The results of a survey of spark plug fouling in 193 late-model passenger cars of four popular makes showed that approximately one-third of the spark plug sets were fouled sufficiently to cause at least a 20 percent decrease in vehicle performance. While the vehicle models and number of cars involved are limited and do not completely represent the car population nor types of service, the survey did provide a sample of the three main price classes of cars. The results indicate that a considerable amount of spark plug fouling exists in vehicles driven in city and city-suburban service. Recognition of spark plug fouling by car owners varied with car make.
Dynamometer and road-test procedures have been devised for studying various factors affecting spark plug fouling. Careful control of test variables has produced acceptable correlation between these procedures. Special instrumentation has been adapted to detect spark plug misfiring by recording pressure fluctuations in the engine exhaust. It is possible to rate the misfiring tendencies of spark plugs from different makes of engines in one particular engine. This is accomplished by duplicating in that engine the temperature-time relationship of the spark-plug insulator tip for each engine make.
Changes in antiknock compound and fuel-sulfur concentration as well as changes in fuel base stock composition affect spark plug fouling. The commercial formulation of a finished fuel, however, is dictated not primarily by spark plug fouling, but by other important considerations, such as fuel octane number. The large effects of changes in the composition of the fuel base stock indicates an important area for future research. Phosphorus fuel additives offer a commercial solution by at least doubling the time to spark plug failure.


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