Known internal combustion engines typically comprise cylinder blocks containing individual cylinders that are closed at one end by an engine cylinder head that is attached to the engine block. In a spark-ignition engine, the cylinder head contains threaded spark plug holes, each of which is open to a respective cylinder. A respective spark plug is threaded into the respective hole to close the hole. Each spark plug includes a central electric terminal that is available for electric connection with a mating terminal of an ignition coil assembly or module.
One general category of ignition coils are individual coils inserted in substantially inside the spark plug insertion hole in the cylinder head of an internal combustion engine. These assemblies have been variously called a pencil coil, a stick coil, a plug hole coil and cigar coil. Generally, such ignition coil assemblies comprise both a wound primary coil and a wound secondary coil concentrically aligned with a ferromagnetic core. Some ignition coil assemblies place the primary coil inside the secondary coil, while others place the secondary coil inside the primary coil, both of which are suitable for use with the present invention.
In operation, an electric current flows through the primary coil creating a large magnetic field. At the proper time in the engine operating cycle for firing a particular spark plug, the electric current is abruptly interrupted, and the rapid change in the magnetic field induces a voltage in the secondary coil sufficiently high to create a spark across gapped electrodes of the spark plug.
Most known pencil coils incorporate electrical, mechanical and thermal isolation between the magnetic core and the closest coil. Typically, the coil is formed on an insulative bobbin, however additional isolation is provided. One known method is the application of heat-shrink tubing around the ferromagnetic core. Another method is a direct casting of rubber or other material inside the bobbin and outside the ferromagnetic core. Unfortunately, both of these methods and their structures have drawbacks.
For example, the bobbin containing the coil typically has a round or cylindrical interior wall. However, the ferromagnetic core may be frustoconical shaped or oval-shaped in its cross-section, and that shape is continued after the plastic sleeve has been heat-shrinked to the ferromagnetic core, resulting in a mis-match of shapes. With direct casting of rubber into the space between the bobbin and ferromagnetic core, the process is very messy. Further, the process of pouring a viscous gel into a tightly constricted space is extremely slow, and also requires significant time and heat for curing. Additionally, this process can result in the trapping of air within the assembly, thereby forming poorly insulated points. In both cases, precise control over the exterior shape of the ferromagnetic core and isolation layer is extremely difficult and potentially expensive.
Accordingly, there exists a need to provide an ignition coil assembly having electrical and thermal isolation between the magnetic core and the bobbin in which the exterior shape is precisely controlled as well as permitting unique and various shapes, while providing a faster, cleaner and more efficient method of manufacturing the ignition coil assembly.