1. Technical Field
The present invention relates generally to ignition coils for developing a spark firing voltage that is applied to one or more spark plugs of an internal combustion engine.
2. Discussion of the Background Art
Ignition coils are known for use in connection with an internal combustion engine such as an automobile engine, and which include a primary winding, a secondary winding, and a magnetic circuit. The magnetic circuit conventionally may comprise a cylindrical-shaped, central core extending along an axis, located radially inwardly of the primary and secondary windings and magnetically coupled thereto. The components are contained in a case formed of electrical insulating material, with an outer core or shield located outside of the case. One end of the secondary winding is conventionally configured to produce a relatively high voltage when a primary current through the primary winding is interrupted. In a common configuration, insulating resin or the like is introduced into the gap between the secondary winding and the case for insulating purposes. The high voltage end is coupled to a spark plug, as known, that is arranged to generate a discharge spark responsive to the high voltage. It is further known to provide relatively slender ignition coil configuration that is adapted for mounting directly above the spark plugxe2x80x94commonly referred to as a xe2x80x9cpencilxe2x80x9d coil.
One problem in the design of ignition coils, particularly pencil coils, involves a relatively high electrical field concentration at a location where the high voltage end of the secondary winding is terminated to a high voltage terminal attached to a secondary winding spool. The relatively high electrical field concentration is magnified by any burr, sharp edge, or solder icicle that may be formed on the high voltage terminal. The high electrical field may also be magnified by a poor position of the high voltage terminal. That is, the terminal may be bent over during manufacture, and may be closer to the case than is desired. This means that there is less insulating resin between the terminal and the case. As a consequence, the increased electrical field concentration, over time, may result in an electrical tree or dendrite forming off of the high voltage terminal propagating through the insulating resin. After the dendrite grows far enough, for example toward ground potential (i.e., through the resin and case to the shield), the high voltage secondary winding will short to ground and the ignition coil will fail.
U.S. Pat. No. 6,208,231 issued to Oosuka et al. entitled xe2x80x9cSTICK-TYPE IGNITION COIL HAVING IMPROVED STRUCTURE AGAINST CRACK OR DIELECTRIC DISCHARGE,xe2x80x9d discloses an ignition coil wherein a high voltage end of the secondary coil is electrically connected to a dummy coil, which is then electrically connected to a terminal plate. A high voltage connector configured for connection to a spark plug is then connected to the terminal plate. Oosuka et al. disclose that since the secondary coil and the terminal plate are electrically connected through not a single connection but rather through the dummy coil, the surface area of the electrically connected portion between the secondary coil and the terminal plate is enlarged so as to avoid the concentration of electrical field. However, Oosuka et al. still disclose that the high voltage end of the dummy coil is electrically connected to the terminal plate by fusing or soldering. Accordingly, it is believed that the same problems described above continue to exist in the design of Oosuka et al.
Accordingly, there is a need for an improved ignition apparatus that minimizes or eliminates one or more of the problems as set forth above.
An object of the present invention is to solve one or more of the problems as set forth above. An ignition apparatus according to the present invention overcomes shortcomings of conventional ignition apparatus by including an electrically conductive cup absent of sharp edges, burrs, or the like, which makes contact with a spring tab extending from the high voltage terminal. The cup also surrounds the high voltage terminal. Because the cup is at the same voltage potential as the high voltage terminal, there will not be an electric field concentration in and around the area of the high voltage terminal. Instead, the electric field concentration will be at a reduced level around the cup. In one embodiment, a 72% reduction in the electric field intensity can be obtained with the arrangement according to the invention. The reduction in electric field concentration reduces or eliminates formation of dendrites which, as described in the Background, may over time result in ignition coil failures. This reduces warranty returns, among other things. In addition, manufacturability is improved, since equipment to remove solder points, and to ensure the desired bend dimensions of the HV terminal can be eliminated.
An ignition apparatus according to the present invention comprises a central core having a main axis, and primary and secondary windings outwardly of the central core. The secondary winding is wound on a secondary winding spool having a high-voltage terminal. A high voltage end of the secondary winding is connected to the high voltage terminal. The apparatus further includes a case outwardly of the core, the spool and the primary and secondary windings. According to the invention, a cup formed of metal material engages the high voltage terminal on an inner surface thereof. The cup is configured to be contacted by a conductive connector that is itself suitable for connection to a spark plug. The cup surrounds the HV terminal, and, being free of sharp edges and the like, reduces electrical field concentrations.
In a preferred embodiment, the cup is formed of aluminum or brass sheet metal material, which is drawn and formed into a cup absent sharp edges, burrs, and other similar artifacts that would result in an increased electric field intensity.
A method of making a high voltage connection including the aforementioned conductive cup is also presented.