Download Ignition Systems

Lecture-16
Prepared under
QIP-CD Cell Project
Internal Combustion Engines
Ujjwal K Saha, Ph.D.
Department of Mechanical Engineering
Indian Institute of Technology Guwahati
1
Introduction
‰ The
combustion in a spark ignition
engine is initiated by an electrical
discharge across the electrodes of a
spark plug, which usually occurs from 100
to 300 before TDC depending upon the
chamber geometry and operating
conditions.
‰ The ignition system provides a spark of
sufficient intensity to ignite the air-fuel
mixture at the predetermined position in
the engine cycle under all speeds and
load conditions.
2
Introduction – contd.
In a four-stroke, four cylinder engine
operating at 3000 rpm, individual cylinders
require a spark at every second revolution,
and this necessitates the frequency of firing
to be (3000/2) x 4 = 6000 sparks per minute
or 100 sparks per second. This shows that
there is an extremely short interval of time
between firing impulses.
‰
3
Introduction – contd.
‰ The internal combustion engines are
not capable of starting by themselves.
Engines fitted in trucks, tractors and other
industrial
applications
are
usually
cranked by a small starting engine or by
compressed air.
‰ Automotive
engines
are
usually
cranked by a small electric motor, which
is better known as a starter motor, or
simply a starter. The starter motor for SI
and CI engines operates on the same
principle as a direct current electric
motor.
4
Ignition System -Requirements
It should provide a good spark between the
electrodes of the plugs at the correct timing
‰
The duration of the spark must be long enough
with sufficient energy to ensure that ignition of the
mixture has a high chance of occurring
‰
The system must distribute this high voltage to
each of the spark plugs at the exact time in
every cycle, i.e., it must have in it a distributing
device
‰
It should function efficiently over the entire
range of engine speed
‰
It should be light, effective and reliable in
service
‰
5
Glow plug ignition
‰ One of the early ignition system employed
was the glow plug ignition used in some
kinds of simple engines like model aircraft.
‰ A glow plug is a coil of nichrome wire that
will glow red hot when an electric current is
passed through it. This ignites the air-fuel
mixture upon contact. The coil is electrically
activated from engine starting, and once it
runs, it will retain sufficient residual heat on
each stroke due to heat generated on the
previous stroke. Glow plugs are also used to
aid starting of diesel engines.
6
Contact ignition
‰ The other method used was the contact
ignition. It consisted of a copper or brass
rod that protruded into the cylinder, and
was heated using an external source.
Heat conduction kept the end of the rod
hot, and ignition takes place when the
combustible mixture comes into its
contact. Naturally this was very inefficient
as the fuel would not be ignited in a
controlled
manner.
This
type
of
arrangement was quickly superseded by
spark ignition.
7
Modern ignition systems
‰ The development of high speed, high
compression internal combustion engine
requires a reliable high-speed ignition
system. This is met by a high-tension ignition
system that uses a spark plug as the source
of ignition. The electrical energy to the spark
plug is supplied by one of the following
systems and is termed accordingly.
1.
2.
3.
Battery ignition system
Magneto ignition system
Electronic ignition system
8
Battery ignition system
Coil
ignition
switch
Ammeter
Secondary
winding
Distributor contacts
Primary
winding
1
2
Contact
breaker
3
capacitor
Distributor
Battery
4
Spark plugs
Contact-breaker
operating cam
9
Battery ignition system
‰ The primary circuit consists of the battery,
ammeter, ignition switch, primary coil
winding, capacitor, and breaker points. The
functions of these components are:
Battery
: provides the power to run the system
Ignition switch
: allows the driver to turn the system on and off
Primary coil
: produces the magnetic field to create the
high voltage in the secondary coil
Breaker points
: a mechanical switch that acts as the
triggering mechanism
: protects the points from burning out
Capacitor
10
Battery ignition system
‰ The
secondary circuit converts magnetic
induction into high voltage electricity to jump
across the spark plug gap, firing the mixture at the
right time. The functions of the components are:
Secondary
coil
Coil wire
Distributor
cap
: the part of the coil that creates the high voltage
electricity
: a highly insulated wire to take the high voltage to
the distributor cap
: a plastic cap which goes on top of the distributor,
to hold the high tension wires in the right order
Rotor
: spins around on the top of the distributor shaft,
and distributes the spark to the right spark plug
Spark plug
leads
Spark plugs
: another highly insulated wire that takes the high
voltage from the cap to the plugs
: take the electricity from the wires, and give it an
air gap in the combustion chamber to jump across,
to light the mixture
11
Magneto ignition system
‰ The
high powered, high speed spark
ignition engines like aircraft, sports and
racing cars use magneto ignition system.
The basic components of a magneto ignition
system consist of a magneto, breaker points,
capacitor, ignition switch, distributor, spark
plug leads, and spark plugs.
‰ Magneto can either be rotating armature
type or rotating magneto type. In the former,
the armature consisting of the primary and
secondary windings all rotate between the
poles of a stationary magneto, while in the
second type, the magneto revolves and the
windings are kept stationary.
12
Magneto ignition system
(with rotating magnets)
Distributor
Spark plugs
Coil
Cam
Primary winding
Secondary winding
Rotating magnet (two-pole)
Contact- breaker
Capacitor
Ignition
switch
13
Electric Circuit
‰ Uses a changing
magnetic field to
generate current
in primary and
secondary
circuits
14
Magnetic Flux
• As magnet
approaches,
induces magnetic
flux in armature
• Breaker points close
and current
dissipates through
primary circuit
15
Spark Fires
• After magnet rotates
past armature flux
reverses direction,
and the breaker
points open
• Change in magnetic
flux produces 170
volts in primary circuit
• Induces 10,000 volts
in secondary circuit,
firing spark plug
16
Breaker Points
• Crankshaft rotation
causes mechanical
actuation of breaker
17
Electronic ignition system
‰ The disadvantage of the mechanical
system is that it requires regular
adjustment to compensate for wear, and
the opening of the contact breakers,
which is responsible for spark timing, is
subject to mechanical variations.
‰ In addition, the spark voltage is also
dependent on contact effectiveness,
and poor sparking may lower the engine
efficiency. Electronic ignition system has
solved these problems.
18
Electronic ignition system
Ignition coil
Electronic
control unit
Spark plugs
Switch
1
2
Sensor coil
3
4
Battery
Armature
Distributor
19
Electronic ignition system – contd.
‰ In this system, the contact breaker points
are replaced by an angular sensor of some
kind - either optical, where a vaned rotor
breaks a light beam, or more commonly
using a hall effect sensor, which responds to
a rotating magnet mounted on a suitable
shaft.
‰ The sensor output processed by a suitable
circuitry is then used to trigger a switching
device such as a thyristor, which switches a
large flow of current through the coil.
20
Electronic ignition system – contd.
‰ The rest of the system (distributor and spark
plugs) remains the same as that of the
mechanical system. The lack of moving parts
compared with the mechanical system leads
to greater reliability and longer service
intervals. In some older cars, it was usually
possible to retrofit an electronic ignition system
in place of the mechanical one.
Ignition coil
Electronic
control unit
Spark plugs
Switch
1
2
Sensor coil
3
4
Battery
Armature
Distributor
21
Spark Plugs
The spark plug ignites
the
air-fuel
mixture
inside the cylinder. This
occurs
when
high
voltage, triggered at
precisely
the
right
instant, bridges the gap
between the center
and
the
ground
electrodes.
It
also
provides a secondary
purpose of helping to
channel some heat
away from the cylinder.
Terminal
‰
Insulator
Electrode
Shell
Reach
Gap
Ground
electrode
22
A cold plug has the
advantage of quicker heat
transfer. It has a shorter
insulator,
and
thereby
allowing heat to travel a
shorter distance.
‰
(a) Cold plug
A hot plug has a longer
insulator, and therefore, heat
travel path from firing tip to
electrode is longer. This
enables it to operate at
higher
temperature
to
compensate for the cooler
running engine.
‰
(b) Ho t plug
23
Firing Order
Firing order indicates the sequence or
order in which the firing impulses occur in a
multi-cylinder spark ignition engine. It is
chosen to give a uniform torque, and
hence a uniform distribution of firing per
revolution of the engine.
‰
This is naturally dictated by the engine
design, the cylinder arrangement and the
crankshaft design. The firing order be such
that there must always be a proper
balance so as to minimize the engine
vibration.
‰
24
Firing Order
‰ As for example, in a four-stroke, four-
cylinder engine, the firing or the ignition in
all the cylinders has to be completed in
two revolutions of the crankshaft. With
crank throws at 1800, the cylinders 1 and
4 will reach TDC at the same time. Now, if
the firing interval is made by 1800, the
firing in cylinder-1 cannot be followed by
cylinder-4. For the same reason, the firing
of cylinder-2 cannot be followed by
cylinder-3. As such, the possible
sequence is 1-2-4-3 or 1-3-4-2.
25
Firing Order
‰ Consider another example of four-
stroke, six-cylinder inline engine, where
cranks are set at 1200, and with the
cylinders 1-6, 2-5 and 3-4 will reach TDC
simultaneously. Here, the possible
sequence is 1-5-3-6-2-4 or 1-4-2-6-3-5.
For radial engines, the cylinders are
usually numbered consecutively. Thus,
for a seven-cylinder radial engine, the
sequence is 1,3,5,7,2,4,6.
26
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27
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