Arc Initiating Processes in the Automotive Distributor

A series of optical and electrical experiments on the transient electrical behavior of the automotive distributor have been carried out. These experiments were guided by the now available increased understanding of corona and other preelectrical breakdown phenomena.lUsing these ideas and simplified models, it was possible to interpret many experimental observations. The objective of this work was to reduce the EMR (Electromagnetic Radiation) associated with the distributor. Since earlier work indicated that most of this EMR was associated with the initial breakdown," this investigation concentrated on arc initiating processes. Major conclusions from this study are listed below. 1. The breakdown is accompanied by an initial current surge which lasts for a few nanoseconds. Unlike the final arc which appears as a ribbon, this transient discharge occupies the whole volume between the anode and the cathode. 2. Breakdown cannot occur until a threshold voltage is exceeded. Even then a separate triggering event is needed. EMR can be reduced considerably by triggering the breakdown near threshold conditions. 3. The source of the EMR is the current surge associated with the discharging of the capacitance of the leads close to rotor. The nonlinear switching process occurs more slowly when breakdown occurs near threshold. It also occurs more slowly as the gap is increased. 4. When silicone coated or nonmetallic cathodes are used, breakdown occurs close to threshold conditions. It is surmized that the triggering electrons are provided by unusual electric field induced emission effects associated with static charging of surfaces. 5. Typically, breakdown with brass cathodes was observed to occur at voltages 50% to 200% above their threshold voltages. Also, the photoemission efficiency of brass cathodes is low, making optical triggering difficult.