The Autoxidation of Biodiesel and its Effects on Engine Lubricants

To investigate the impact of biodiesel on automotive engines during usage, the chemistry of two significant biodiesel components, methyl linoleate and oleate were examined. Both were oxidised in bench top reactors at temperatures between 100 – 170 oC to represent the different conditions across the engine. Products were identified and quantified by GC-FID and GC-MS to determine the main degradation mechanisms. Methyl oleate and linoleate were also oxidised with squalane in the reactors to simulate the effects of fuel dilution in the engine lubricant. At lower temperatures methyl linoleate was shown to enhance the rate of squalane oxidation, but as the temperature increased, the pro-oxidant character decreased to become inhibiting by 170 oC, with the temperature at which this crossover occurred measured at 158  5 oC – methyl oleate had no effect however. This temperature dependent behaviour is attributed to the weak C-H bond of the doubly allylic system (the only feature not common to both molecules) and the subsequent reversibility of O2 addition to the methyl linoleate radical formed via hydrogen abstraction. Studies of a similar molecule, 1,4-pentadiene, revealed ceiling temperatures for this specific reaction as 173  6 oC which was in good agreement with the experimental value. The effects of lubricant additives (two antioxidants and a detergent) on the degradation of squalane-methyl linoleate mixtures was also examined. It was shown that at all temperatures the antioxidant could delay the onset of the reactions, but have no effect on them once they started. Similarly, methyl linoleate could reduce the effectiveness of the antioxidants at all measured temperatures showing that the reversible addition of O2 to allylic radicals was not relevant for these reactions. Two possible mechanisms for this behaviour are discussed along with potential methods for testing them in future experiments. No noticeable effects were observed for the detergent.