4 Ignition System
-
Upload
mohammed-gomaa -
Category
Documents
-
view
252 -
download
4
Transcript of 4 Ignition System
-
8/13/2019 4 Ignition System
1/51
Ignition Systems in Spark-Ignition Engines
By
Dr. Amr Ibrahim
Dr. Amr Ibrahim, 2010, all rights reserved. The commercial use of these slides without
permission is strictly prohibited.
-
8/13/2019 4 Ignition System
2/51
What is the main function of an ignition system?
The ignition system is responsible for generating the spark at the
optimum time in order to initiate the combustion of the air-fuel
mixture.
What is the optimum spark timing?
The spark is usually produced before the
piston reaches the TDC position by the end of
compression stroke
The spark timing may vary from 5 to 40
degrees before TDC
The spark timing is usually experimentally
selected in order for the engine to produce
the maximum torque. This timing also gives
maximum brake power & minimum brake
specific fuel consumption.
-
8/13/2019 4 Ignition System
3/51
Types of ignition systems:
mechanical (contact point) ignition system
electronic (transistorized) ignition system
distributor less ignition system
Retarded spark timing is used atengine idlingto bring the ignition
point closer to the TDC to avoid misfire and ensures engine smooth
operation
The optimum spark timing varies with engineload and speed
-
8/13/2019 4 Ignition System
4/51
The Mechanical ignition system
-
8/13/2019 4 Ignition System
5/51
battery & alternator
ignition switch
Ignition coil
ignition distributor
spark plugs
cables
The mechanical ignition system consists of:
-
8/13/2019 4 Ignition System
6/51
Engine battery
it is an electrochemical device
it produces electric current for starting motor and ignition
system while starting the engine
it also supplies current to electrical accessories when the
alternator is not handling the load
-
8/13/2019 4 Ignition System
7/51
The chemicals in the battery are:
o sponge lead (a solid)
o lead oxide (a paste)
o sulfuric acid mixed with water (electrolyte)
-
8/13/2019 4 Ignition System
8/51
PbO2 + Pb + 2H2SO4 2PbSO4 + 2H2O
Chemical reaction during discharging process:
2PbSO4 + 2H2O PbO2 + Pb + 2H2SO4
Chemical reaction during charging process:
the specific gravity of the battery electrolyte
ranges from about 1.265 for fully charged battery
to 1.11 for completely discharged battery
the state of charge of a battery can be
determined using a hydrometer
-
8/13/2019 4 Ignition System
9/51
The spark plug
it generates the spark within the cylinder
it has two metal electrodes (central and ground
electrodes)
there is an air gap between the central and ground
electrodes
when the central electrode is supplied with a high
voltage current, the current passes from the central to
ground electrode in the form ofa spark
-
8/13/2019 4 Ignition System
10/51
the high voltage current is supplied to the terminal from the
ignition coil
most spark plugs have electrodes made of nickel and chrome alloysthe wider the gap, the higher the voltage required to produce it
the gap may vary from 0.9 to 2 mm
-
8/13/2019 4 Ignition System
11/51
The ignition coil
it is a step up transformer
it converts the small battery voltage
of 12 volts to a high voltage of 25,000
volts
in some electronic ignition systems,the voltage may go up to 47,000 volts
or higher
-
8/13/2019 4 Ignition System
12/51
the ignition coil consists of two coils:the primary and secondary coils
the secondary coil is made of a winding of many thousands of turns
(e.g. 20,000 turns) of a fine wire
the primary coil is made of a few hundred turns (e.g. 200 turns) of
relatively heavy wire
the secondary coil is wrapped around an iron core
the primary coil is wrapped around the secondary coil (the two coils
are electrically insulated)
-
8/13/2019 4 Ignition System
13/51
The ignition distributor
it receives the high voltage current fromthe ignition coil and distribute it to spark
plugs according to the firing order
it contains two mechanisms that can vary
the spark timing according to engine speedand load
its shaft speed is equivalent to the
camshaft speed (i.e. half the crankshaft
speed) in 4-stroke cycle engines
the distributor shaft is driven by a gear on
the camshaft
-
8/13/2019 4 Ignition System
14/51
1. The rotor
it receives the high voltage current via thecentral terminal in the cap and distribute it
via the side terminals
What is under the distributor cap?
-
8/13/2019 4 Ignition System
15/51
2. The breaker cam & contact points
the contact points are operated by a breaker camas the cam rotates, the points open & close
the cam has as many lobes as no. of cylinders
Open contact points Closed contact points
-
8/13/2019 4 Ignition System
16/51
3. The condenser
-
8/13/2019 4 Ignition System
17/51
The wiring diagram:
-
8/13/2019 4 Ignition System
18/51
How the high voltage is produced?
the basic principle behind generating the high voltage in the ignition
coil is Faradays law:
dt
dE
the contact points operate as a mechanical switch in order to
interrupt the primary current in the primary circuit and produce a
change in the magnetic field
when the contact points open the primary circuit, a high voltage isproduced in the secondary circuit and a spark is generated
-
8/13/2019 4 Ignition System
19/51
The electric circuit when the contact points are closed:
-
8/13/2019 4 Ignition System
20/51
What happens when the contact points close the primary circuit?
the current flows from the battery to the primary coil
the flowing current produces a change in the magnetic field which
cuts the turns of the primary coil and secondary coil
this produces a self electric motive force in the primary coil which
opposes the flowing current
that results in a slow growth of the primary current and a small
d/dt is produced
when the secondary coil is subjected to a small d/dt, a small
voltage is produced in the secondary coil
during this period, the rotor in the distributor is rotating betweenthe side terminals
-
8/13/2019 4 Ignition System
21/51
The electric circuit when the contact points are open:
-
8/13/2019 4 Ignition System
22/51
What happens when the contact points open the primary circuit?
when the primary circuit is suddenly open, the primary current
diminishes producing a change in the magnetic field
the primary current rushes to the condenser and instantly charges it
once the condenser is charged, it is discharged again
when the condenser is being discharged, its current flows into an
opposite direction to the primary current causing a rapid collapse of theprimary current which leads to high d/dt
the high d/dt produces a high voltage in the secondary coil
at that time, the rotor in the distributor is in contact with a side
terminal and a spark is generated
-
8/13/2019 4 Ignition System
23/51
The dwell angle
The dwell angle is the angle of rotation of the distributor shaft
through which the primary circuit is closed.
if this angle is too small, the stored magnetic energy in the
primary circuit will not be sufficient to produce the required high
voltage in the secondary circuit
if this angle is too high, high ignition coil temperature and
perhaps coil damage could occur
the dwell angle may range from 30 to 60 degrees
-
8/13/2019 4 Ignition System
24/51
Contact points gap
the contact points gap is the distance between the two contact
points when the are fully open
the gap may range from 0.3 to 0.4 mm
the wider the gap, the smaller the dwell angle, and vice versa
rubbing block
during engine operation, wear occursin the rubbing block and gap decreases
the gap has to be readjusted by
relocating the position of the fixed
contact point
fixed point
moving point
-
8/13/2019 4 Ignition System
25/51
Adjusting the spark timing
if the spark timing istoo advanced, the combustion will start too
early and the compression work will increaseif the spark timing istoo retarded, the combustion will complete
too late and the peak cylinder pressure will be reduced causing a
decrease in the expansion work
The optimum spark timing is usually experimentally selected in
order for the engine to produce themaximum torque
The optimum spark timing varies with engineload and speed
The ignition system must be capable of changing the spark timing
with load and speed in order for the engine to produce the
maximum torque
-
8/13/2019 4 Ignition System
26/51
During engine motoring (i.e. with no combustion), the in-cylinder
pressure increases with crank angle till it reaches its maximum at the
TDC
-
8/13/2019 4 Ignition System
27/51
the period from point 1 to point 2 is called the flame development
period or ignition delay
experimentally, it was found that the maximum torque is obtained
when the spark timing is adjusted so that the maximum pressure is
produced 10 to 16 degrees after TDC
point 1: spark
timing
point 2: rapid
combustion starts
point 3: maximum
pressure occurs
after the TDC
point 4:
combustion ends
-
8/13/2019 4 Ignition System
28/51
Setting the spark timing according to engine load
o during part load operation, less air and fuel is admitted into the
cylinder compared to the full load operation
o the decrease in the inlet mixture density during part load
operation leads to a decrease in engine flame speed
o also, during engine part load operation, the engine may use lean
air-fuel mixture
o the use of lean air-fuel mixture decreases the flame speed
compared to the use of rich mixture
The flame speed during engine part load operation is slowerthan the
flame speed during full load operation due to:
-
8/13/2019 4 Ignition System
29/51
therefore, the spark timing must be advanced during engine part load
operation in order for the combustion to complete at the appropriate
time
advancing the spark timing during part load operation ensures that
the maximum cylinder pressure occurs at the optimum time for
maximum engine torque operation
the vacuum advance mechanism is
responsible for advancing the spark timing
during engine part load operation
-
8/13/2019 4 Ignition System
30/51
The vacuum advance mechanism is connected to a small port located
abovethe throttle valve
-
8/13/2019 4 Ignition System
31/51
-
8/13/2019 4 Ignition System
32/51
when the throttle valve moves past the vacuum port, the intake
manifold vacuum pulls the diaphragm
this rotates the breaker plate so the contact points open earlier and
the spark timing is advanced
the spark advance increases as the vacuum acting on the diaphragm
increases
during engine idling & wide-open-throttle operation, there is no
vacuum acting on the diaphragm
-
8/13/2019 4 Ignition System
33/51
Setting the spark timing according to engine speed
experimentally, it was found that the total combustion angle increaseswith the increase of engine speed
therefore, the spark timing must be advanced with the increase of
engine speed in order for the combustion to complete at the
appropriate time and the maximum pressure to occur at the optimum
time
the centrifugal advance mechanism is responsible for advancing the
spark timing with the increase of engine speed
the centrifugal advance mechanism is located inside the ignition
distributor
-
8/13/2019 4 Ignition System
34/51
The centrifugal advance mechanism
-
8/13/2019 4 Ignition System
35/51
-
8/13/2019 4 Ignition System
36/51
as the engine speed increases, the movement of the weights under
centrifugal force advances the angular position of the cam relative to
the driveshaftin this cam new position, the contact points open earlier and the
spark is advanced
-
8/13/2019 4 Ignition System
37/51
During operation, both the vacuum advance mechanism and centrifugal
advance mechanism advance the spark timing according to engine load
and speed
-
8/13/2019 4 Ignition System
38/51
Electronic (Transistorized) Ignition System
-
8/13/2019 4 Ignition System
39/51
What is the main difference between the mechanical and
electronic ignition systems?
in the mechanical ignition system, the primary current in theprimary circuit is interrupted by a mechanical switch (the contact
breaker points)
in the electronic ignition system, the primary current is interrupted
byan electronic switch
-
8/13/2019 4 Ignition System
40/51
What is the main component of an electronic switch?
o it is the transistor
the transistor is a semiconductor and has
three terminals: a collector, a base , and an
emitter
when an electric current flows to its base, it
becomes a conductor
-
8/13/2019 4 Ignition System
41/51
The advantages of using the transistor as an electronic switch
compared to the contact points:
the transistor can last for longer time (no mechanical wear)
the transistor can interrupt higher voltage current
the transistor switches the current flow faster than the mechanical
switch
The basic principle of the transistorized ignition system:
when the current flows to the transistor base, the transistor becomes a
conductor and the primary current flows through the primary circuit
when there is no current flowing to the transistor base, the transistor
becomes an insulator and the primary circuit is switched off
when the primary circuit is switched off, a high voltage is produced in
the secondary circuit and a spark is generated
-
8/13/2019 4 Ignition System
42/51
Generating the transistor base current:
the transistor base current is generated by a signal generator
located in the ignition distributor
the signal generator type could differ from an engine to another
(e.g. magnetic, Hall effect, or optical type)
-
8/13/2019 4 Ignition System
43/51
The magnetic type signal generator
-
8/13/2019 4 Ignition System
44/51
-
8/13/2019 4 Ignition System
45/51
-
8/13/2019 4 Ignition System
46/51
Advancing the spark timing based on engine load & speed:
the vacuum advance mechanism is used to advance the spark timing
according to engine load by shifting the position of the pick up coil
-
8/13/2019 4 Ignition System
47/51
the centrifugal advance mechanism advances the spark timing
according to engine speed by changing the angular position of the
signal rotor relative to the driveshaft
-
8/13/2019 4 Ignition System
48/51
sometimes in electronic ignition systems, both the ignition coil
and distributor are integrated in one assembly
-
8/13/2019 4 Ignition System
49/51
Electronic ignition system using an ECU
the ECU switches on and off the primary circuit
the ECU controls the spark timing according to engine operating
conditions detected by engine sensors
the distributor mainly distributes the high voltage current to engine
cylinders
-
8/13/2019 4 Ignition System
50/51
the electronic control of the spark timing via the ECU can achieve spark
timings closer to the ideal ignition timings compared to the mechanical
ignition system
ESA = electronic spark advance
-
8/13/2019 4 Ignition System
51/51
Distributor less (or direct) ignition system
in this system, an ignition coil may be used for each cylinder or one
ignition coil may be used for each two cylinders
the ECU controls each ignition coil and the spark timing is
determined according to engine operating conditions detected by
engine sensors