ignition system or coil ignition system.
Battery
ignition system consists of a battery of 6 or 12 volts, ignition switch,
induction coil, contact breaker, condenser, distributor and spark plugs.
(b) Magneto
ignition system.
(c) Electronic
ignition system.
BATTERY OR COIL IGNITION SYSTEM
The primary
circuit consists of battery, switch, primary winding and contact breaker point
which is grounded. A condenser is also connected in parallel to the contact
breaker points. One end of the condenser is grounded and other connected to the
contact breaker arm. It is provided to avoid sparking at contact breaker points
so as to increase their life. The secondary ignition circuit consists of
secondary winding distributors and spark plugs. All spark plugs are grounded.
The ignition
coil steps up 12 volts or 6-volt supply to a very high voltage which may range
from 20,000 to 30,000 volts. A high voltage is required for the spark to jump
across the spark plug gas. This spark ignites the air-fuel mixture as the end
of compression stroke. The rotor of the distributor revolves and distributors
the current to the four segments which send the current to different spark
plugs. For a 4-cylinder engine the cam of the contact breaker has four lobes.
Therefore, it makes and breaks the contact of the primary circuit four times in
every revolution of cam. Because of which current is distributed to all the
spark plugs in some definite sequence.
The primary
winding of ignition coil has less number of turns (e.g. 200 turns) of thick
wire. The secondary winding has relatively large number of turns (e.g. 20,000
turns) of thin wire.
When ignition
switch in turned on, the current flows from battery to the primary winding.
This produces magnetic field in the coil. When the contact point is open, the
magnetic field collapses and the movement of the magnetic field induces current
in the secondary winding of ignition coil. As the number of turns in secondary
winding are more, a very high voltage is produced across the terminals of
secondary.
The distributor
sends this high voltage to the proper spark plug which generates spark for
ignition of fuel-air mixture. High voltage current is passed to all spark in a
definite order so that combustion of fuel-air mixture takes place in all
cylinders of the engine.
A ballast
register is connected in series in primary circuit to regulate the current. At
the time of starting this register is bypassed so that more current can flow in
this circuit.
The breaker
points are held by a spring except when they are forced apart by lobes of the
cam.
MAGNETO :
Magneto is a kind of generator to provide electrical energy to run the ignition system. It is replacement of battery for ignition. When it is rotated by the engine, it produces high voltage current to be supplied to spark plugs through the distributor.
MAGNETO-IGNITION SYSTEM
This system
consists of a magneto in place of a battery. The magneto produces and supplies
current in primary winding. Rest of the system is same as that in battery
ignition system. The magneto consists of a fixed armature having primary and
secondary windings and a rotating magnetic assembly. This rotating assembly is
driven by the engine.
Rotation of
magneto generates current in primary winding having small number of turns.
Secondary winding having large number of turns generates high voltage current
which is supplied to distributor. The distributor sends this current to
respective spark plugs. The magneto may be of rotating armature type or
rotating magnet type. In rotating armature type magneto, the armature having
primary and secondary windings and the condenser rotates between the poles of a
stationary horse shoe magnet. In magneto, the magnetic field is produced by
permanent magnets.
ELECTRONIC IGNITION SYSTEMS
Electronic ignition systems use some solid-state devices like transistor and capacitors, etc. to generate right sparking voltage at right time. These systems have overcome the limitations of conventional ignition systems. Modern automobiles make use of these systems. Two systems, common in use, are:
(a) Capacitive
discharge ignition, and
(b)
Transistorized coil ignition.
These systems
are more reliable and require less maintenance. Wear and tear of components is
reduced and life of spark plugs is increased with the use of electronic
ignition.
CHARGING SYSTEM
Charging system is required to recharge the battery which is an important component of electrical system of an automobile. Charging is required as the capacity of a battery to supply current is limited to the energy stored in it in the form of chemical energy. Battery supplies the current to run the starting motor, various lights and horn, etc.
The charging
system generates electricity to recharge the battery and run other electrical
components.
COMPONENTS OF A CHARGING SYSTEM
Charging system
consists of:
GENERATOR OR DYNAMO
It converts
mechanical energy into electrical energy.
REGULATOR
It controls the
generator output according to the need. It controls the current or voltage.
RELAY
It is used to
control the flow of current between generator and battery. It acts as circuit
breaker.
STARTING SYSTEM
The starting
system of an automobile is used to start the internal combustion engine. Both
SI and CI engines cannot start by itself. These engines need to be cranked by a
starting motor. This motor is also called a starter or cranking motor. Cranking
of any engine means rotating its crank shaft. Rotation of crank shaft causes
the piston to reciprocate. When piston reciprocates, suction, compression, expansion
and exhaust strokes of engine are completed. Thus, engine completes its working
cycle and it starts running.
Starting motor
produces necessary torque to rotate the crank shaft through a suitable gear
COMPONENTS OF STARTING SYSTEM
(a) Starting Motor:
Starting motor to produce rotation of crank shaft.
(b) Drive Mechanism:
Drive mechanism to transfer rotary motion of starter to the crank shaft of the
engine.
(c) The
ignition switch to start motor.
FUNCTIONS OF COMPONENTS USED IN CIRCUITS
Functions of
various components used in battery ignition and magneto-ignition systems are
discussed here in brief.
BATTERY
The battery
supplies the necessary current to the primary winding of ignition coil which is
converted into high voltage current to produce spark. It also supplied current
to run the starting motor when engine is cranked for starting. A battery stores
energy in the form of chemical energy and supplies it for running lights and
other accessories of an automobile. Lead-acid battery is commonly used in most
of the automobiles.
IGNITION OR INDUCTION COIL
The ignition
coil is step up transformer to increase the voltage form 12 volt or 6 volt to
20000-30000 volts. It consists of a primary winding and a secondary winding
wound on a laminated soft iron core. Primary winding contains about 300 turns
made of thick wire. Secondary consists of about 20000 turns of thin wire. In a
can type coil, secondary is wound on the soft core over which primary is wound.
This assembly is housed in a steel casing fitted with a cap. The cap is made of
insulating material. The terminals for electrical connections are provided in
cap. This type of coil is shown in Figure 3.3. To save the windings from
moisture and to improve insulation, windings are dipped in oil.
One primary
terminal is connected to ignition switch and other to the contact breaker.
Secondary terminal is connected to the distributor.
Contact Breakers
Contact breaker is required to make contact and break contact of the primary circuit of ignition system. It consists of two contact breaker points .One point remains fixed while the other can move. A cam is sued to move the movable point. As cam moves, the contact is made and broken alternately. Primary circuit breaks when the breaker points open. Magnetic field collapses due to this. This produces high voltage current in the secondary winding which is supplied to the distributor. This current is distributor to proper spark plug where it produces spark for ignition of fuel-air mixture.
CONDENSER
It absorbs and
store the inductive current generated in the coil. If condenser is not
provided, the induced current will cause arcing at the breaker points. This
will cause burning of the breaker points.
DISTRIBUTOR
The distributor
sends the high voltage current, generated in the secondary winding, to the
proper spark plug at proper time. If the automobile is having a four-cylinder
engine, it will have four spark plugs.
The cap of the
distributor is connected to the secondary winding of coil. It has a rotor which
rotates and comes in contact with the terminals placed around the rotor. As the
rotor comes in contact with the terminals, the current is passed to the
respective spark plug at proper time when spark is needed.
IGNITION SWITCH
The function of
the ignition switch is to connect the battery and starting motor in the
automobiles having self-starting system. Example: In car, jeep, etc.
Its function is
to connect battery to induction coil in the battery ignition system.
SPARK PLUGS
The function of the spark plug is to produce spark between its electrodes. This spark is used to ignite the fuel-air mixture in the spark ignition (SI) engines.