The Role of NOx in Engine Lubricant Oxidation

Fuel economy trends like engine downsizing and reduced piston ring tension have created challenging environments for modern engine oils which are expected to deliver more from less. Downsizing has led to smaller sumps meaning longer contact times of the lubricant with NOx in blow-by gasses. Fuel economy requirements have also demanded greater engine efficiency with increasing sump temperatures and blow-by rates due to reduced ring tension. These trends promote nitro-oxidation driven by the action of NOx and air on the lubricant. Nitro-oxidation has often been overlooked as a mechanism of oil oxidation in real world engines. Indeed, in bench oxidation tests purporting to protect modern engines against lubricant oxidation, the emphasis is almost exclusively on iron catalysed oxidation. This paper will show that a simple bench nitro-oxidation test is capable of reproducing trends in nitrate ester formation and consumption seen in real engines and demonstrate that lubricant oxidation and viscosity increase is accounted for by the action of NOx without the use of iron catalysts. This demonstrates that the initiation of lube oil oxidation in real engines is due to the action of NOx and it is only later in the oil life cycle that iron plays any significant role. This has important implications for the widespread use of iron-catalysed bench oxidation tests as guardians for oxidation performance in the real world. Because the oxidation observed in iron catalysed bench oxidation tests does not include NOx, the initiation and propagation of oxidation proceeds via a different mechanism and requires a different anti-oxidant solution.