A critical evaluation of laboratory test methods used to assess coke formation tendency in synthetic lubricants

Lubricating oil in combustion engines undergoes thermal degradation under high temperatures and forms solid deposits. These solid deposits, also referred as coke are insidious, black, and carbonaceous solids. To mitigate the problems associated with oil coking, an effective testing methodology must be developed to characterize the coke formation qualitatively and quantitatively. Testing methodologies have been developed to measure coking tendency however some of the international standards such as the SAE ARP 6166 use visual inspection methods to quantify coke. Such methods are unsuitable for advanced research as they are prone to error in human judgment. This paper intends to bridge this gap and discusses a test methodology that can measure the formed coke quantitatively and qualitatively. The formation of coke has been studied under different laboratory methods such as static immersion test, thin film oxidation test, and dynamic test to replicate the various conditions in which coke forms inside an actual operating engine. In a static immersion test, a metal sample is submerged in oil and heated to form coke. In the thin film oxidation test, an oil drop is deposited and heated over the metal sample. In the case of dynamic test measurement, the oil is sprayed over a heated metal sample. The lubricant coke has been characterized using its thickness, mass, and chemical composition. One of the key challenges associated with accurate coke measurement is the brittle nature of coke. It has been concluded that the thin film oxidation test method is the fastest method to assess the coke deposit, followed by the static immersion test and the dynamic test method.