Cylinder coils are a component used in a certain type of automotive ignition system known as a coil-on-plug (COP). This configuration is a type of direct ignition (DI) distributor-less ignition system (DIS). Instead of a single ignition coil that provides spark to each cylinder via a distributor, these systems have a separate coil to fire each spark plug. In a coil-on-plug system, an ignition coil is integrated into the top of each spark plug boot. Other DI systems also have a separate coil pack for each cylinder, and each one is located near the spark plug it fires.
There are both benefits and drawbacks to ignition systems that use cylinder coils. Direct ignition systems typically have far fewer components to break down or require servicing, since they lack distributors. A distributor is a mechanical component that is typically driven off the camshaft and used to individually fire each spark plug. This may be accomplished through ignition parts, such as an ignition control module, coil, distributor, and rotor. The use of cylinder coils removes the need for the distributor, cap, rotor, plug wires, and certain other components as well.
Many of the ignition parts that cylinder coils make unnecessary are wear items that must be replaced at regular intervals. This can result in a savings of both time and money in maintenance operations, since systems with cylinder coils lack components like distributor caps, rotors, and plug wires. A potential drawback to this is that when a cylinder coil does require replacement, it is often very expensive.
One typical configuration involves the plug boot portion of a cylinder coil passing through the top of a valve cover. These types of engines usually have a metal spark plug tube which is sealed to the top of the valve cover by an o-ring. If the seal fails, oil may leak into the spark plug tube and contaminate the rubber boot. This can cause a misfire condition, and requires the often costly replacement of the affected cylinder coil. Some applications allow a damaged spark plug boot to be replaced with a new one, though other cylinder coils are integrated with the boot to the point where the entire unit must be replaced.
Since DI systems lack a distributor, a variety of other components can play a part in firing a cylinder coil at the correct time. Many systems make use of a crankshaft position (CKP) sensor that sends a signal to the electronic control unit (ECU). The computer can then determine the proper timing to fire each coil. Other systems may also use a camshaft position (CMP) sensor in determining the proper ignition timing.