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IGNITION SYSTEM
IGNITION SYSTEM
 In principle, a conventional ignition system should provide
sufficiently large voltage across the spark plug electrodes to
affect the spark discharge.
 Further, it should supply the required energy for the spark
to ignite the combustible mixture adjacent to the plug
electrodes under all operating conditions.
 The design of a conventional ignition system should take
these factors into account to provide the spark of proper
energy and duration at the appropriate time.
 As air is a 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
air-fuel mixture in the combustion chamber of an engine it
is called the spark-ignition system.
CLASSIFICATION OF IGNITION SYSTEM
The ignition systems are classified depending upon how
the primary energy for operating the circuit is made
available as:
(i) battery ignition systems
(ii) magneto ignition systems
In modern automobiles the following two types are in
common use.
(i) Transistorized coil ignition system (TCI system)
(ii) Capacitive discharge ignition system (CDI system)
REQUIREMENTS OF AN IGNITION SYSTEM
(i) It should provide a good spark between the
electrodes of the plugs at the correct timing.
(ii) It should function efficiently over the entire
range of engine speed.
(iii) It should be light, effective and reliable in
service.
(iv) It should be compact and easy to maintain.
(v) It should be cheap and convenient to handle.
(vi) The interference from the high voltage source
should not affect the functioning of the radio and
television receivers inside an automobile.
BATTERY IGNITION SYSTEM
 Most of the modern spark-ignition engines use battery
ignition system.
 In this system, the energy required for producing spark is
obtained from a 6 or 12 volt battery.
 The construction of a battery ignition system is extremely
varied.
 It depends on the type of ignition energy storage as well as
on the ignition performance which is required by the
particular engine.
 The reason for this is that an ignition system is not an
autonomous machine, that is, it does not operate
completely by itself, but instead it is but one part of the
internal combustion engine, the heart of the engine
BATTERY IGNITION SYSTEM
The essential components of the system are:
(i) battery
(ii) ignition switch
(iii) ballast resistor
(iv) ignition coil
(v) contact breaker
(vi) capacitor
(vii) distributor
(viii) spark plug
BATTERY IGNITION SYSTEM
OPERATION OF A BATTERY IGNITION SYSTEM
OPERATION OF A BATTERY IGNITION SYSTEM
 When the ignition switch is closed, the primary
winding of the coil is connected to the positive
terminal post of the storage battery. If the primary
circuit is closed through the breaker contacts, a
current flows, the so called primary current.
 This current, flowing through the primary coil, which
is wound on a soft iron core, produces a magnetic field
in the core.
 A cam driven by the engine shaft, is arranged to open
the breaker points whenever an ignition discharge is
required.
 When the breaker points open, the current which had
been flowing through the points now flows into the
condenser, which is connected across the points.
 As the condenser becomes charged, the primary
current falls and the magnetic field collapses.
OPERATION OF A BATTERY IGNITION SYSTEM
 The collapse of the field induces a voltage in the primary
winding, which charges the condenser to a voltage much
higher than battery voltage.
 The condenser then discharges into the battery, reversing
the direction of both the primary current and the magnetic
field.
 The rapid collapse and reversal of the magnetic field in the
core induce a very high voltage in the secondary winding of
the ignition coil.
 The secondary winding consists of a 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 a rotating switch called the distributor, which
is located in the secondary or high tension circuit of the
ignition system.
LIMITATIONS OF A BATTERY IGNITION SYSTEM
(i) The primary voltage decreases as the engine speed
increases due to the limitations in the current switching
capability of the breaker system.
(ii) Time available for build-up of the current in the primary
coil and the stored energy decrease as the engine speed
increases due to the dwell period becoming shorter.
(iii) Because of the high source impedance (about 500 kΩ)
the system is sensitive to side-tracking across the spark plug
insulator.
(iv) The breaker points are continuously subjected to
electrical as well as mechanical wear which results in short
maintenance intervals. Increased currents cause a rapid
reduction in breaker point life and system reliability.
MAGNETO IGNITION SYSTEM
OPERATION OF A MAGNETO IGNITION SYSTEM
 A schematic diagram of a high tension magneto
ignition system is shown in Fig.
 The high tension magneto incorporates the windings
to generate the primary voltage as well as to step up the
voltage and thus does not require a separate coil to
boost up the voltage required to operate the spark
plug.
 Magneto can be either rotating armature type or
rotating magnet type
OPERATION OF A MAGNETO IGNITION SYSTEM
 The working principle of the magneto ignition system
is exactly the same as that of the coil ignition system.
 With the help of a cam, the primary circuit flux is
changed and a high voltage is produced in the
secondary circuit.
 The variation of the breaker current with speed for the
coil ignition system and the magneto ignition system
is shown in Fig.
 It can be seen that since the cranking speed at start is
low the current generated by the magneto is quite
small.
 As the engine speed increases the flow of current also
increases.
Transistorized Coil Ignition (TCI) System
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:
(i) reduced ignition system maintenance
(ii) reduced wear of the components
(iii) increased reliability
(iv) extended spark plug life
(v) improved ignition of lean mixtures
Transistorized Coil Ignition (TCI) System
 The contact breaker and the cam assembly of the conventional
ignition system are replaced by a magnetic pulse generating
system which detects the distributor shaft position and sends
electrical pulse to an electronic control module
 The module switches off the flow of current to the primary coil
inducing a high voltage in the secondary winding which is
distributed to the spark plugs as in the conventional breaker
system.
 The control module contains timing circuit which later closes
the primary circuit so that the buildup of the primary circuit
current can occur for the next cycle.
 A magnetic pulse generator where a gear shaped iron rotor driven
by the distributor shaft rotates past the pole of a stationary
magnetic pickup, is generally used.
 The number of teeth on the rotor is equal to the number of
cylinders. The magnetic field is provided by a permanent
magnet.
Transistorized Coil Ignition (TCI) System
 As each rotor tooth passes the magnet pole it first increases
and then decreases the magnetic field strength ψ linked
with the pickup coil wound on the magnet, producing a
voltage signal proportional to dψ dt .
 In response to this the electronic module switches off the
primary circuit coil current to produce the spark as the
rotor tooth passes through alignment and the pickup coil
voltage abruptly reverses and passes through zero.
 The increasing portion of the voltage waveform, after this
voltage reversal, is used by the electronic module to
establish the point at which the primary coil current is
switched on for the next ignition pulse.
Transistorized Coil Ignition (TCI) System
Capacitive Discharge Ignition (CDI) System
 In this system, a capacitor rather than an induction
coil is used to store the ignition energy.
 The capacitance and charging voltage of the capacitor
determine the amount of stored energy.
 Ignition transformer steps up the primary voltage
generated at the time of spark by the discharge of the
capacitor through the thyristor to the high voltage
required at the spark plug.
 The CDI trigger box contains the capacitor, thyristor
power switch, charging device (to convert battery
voltage to charging voltage of 300 to 500 V by means of
pulses via a voltage transformer), pulse shaping unit
and control unit.
Capacitive Discharge Ignition (CDI) System
Capacitive Discharge Ignition (CDI) System
 The advantage of using this system is that it is
insensitive to electrical shunts resulting from spark
plug fouling.
 Because of the fast capacitive discharge, the spark is
strong but short (0.1 to 0.3 ms) which leads to ignition
failure during lean mixture operating conditions.
 This is the main disadvantage of the CDI system.
SPARK ADVANCE MECHANISM
 It is obvious from the above discussion that the point in the cycle
where the spark occurs must be regulated to ensure maximum
power and economy at different speeds and loads and this must
be done automatically.
 The purpose of the spark advance mechanism is to assure that
under every condition of engine operation, ignition takes place
at the most favorable instant in time, i.e., most favorable from a
standpoint of engine power, fuel economy, and minimum
exhaust dilution.
 By means of these mechanisms the advance angle is accurately
set so that ignition occurs before the top dead-center point of the
piston.
 The engine speed and the engine load are the control quantities
required for the automatic adjustment of the ignition timing.
Most of the engines are fitted with mechanisms which are
integral with the distributor and automatically regulate the
optimum spark advance to account for change of speed and load.
Effect of spark advance on P-Ɵ diagram
Sliding contact type
centrifugal advance mechanism
Centrifugal Advance Mechanism
 The centrifugal advance mechanism controls the ignition
timing for full-load operation.
 The adjustment mechanism is designed so that its
operation results in the desired advance of the spark.
 The cam is mounted, movably, on the distributor shaft so
that as the speed increases, the flyweights which are swung
farther and farther outward, shift the cam in the direction
of shaft rotation.
 As a result, the cam lobes make contact with the breaker
lever rubbing block somewhat earlier, thus shifting the
ignition point in the early or advance direction.
 Depending on the speed of the engine, and therefore of the
shaft, the weights are swung outward a greater or a lesser
distance from the center.
 They are then held in the extended position, in a state of
equilibrium corresponding to the shifted timing angle, by a
retaining spring which exactly balances the centrifugal force.
 The weights shift the cam either on a rolling contact or sliding
contact basis; for this reason we distinguish between the rolling
contact type and the sliding contact type of centrifugal advance
mechanism.
 The beginning of the timing adjustment in the range of low
engine speeds and the continued adjustment based on the full
load curve are determined by the size of the weights, by the
shape of the contact mechanisms (rolling or sliding contact
type), and by the retaining springs, all of which can be widely
differing designs.
 The centrifugal force controlled cam is fitted with a lower limit
stop for purposes of setting the beginning of the adjustment, and
also with an upper limit stop to restrict the greatest possible full-
load adjustment.
Centrifugal Advance Mechanism
Vacuum advance mechanism
 Vacuum advance mechanism shifts the ignition point under part load
operation.
 The adjustment system is designed so that its operation results in the
prescribed part-load advance curve. In this mechanism the adjustment
control quantity is the static vacuum prevailing in the carburettor, a
pressure which depends on the position of the throttle valve at any
given time and which is at a maximum when this valve is about half
open.
 The diaphragm of a vacuum unit is moved by changes in gas pressure.
 The position of this diaphragm is determined by the pressure
differential at any given moment between the prevailing vacuum and
atmospheric pressure.
 The beginning of adjustment is set by the pre-established tension on a
compression spring.
 The diaphragm area, the spring force, and the spring rigidity are all
selected in accordance with the part-load advance curve which is to be
followed and are all balanced with respect to each other.
Vacuum advance mechanism
 The diaphragm movement is transmitted through a vacuum
advance arm connected to the movable breaker plate, and this
movement shifts the breaker plate an additional amount under
part-load conditions in a direction opposite to the direction of
rotation of the distributor shaft.
 Limit stops on the vacuum advance arm in the base of the
vacuum unit restrict the range of adjustment. The vacuum
advance mechanism operates independent of the centrifugal
advance mechanism.
 The mechanical interplay between the two advance
mechanisms, however, permits the total adjustment angle at any
given time to be the result of the addition of the shifts provided
by the two individual mechanisms.
 In other words, the vacuum advance mechanism operates in
conjunction with the centrifugal advance mechanism to provide
the total adjustment required when the engine is operating
under part load.
Vacuum advance mechanism
FIRING ORDER
 Every engine cylinder must fire once in every cycle.
 This requires that for a four-stroke four-cylinder engine the
ignition system must fire for every 180 degrees of crank
rotation. For a six-cylinder engine the time available is only
120 degrees of crank rotation.
 The order in which various cylinders of a multi-cylinder
engine fire is called the firing order.
 The number of possibilities of firing order depends upon
the number of cylinders and throws of the crankshaft.
 It is desirable to have the power impulses equally spaced
and from the point of view of balancing this has led to
certain conventional arrangements of crankshaft throws.
 There are three factors which must be considered
before deciding the optimum firing order of an engine.
 These are:
(i) engine vibrations
(ii) engine cooling and
(iii) development of back pressure
FIRING ORDER
 Consider that the cylinder number 1 of the four-cylinder
engine, shown in Fig, is fired first.
 Pressure, p, generated in the cylinder number 1 will give
rise to a force equal to pA[b/(a+b)] and pA[a/(a+b)] on the
two bearings A and B respectively.
 The load on bearing A is much more than load on bearing
B.
 If the next cylinder fired is cylinder number 2, this
imbalance in load on the two bearings would further
aggravate the problem of balancing of the crankshaft
vibrations.
 If we fire cylinder number 3 after cylinder number 1, the
load may be more or less evenly distributed.
FIRING ORDER
 Further, consider the effect of firing sequence on engine
cooling.
 When the first cylinder is fired its temperature increases.
 If the next cylinder that fires is number 2, the portion of
the engine between the cylinder number 1 and 2 gets
overheated.
 If then the third cylinder is fired, overheating is shifted to
the portion between the cylinders 2 and 4.
 Thus we see that the task of the cooling system becomes
very difficult because it is then, required to cool more at
one place than at other places and this imposes great strain
on the cooling system.
 If the third cylinder is fired after the first the overheating
problem can be controlled to a greater extent.
FIRING ORDER
 Next, consider the flow of exhaust gases in the exhaust pipe.
 After firing the first cylinder, exhaust gases flow out to the
exhaust pipe.
 If the next cylinder fired is the cylinder number 2, we find that
before the gases exhausted by the first cylinder go out of the
exhaust pipe the gases exhausted from the second cylinder try to
overtake them.
 This would require that the exhaust pipe be made bigger.
Otherwise the back pressure in it would increase and the
possibility of back flow would arise.
 If instead of firing cylinder number 2, cylinder number 3 is fired
then by the time the gases exhausted by the cylinder 3 come into
the exhaust pipe, the gases from cylinder 1 would have sufficient
time to travel the distance between cylinder 1 and cylinder 3 and
thus, the development of a high back pressure is avoided.
FIRING ORDER
 For four-cylinder engines the possible firing orders are:
 1 − 3 − 4 − 2 or 1 − 2 − 4 – 3
 The former is more commonly used in the vertical
configuration of cylinders. For a six-cylinder engine
the firing orders can be:
 1 − 5 − 3 − 6 − 2 − 4 or 1 − 5 − 4 − 6 − 2 − 3 or
 1 − 2 − 4 − 6 − 5 − 3 or 1 − 2 − 3 − 6 − 5 − 4
FIRING ORDER
Spark Plug
 The spark plug provides the two electrodes with a proper
gap across which the high potential discharges to generate
a spark and ignite the combustible mixture within the
combustion chamber.
 A spark plug consists essentially of a steel shell, an
insulator and two electrodes.
 The central electrode to which the high tension 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
and thereby is automatically grounded when the plug is
installed on the cylinder head of the engine.
 The electrodes are usually made of high nickel alloy to
withstand the severe erosion and corrosion to which they
are subjected in use.
Spark Plug
 The tips of the central electrode and the insulator are
exposed to the combustion gases.
 This results in the insulators having a tendency to
crack from the high thermal and mechanical stresses.
 Some insulators are also seriously affected by moisture
and by abnormal surface deposits.
 Since, the central electrode and 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 and
thereby prevent pre-ignition.
Spark Plug
Heat transfer path of hot and cold spark plugs
Hot and Cold spark plugs
 Spark plugs are usually classified as hot plugs or cold plugs
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
Unit 4(ICE &GT)ignition system.pptx

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Unit 4(ICE &GT)ignition system.pptx

  • 2. IGNITION SYSTEM  In principle, a conventional ignition system should provide sufficiently large voltage across the spark plug electrodes to affect the spark discharge.  Further, it should supply the required energy for the spark to ignite the combustible mixture adjacent to the plug electrodes under all operating conditions.  The design of a conventional ignition system should take these factors into account to provide the spark of proper energy and duration at the appropriate time.  As air is a 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 air-fuel mixture in the combustion chamber of an engine it is called the spark-ignition system.
  • 3. CLASSIFICATION OF IGNITION SYSTEM The ignition systems are classified depending upon how the primary energy for operating the circuit is made available as: (i) battery ignition systems (ii) magneto ignition systems In modern automobiles the following two types are in common use. (i) Transistorized coil ignition system (TCI system) (ii) Capacitive discharge ignition system (CDI system)
  • 4. REQUIREMENTS OF AN IGNITION SYSTEM (i) It should provide a good spark between the electrodes of the plugs at the correct timing. (ii) It should function efficiently over the entire range of engine speed. (iii) It should be light, effective and reliable in service. (iv) It should be compact and easy to maintain. (v) It should be cheap and convenient to handle. (vi) The interference from the high voltage source should not affect the functioning of the radio and television receivers inside an automobile.
  • 5. BATTERY IGNITION SYSTEM  Most of the modern spark-ignition engines use battery ignition system.  In this system, the energy required for producing spark is obtained from a 6 or 12 volt battery.  The construction of a battery ignition system is extremely varied.  It depends on the type of ignition energy storage as well as on the ignition performance which is required by the particular engine.  The reason for this is that an ignition system is not an autonomous machine, that is, it does not operate completely by itself, but instead it is but one part of the internal combustion engine, the heart of the engine
  • 6. BATTERY IGNITION SYSTEM The essential components of the system are: (i) battery (ii) ignition switch (iii) ballast resistor (iv) ignition coil (v) contact breaker (vi) capacitor (vii) distributor (viii) spark plug
  • 8. OPERATION OF A BATTERY IGNITION SYSTEM
  • 9. OPERATION OF A BATTERY IGNITION SYSTEM  When the ignition switch is closed, the primary winding of the coil is connected to the positive terminal post of the storage battery. If the primary circuit is closed through the breaker contacts, a current flows, the so called primary current.  This current, flowing through the primary coil, which is wound on a soft iron core, produces a magnetic field in the core.  A cam driven by the engine shaft, is arranged to open the breaker points whenever an ignition discharge is required.  When the breaker points open, the current which had been flowing through the points now flows into the condenser, which is connected across the points.  As the condenser becomes charged, the primary current falls and the magnetic field collapses.
  • 10. OPERATION OF A BATTERY IGNITION SYSTEM  The collapse of the field induces a voltage in the primary winding, which charges the condenser to a voltage much higher than battery voltage.  The condenser then discharges into the battery, reversing the direction of both the primary current and the magnetic field.  The rapid collapse and reversal of the magnetic field in the core induce a very high voltage in the secondary winding of the ignition coil.  The secondary winding consists of a 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 a rotating switch called the distributor, which is located in the secondary or high tension circuit of the ignition system.
  • 11. LIMITATIONS OF A BATTERY IGNITION SYSTEM (i) The primary voltage decreases as the engine speed increases due to the limitations in the current switching capability of the breaker system. (ii) Time available for build-up of the current in the primary coil and the stored energy decrease as the engine speed increases due to the dwell period becoming shorter. (iii) Because of the high source impedance (about 500 kΩ) the system is sensitive to side-tracking across the spark plug insulator. (iv) The breaker points are continuously subjected to electrical as well as mechanical wear which results in short maintenance intervals. Increased currents cause a rapid reduction in breaker point life and system reliability.
  • 13. OPERATION OF A MAGNETO IGNITION SYSTEM  A schematic diagram of a high tension magneto ignition system is shown in Fig.  The high tension magneto incorporates the windings to generate the primary voltage as well as to step up the voltage and thus does not require a separate coil to boost up the voltage required to operate the spark plug.  Magneto can be either rotating armature type or rotating magnet type
  • 14. OPERATION OF A MAGNETO IGNITION SYSTEM  The working principle of the magneto ignition system is exactly the same as that of the coil ignition system.  With the help of a cam, the primary circuit flux is changed and a high voltage is produced in the secondary circuit.  The variation of the breaker current with speed for the coil ignition system and the magneto ignition system is shown in Fig.  It can be seen that since the cranking speed at start is low the current generated by the magneto is quite small.  As the engine speed increases the flow of current also increases.
  • 15. Transistorized Coil Ignition (TCI) System 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: (i) reduced ignition system maintenance (ii) reduced wear of the components (iii) increased reliability (iv) extended spark plug life (v) improved ignition of lean mixtures
  • 17.  The contact breaker and the cam assembly of the conventional ignition system are replaced by a magnetic pulse generating system which detects the distributor shaft position and sends electrical pulse to an electronic control module  The module switches off the flow of current to the primary coil inducing a high voltage in the secondary winding which is distributed to the spark plugs as in the conventional breaker system.  The control module contains timing circuit which later closes the primary circuit so that the buildup of the primary circuit current can occur for the next cycle.  A magnetic pulse generator where a gear shaped iron rotor driven by the distributor shaft rotates past the pole of a stationary magnetic pickup, is generally used.  The number of teeth on the rotor is equal to the number of cylinders. The magnetic field is provided by a permanent magnet. Transistorized Coil Ignition (TCI) System
  • 18.  As each rotor tooth passes the magnet pole it first increases and then decreases the magnetic field strength ψ linked with the pickup coil wound on the magnet, producing a voltage signal proportional to dψ dt .  In response to this the electronic module switches off the primary circuit coil current to produce the spark as the rotor tooth passes through alignment and the pickup coil voltage abruptly reverses and passes through zero.  The increasing portion of the voltage waveform, after this voltage reversal, is used by the electronic module to establish the point at which the primary coil current is switched on for the next ignition pulse. Transistorized Coil Ignition (TCI) System
  • 20.  In this system, a capacitor rather than an induction coil is used to store the ignition energy.  The capacitance and charging voltage of the capacitor determine the amount of stored energy.  Ignition transformer steps up the primary voltage generated at the time of spark by the discharge of the capacitor through the thyristor to the high voltage required at the spark plug.  The CDI trigger box contains the capacitor, thyristor power switch, charging device (to convert battery voltage to charging voltage of 300 to 500 V by means of pulses via a voltage transformer), pulse shaping unit and control unit. Capacitive Discharge Ignition (CDI) System
  • 21. Capacitive Discharge Ignition (CDI) System  The advantage of using this system is that it is insensitive to electrical shunts resulting from spark plug fouling.  Because of the fast capacitive discharge, the spark is strong but short (0.1 to 0.3 ms) which leads to ignition failure during lean mixture operating conditions.  This is the main disadvantage of the CDI system.
  • 22. SPARK ADVANCE MECHANISM  It is obvious from the above discussion that the point in the cycle where the spark occurs must be regulated to ensure maximum power and economy at different speeds and loads and this must be done automatically.  The purpose of the spark advance mechanism is to assure that under every condition of engine operation, ignition takes place at the most favorable instant in time, i.e., most favorable from a standpoint of engine power, fuel economy, and minimum exhaust dilution.  By means of these mechanisms the advance angle is accurately set so that ignition occurs before the top dead-center point of the piston.  The engine speed and the engine load are the control quantities required for the automatic adjustment of the ignition timing. Most of the engines are fitted with mechanisms which are integral with the distributor and automatically regulate the optimum spark advance to account for change of speed and load.
  • 23. Effect of spark advance on P-Ɵ diagram
  • 24. Sliding contact type centrifugal advance mechanism
  • 25. Centrifugal Advance Mechanism  The centrifugal advance mechanism controls the ignition timing for full-load operation.  The adjustment mechanism is designed so that its operation results in the desired advance of the spark.  The cam is mounted, movably, on the distributor shaft so that as the speed increases, the flyweights which are swung farther and farther outward, shift the cam in the direction of shaft rotation.  As a result, the cam lobes make contact with the breaker lever rubbing block somewhat earlier, thus shifting the ignition point in the early or advance direction.  Depending on the speed of the engine, and therefore of the shaft, the weights are swung outward a greater or a lesser distance from the center.
  • 26.  They are then held in the extended position, in a state of equilibrium corresponding to the shifted timing angle, by a retaining spring which exactly balances the centrifugal force.  The weights shift the cam either on a rolling contact or sliding contact basis; for this reason we distinguish between the rolling contact type and the sliding contact type of centrifugal advance mechanism.  The beginning of the timing adjustment in the range of low engine speeds and the continued adjustment based on the full load curve are determined by the size of the weights, by the shape of the contact mechanisms (rolling or sliding contact type), and by the retaining springs, all of which can be widely differing designs.  The centrifugal force controlled cam is fitted with a lower limit stop for purposes of setting the beginning of the adjustment, and also with an upper limit stop to restrict the greatest possible full- load adjustment. Centrifugal Advance Mechanism
  • 28.  Vacuum advance mechanism shifts the ignition point under part load operation.  The adjustment system is designed so that its operation results in the prescribed part-load advance curve. In this mechanism the adjustment control quantity is the static vacuum prevailing in the carburettor, a pressure which depends on the position of the throttle valve at any given time and which is at a maximum when this valve is about half open.  The diaphragm of a vacuum unit is moved by changes in gas pressure.  The position of this diaphragm is determined by the pressure differential at any given moment between the prevailing vacuum and atmospheric pressure.  The beginning of adjustment is set by the pre-established tension on a compression spring.  The diaphragm area, the spring force, and the spring rigidity are all selected in accordance with the part-load advance curve which is to be followed and are all balanced with respect to each other. Vacuum advance mechanism
  • 29.  The diaphragm movement is transmitted through a vacuum advance arm connected to the movable breaker plate, and this movement shifts the breaker plate an additional amount under part-load conditions in a direction opposite to the direction of rotation of the distributor shaft.  Limit stops on the vacuum advance arm in the base of the vacuum unit restrict the range of adjustment. The vacuum advance mechanism operates independent of the centrifugal advance mechanism.  The mechanical interplay between the two advance mechanisms, however, permits the total adjustment angle at any given time to be the result of the addition of the shifts provided by the two individual mechanisms.  In other words, the vacuum advance mechanism operates in conjunction with the centrifugal advance mechanism to provide the total adjustment required when the engine is operating under part load. Vacuum advance mechanism
  • 30. FIRING ORDER  Every engine cylinder must fire once in every cycle.  This requires that for a four-stroke four-cylinder engine the ignition system must fire for every 180 degrees of crank rotation. For a six-cylinder engine the time available is only 120 degrees of crank rotation.  The order in which various cylinders of a multi-cylinder engine fire is called the firing order.  The number of possibilities of firing order depends upon the number of cylinders and throws of the crankshaft.  It is desirable to have the power impulses equally spaced and from the point of view of balancing this has led to certain conventional arrangements of crankshaft throws.
  • 31.  There are three factors which must be considered before deciding the optimum firing order of an engine.  These are: (i) engine vibrations (ii) engine cooling and (iii) development of back pressure FIRING ORDER
  • 32.  Consider that the cylinder number 1 of the four-cylinder engine, shown in Fig, is fired first.  Pressure, p, generated in the cylinder number 1 will give rise to a force equal to pA[b/(a+b)] and pA[a/(a+b)] on the two bearings A and B respectively.  The load on bearing A is much more than load on bearing B.  If the next cylinder fired is cylinder number 2, this imbalance in load on the two bearings would further aggravate the problem of balancing of the crankshaft vibrations.  If we fire cylinder number 3 after cylinder number 1, the load may be more or less evenly distributed. FIRING ORDER
  • 33.  Further, consider the effect of firing sequence on engine cooling.  When the first cylinder is fired its temperature increases.  If the next cylinder that fires is number 2, the portion of the engine between the cylinder number 1 and 2 gets overheated.  If then the third cylinder is fired, overheating is shifted to the portion between the cylinders 2 and 4.  Thus we see that the task of the cooling system becomes very difficult because it is then, required to cool more at one place than at other places and this imposes great strain on the cooling system.  If the third cylinder is fired after the first the overheating problem can be controlled to a greater extent. FIRING ORDER
  • 34.  Next, consider the flow of exhaust gases in the exhaust pipe.  After firing the first cylinder, exhaust gases flow out to the exhaust pipe.  If the next cylinder fired is the cylinder number 2, we find that before the gases exhausted by the first cylinder go out of the exhaust pipe the gases exhausted from the second cylinder try to overtake them.  This would require that the exhaust pipe be made bigger. Otherwise the back pressure in it would increase and the possibility of back flow would arise.  If instead of firing cylinder number 2, cylinder number 3 is fired then by the time the gases exhausted by the cylinder 3 come into the exhaust pipe, the gases from cylinder 1 would have sufficient time to travel the distance between cylinder 1 and cylinder 3 and thus, the development of a high back pressure is avoided. FIRING ORDER
  • 35.  For four-cylinder engines the possible firing orders are:  1 − 3 − 4 − 2 or 1 − 2 − 4 – 3  The former is more commonly used in the vertical configuration of cylinders. For a six-cylinder engine the firing orders can be:  1 − 5 − 3 − 6 − 2 − 4 or 1 − 5 − 4 − 6 − 2 − 3 or  1 − 2 − 4 − 6 − 5 − 3 or 1 − 2 − 3 − 6 − 5 − 4 FIRING ORDER
  • 37.  The spark plug provides the two electrodes with a proper gap across which the high potential discharges to generate a spark and ignite the combustible mixture within the combustion chamber.  A spark plug consists essentially of a steel shell, an insulator and two electrodes.  The central electrode to which the high tension 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 and thereby is automatically grounded when the plug is installed on the cylinder head of the engine.  The electrodes are usually made of high nickel alloy to withstand the severe erosion and corrosion to which they are subjected in use. Spark Plug
  • 38.  The tips of the central electrode and the insulator are exposed to the combustion gases.  This results in the insulators having a tendency to crack from the high thermal and mechanical stresses.  Some insulators are also seriously affected by moisture and by abnormal surface deposits.  Since, the central electrode and 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 and thereby prevent pre-ignition. Spark Plug
  • 39. Heat transfer path of hot and cold spark plugs
  • 40. Hot and Cold spark plugs  Spark plugs are usually classified as hot plugs or cold plugs 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