Ignition coil

An ignition coil (also called a spark coil) is an induction coil in an automobile's ignition system that transforms the battery's 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 (condenser). 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 (or pair of cylinders), eliminating fault-prone spark plug cables and a distributor to route the high voltage pulses.

Aceon Bright Ignition Coil
Bosch ignition coil in a Saab 96.
Dual ignition coils (blue cylinders, top of picture) on a Saab 92.

Ignition systems are not required for diesel engines which rely on compression to ignite the fuel/air mixture.

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 connected from the power source (e.g. the car's battery) to the primary winding is closed (by a contact breaker or transistor, 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 distributor and spark plug wires), before reaching the spark plug, 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 systems operate using the same principal 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.

A modern 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 older wasted spark systems for four-stroke engines, the secondary winding of the ingnition coil has two output terminals, both of which connect to a spark plug. The reverse pulse triggers the spark plug in a cylinder contains no air/fuel mixture (since that cylinder is out of phase by 360 degrees).[1]

Materials

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. Later, ignition coils were instead cast in filled epoxy resins, 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.

Use in cars

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 engine speeds (RPM), making starting easier.[2]

Early systems
Diagram of the Citroën 2CV wasted spark ignition system

Most older ignition coil systems used a single coil shared by all the spark plugs (via a distributor). Some exceptions occured, such as the Saab 92 and the Wartburg 353 using a separate coil for each cylinder and the 1948 Citroën 2CV using a wasted spark system with a double-ended ignition coil and no distributor.

Modern systems
Coil-pack of an Opel engine
Main article: Ignition system

Since the 1990s, ignition systems have mostly switched to a design where 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 (for example two coils in a four-cylinder engine, or three coils in a six-cylinder engine). 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, known as direct ignition (DI) or coil-on-plug. Where one coil serves two spark plugs (in two cylinders), 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.

Direct and distributorless ignition

Modern engine designs have abandoned the high-voltage distributor and coil, instead performing the distribution function in the primary circuit electronically and applying the primary (low-voltage) pulse to individual coils for each spark plug, or one coil for each pair of companion cylinders in an engine (two coils for a four-cylinder, three coils for a six-cylinder, four coils for an eight-cylinder, and so on).

In traditional remote distributorless systems, the coils are mounted together in a transformer oil filled coil pack, or separate coils for each cylinder, which are secured in a specified place in the engine compartment with wires to the spark plugs, similar to a distributor setup. General Motors, Ford, Chrysler, Hyundai, Subaru, Volkswagen and Toyota are among the automobile manufacturers known to have used coil packs. Coil packs by Delco for use with General Motors engines allow removal of the individual coils in case one should fail, but in most other remote distributorless coil pack setups, if a coil were to fail, replacement of the whole pack would be required to fix the problem.

Both direct and remote distributorless systems also allow finer levels of ignition control by the engine computer, which helps to increase power output, decrease fuel consumption and emissions, and implement features such as cylinder deactivation. Spark plug wires, which need routine replacement due to degradation, are also eliminated when the individual coils are mounted directly on top of each plug, since the high voltages and fields exist only over a very short distance from the coil to the plug.

Coil-on-plug systems

Since the early 2000s, many cars have used a 'coil-on-plug' ignition system, whereby a small ignition coil is located directly above the spark plug for each cylinder. This design means that high-voltage electricity is only present in the small distance between each coil and the spark plug.

Wasted spark
Main article: Wasted spark

The distributor can be eliminated on four-stroke engines by using the wasted spark principle. An ignition pulse is delivered to two cylinders at the same time, chosen so that one cylinder is in an exhaust stroke while the other is about to begin the power stroke. The spark in the cylinder on the exhaust phase is wasted. Each end of the ignition coil winding is connected to a spark plug and they fire in pairs.

A single-cylinder engine has only one spark plug and so needs no distributor. Ignition systems on such engines may produce a wasted spark during the exhaust stroke.

See also


References
  1. Horst Bauer (ed)., Automotive Handbook 4th Edition, Robert Bosch GmBH, 1996, ISBN 0-8376-0333-1 pg.439-440
  2. V. A. W. Hillier, Hillier's Fundamentals of Automotive Electronics, Nelson Thornes, 1996 ISBN 0-7487-2695-0, page 167
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