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The role of NOx in engine lubricant oxidation
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
Engine technology trends like downsizing, direct injection and effective lean NOx aftertreatment have created challenging environments for lubricating oils. Longer contact times of the lubricant with fuel and NOx, higher sump temperatures and higher NOx levels in blow-by gas promote nitration-oxidation driven by the action of NOx and air on hydrocarbons. Nitration-oxidation has often been overlooked as a mechanism of oil oxidation in real world engines. Indeed, the emphasis is almost exclusively on iron catalysed oxidation in bench tests purporting to protect modern engines against lubricant oxidation. This paper will demonstrate that a proprietary bench nitration-oxidation test is capable of reproducing trends in nitrate ester formation and consumption seen in real engines, which also fully explain the resulting impact on lubricant oxidation without the use of iron catalysts.