FUNDAMENTAL STUDIES RELATED TO SPARK PLUG DEPOSITS 580108

To learn about ways of controlling spark plug fouling, research was done in three areas: (1) chemical and physical properties of spark plug deposits, (b) development of an instrument to measure a basic physical property, viz. insulator leakage resistance, as a criterion of spark plug fouling, and (c) engine tests simulating car road performance and aiding the other studies.

Ease of deposit fusibility was found to correlate with low resistance and a high degree of fouling. Electron microscopic examination showed that fusibility varied with the area of the ceramic insulator on which the deposit was laid and with the grain structure in a given area. Lead halides are most undesirable in this respect.

Spark plugs whose leakage resistances were less than one megohm at normal engine operating temperatures were generally fouled. The corresponding resistances of new plugs were usually around 1000 megohms. Resistances were measured both on plugs in the running engine and on plugs that had been removed from an engine and were placed in a hot inert atmosphere.

Engine tests gave a measure of plug fouling in terms of power loss on acceleration. In the course of this work it was observed that greatly fouled plugs could be responsible for unusually high octane requirements.

As a result of these studies means of spark plug fouling control by variation in fuel composition have been suggested.