spark ignition system for automobile

1. University of Engineering and Technology
Lahore, Pakistan
by
Dr. Muhammad Mahmood Aslam Bhutta
PakistanLahore

2. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Objectives of Ignition System
1. Generate heat to Initiate the combustion of
the air fuel mixture
2. Quantity of heat must be enough to create a
self sustaining flame

3. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Combustion in SI Engines
In an SI engine, combustion ideally consists of an
exothermic subsonic flame progressing through a
premixed homogeneous air-fuel mixture.

4. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Ignition and Flame Development
Combustion is initiated by an electrical discharge across the
electrodes of a sparkplug.
This high-temperature plasma discharge between the electrodes
ignites the air-fuel mixture in the immediate vicinity, and the
combustion reaction spreads outward from there.
Flame can generally be detected at about 6° of crank rotation after
spark plug firing.
The spread of the flame front is greatly increased by induced
turbulence and swirl within the cylinder.

5. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Maximum voltage can be greater than 40,000 volts lasting for about
10-8 seconds. The total energy delivered during one discharge is
generally about 30 to 50 mJ.
This gives a peak temperature in the order of 60,000 K.
A stoichiometric mixture of hydrocarbon fuel requires about 0.2 mJ
(0.2 X 10-3J) of energy to ignite self-sustaining combustion. This
varies to as much as 3 mJ for non-stoichiometric mixtures.
The discharge of a spark plug delivers 30 to 50 mJ of energy, most of
which, however, is lost by heat transfer.

6. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Spark Ignition Systems
.
A number of systems are available, like
1. Simple magneto ignition systems
Most small engines and some larger ones uses a magneto
driven by the engine crankshaft to generate the needed
voltage for ignition coil
2. Battery ignition systems with coil ignition with or without
transistors
In this system a battery-coil combination is used . Most
automobiles use a 12-volt electrical system, including a 12-
volt battery. This low voltage is multiplied many times by the
coil that supplies the very high potential delivered to the spark
plug.
3. Capacitor discharge systems
Some systems use a capacitor to discharge across the spark
plug electrodes at the proper time

7. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Direct ignition
Some engines have a separate high-voltage generation system for
each spark plug, known as direct ignition system
The timing information is usually collected from the flywheel

8. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Magneto Ignition Systems
A Magneto is used for current is
induced in the LT winding by
the changing magnetic field.
A high voltage is induced in the
HT winding, it is significant only
when the rate of change of the
current is large in the LT winding
at the instant when the contact
breaker opens.

9. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan

10. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
A high voltage (200—300 V) is generated in the LT windings and this
energy is stored in the capacitor. Without the capacitor there would be
severe arcing at the contact breaker. Once the spark has ended, the
capacitor discharges.

11. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
• As engine speeds increase, the dwell period becomes shorter and
spark energy will be reduced.
• Such a system can produce up to 400 sparks per second; beyond
this the spark energy becomes too low and this lead to misfiring.
• For higher spark rates, twin coil/contact breaker or alternative
electronic systems can be used.
CB point Ignition System Overview

12. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
In this type of system battery voltage is used to drive a charging circuit
that raises the capacitor voltage to about 500 V.
At ignition, the energy stored in the capacitor is discharged through an
ignition transformer, the circuit being controlled by a thyristor.
Capacitive Discharge Ignition (CDI)
The short duration
(about 0.1 ms) spark
is generated; the rapid
discharge makes this
ignition system less
susceptible to spark
plug fouling

13. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Distributer

14. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Distributor
A distributor is used to distribute the
high voltage from the coil to the
correct cylinder.
The central shaft is driven at half engine
speed (for four-stroke cycles) and
rotor arm directs the HT voltage to
the appropriated sparking plug via
the distributor cap.
For a four-cylinder engine there is a
four-lobed cam, number of the lobes
of the cam depends upon the
number of cylinder connected to the
distributer. It operates the contact
breaker which is to interrupt the
current flowing in the primary circuit
of the ignition coil.

15. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
The contact breaker and capacitor are mounted on a plate that can
rotate a limited amount around the cams relative to the base plate.
The position of this plate is controlled by the vacuum unit, a spring-
controlled diaphragm that is connected to the inlet manifold.
The cams are on a hollow shaft that can rotate around the main shaft.
The relative angular position of the two shafts is controlled by the
spring loaded flyweights.

16. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Spark Plug
The gap distance between electrodes on a
modern spark plug is about 0.7 to 1.7 mm.
Smaller gaps are acceptable if there is a
rich air-fuel mixture or if the pressure is
high (high inlet pressure by turbocharging
or a high compression ratio).
If the electrode’s temperature is too hot,
pre-ignition will occur.
If the electrode’s temperature is too low,
carbon deposits will build up on the central
insulator, so causing electrical breakdown.

17. DR. Muhammad Mahmood Aslam Bhutta Lahore,Pakistan
Types of spark plugs
For older engines with worn piston rings that burn an excess of oil,
hotter plugs are recommended to avoid fouling.
Modern spark plugs are made with better, more expensive materials,
and have a much greater life span than those of a decade ago. Some
quality spark plugs with platinum-tipped electrodes are made to last
160,000 km (100,000 miles) or more.
Spark plugs with several electrodes and two or more simultaneous
sparks are now available. These give a more consistent ignition and
quicker flame development.
• A cool-running engine requires a ‘hot’
or ‘soft’ sparking plug with a long heat
flow path in the central electrode.
• A hot-running engine (e.g. a high
compression ratio engine) requires a
‘cold’ or ‘hard’ sparking plug with a
much shorter heat flow path.