Factors affecting the decomposition of biodiesel under simulated engine sump oil conditions

Abstract

Biodiesel is a renewable fuel derived from plant, waste, or algal oils. It is synthesized by the transesterification of triglycerides with an alcohol to yield fatty acid alkyl esters. These esters are prone to oxidative deterioration, yielding a variety of products which increase the viscosity of the fuel beyond acceptable levels. A proportion of the fuel used will always find its way into the vehicle's lubricating oil with dilution becoming increasingly significant on vehicles equipped with a particulate filter because late injections lead to increased wall wetting. Biodiesel will also accumulate in the lubricating oil but, unlike mineral diesel, the biodiesel will not evaporate at the normal operating temperature of the oil and will instead accumulate. As the biodiesel oxidizes within the oil, degradation products will eventually form, leading to a significant increase in the oil viscosity with a potential impact on base engine durability. This study investigates the relative effects of a number of factors likely to be relevant to biodiesel oxidation within a simulated engine lubrication oil environment. The findings of this study suggest that determining the point at which oxidation occurs is inherently difficult and the impact of various factors can vary depending on the oxidation indicator examined. The most significant factors in total oxidation were found to be the temperature and air flowrate with other factors, such as the presence of iron and pro-oxidant, having a significant impact only after initial oxidation reactions had occurred.