Why, What, and How: Engine Varnish

Abstract

Varnish formation, generated in a spark-ignited engine operating under various modes of speed, temperature, compression ratio, spark-advance, and air-fuel ratios, was investigated to establish the interrelationship between fuels, lubricants, additives, and engine operating conditions with respect to their effect and contribution to varnish deposition. The varnish precursors were identified to be primarily fuel aromatic compound. A series of engine tests was then conducted with blends of fuel aromatics and alkylate to determine their varnishing proclivities. Thus it was established that the varnishing characteristics of fuels are dependent not only on the total aromatics but even more so on the individual compounds. It was also determined that finished lubricants act as varnish inhibitors and thus determine to a high degree the engine varnish formation. On the other hand, of all engine variables, only the compression ratio and jacket temperature were found to be also of importance, although their total effect on varnish formation was smaller than that of the fuel or lubricant. Subsequently, the varnish deposits were removed from the engine parts, and a chemical analysis was made using modern instrumental techniques. The results showed the engine varnish composition is mostly alkane in character and contains condensed hydrocarbons, hydroxy-containing organic compounds, as well as ketones and nitro compounds. In addition, it was established that lead salts are also part of varnish but are not a prerequisite to varnish formation. Knowing the chemical composition of the varnish precursors, the composition of the varnish deposits, and the engine conditions under which these deposits were obtained, the chemical mechanism of varnish formation was defined by means of chemical equations. The final analysis of varnish formation is shown in the form of a computer-derived mathematical equation defining the relative importance of all variables affecting engine varnish deposition. Data are provided which show that piston varnish formation is a chemical process which occurs on the piston rubbing surfaces as the blowby travels from the combustion chamber to the engine sump.