EFFECT OF OIL VOLATILITY AND ADDITIVES ON ENGINE OCTANE REQUIREMENT

The octane requirement of an engine to suppress either spark knock or audible surface ignition is influenced greatly by the motor oil used. Each component of the motor oil contributes. The volatility of the base oil, the organic portions of detergents and oxidation inhibitors, and the type of viscosity-index improver-all have a direct bearing on the requirement. Multi-graded oils are not necessarily more effective in reducing octane requirement than single-graded oils; much depends on the base oil and V.I. improver. Generally, effects of components are algebraically additive.

A modified detergent with a specific V.I. improver reduces both spark knock and surface ignition. With proper selection of lubricant, engines that are surface-ignition limited can be made spark-knock limited. In field tests, although higher-speed commuter driving reduced the requirement level, the benefits of the modified detergent were found to be equivalent under both city and commuter driving. Expressed in performance numbers, benefits observed in the field were similar to those observed in the laboratory regardless of differences in engine design, compression ratio, and engine operation.