ABSTRACT
Nitrogen oxides, NOx, play a role in degradation of lubricating oils in internal combustion engines. It is well established that NOx reactions contribute to sludge and varnish formation (1 2 3 4 5 6 7 8–9) and to oil oxidation and thickening (10, 11, 12). Therefore, evaluation of engine oils in engine test, such as ASTM Sequences VE (13) and IIIE (14), is performed under conditions producing high levels of NOx or high levels of blowby in order to accelerate degradation and increase test severity.
Nitrogen oxides, formed during combustion, come into contact and react with oil components in the piston/cylinder areas and enter the crankcase environment as a part of blowby gases which leak past the rings. The blowby gases travel through the engine and come into contact with the oil in different locations depending upon the type of ventilation system.
Despite the fact that a causal relationship between NOx and engine oil degradation, insolubles accumulation, and sludge and varnish formation has been established, the mechanisms by which these effects occur have not been fully elucidated.
It is generally accepted that deposit formation in engines is initiated by formation of precursors resulting from interactions of nitrogen dioxide, NO2, with unsaturated fuel derived components of blowby (1, 5). Interactions of NOx with additive systems and base oil components have not, however, been investigated in detail as a source of lubricant degradation and deposit formation. Therefore, this work was aimed at developing an understanding of such interactions using a model system. Hexadecane was utilized as a model lubricant and 2,6-di-tert- butyl-4-methylphenol (MPH) as a radical trapping antioxidant. This model system was oxidized in the presence of NO2. The effects of NO2 on oxidation processes and the fate of NOx in these systems have been investigated. The significance of results obtained in this study with respect to mechanisms of oil oxodation in internal combustion engines is also discussed.
