ignition system

IGNITION SYSTEM-SI ENGINE
……the ultimate spark creator in petrol
engine….
BY
S K Singh
Centre for Energy Studies
IIT Delhi

BASICS OF IGNITION SYSTEM
SI engine – low compression ratio and high self ignition temperature
of fuel – so for combustion ignition source is must.
Principle: A conventional ignition system should provide sufficiently
large voltage across the spark plug electrodes to affect the spark
discharge.
Air is poor conductor of electricity- an air gap in an electric circuit
acts as a high resistance. But when a high voltage is applied across
the electrodes of a spark plug, it produces a spark across the gap.
When such a spark is produced to ignite a homogeneous A/F
mixture in the combustion chamber of an engine it is called the
spark ignition engine (or SI Engine).

BASIC REQUIREMENT OF IDEAL IGNITION SYSTEM
• Supply necessary energy within a small volume in a time sufficiently
short to ensure that minimum energy is lost other than that needed
to establish the flame under all conditions of operation.
• For A/F range 12-13:1 a spark under 10 millijoules is sufficient to
initiate the combustion.
• The system must produce and distribute high voltage of the order of
20000 volts to 22000 volts for generating spark between the
electrode gap
• The system must have a source of electrical energy (may be a
battery or a generator or a magneto).
• Battery voltage is usually 6 to 12 volts. This low voltage is boosted
by means of an induction coil, contact breaker and a condenser to a
value sufficient to meet the breakdown voltage and to release
sufficient energy for spark generation.

TYPES OF IGNITION SYSTEM
1. Battery Ignition system
2. Magneto ignition system
3. Electronic ignition system

COMPONENTS OF BATTERY IGNITION SYSTEM

1- A Ballast resistor: In series with primary winding – to regulate
primary current. The objective of BR is to prevent injury to the spark
coil by overheating if the engine should be operated for a long time
at low speed, or stalled with breaker in closed position.
This coil is made of iron wire, and iron has the property that its
electrical resistance increases very rapidly if a certain
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 primary circuit
holds the primary current down to safe value.
For starting this resistor is by passed so that more current can
flow in the primary circuit.
COMPONENTS OF THE IGNITION SYSTEM

2- A cam rotating at camshaft speed operates the contact
breaker points and causes them to open and close.
3- Ignition coil to step up the 6 volts or 12 volts of the
battery to a high tension voltage sufficient to promote
and electric spark across the electrodes of the spark
plug.
4- A distributor to distribute the high voltage to different
spark plugs at correct time
5- A battery : 6 volts or 12 volts (Lead acid – light duty
commercial vehicles : Alkaline battery – heavy duty
commercial vehicles)
6- Contact breaker points (CB point) : Made of hard
material (tungsten) with 3 mm diameter.
COMPONENTS OF BATTTERY IGNITION SYSTEM

IGNITION COIL CONSTRUCTIONAL DETAILS
The Ignition coil consist of two insulated conducting coil-
primary and secondary.
The primary winding is connected to the battery through an
ignition switch and the contact breaker.
The secondary winding is connected to the spark plugs
through the distributor
In order to boost the voltage the primary windings has a few
hundred turns (100-300) of relatively thick wire (20 gauge)
while the secondary windings contain several thousand turns
(20000-22000) of thin wire (38-40 gauge enameled copper wire
sufficiently insulated to withstand high voltage). More heat is
generated in primary than secondary and so primary winding
is wounded over secondary to dissipate heat.
The two wires are also interconnected allowing the secondary
windings to be earthed through the primary circuit

WORKING OF BATTERY IGNITION SYSTEM
• When ignition switch closed (switch on) – primary winding is
connected to positive terminal post of storage battery. If
primary circuit is closed through the CB point, a current flows –
which is called primary current
• Primary current produces a magnetic field in core.
• A cam driven CB opening mechanism opens it when spark is
required
• When CB points are open, the current which had been flowing
now flows into the condenser.
• As the condenser becomes charged, the primary current falls
and magnetic field collapses.
• The collapse of the field induces a voltage in the primary
winding, which charge the condenser to a voltage much higher
than the battery voltage.
• The condenser then discharges into the battery, reversing the
direction of both the primary current and the magnetic field.

WORKING OF BATTERY IGNITION SYSTEM
• The rapid collapse and reversal of the magnetic field in the core
induce a very high voltage in the secondary winding.
• The secondary winding consists of large number of turns of
very fine wire wound on the same core with the primary.
• The high secondary voltage is led to the proper spark plug by
means of rotating switch (distributor).

2-MAGNETO IGNITION SYSTEM WORKING
Magneto- special type of electric generator
It is mounted on the engine and replaces all the components
of the coil ignition system except the spark plug and cb point
- A magneto when rotated by the engine, is capable of
producing very high voltage and does not need a battery
as a source of external energy
- The high tension magneto incorporates in itself
windings to generate as well as to step up the voltage
and thus does not require a separate coil to boost up
the voltage required to generate the spark.
- Magneto can be rotating armature type or rotating
magneto type
- The operation of the magneto ignition system is exactly
same as that of the battery coil ignition system. In MIS
as the breaker points are opened and closed with the
help of a cam, the primary circuit flux is charged and
a high voltage is produced in the secondary
circuit

Magneto Vs Battery Ignition system
S.No Battery Ignition system Magneto Ignition system
1 Requires battery: Difficult to
start the engine when battery is
discharged
No battery, no discharge
2 More maintenance problems
due to battery
Less maintenance
3 Current for primary circuit is
obtained from battery
Current is generated by magneto
4 A good spark is available at spark
plug even at low engine speed.
During starting quality of spark is
poor due to low speed.
5 Efficiency of system decreases
with reduction in spark intensity
as engine speed rises.
Efficiency of system increases with
increase in engine speed due to
high intensity spark
6 Occupies more space Occupies less space
7 Used in cars and LCV’s Used in Racing cars and two
wheelers

LIMITATIONS OF BREAKER SYSTEMS
AND NEED OF MODERN IGNITION
SYSTEMS
• Contact Breaker systems: Available voltage decreases
as engine speed increases due to limitations in current
switching capability of the breaker system and the
decreasing time available to build up the stored energy
in the primary coil.
• Due to high current load, the breaker points are
subjected to electrical wear in addition to
mechanical wear which requires frequent
maintenance
• So there is a need to develop advanced (Modern)
ignition systems (like TCI and CDI)

TRANSISTORISED IGNITION SYSTEM
OR
TRANSISTOR ASSISTED CONTACT SYSTEM (TAC)

The inherent property of transistor is to interrupt a relatively high current
carrying circuit, i.e its ability to control a much larger current in the collector
circuit with a small current in the base circuit, makes it ideal replacement for
the breaker points and condenser of a conventional ignition system.
The emitter (E) of the transistor is connected to the ignition coil through a
ballast resistor and the collector (C) to the battery. When the cam operated
contact point opens the base current and , thereby, the primary circuit current
is interrupted and the normal induction coil operation follows.
The current through the contact breaker is reduced by a large amount due to
the fact that the contact breaker has to switch only the base current
(which amounts to only 100 milliamps for the
point saver module (versus 4 amps in the
conventional battery ignition system).
The breaker points now control a low current, non inductive circuit
that produces minimal arcing, thus the condenser is no longer
required.
The life of cb point is greatly increased.

Contd..
The power transistor functioning as a relay in the primary,
grounds the circuit when points are closed and opens the
circuit when the points break, thus mimicking the action of the
breakers.
It can easily handle the back emf generated by the coil without
even getting warm.
Switching action is extremely rapid due to the nature of
semiconductor action and the elimination of condenser from
the circuit (coil field collapse time is a function of time required
to charge the condenser).
The faster switching produces a faster voltage rise time (by a
factor of 5) at the coil. The effect is a higher secondary voltage
(with shorter current duration) delivered to the spark plug.

Transistorized Ignition
System advantages
• In automotive applications, the transistorized
coil ignition systems which provide a higher
output voltage and use electronic triggering to
maintain the required timing are fast replacing the
conventional ignition systems. These systems are
also called high energy electronic ignition
systems. These have the following advantages:-
1. Reduced ignition system maintenance
2. Reduced wear of the components.
3. Increased reliability
4. Extended spark plug life
5. Improved ignition of lean mixtures

disadvantages
Source impedance: Opposition exhibited by output terminals to an AC of a
particular frequency as a result of resistance, inductance and capacitance

4-HIGH VOLTAGE CAPACITIVE DISCHARGE IGNITION
OR
CAPACITIVE DISCHARGE IGNITION (CDI)
In CDI ignition system, a capacitor is used as a means of energy storage. It
is charged to a high voltage (about 300volts) by means of a transformer and
at the moment of ignition, discharged by the thyristor through the primary
circuit which generates a high voltage pulse in the secondary circuit to fire
the spark plug
CDI Trigger box contains – Capacitor, thyristor power switch (SCR), charging
device (to Convert battery voltage to the charging voltage of 300 to 500 V by
means of pulse Via a voltage transformer
Converts 6 v dc of
battery into 250 to 350
volts 6v
SCR: Silicon
controlled
rectifier

Working principle of CDI system
• A battery of 6 volts usually connected to a transistorized dc
to dc converter which is designed to give high voltage in the
range of 250-350 volts from battery.
• The condenser is charged to this output voltage of the
converter through the charging resistance.
• The resistance is also designed that it controls the required
current in the SCR (silicon controlled rectifier) and it also
increases the damping in the spark plug.
• The condenser is discharged when the SCR triggering
device sends a pulse and produce a high voltage in the
secondary winding which subsequently jumps across the
air gap between the electrodes of the spark plug producing
a spark.

• Capacitive discharge ignition systems work by storing energy in an
external capacitor, which is then discharged into the ignition coil
primary winding when required. This rate of discharge is much higher
than that found in inductive systems, and causes a corresponding
increase in the rate of voltage rise in the secondary coil winding.
• This faster voltage rise in the secondary winding creates a spark that
can allow combustion in an engine that has excess oil or an over
rich fuel air mixture in the combustion chamber. The high initial spark
voltage avoids leakage across the spark plug insulator and
electrodes caused by fouling, but leaves much less energy available
for a sufficiently long spark duration; this may not be sufficient for
complete combustion in a “lean burn” turbocharged engine resulting
in misfiring and high exhaust emissions.
• Ignition in lean fuel mixtures by capacitor discharge systems can
sometimes only be accomplished by the use of multi-spark ignition,
where the ignition system duplicates the prolonged spark of inductive
spark systems by sparking a number of times during the cycle. This adds
greater stress onto the high-tension leads and can cause considerable
spark plug wear and possible failure.
CDI SYSTEM Contd..

CDI SYSTEM
• Capacitor discharge ignition (CDI) or thyristor ignition is
a type of automotive electronic ignition system which is
widely used in motorcycles, lawn mowers, chain saws,
small engines, turbine powered aircraft, and some cars.
• It was originally developed to overcome the long charging
times associated with high inductance coils used in
inductive ignition systems, making the ignition system
more suitable for high engine speeds (for small engines,
racing engines and rotary piston engines).
• Capacitor discharge ignition uses capacitor discharge
current output to fire the spark plugs.

• Electronic capacitor discharge ignition (CDI) systems
have been common on large industrial engines because the
technology has been in use since the 1960's.
• An advantage of the capacitor discharge ignition system is
that the energy storage and the voltage ‘step up'
functions are accomplished by separate circuit
elements allowing each one to be optimised for its job.

ADVANTAGES OF CDI SYSTEM OVER TCI SYSTEM

Advantages and Disadvantages of CDI
• A CDI system has a short charging time, a fast voltage
rise (between 3 ~ 10 kV/μs) compared to typical
inductive systems (300 ~ 500 V/μs) and a short spark
duration limited to about 50-80 µs. The fast voltage rise
makes CDI systems insensitive to shunt resistance, but
the limited spark duration can for some applications be
too short to provide reliable ignition. The insensitivity to
shunt resistance and the ability to fire multiple sparks can
provide improved cold starting ability.

VACUUM ADVANCE
The vacuum adavnce on spark timing is necessary because the lean mixtures require an
earlier spark timing than the rich mixtures. Therefore as the throttle is closed the spark
must be advanced for optimum performance

Vacuum advance
• When an engine is at idle, it experiences high manifold
vacuum. If you have ever popped a vacuum line off of an
idling automobile engine, then you have heard the loud
hiss that results. That is manifold vacuum suction. Because
the fuel is traveling so slowly and the engine is under
no load, the vacuum in the engine operates the advance
on the distributor, which then advances the timing so the
engine idles smoothly and the spark is happening at the
correct time. When the accelerator is depressed hard or
when the vehicle is under a good load, there is essentially
zero manifold vacuum. Therefore, the advance mechanism
on the distributor is not actuated at all, resulting in no timing
advance.

Vacuum advance mechanism
• Vehicles equipped with vacuum advanced distributors (usually older
models) have a "pod" on the side of the distributor, with a rubber hose
attached to it. The hose connects directly to the intake manifold of the
engine so that it receives the full vacuum, or lack there of.
• At idle, when the manifold vacuum is high, the tube attached to the
advance pod will suck in, causing the distributor to advance the
timing. There are springs inside the distributor which hold the
timing mechanism in place. They remain in that position until the
manifold vacuum affects them.
• Newer automobiles sometimes use a centrifugal system that uses
weights instead of vacuum. The problem with them is that they are
based only on engine RPM (revolutions per minute) and cannot detect
whether or not the engine is under a load or not. Because of that, the
vacuum advanced systems are much more efficient at advancing or
retarding the timing to the engine's needs.

centrifugal advance mechanism
The centrifugal advance mechanism advances the ignition
spark as engine speed increases. This is accomplished by
two weights attached to the cam. As the engine's speed and
that of the distributor shaft increase, the weights move
outward by centrifugal force, thereby turning the cam and
causing the ignition to advance.

CENTRIFUGAL ADVANCE CONTD…..
• As speed increases, centrifugal force on the weights moves
them outwards against spring tension.
• This movement causes the distributor cam or trigger wheel
to move ahead. With this design, the higher the engine speed,
the faster the distributor shaft turns, the
farther out the advance weights move, and the farther
ahead the cam or trigger wheel is moved forward or advanced.
At a preset engine speed, the lever strikes a stop and
centrifugal advance reaches maximum.
• The action of the centrifugal advance causes the contact points
to open sooner, or the trigger wheel and pickup coil turn off
the ECU sooner. This causes the ignition coil to fire with the
engine pistons not as far up in the cylinders.

COMPUTERIZED ADVANCE
• The computerized advance, also known as an electronic spark
advance system, uses various engine sensors and a computer to
control ignition timing. The engine sensors check various operating
conditions and sends electrical data to the computer
• The computer an change ignition timing for maximum engine efficiency.
Ignition system engine sensors include the following:-
1. ENGINE SPEED SENSOR (reports engine speed to the computer)
2. CRANKSHAFT POSITION SENSOR (reports piston position)
3. THROTTLE POSITION SWITCH (notes the position of the throttle)
4. INLET AIR TEMPERATURE SENSOR (checks the temperature of
the air entering the engine)
5. ENGINE COOLANT TEMPERATURE SENSOR (measures the
operating temperature of the engine)
6. DETONATION SENSOR (allows the computer to retard timing when
the engine knocks or pings)
7. INTAKE VACUUM SENSOR (measures engine vacuum, an indicator of
load) The computer receives different current or voltage levels (input
signals) from these sensors.

• It is programmed to adjust ignition timing based on engine conditions.
The computer may be mounted on the air cleaner, under the dash, on
a fender panel, or under a seat. The following is an example of the
operation of a computerized advance. A vehicle is traveling down the
road at 50 mph; the speed sensor detects moderate engine
speed.
• The throttle position sensor detects part throttle and the air inlet and
coolant temperature sensors report normal operating temperatures.
The intake vacuum sensor sends high vacuum signals to the
computer. The computer receives all the data and calculates that the
engine requires maximum spark advance. The timing would occur
several degrees before TDC on the compression stroke. This action
assures that high fuel economy is attained on the road.
• If the operator began to pass another vehicle, intake vacuum sensor
detects a vacuum drop to near zero and a signal is sent to the
computer. The throttle position sensor detects a wide, open throttle
and other sensor outputs say the same.
• The computer receives and calculates the data, then, if required,
retards ignition timing to prevent spark knock.

Mechanical Fuel Pump
• A mechanical fuel pump is a device that delivers fuel to the engine.
Due to the fact that mechanical fuel pumps generally work best on
carbureted engines, they are typically found only on older vehicles. A
carburetor squats over the engine on many older cars and delivers fuel
into the combustion chambers via intake ports. Some older cars ran
the fuel lines below the gas tank so that gravity does the work of
carrying gasoline from the tank to the carburetor. However, many older
models needed a little additional help to get the fuel from the tank to
the carburetor. The mechanical fuel pump, which was usually located
on the side of the engine, was therefore designed to run off of the
engine’s momentum and provide the carburetor with a steady supply of
fuel.

FIGURE A mechanical fuel pump.

• A mechanical fuel pump mounts to the side of the engine. A lever or
push rod on the pump passes through an opening in the side of
the engine and lines up with a special lobe on the camshaft. As the
camshaft turns, the lobe moves the lever up and down, raising and
lowering a flexible diaphragm inside the pump. With the motion of the
diaphragm, gasoline is drawn down the fuel lines and into the
pump. From there, the gasoline is pushed into the carburetor, which
uses the vacuum of the engine to pull fuel into the combustion
chambers.
Because diaphragm fuel pumps run directly
off of the engine, they will only work when
the engine is running or being started. This
prevents dangerous buildups of fuel from
occurring in the lines. Mechanical fuel pumps
also do not pressurize the system very
highly:-
Most carbureted fuel systems run as low as 4
pounds per square inch (psi), and rarely ever
more than 15 psi.

• Since diaphragm fuel pumps are located right on the side of the
engine, replacing this part is relatively easy and quick. However,
despite the low pressure levels of carbureted fuel systems, if you
disconnect the line that feeds into a mechanical fuel pump, it will
drip fuel. The fuel line is below the gas tank, so gravity will
continuously pull fuel down the line if you do not plug it. You will
also need to be sure that you correctly line the lever or push rod up
with the engine’s camshaft when installing the new fuel pump, so that
you do not inadvertently damage the pump when you turn the engine
over.
• Eventually electronic fuel injection systems, which were favored for
greater accuracy and control, replaced carburetors. When this
happened, cars also began to require fuel systems that ran under
high pressure, generally between 40 and 60 psi. Because a
mechanical fuel pump cannot support a system under that much
pressure, they were eventually cast aside in favor of the higher
performing electronic fuel pump.

Decline of mechanical pumps
• As engines moved away from carburetors and towards fuel
injection, mechanical fuel pumps were replaced with electric fuel
pumps, because fuel injection systems operate more efficiently at
higher fuel pressures (40-60psi) than mechanical pumps can
generate. Electric fuel pumps are generally located in the fuel tank,
in order to use the fuel in the tank to cool the pump and to ensure
a steady supply of fuel.
• Another benefit of an in-tank mounted fuel pump is that a suction
pump at the engine could suck in air through a (difficult to diagnose)
faulty hose connection, while a leaking connection in a pressure
line will show itself immediately. A potential hazard of a tank-
mounted fuel pump is that all of the fuel lines are under high pressure,
from the tank to the engine. Any leak will be easily detected, but is
also hazardous.
• Electric fuel pumps will run whenever they are switched on, which
can lead to extremely dangerous situations if there is a leak due to
mechanical fault or an accident. Mechanical fuel pumps are much
safer, due to their lower operating pressures and because they 'turn
off' when the engine stops running.

• WARNING: A leaky fuel pump is dangerous because the
fuel may ignite and start a fire!
• Another way to check the pump is to disconnect the fuel
line at the carburetor and place the end of the line into a
container. Crank the engine to see if the pump is pushing
any fuel through the line. Strong steady spurts of fuel
mean the pump is working. No fuel or a weak stream
means a bad pump, a plugged fuel filter, fuel line
blockage or no fuel in the tank.
• WARNING: Do not smoke near gasoline, and do not allow
any sparks near the carburetor or open fuel line as this
may ignite the fuel causing a fire! Do not spill gasoline on
a hot engine. Wait until the engine has cooled to work on
the fuel system. Also, avoid skin contact with gasoline
and do not breathe the vapors.

• The fuel pump is a submersible pump
with a permanent magnet electric
motor. Fuel enters the pump inlet tube
after passing through a sock style filter
and is pushed through the pump by the
motor to the outlet.
Electronic Fuel Pump
It consists of a motor, a vane roller pump, a fuel damper,
and a relief valve to prevent system damage from over
pressure. The fuel pump shares its mounting cage with the fuel
gauge sending unit.

• Long time back when the diesel
engines had not much evolved
like today, to extract best
performance and long life
adequate fuel pressure in the
lines was necessary for diesel
pump & nozzles to work at their
optimum. Over a period of
time, the pump used to get
weak leading to drop in fuel
pressure in lines and laborious
working of the engine. This
used to result in fuel pump
calibration that was a costly
affair.
FIGURE A typical fuel pump model assembly, which
includes the pickup strainer and fuel pump, as well as the
fuel-pressure sensor and fuel level sensing unit.

• To ensure optimum performance,
additional electric fuel pump, either in
the tank or near diesel filter intake
used to be installed; not in all but in
some where the owners were
knowledgeable or the mechanic
suggested so. In addition to maintaining
proper fuel pressure, one problem that
was the bane of all diesel owners was
of air interlock that was due to, no fuel,
damaged fuel line, frayed 'o' ring of the
filters etc., that resulted in stalling,
misfiring etc. Electric fuel pump
removed the air interlock problem.
FIGURE A roller cell-type electric fuel pump.

SPARK PLUG OPERATING TEMPERATURE

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