Biodiesel Impact on Wear Protection of Engine Oils

Pure biodiesel fuel (B100) is typically made of fatty acid methyl esters (FAME). FAME has different physical properties as compared to mineral diesel such as higher surface tension, lower volatility and higher specific gravity. These differences lead to a larger droplet size and thus more wall impingement of the fuel during injection in the combustion chamber. This results in higher levels of fuel dilution as the oil is scraped down into the crankcase by the scraper ring. The lower volatility also makes biodiesel more difficult to evaporate once it enters the crankcase. For these reasons, levels of fuel dilution in biodiesel fueled engines are likely to be higher compared to mineral diesel fueled engines. When in-cylinder dosing is applied to raise the exhaust temperature required for the regeneration of Diesel Particulate Filters (DPF's), biodiesel dilution in the engine oil may be elevated to high levels. Besides changing the viscometric properties of the lubricant, biodiesel and its degradation products could also interact with the lubricant additives and impact their performance. The partially oxidized biodiesel components may compete with ZDDP antiwear additives on metal surfaces. In this study, the wear characteristics of the biodiesel contaminated oil are evaluated by Electrical Contact Resistance (ECR), High Frequency Reciprocating Rig (HFRR) and four-ball tests. The formation of a complex between oxidized biodiesel components and ZDDP is studied with FTIR and ³¹P-NMR techniques. Based on wear bench test and model compound studies, it is concluded that biodiesel fuel can lead to increased engine wear caused by the interaction of oxidized biodiesel compounds and ZDDPs.