An ignition coil is an induction coil in an
automobile’s ignition system which transforms the battery’s low voltage to the thousands
of volts needed to create an electric spark in the spark plugs to ignite the fuel. Some
coils have an internal resistor while others rely on a resistor wire or an external resistor
to limit the current flowing into the coil from the car’s 12-volt supply. The wire that
goes from the ignition coil to the distributor and the high voltage wires that go from the
distributor to each of the spark plugs are called spark plug wires or high tension leads.
Originally, every ignition coil system required mechanical contact breaker points, and a capacitor.
More recent electronic ignition systems use a power transistor to provide pulses to the
ignition coil. A modern passenger automobile may use one ignition coil for each engine
cylinder, eliminating fault-prone spark plug cables and a distributor to route the high
voltage pulses. Ignition systems are not required for diesel
engines which rely on compression to ignite the fuel/air mixture. Basic principles
An ignition coil consists of a laminated iron core surrounded by two coils of copper wire.
Unlike a power transformer, an ignition coil has an open magnetic circuit – the iron core
does not form a closed loop around the windings. The energy that is stored in the magnetic
field of the core is the energy that is transferred to the spark plug.
The primary winding has relatively few turns of heavy wire. The secondary winding consists
of thousands of turns of smaller wire, insulated for the high voltage by enamel on the wires
and layers of oiled paper insulation. The coil is usually inserted into a metal can
or plastic case with insulated terminals for the high voltage and low voltage connections.
When the contact breaker closes, it allows a current from the battery to build up in
the primary winding of the ignition coil. The current does not flow instantly because
of the inductance of the coil. Current flowing in the coil produces a magnetic field in the
core and in the air surrounding the core. The current must flow long enough to store
enough energy in the field for the spark. Once the current has built up to its full
level, the contact breaker opens. Since it has a capacitor connected across it, the primary
winding and the capacitor form a tuned circuit, and as the stored energy oscillates between
the inductor formed by the coil and the capacitor, the changing magnetic field in the core of
the coil induces a much larger voltage in the secondary of the coil. More modern electronic
ignition systems operate on exactly the same principle, but some rely on charging the capacitor
to around 400 volts rather than charging the inductance of the coil. The timing of
the opening of the contacts must be matched to the position of the piston in the cylinder.
The spark must occur after the air/fuel mixture is compressed. The contacts are driven off
a shaft that is driven by the engine crankshaft, or, if electronic ignition is used, a sensor
on the engine shaft controls the timing of the pulses.
The amount of energy in the spark required to ignite the air-fuel mixture varies depending
on the pressure and composition of the mixture, and on the speed of the engine. Under laboratory
conditions as little as 1 millijoule is required in each spark, but practical coils must deliver
much more energy than this to allow for higher pressure, rich or lean mixtures, losses in
ignition wiring, and plug fouling and leakage. When gas velocity is high in the spark gap,
the arc between the terminals is blown away from the terminals, making the arc longer
and requiring more energy in each spark. Between 30 and 70 millijoules are delivered in each
Formerly, ignition coils were made with varnish and paper insulated high-voltage windings,
inserted into a drawn-steel can and filled with oil or asphalt for insulation and moisture
protection. Coils on modern automobiles are cast in filled epoxy resins which penetrate
any voids within the winding. A single-spark system has one coil per spark
plug. To prevent premature sparking at the start of the primary pulse, a diode or secondary
spark gap is installed in the coil to block the reverse pulse that would otherwise form.
In a coil meant for a dual-spark system, the secondary winding has two terminals isolated
from the primary, and each terminal connects to a spark plug. With this system, no extra
diode is needed since there would be no fuel/air mixture present at the inactive spark plug.
In a low-inductance coil, fewer primary turns are used, so primary current is higher. This
is not compatible with the capacity of mechanical breaker points, so solid-state switching is
used. Use in cars
Early gasoline internal combustion engines used a magneto ignition system, since no battery
was fitted to the vehicle; magnetos are still used in piston-engine aircraft. The voltage
produced by a magneto is dependent on the speed of the engine, making starting difficult.
A battery-operated coil can provide a high-voltage spark even at low speeds, making starting
easier. When batteries became common in automobiles for cranking and lighting, the ignition coil
system displaced magneto ignition. In older vehicles, a single coil would serve
all the spark plugs via the ignition distributor. Notable exceptions are the Saab 92, some Volkswagens,
and the Wartburg 353 which have one ignition coil per cylinder. The flat twin cylinder
1948 Citroën 2CV used one double ended coil without a distributor, and just contact breakers,
in a wasted spark system. Hitachi Automotive Systems, a Japanese company
is a top manufacturer of world class ignition coils used in automotives.
Modern ignition systems In modern systems, the distributor is omitted
and ignition is instead electronically controlled. Much smaller coils are used with one coil
for each spark plug or one coil serving two spark plugs. A large ignition coil puts out
about 40 kV, and a small one such as from a lawn mower puts out about 15 kV. These
coils may be remotely mounted or they may be placed on top of the spark plug. Where
one coil serves two spark plugs, it is through the “wasted spark” system. In this arrangement,
the coil generates two sparks per cycle to both cylinders. The fuel in the cylinder that
is nearing the end of its compression stroke is ignited, whereas the spark in its companion
that is nearing the end of its exhaust stroke has no effect. The wasted spark system is
more reliable than a single coil system with a distributor and less expensive than coil-on-plug.
Where coils are individually applied per cylinder, they may all be contained in a single molded
block with multiple high-tension terminals. This is commonly called a coil-pack.
A bad coil pack may cause a misfire, bad fuel consumption or loss of power.
Related coils An Oudin coil is a disruptive discharge coil.
Low tension coil See also
Flyback converter Flyback transformer
Electromagnetism Faraday’s law of induction
Induction coil Ignition system
Magnetic field Transformer
Patents U.S. Patent 609,250, “Electrical Igniter for
Gas Engines”, Nikola Tesla, 1898. U.S. Patent 1,391,256 – Induction coil structure
– Arthur Atwater Kent – 1921 U.S. Patent 1,474,152 – Induction coil – Arthur
Atwater Kent – 1923 U.S. Patent 1,474,597 – Induction coil – Arthur
Atwater Kent – 1923 U.S. Patent 1,569,756 – Ignition coil – Arthur
Atwater Kent – 1926 U.S. Patent 1,723,908 – Ignition system – Ernst
Alexanderson – 1929 References
^ Horst Bauer., Automotive Handbook 4th Edition, Robert Bosch GmBH, 1996, ISBN 0-8376-0333-1
pg.439-440 ^ V. A. W. Hillier, Hillier’s Fundamentals
of Automotive Electronics, Nelson Thornes, 1996 ISBN 0-7487-2695-0, page 167
External links Interactive tutorial on Ignition Coils at
the National High Magnetic Field Laboratory.