Types of ignition coils
Current technology coils are of several basic types. Pencil coils are typically an inch in diameter and up to four inches long. These coils are designed to fit in between the cams of DOHC and SOHC engines. The normal practice for pencil coils is to include the switching electronics needed to control the coil, in a small pocket near the outer end of the coil package.
Pencil coils have the advantage that they are made to fit within the small area available between the cams. While they package very well on to the engine, pencil coils have the disadvantage of their location. That part of the engine typically operates at very high temperature. The metal surrounding the coil can interfere with the magnetic action of the coil, reducing its output. This is a major consideration in direct injection engines. Pencil coils are also more expensive to manufacture, and possibly less reliable in actual use, than conventional coils.
Coil Near Plug (CNP) applications use a short, plastic insulated stick or a rubber spark plug boot to reach down into the engine and the spark plug. This keeps the transformer portion of the coil up and away from the metal of the cylinder head. While CNPs do not package as neatly as pencil coils, their operating temperature is lower and thus their reliability is greater. CNPs are perhaps one-third less expensive to manufacture than comparable coils. Some CNPs include integrated electronic packages while others do not.
Still in use on some engines are the coil pack type ignition coils. Inside the coil packs are double ended coils that can be connected to simultaneously fire the plug for one cylinder under compression and also a second cylinder on its exhaust stroke. The spark that occurs on the cylinder under “exhaust” is a wasted spark that serves no purpose. Still, this arrangement allows a single coil pack to serve a pair of cylinders. On the next rotation of the engine, what was a wasted spark will become a useful spark when that cylinder is under compression.
The disadvantage of coil pack type systems is that they still use spark plug wires and boots to conduct the spark from the coil to the spark plugs. Spark plug wires and boots have long had warranty-related problems and are being design-eliminated from most current technology engine designs. Coil packs are also comparatively big and heavy as compared to CNPs.
There are still millions of road vehicles and agricultural vehicles that still use a distributor, distributor cap, rotor and spark plug wires. With this type of system, cylindrical, oil-filled coils may be used. In this type of system, the coil is designed to supply sparks to as many as eight cylinders. The cap and rotor act as a high voltage switch that distributes the output of the coil to the required spark plug. The cap, rotor and spark plug wires are all subject to wear and contamination issues. Also, the coil in this type of original-equipment system doesn’t have time to store a full amount of spark energy for every combustion event. The net result is that these systems have lower output voltages and lower reliability than the newer systems that replaced them.
Ignition coil construction
The basic construction of an ignition coil begins with two coils of wire. The primary consists of approximately 100 turns of what is called magnet wire. Really, there is nothing special about “magnet” wire other than being intended for the making of electromagnetic assemblies such as transformers or coils. For the typical ignition coil, the primary is a single strand of copper wire of approximately 22 gauge. The wire is insulated with a thin layer of polyester insulation.
This insulation allows the primary to be wound into a coil shape (to maximize inductance) without the individual turns shorting out to their neighbors. Winding the coil in a circular, tube like shape helps to concentrate the inductance.
The secondary of the coil is typically wound on to a plastic bobbin that has been divided into sections or bays. The wire used in the secondary is much finer than the primary wire. This too is magnet wire. Typically, this type of wire is a single strand, 42 to 44 gauge wire that is coated with a dozen or more very thin layers of insulation. For comparison, this wire is smaller in diameter than the average human hair. It depends on the type of coil being made, but as many as 10,000 turns of the fine wire is wound onto the secondary bobbin. The secondary coil will have a resistance of between 6,000 and 10,000 ohms. The primary coil mounts inside the secondary coil. Stretched out, there might be as much as half a mile of wire wound onto the secondary bobbin.
Inside both the primary and the secondary coils is the lamination stack. The lamstack can be 20 to 30 layers of thin electrical grade steel that is laid out in a rectangular path. Often, two “C” or “U” shaped pieces are made so that they can be assembled, face to face, with one side routed through the primary and secondary coils. The two C’s do not quite make a full path. A gap of roughly 1mm (0.039 inch) is left in the otherwise rectangular path. This gap is important to the storage of magnetic energy in the primary of the coil.