New techniques for suppression of automobile ignition noise

Automobile ignition noise is due to fast rise-time (impulsive) currents that flow in the vehicle's wiring. The major sources of these impulses are the breakdown in the distributor of the gap between the rotor and the posts, the breakdown of the spark plug gap, and the closure of the breaker points. Each of these sources can be treated individually to suppress the ignition noise. Concepts have been developed and prototypes built and tested for incorporating simple low-pass filters directly into the spark plug (by appropriate sizing of the plug's shell and inner electrode) and into the distributor. The intent of the work was to take an individual vehicle, already suppressed by the techniques used in mass production at the time (1972) in the United States, and to improve that suppression by at least 10 dB over the frequency range 30 to 500 MHz. The amount of suppression improvement obtained, determined by comparison of frequency-scanned peak field strength, ranged from about 13 dB to about20 dB depending upon the frequency. Similar improvements were obtained on a vehicle selected at random. Preliminary measurements (made only on the first vehicle) indicate no undesirable effects upon the vehicle's operation or its exhaust emissions at three conditions. The cost of incorporating these suppression techniques into mass-produced automobiles is not known exactly but is believed by the authors to be nominal.