Spark Plugs Do Tell A Story!
Below is an overview of this system's operation
Spark Plugs
A spark plug is a device, inserted into the combustion chamber of an engine,
containing a side electrode and insulated center electrode spaced to provide a gap for firing
an electrical spark to ignite air-fuel mixtures.
The high-voltage burst from the coil via the distributor is received at the
spark plug's terminal and conducted down a center electrode protected by a porcelain insulator.
At the bottom of the plug, which projects into the cylinder, the voltage must be powerful enough
to jump a gap between the center and side electrodes through a thick atmosphere of fuel mixture.
When the spark bridges the gap, it ignites the fuel in the cylinder.
Spark Plug Wear
The spark plugs ignite the fuel mixture in the cylinders by means of a burst
of high-voltage electricity carried from the distributor. The ability of the spark to ignite
the fuel is badly affected if the plugs are damaged or the spark gaps are abnormal. It is therefore
important to examine used spark plugs closely and to clean them periodically. The gaps of old
and new plugs should also be checked before installing them. There are three basic types of
spark plug fouling: "carbon" fouling, "high speed" or "lead"
fouling, and "oil/carbon" fouling.
Carbon fouling is caused from low-speed operation or a fuel mixture that
is too rich. It causes missing or roughness and creates soft black soot that is easily removed.
Lead fouling is caused by tetraethyl lead used in some fuels and by extended high speed operation.
Lead compounds which are added to the gasoline have a bad effect on some spark plug insulators.
At high temperatures, it is a good conductor and may give good results under light loads, but
often fails under full loads and high combustion temperatures. In some cases, it is possible
to run the engine at a speed just below the point where missing will occur; then, increase
the speed (always keeping below the missing speed) to burn off the lead fouling. Lead fouling
appears as a heavy, crusty formation, or as tiny globules.
The third type of fouling is found on engines that are so badly worn that
excess oil reaches the combustion chamber past the piston ring, or the valve guides.
In all cases of fouling or wear, it is best to replace the plugs. To avoid
having to replace plugs one at a time as they wear out, always replace the entire set, even
though only one plug may be bad. Plugs should normally be replaced about every 12,000 miles.
Coil
The coil is a compact, electrical transformer that boosts the battery's 12
volts to as high as 20,000 volts. The incoming 12 volts of electricity pass through a primary
winding of about 200 turns of copper wire that raises the power to about 250 volts. Inside
the distributor, this low-voltage circuit is continuously broken by the opening and closing
of the points, each interruption causing a breakdown in the coil's electromagnetic field. Each
time the field collapses, a surge of electricity passes to a secondary winding made up of more
than a mile of hair-like wire twisted into 25,000 turns. At this point, the current is boosted
to the high voltage needed for ignition and is then relayed to the rotor.
Ignition Circuit
The distributor is separated into three sections: the upper, middle, and
lower. In the middle section, the corners of the spinning breaker cam strike the breaker arm
and separate the points some 160 miles an hour. (standard ignition) High-voltage surges generated
by the action of the coil travel to the rotor that whirls inside a circle of high-tension terminals
in the distributor cap. At each terminal, current is transferred to wires that lead to the
spark plugs. Two other devices - the vacuum advance and the centrifugal advance - precisely
coordinate the functions of the points and the rotor assembly as the requirements of the engine
vary.
An ignition circuit consists of two sub-circuits: the primary, which carries
low voltage; and the secondary, which carries high voltage. The primary circuit, controlled
by the ignition key, releases 12 volts of electricity from the battery or alternator through
the coil to a set of breaker points in the lower part of the distributor, or to the relay in
electronic ignition applications. When the points or relay are closed, current flows through
the chassis back to the battery, completing the circuit. When the points or relay are open,
the flow stops, causing a high-voltage surge to pass from the coil through a rotor in the top
of the distributor to the spark plugs. Once the car has started, the voltage regulator protects
the battery from being overcharged by the alternator. The condenser absorbs part of the low-voltage
current when the points are open.
Computerized and Electronic Ignition
In an electronic ignition, a rotating reluctor and magnetic-pickup coil replace
the traditional cam, breaker points and condenser in the distributors of cars equipped for
electronic ignition. This system reduces the time between tune-ups. The high spots of the reluctor
interrupt the magnetic field of the pickup coil and the permanent magnet. These interruptions,
or pulses, are transmitted from the pickup to a nearby electronic control unit. There, the
pulses signal a transistor to break the low-voltage sub-circuit and release high voltage from
the coil to the spark plugs.
The short-lived electronic ignition system was a transition from the points
and condenser system to the computerized ignition system. It came into widespread use in the
mid-1970s, but there are still a few engines that use electronic ignition.
Starting Circuit
The starter circuit is activated when the ignition switch is turned on. This
opens a second switch in the solenoid, permitting a second flow of electricity from the battery
to the starter motor.
The engine cranking circuit is made up of a battery, starting motor, ignition
switch, and electrical wiring. When the ignition switch is placed in the "start"
position, the solenoid windings are energized and the resulting shift lever movement causes
the drive pinion gear to engage the flywheel ring gear, and cranking takes place. When the
engine starts, an overrunning clutch (part of the drive assembly) protects the armature from
too much speed until the switch is opened. At this time, a return spring causes the pinion
gear to disengage from the flywheel.
Spark Plug Wires
The spark plug wire carries 20,000 or more volts from the distributor cap
to the spark plug. Spark plug wires are made of various layers of materials. The fiber core,
inside the spark plug wire carries the high voltage. The older design of spark plug wires used
a metallic wire to carry the high voltage. This caused electrical interference with the radio
and TV reception. Some spark plug wires have a locking connection at the distributor cap. The
distributor cap must first be removed and the terminals be squeezed together, and then the
spark plug wire can be removed from the distributor cap.
To reduce interference with radio and TV reception, ignition systems are
provided with resistance in the secondary circuit. Resistor spark plugs or special resistor
type ignition cable may be used.
To work effectively in modern ignition systems, it is important that the
resistor ignition cable is capable of producing a specifically designed resistance. The cable
must also have enough insulation so that it can withstand heat, cold, moisture, oil, grease,
and chafing. High tension electricity passing through a cable builds up a surrounding electrical
field. The electrical field frees oxygen in the surrounding air to form ozone, which will attach
to the rubber insulation if it is not properly protected. Ozone causes the rubber to deteriorate
and lose its insulating qualities. Electrical losses will seriously weaken the spark at the
plug gap.
Distributor Cap
As the rotor rotates inside the cap, it receives the high voltage from the
ignition coil, then passes it to the nearest connection, which is a metal projection in the
cap, which is connected to a spark plug.
The distributor cap should be checked to see that the sparks have not been
arcing from point to point within the cap. The inside of the cap must be clean. The firing
points should not be eroded, and the inside of the towers must be clean and free from corrosion.
Distributor Rotor
A distributor rotor is designed to rotate and distribute the high tension
current to the towers of the distributor cap. The firing end of the rotor, from which the high
tension spark jumps to each of the cap terminals, should not be worn. Any wear will result
in resistance to the high tension spark. The rotor with a worn firing end will have to be replaced.
Rotors are mounted on the upper end of the distributor shaft. In this connection,
the rotor must have a snug fit on the end of the shaft. On another design, two screws are used
to attach the rotor to a plate on the top of the distributor shaft. Built-in locators on the
rotor, and holes in the plate, insure correct reassembly. One locator is round; the other is
square.
The rotor is driven directly by the camshaft, but is "advanced"
(turned) by the centrifugal advance mechanism. Advancing the spark timing allows the engine
to run efficiently. A vacuum advance is also fitted on some cars for the same reason.
Condenser
Primary current produces a magnetic field around the coil windings. This
does not occur instantly, because it takes time for the current and the magnetic field to reach
maximum value. The time element is determined by the resistance of the coil winding or the
length of time the distributor contacts are closed. The current does not reach the maximum
because the contacts remain closed for such a short time, and more so at higher engine speeds.
When the breaker points begin to open, the primary current will continue to flow. This condition
in a winding is increased by means of the iron core. Without an ignition condenser, the induced
voltage causing this flow of current would create an arc across the contact points and the
magnetic energy would be consumed in this arc. As a result, the contact points would be burned
and ignition would not occur. The "condenser" prevents the arc by making a place
for the current to flow. As a result of condenser action, the magnetic field produced and continued
by the current flow will quickly collapse. It is the rapid cutting out of magnetic field that
induces high voltage in the secondary windings. So, if the condenser should go bad, the high
voltage needed to jump the gap at the spark plugs will not be possible. This could cause a
no-start condition or a driving problem.
Breaker Point (Standard) Ignition
The ignition distributor makes and breaks the primary ignition circuit. It
also distributes high tension current to the proper spark plug at the correct time. The distributor
is driven at one half crankshaft speed on four cycle engines. It is driven by the camshaft.
Distributor construction varies with the manufacturers, but the standard model is made of a
housing into which the distributor shaft and centrifugal weight assembly are fitted with bearings.
In most cases, these bearings are bronze bushings.
In standard ignition, the contact set is attached to the movable breaker
plate. A vacuum advance unit attached to the distributor housing is mounted under the breaker
plate. The rotor covers the centrifugal advance mechanism, which consists of a cam actuated
by two centrifugal weights. As the breaker cam rotates, each lobe passes under the rubbing
block, causing the breaker points to open. Since the points are in series with the primary
winding of the ignition coil, current will pass through that circuit when the points close.
When the points open, the magnetic field in the coil collapses and a high tension voltage is
induced in the secondary windings of the coil by the movement of the magnetic field through
the secondary windings.
The design is to provide one lobe on the breaker cam for each cylinder of
the engine; i.e., a six cylinder engine will have a six lobe cam in the distributor; and an
eight cylinder engine will have an eight lobe cam, so every revolution of the breaker came
will produce one spark for each cylinder of the engine. However, on a four cycle engine, each
cylinder fires every other revolution so the distributor shaft must revolve at one half crankshaft
speed. After the high tension surge is produced in the ignition coil by the opening of the
breaker points, the current passes from the coil to the center terminal of the distributor
cap. From there, it passes down to the rotor mounted on the distributor shaft and revolves
with it. The current passes along the rotor, and jumps the tiny gap to the cap electrode under
which the rotor is positioned at that instant. This cap electrode is connected by high tension
wiring to the spark plug. As the rotor continues to rotate, it distributes current to each
of the cap terminals in turn.
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