The Degradation of Lubricant and Fuel Due to Nitrogen Dioxide

Due to the recent trends to improve fuel economy and reduce emissions there have been changes to engine design, resulting in higher temperatures and more nitrogen
dioxide in the engine, as well as increased biodiesel usage. Overall, this has led to a more severe environment for the engine lubricant, increasing degradation; an understanding of the mechanism is required to prevent this degradation.

Squalane has been used as a model base oil and degraded in an environment of nitrogen dioxide at 150 oC. GC x GC has been used to separate the resultant product mixture, and time of flight mass spectrometry identified products as the squalane alcohol, ketones and alkane fragments. Nitrogen containing products have been identified using a GC x GC with nitrogen chemiluminescence detector, the highest concentration of product has been identified as nitromethane. Other nitroalkanes have been proposed based on retention index. The experiments have been repeated for mixtures of nitrogen dioxide and oxygen and no unique products have been identified.

The impact of biodiesel degradation has been assessed using methyl linolate as a model compound and applying the same techniques as used for the base oil model system.
The products have been identified as being due to fragmentation of the methyl linolate and the formation of ketones. The largest nitrogen containing product concentration being the nitromethane, other nitoalkanes which have been identified.

This experimental evidence has led to the elucidation of the mechanism for nitration which is presented and compared with the autoxidation mechanism. The nitration is
hydrogen abstraction initiated by NO2. The second stage is the addition of NO2 to form an alkyl nitrite. The CO-NO bond is comparable to a peroxide, therefore decays
to an alkoxy radical. The alkoxy radical can then follow the established mechanisms in autoxidation to form the observed products.