CHAPTER 6

ENGINE SYSTEMS

1.1 Ignition System

An ignition system in an IC engine is meant for igniting the air-fuel mixture
inside the cylinder. The ignition has to be performed at a pre-set timing. If it is not
done at a correct time the engine may not produce sufficient power, or may run
erratically, or may not run at all the ignition of mixture requires a spark in spark
ignition engines. The spark can be produced only at a curtain voltage is produced
by an ignition coil which is supplied with a low voltage form battery. The
combination of all devices required to produce an electric spark of desired intensity
and at proper moment is called the spark ignition system.

6.2 Requirements of an efficient spark ignition system

1. Spark must occur continuously & timely.

2. The spark should be any misfire.
3. The spark should be strong enough to ignite the charge.
4. The ignition system should be simple, cheap & contained as a comport unit.
5. The system should operate efficiently at all the engine speed.
6. It should not cause interference with the radio & television transmission.

6.3Types of spark ignition systems

6.3.1 Battery ignition system
 This system generally employs a battery of 12 or 24 volts, & this voltage is
raised to an extremely high value of 15000 to 24000 volts for producing sparks.

Main components of the system are.

1. Battery
2. Switch
3. Conductor
4. Coil
5. Contact breaker
6. Conducer
7. Advance
8. Cam
9. Distributor
10. Spark plug.
 Fig. 6.1: Layout of battery ignition system showing various components

Battery marked 1, supplies 12 or 24 volts direct current to the system. When

the ignition switch 2 is turned on switch marked 2, serves to disconnect the ignition
system devices from the current sources when the engines is to be stopped.
Conductor marked 3, carries low voltage current from the battery to primary
winding of the ignition coil. Coil marked as 4, liens form low-voltage current
received from the storage battery into high-voltage current which is necessary to
ignite the combustible mixture these are two winding. One is primary, lesser
number of turns but the wire is thick & secondary winding with more number of
turns but wire is thin. Due to these two kinds of winding, the induction coil
operates on the principle of an ordinary atelic transformer in raising the voltage
from a low to a very high value. Contact breaker marked 5, makes & breaks the
flow of current repeatedly between ignition coil & the distribution condenser
marked 6, is connected in parallel across the breaker contact to eliminate the
harmful effect of self in diction currents in the primary winding which would
otherwise cause an undesired electric spark between the contact. Advance marked
7, sets automatically the in start at which a spark is to be generated it is required
because sparking time depends on speed of the vehicle engine & the load on it.
Distributor marked 9, consists essentially a rotor & a stator. Rotation to the rotor is
impaled by camshaft through meshing gears of which one is mounted on the
camshaft & the other is on rotor spindle. One revolution of the rotor brings current
to all spark plug of different cylinders. Conductor marked 10, connects stationary
electrodes with the spark plugs & carries extremely high voltage required for
sparking. Spark plug marked 11m is device used to produce the spark.
 6.3.2 Magneto Ignition System

Fig. 6.2: Layout of magneto ignition engine showing various parts

A Magneto ignition system is shown in fig. the battery in a coil ignition

system is replaced by a magneto assembly for producing low voltage current other
parts are similar to battery ignition system in their construction & working the
magneto, in this case, consists of a fixed armature having a primary & a secondary
winding and also a rotating assembly of magnets driven form the engine the
arrangement shown in this figures is for a four-cylinder engine whose firing order
is 1- 3-4-2 as marked on the distributor the sequence of spark plug is 1-2-3-4 on
the cylinders 1-2-3-4 in which cylinder 1 is in front of the block while cylinder 4 is
in rear.

6.3.3 Comparison between Coil & Magneto ignition systems

The high tension voltage characterizes of a coil ignition system is different

from that of the magneto ignition system. Where as in coil ignition this voltage
decrease with increase in the engine speed the current values at the moment of
break of the contact breakers for the two systems are shown in fig. it shows that the
coil ignition gives a higher current at lower speeds but the magneto current is more
at higher engine speeds.

Fig. 6.3:
Comparison of performance between coil and magneto ignition system at different engine
speed versus (a) voltage developed (b) current at break at the contact breaker

6.3.4 Electric Ignition Systems

The coil ignition system is an electro-mechanical arrangement which suffers

from the following limitations.

1. The contact breakers points are switched on and off frequently to arrow flow
of large inductive current it not only requires regular dining & resetting but
also causes wear and burning of the points.
2. The contact breakers point does not function well at higher speeds because
of its inertial effect.
3. The dwell period in contact breakers point at higher speeds is insufficient to
build-up fail strength magnetic field which results in a weak spark.
4. The inductive circuit restricts satisfactory sparking rate above 300-400
sparks per second. This means that in a six-cylinder engine the sparking will
be erratic above 9000-12000 rpm. Since the racing car engines run above
 this speed, hence coil ignition system is not fit for their satisfactory
operation.
To obviate these limitations, the electronic ignition is a good answer.

Merits of electronic ignition system employs solid-state electronic device

made from semiconductor. Transistor & thruster are such devices which are rugged
and comport limits & are capable of switching the electricity without opening &
closing the contact the timely sized trausistor behaves as a relay & can operate as a
switch also at speeds much higher than those required for an ignition system.

Types: - Electronic ignition systems can be of different kinds as given below

1. Semi transistorized (or hybrid) coil ignition system (TCI system)

2. Fully transistorized (or contact breaker less) coil ignition system (TCI
system)
3. Capacitive discharge ignition (CDI) system.

1.3.5 Semi-transistorized (or hybrid) Ignition System

In this case, a transistor unit is interposed between the ignition coil & the
contact-breaker the function of transistor is to interrupt the primary current which
is being done by C.B points in coil ignition system. In this case, the C.B. points
serve to carry a very small non-inductive control current of transistor this helps in
much reduced wear of points, absence of arcing & improved low & high speed
performances the transistor-assisted contact results in a better starting also. Also
because the transistor can handle a much higher main current than could the
contact breaker points when used in the normal way, the design of the coil can be
modified to improve its performance at high speed.
 Fig. 6.4: Simplified diagram of a semi transistorized ignition system (a) when the points are
closed and (b) when the points are open

The contact breaker & condenser can be dispensed with and contact
adjustments are required only at 40,000 km intervals, solely to compensate for
mechanical wear of the operating mechanism.
1.3.6 Fully transistorized (or contact breaker less) ignition system

This is more advanced system & is vastly used on racing engines. It employs
two stages of transistor switching in conjunction with a special design of coil, &
eliminates the needs of C.B. points that is why the system is also known as
“breaker less” it has been employed on Volvo 740 car engine.

Fig. 6.5: A simplified diagram of a fully transistorized ignition system

In this arrangement the transistor are triggered by electrical pulses produced

by an electromagnetic pickup, for that the pole pieces are used around the engine
flywheel this system is capable of producing well-timed sparks up to 1000 per
second which is sufficiently enough to operate the engines at 15000 rpm. A typical
circuit is shown which is used on contests classic car engine. A spark is generated
in it when one of the probes is released.
6.3.7 CDI Ignition

Capacitor discharge ignition is generally used on engines of 2 wheelers. In

this system a capacitor is used to store the ignition energy the amount of stored
energy is decided by the capacitance used & the charging voltage of capacitor. The
ignition transformer steps up the primary voltage which is generated at the time of
spark by the discharge of capacitor through the thyristor, to a high voltage
necessary at the spark plug. It works in conjunction with a magneto & employs a
CDI unit it performs the following functions.

Fig. 6.6: A CDI system (a) layouts (b), (c) Working

 1. As the rotor turns, current is produced in the source coil & then stored
in the ignition condenser.
2. At the same time, the pickup coil sends a pulse to activate the thyristor.
3. Then the ignition conductor discharge the stored current rapidly this
induces a high voltage in ignition coil, there by a strong spark is
produced.

6.3.8 Advantage of CDI system over battery ignition system

A CDI system processes following advantage over battery ignition system.

1. The condenser of CDI can store many thousand times more energy. It
therefore allows a high output voltage at higher speeds when sparking
rates are also high.
2. The internal resistance is small it therefore allow a faster rise in voltage.
Hence the system does not get side tracked.
3. Since the breaker points serve as a trigger only, this system is free from
frequent maintenance of C.B points.
4. This system draws less current from the battery at low speeds, which
increases with increase in engine speed. Due to this feature, the low speed
efficiency is increased & the cold starting also become easier.
5. The output voltage is relatively independent of engine speed this
eliminates the chances of misfire even when the spark play is fouled.
Fig. 6.7: Merits of a CDI system showing (a) Current drawn from the battery and (b) Output
voltage

6.3.9 The components used for ignition systems

1. Battery
To provide electrical energy for ignition, a storage battery is used. It is
charged by a dynamo driven by the engine. Owing to the electrochemical
reactions, it is able to convert the chemical energy into electrical energy. The
battery must be mechanically strong to with stand the strains to which it is
constantly subjected to given reason able care & attention two years or more
trouble free life may be obtained from a battery.
Two types of batteries are used for spark ignition engines, the lead acid
battery & the alkaline battery the former is used in light duty commercial
vehicles & the later on heavy duty commercial vehicles.
2. Ignition Switch
Battery is connected to the primary winding of the ignition coil through on
ignition switch & ballast resistor with the help of the ignition switch the
ignition system can be turned on or off.
3. Ballast Resistor
A ballast resistor is provided in series with the primary winding to regulate
the primary current the object of this is to prevent injury to the spark coil by
overheating if the engine should be operated for a long time at low speed, or
should be started with the breaker in the closed position this coil is made of
iron wire, & iron has the property that its electrical resistance increases very
rapidly if a curtain temperature is exceeded the coil is therefore made of wire
of such size that if the primary current flows nearly continuously, the ballast
coil reaches a temperature above that where this rapid increase in resistance
occurs. This additional resistance in the primary circuit holds the primary
current down to a safe value for starting from cold this resistor is by passed
to allow more current to flow in the primary circuit.
4. Ignition Coil
Ignition coil is the source of ignition energy in the conventional ignition
system this coil stores the energy in its magnetic field & delivers it at the
appropriate time in the form of a ignition pulse through the high tension
ignition cables to the respective spark plug.
The purpose of ignition coil is to step up the 6 to 12 volts of the battery to a
high voltage sufficient to induce an electric spark across the electrodes of the
spark plug.
 Fig. 6.8: Ignition Coil

The ignition coil consists of a magnetic core of soft iron wire or sheet & two
insulated conducting coils, called primary & the secondary windings.
5. Contact Breaker
This is a mechanical device for making & breaking the primary circuit of the
ignition coil it consist essentially of a fixed metal point bears which is being
on a spring loaded pivoted arm.
 Fig. 6.9: Contact Breaker

When points are closed the current flows & when they are open, the circuit
is broken & the flow of current stops.

6. Capacitor
The principal of construction of the ignition capacitor is the same as that of
every electrical capacitor, which is very simple two metal plates separated
by an insulating material are placed face to face the insulation is often only
air. But in most cases it consists of some high quality insulation materials
suitcase for the particular technical requirements, capable of withstanding
electrostatic stresses without suffering damages the metal plates themselves
are usually replaced by metal foil or by metallic layers deposited by
evaporation on the insulation material itself. In order to save space, these
thin strips, for example, consisting of two strips of aluminum foil &several
layers of special capacitor paler are rolled up in a solid roll contacts are
attached to the two metal strips & the entire roll is first impregnated in an
oily or waxy material to improve the insulating properties of the paper &
 then the roll is inserted into a metal shell for protection against moisture,
external physical contact & damage.

7. Distributor
The function of the distributor is to distribute the ignition surges to the
individual spark plugs in the correct sequence & at the correct instant in
time. Depending on whether a particular engine has 4,6 or 8 cylinder, there
are 4,6 or 8 ignition pulse generated for every rotation of the distributor
shaft. The use of a distributor represents a considerable simplification in a
battery ignition system because in most cases we wale t use only a single
ignition circuit the contact breaker & the spark advance mechanism are
combined with the distributor in a single unit because of the absolute
necessity that the distributors operate in synchronism with the crankshaft.
There are two types of distributors, the brush type & the gap type in the
former, carbon brush called by the rotor arm slides over metallic segments
embedded in the distributor cap of molded insulating material, thus
establishing electrical connection between the secondary winding of the coil
& the spark plug, while in latter the electrode of the rotor arm pass close to,
but does not actually contact the segments in the distribution cap with the
latter type of distributor, there will not be any appreciable wear of the
electrodes.

8. Spark plug
The spark plug provides the two electrodes with a proper gap across which
the high potential discharges to generate a spark & ignite the combustible
mixture within the combustion chamber.
A spark plug consists essentially of a steel shaft an insulator & two
electrodes the control electrode to which the high knsion supply from the
ignition coil is connected, is well insulated with porcelain or other ceramic
materials the other electrode is welded to the steel shell of the plug &
thereby is automatically grounded when the plug is instated on the cylinder
head of the engine. The electrodes are usually made of high nickel alloy to
with stand the severe erosion and corrosion to which they are subjected in
use.

Fig. 6.10: Schematic of a typical spark plug

The tips of the central electrode & the insulator are exposed to the
combustion gases. This results in the insulator having a tendency to crack
from the high thermal & mechanical stresses. Some insulators are also
seriously affected by moisture and by abnormal surface deposits. Since the
central electrode & the insulator are subjected to the high temperature of the
combustion gases, the heat must flow from the insulator to the steel shell
which is in contact with the relatively cool cylinder head in order to cool the
electrodes & there by prevent pre ignition.
Spark plug are usually classified as hot plug or cold plug depending upon
the relative operating temperature range of the tip of the high tension
electrode. The operating temperature is governed by the amount of heat
transferred which in turn depends on the length of the heat transfer path from
the tip to the cylinder head and on the amount of surface area exposed to the
combustion gases. A cold plug has a short heat transfer path and a small area
exposed to the combustion gases as compared to a hot plug.
The type of spark plug used in an engine depends on the particular engine
requirement. Every engine manufacture determines the type of plug, cold or
hot, that is best switch to his engine.