Abnormal Combustion Induced by Combustion Chamber Deposits Derived from Engine Oil Additives in a Spark-Ignited Engine

Although metallic compounds are widely known to affect combustion in internal combustion engines, the potential of metallic additives in engine oils to initiate abnormal combustion has been unclear. In this study, we investigated the influence of combustion chamber deposits derived from engine oil additives on combustion in a spark-ignited engine. We used a single-cylinder four-stroke engine, and measured several combustion characteristics (e.g., cylinder pressure, in-cylinder ultraviolet absorbance in the end-gas region, and visualized flame propagation) to evaluate combustion anomalies. To clarify the effects of individual additive components, we formed combustion products of individual additives in a combustion chamber prior to measuring combustion characteristics. We tested three types of metallic additives: a calcium-based detergent, a zinc-based antiwear agent, and a molybdenum-based friction modifier. Measurements of combustion characteristics after deposit formation revealed that the deposits derived from the calcium and zinc compounds facilitated auto-ignition and increased knock intensity. However, the deposits from molybdenum compounds showed no significant impact on auto-ignition. Our results showed that the chemical composition of combustion chamber deposits has specific effects relating to abnormal combustion in spark-ignited engines.