Ignition Coil FAQ

Frequently Asked Questions about Ignition Coils

What does an ignition coil do?

The coil is a compact, electrical transformer that boosts the battery’s 12 volts to as high as 40,000 volts.

How does an ignition coil work?

To understand how ignition coils operate requires that you understand some basic principles of electronics.
You probably didn’t think that you were going to get a short Physics lesson, but here goes:

Faraday’s Law and Auto Ignition

How do you obtain 40,000 volts across a sparkplug in an automobile when you have only 12 volts DC to start with? The essential task of firing the sparkplugs to ignite a gasoline-air mixture is carried out by a process which employs Faraday’s law.

The primary winding of the ignition coil is wound with a small number of turns and has a small resistance. Applying the battery to this coil causes a sizable DC current to flow. The secondary coil has a much larger number of turns and therefore acts as a step-up transformer. But instead of operating on AC voltages, this coil is designed to produce a large voltage spike when the current in the primary coil is interrupted. Since the induced secondary voltage is proportional to the rate of change of the magnetic field through it, opening a switch quickly in the primary circuit to drop the current to zero will generate a large voltage in the secondary coil according to Faraday’s Law. The large voltage causes a spark across the gap of the sparkplug to ignite the fuel mixture. For many years, this interruption of the primary current was accomplished by mechanically opening a contact called the “points” in a synchronized sequence to send high voltage pulses through a rotary switch called the “distributer” to the sparkplugs. One of the drawbacks of this process was that the interruption of current in the primary coil generated an inductive back-voltage in that coil which tended to cause sparking across the points. The system was improved by placing a sizable capacitor across the contacts so that the voltage surge tended to charge the capacitor rather than cause destructive sparking across the contacts. Using the old name for capacitors, this particular capacitor was called the “condenser”.
More modern ignition systems use a transistor switch instead of the points to interrupt the primary current.

The transistor switches are contained in a solid-state Ignition Control Module. Modern coil designs produce voltage pulses up in the neighborhood of 40,000 volts from the interruption of the 12 volt power supplied by the battery.
Some modern engines have multiple ignition coils mounted directly on the sparkplugs. Instead of single voltage pulses, they may under some engine conditions produce three voltage pulses. The coil arrangement shown is on a Dodge engine.

How is an ignition coil constructed and what materials are used?


The potting materials have changed dramatically since the ignition coils inception; originally ignition coils were paper insulated wiring coils mounted in wood or steel boxes. Later the ignition coil was built into the familiar conical shape and filled and formed with a phenolic plastic potting material to protect it from vibration and heat.
The potting materials have changed from phenolic plastic to epoxy’s, urethane polymers, and most recently to DuPont Rynite® and Thermx®. The materials may look like the same black plastic, but they have evolved dramatically.
Some early ignition coils were oil cooled (the windings were submerged in oil), which could leak and cause overheating and coil failure; but all modern ignition coils are now dry due to improvements in core materials, coil winding materials, and potting materials that can withstand higher under hood temperatures, vibration, and higher secondary voltage.


Primary and secondary coil windings use an insulated copper or copper coated aluminum wire. Originally the windings were insulated with paper or cloth. Most modern coil windings are now insulated with polyurethane or polyamide enamel composites.


All ignition coils have a magnetic core made of ferromagnetic metal such as iron, or ferromagnetic compounds such as ferrites to concentrate the strength and increase the effect of magnetic fields produced by the electric current.

The presence of the magnetic core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core.

What are common ignition system designs and technological generations?


1910 – 1985
(All Makes) (Point Style and Electronic Ignition Systems)



1969 – 1985
(G.M. Electronic Ignition)


1985 – Present
(Multiple Makes and Models)


1994 – Present
(1 per cylinder)(Multiple Makes and Models)


2012 & Up
(starting with hybrids)

In the configuration shown, the spark current is used to create a bias voltage, eliminating the need for an additional voltage source. The measured spark gap current after the spark event reflects the combustion process. Related parameters are extracted through signal processing.

Some modern COP (Coil on Plug) systems (2102 & UP) monitor Ionization current to determine a number of factors such as misfire detection, knock detection, fuel compensation, and proper timing advance for the next firing sequence, etc. These COP systems are capable of inducing over 50,000 volts if required (dictated by the required ionization current).
Delphi’s Ionization Current Sensing Ignition Subsystem (Ion Sense) is a technology based on the principle that electrical current flow in an ionized gas (e.g. during combustion) is proportional to the flame electrical conductivity. By placing a direct current bias on the spark plug electrodes, the conductivity can be measured.
With Ion Sense technology, the conventional spark plug acts as an intrusive sensor in the cylinder to obtain information about each combustion event with minimal influence due to environmental conditions such as vibration, mechanical noise, and temperature. Optimized individual cylinder knock control helps increase engine efficiency and reduce fuel consumption. Through Ion Sense technology, misfire detection is OBD II capable and provides very high reliability and robustness compared to many other detection methods.
Advanced features of Ion Sense Subsystems, such as compensation of combustion due to fuel variation, are also available to help reduce cold-start HC tailpipe emissions.

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