An ignition coil is used in the

ignition system Ignition systems are used by heat engines to initiate combustion by igniting the fuel-air mixture. In a spark ignition versions of the internal combustion engine (such as petrol engines), the ignition system creates a spark to ignite the fuel-ai ...

of a

spark-ignition engine A spark-ignition engine (SI engine) is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug. This is in contrast to compression-ignition engines, ty ...

to transform the battery voltage to the much higher voltages required to operate the

spark plug A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air ...

(s). The spark plugs then use this burst of high-voltage electricity to ignite the air-fuel mixture. The ignition coil is constructed of two sets of coils wound around an iron core. Older engines often use a single ignition coil which has its output directed to each cylinder by a

distributor A distributor is an electric and mechanical device used in the ignition system of older spark-ignition engines. The distributor's main function is to route electricity from the ignition coil to each spark plug at the correct time. Design ...

, a design which is still used by various

small engine A small engine is the general term for a wide range of small-displacement, low-powered internal combustion engines used to power lawn mowers, generators, concrete mixers and many other machines that require independent power sources. These engin ...

s (such as lawnmower engines). Modern car engines often use a distributor-less system (such as ''coil-on-plug''), whereby every cylinder has its own ignition coil.

Diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

s use compression ignition and therefore do not have ignition coils.

Design

An ignition coil consists of an iron core surrounded by two coils (''windings'') made from copper wire. The primary winding has relatively few turns of heavy wire, while the secondary winding consists of thousands of turns of smaller wire and is insulated from the high voltage by enamel on the wires and layers of oiled paper insulation. When the

electrical circuit An electrical network is an interconnection of electrical components (e.g., battery (electricity), batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e. ...

connected from the power source (e.g. the car's battery) to the primary winding is closed (by a

contact breaker A contact breaker (or "points") is a type of electrical switch, found in the ignition systems of spark-ignition internal combustion engines. The switch is automatically operated by a cam driven by the engine. The timing of operation of the switch ...

transistor A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

), current flows through the primary winding, which produces a magnetic field around the core. This current flow lasts for a period of time to build up energy in the coil. Once the coil is charged, the circuit is opened, and the resulting oscillation in the magnetic field induces a high voltage in the secondary winding. This high-voltage electricity travels through several components (such as a

and spark plug wires), before reaching the

, where it is used to ignite the air/fuel mixture. The timing of the circuit opening must be coordinated with the rotation of the engine, so that the burst of high-voltage electricity is produced at the optimal time to ignite the air/fuel mixture. Modern

electronic ignition Ignition systems are used by heat engines to initiate combustion by igniting the fuel-air mixture. In a spark ignition versions of the internal combustion engine (such as petrol engines), the ignition system creates a spark to ignite the fuel-air ...

systems operate using the same principle of charging an electric circuit, however they use a capacitor charged to around 400 volts, rather than using the induction charging of an ignition coil. Typical output voltages for modern ignition coils vary from 15 kV (for a lawnmower engine) to 40 kV (for a larger engine)https://performancedistributors.com/product/chevrolet-screamin-demon-coils/. A modern single-spark system has one coil per spark plug. To prevent premature sparking at the start of the primary pulse, a

diode A diode is a two-Terminal (electronics), terminal electronic component that conducts electric current primarily in One-way traffic, one direction (asymmetric electrical conductance, conductance). It has low (ideally zero) Electrical resistance ...

or secondary spark gap is installed in the coil to block the reverse pulse that would otherwise form. In older wasted spark systems for four-stroke engines, the secondary winding of the ignition coil has two output terminals, both of which connect to a spark plug. The reverse pulse triggers the spark plug in a cylinder containing no air/fuel mixture (since that cylinder is out of phase by 180 degrees).Horst Bauer (ed)., Automotive Handbook 4th Edition, Robert Bosch GmBH, 1996, pg.439-440

Materials

Formerly, ignition coils were made with

varnish Varnish is a clear Transparency (optics), transparent hard protective coating or film. It is not to be confused with wood stain. It usually has a yellowish shade due to the manufacturing process and materials used, but it may also be pigmente ...

and paper insulated high-voltage windings, inserted into a drawn-steel can and filled with transformer oil or asphalt for insulation and moisture protection. Later, ignition coils were instead cast in filled

epoxy resin Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also co ...

s, which penetrate any voids forming within the windings. The ignition coil is usually inserted into a metal can or plastic case with insulated terminals for the high voltage and low voltage connections.

History

Predecessor

Early cars used a magneto ignition system, due to the lack of an electric power source (e.g. battery) in the car. Ignition coils replaced magneto ignition in new cars as batteries became a common inclusion in cars (for cranking and lighting). Compared with magneto ignition, an ignition coil system can provide a high-voltage spark at low engine speeds (RPM), making starting easier.V. A. W. Hillier, ''Hillier's Fundamentals of Automotive Electronics'', Nelson Thornes, 1996 , page 167

Early systems

Most older ignition coil systems used a single coil shared by all the spark plugs (via a

). There were some exceptions, such as the

Saab 92 The Saab 92 was the first production car from Saab. The design was very aerodynamic for its time, with a drag coefficient (''cx'' or ''cw'') of 0.30. The entire body was stamped out of one piece of sheet metal and then cut to accommodate doors ...

and the Wartburg 353 using a separate coil for each cylinder and the 1948

Citroën 2CV The Citroën 2CV (, , lit. "two horses", meaning "two Tax horsepower#France, ''taxable'' horsepower") is an economy car produced by the French company Citroën from 1948 to 1990. Introduced at the 1948 Paris Paris Auto Show, Salon de l'Automobi ...

using a wasted spark system with a double-ended ignition coil and no distributor.

Distributorless systems

Since the 1990s, ignition systems have mostly switched to a design where the distributor is omitted and ignition is instead electronically controlled. In these distributor-less systems, multiple smaller ignition coils are used, usually in the form of one coil for each cylinder or a wasted spark system with one coil for each pair of cylinders. The ignition coils for these can be combined into a single casing (a ''coil pack'') and located away from the spark plugs; however it is increasingly common for coil-on-plug systems to be used, whereby the individual ignition coils are small units attached directly to the top of each spark plug. An advantage of coil-on-plug systems is that in the event of a fault, a single ignition coil can be replaced rather than, unnecessarily, those for all of the cylinders.

References

{{DEFAULTSORT:Ignition Coil Ignition systems Electromagnetic coils Engine components