All ignition systems consist
of two interconnected electrical circuits: a primary (low voltage) circuit and a
secondary (high voltage) circuit.
Depending on the exact type
of ignition system, components in the primary circuit include the following.
- ignition switch
- ballast resistor or
resistance wire (some systems)
- starting by-pass (some
- ignition coil primary
- triggering device
- switching device or control
The secondary circuit
includes these components.
- ignition coil secondary
- distributor cap and rotor
- ignition cables
- spark plugs
Primary Circuit Operation
When the ignition switch is
on, current from the battery flows through the ignition switch and primary
circuit resistor to the primary winding of the ignition coil. From here it
passes through some type of switching device and back to ground. the switching
device can be electronically or mechanically controlled by the triggering
device. The current flow in the ignition coil's primary winding creates a
magnetic field. The switching device or control module interrupts this current
flow at predetermined times. When it does, the magnetic field in the primary
winding collapses. This collapse generates a high-voltage surge in the secondary
winding of the ignition coil. The secondary circuit winding of the ignition
coil. The secondary circuit of the system begins at this point.
The secondary circuit in the
ignition system carries high voltage to the spark plugs. The exact manner in
which the secondary circuit delivers these high-voltage surges depends on the
system design. Until 1984 all ignition systems used some type of distributor to
accomplish this job. However, in an effort to reduce emissions, improve fuel
economy, and boost component reliability, many auto manufacturers are using
In a system using a
distributor, high voltage from the secondary winding passes through an ignition
cable running from the coil to the distributor. The distributor then distributes
the high voltage to the individual spark plugs through a set of ignition cables.
The cables are arranged in the distributor cap according to the firing order of
the engine. A rotor, which is driven by the distributor shaft, rotates and
completes the electrical path from the secondary winding of the coil to the
individual spark plugs. The distributor delivers the spark to match the
compression stroke of the piston. The distributor assembly may also have the
capability of advancing or retarding ignition timing.
Distributorless or direct
ignitions have no distributor, rather spark distribution is controlled by the
vehicle's computer. Instead of a single ignition coil for all cylinders, each
cylinder may have its own ignition coil, or two cylinders may share one coil.
The coils are wired directly to the spark plug they control. An ignition control
module, tied into the vehicle's computer control system, controls the firing
order and the spark timing and advance. This module is typically located under
the coil assembly. It may also be integrated into a special housing that
contains most of the system's ignition parts.
All ignition systems share a
number of common components. Some, such as the battery and ignition switch,
perform simple functions. The battery supplies low-voltage current to the
ignition primary circuit. The current flow when the ignition switch is in the
start or run position. Full-battery voltage is always present at the ignition
switch, as if it were directly connected to the battery.
To generate a spark to begin
combustion, the ignition system must deliver high voltage to the spark plugs.
Vehicles manufactured in recent years may require a voltage level between 30,000
and 60,000 volts to force a spark across the air gap of a spark plug. Since the
battery typically delivers only 10 to 12 volts, a method of stepping up the
voltage must be used. Multiplying battery voltage is the job of a coil.
The ignition coil is a pulse
transformer. It transforms battery voltage into short bursts of high voltage. As
explained previously, when a wire is moved through a magnetic field, voltage is
induced in the wire. The inverse of this principle is also true - when a
magnetic field moves across a wire, voltage is induced in the wire.
If a wire is bent into loops
forming a coil and a magnetic field is passed through the coil, an equal amount
of voltage is generated in each loop of wire. The more loops of wire in the
coil, the greater the total voltage induced.
Also, the faster the magnetic
field moves through the coil, the higher the voltage induced in the coil. If the
speed of the magnetic field is doubled, the voltage output doubles.
An ignition coil uses these
principles and has two coils of wire wrapped around an iron core. An iron or
steel core is used because it has low inductive reluctance. In other words, iron
freely expands or strengthens the magnetic field around the windings. The first,
or primary, coil is normally composed of 100 to 200 turns of 20-gauge wire. This
coil of wire conducts battery current. When a current is passing through the
primary coil, it magnetized the iron core. The strength of the magnet depends
directly on the number of wire loops and the amount of current flowing through
those loops. The secondary coil of wires may consist of 15,000 to 25,000, or
more, turns of very fine copper wire.
Because of the effects of
counter EMF on the current flowing through the primary winding, it takes some
time for the coil to become fully magnetized or saturated. The CEMF that opposes
current flow in the winding is called inductive reactance. Therefore, primary
current flow is present for some time between firings of the spark plugs. When
the primary coil circuit is suddenly opened, the magnetic field instantly
collapses. The sudden collapsing of the magnetic field produces a very high
voltage is used to push current across the gap of the spark plug.
The number of ignition coils
used in an ignition system varies depending on the type of ignition system found
on a vehicle. In most ignition systems with a distributor, only one ignition
coil is used. The high voltage of the secondary winding is directed, by the
distributor, to the various spark plugs in the system. Therefore, there is one
secondary circuit with a continually changing path.
While distributor systems
have a single secondary circuit with a continually changing path,
distributorless (DIS) systems have several secondary circuits, each with an
Every type of ignition system
uses spark plugs. The spark plugs provide the crucial air gap across which the
high voltage from the coil flows in the form of an arc. The three main parts of
a spark plug are the steel core, the ceramic core, or insulator, which acts as a
heat conductor; and a pair of electrodes, one insulated in the core and the
other grounded on the shell. The shell holds the ceramic core and electrodes in
a gas-tight assembly and has the threads needed for plug installation in the
engine. An ignition cable connects the secondary to the top of the plug. Current
flows through the center of the plug and arcs from the tip of the center
electrode to the ground electrode. The resulting spark ignites the air/fuel
mixture in the combustion chamber. Most automotive spark plugs also have a
resistor between the top terminal and the center electrode. This resistor
reduces the amount of current and, therefore, reduces the amount of radio
interference caused by the spark plug. The resistor, like all other resistances
in the secondary, increases the voltage needed to jump the gap of the spark
Spark plugs come in many
different sizes and designs to accommodate different engines. To fit properly,
spark plugs must be of the proper size and reach. Another design factor that
determines the usefulness of a spark plug for a specific application is its heat
range. The desired heat range depends on the design of the engine and on the
type of driving conditions the vehicle is subject to. Once a technician selects
a spark plug with the correct size, reach, and heat range for a particular
application, there is one more spark plug characteristic that must be checked
and adjusted - the spark plug air gap. Although the size, reach, and heat range
of a spark plug are already determined by the manufacturer, the technician has
the responsibility o properly gap the plug.
Size Automotive spark
plugs are available in either 14- or 18-millimeter diameters. All 18-millimeter
plugs feature tapered seats that match similar seats in the cylinder head and
need no gaskets. The 14-nukkuneter variety can have either a flat seat that
requires a gasket or a tapered seat that does not. The latter is the most
commonly used. All spark plugs have a hex-shaped shell that accommodates a
socket wrench for installation and removal. The 14-millimeter, tapered seat
plugs have shells with a 5/8-inch hex; 14-millimeter gasketed and 18-millimeter
tapered seat plugs have shells with a 13/16-inch hex.
Reach One of the most
important design characteristics of spark plugs is the reach. This refers to the
length of the shell from the contact surface at the seat to the bottom of the
shell, including both threaded and nonthreaded sections. Reach is crucial. The
plug's air gap must be properly placed in the combustion chamber so it can
produce the correct amount of heat. Installing plugs with too short a reach
means the electrodes are in a pocket and the arc is not able to adequately
ignite the air/fuel mixture. In addition, the exposed threads in the cylinder
head will accumulate carbon deposits. If the reach is too long, the exposed plug
threads can go so hot they will ignite the air/fuel mixture at the wrong time,
causing preignition. Preignition is a term used to describe abnormal combustion,
which is caused by something other than the heat of the spark.
Heat Range Spark plugs
are available in different heat ranges. A heat range indicates how well a spark
plug can conduct heat away from its tip. A cooler plug transfers hear rapidly,
resulting in lower tip temperatures. A hotter plug transfers heat slowly,
resulting in higher tip temperatures. The shape of the porcelain insulator and
its point of contact with the outer metal shell determines spark plug heat
range. A spark plug's heat is transferred from the core to the shell to the
cylinder head to the engine's coolant, which moves the heat away from the head.
Installing a plug with the
correct heat range is important because the plug must remain hot enough to burn
away fouling deposits while the engine is idling, yet cool enough at higher
speed to prevent preignition and electrode wear. The heat range is indicated by
a code imprinted on the side of the plug, usually on the porcelain insulator.
Spark Plug Air Gap The
correct spark plug air gap is essential to achieve optimum engine performance
and long plug life. A gap that is too wide requires higher voltage to jump the
gap. If the required voltage is greater than what is available, the result is
misfiring. Misfiring results from the inability of the ignition to jump the gap
or the inability to maintain the spark. On the other hand, a gap that is too
narrow requires lower voltages, which leads to rough idle and prematurely burned
electrodes, due to higher current flow. Always set the gap according to the
manufacturer's specifications. New electronic ignition systems call for wider
air gap than older systems due to higher available voltage and leaner air/fuel
Spark plug, or ignition
cables make up the secondary wiring. These cables carry the high voltage from
the distributor or the multiple coils to the spark plugs. The cables are not
solid wire, rather they contain fiber cores that act as resistors in the
secondary circuit. the cut down on radio and television interference, increase
firing voltages, and reduce spark plug wear by decreasing current. Insulated
boots on the ends of the cables strangthen the connections as well as prevent
dust and water infiltration and voltage loss.