Investigation and analysis into the impact of rapeseed methyl ester biodiesel on the diesel oxidation catalyst performance

Abstract

The term ‘biodiesel’ refers, in this paper, to the fatty acid alkyl esters derived from vegetable, animal or waste oil feedstocks. Numerous economic and environmental drivers, such as the need for fuel security, the requirement to reduce global carbon dioxide emissions and the volatile price of crude oil, are leading to the production of alternative transport fuels such as biodiesel, and their consumption, in increasing quantities. Most published research has focused on the impact of biodiesel on the engine performance and the emissions and, to a lesser degree, on the intention of understanding the impact on after-treatment systems. Where the after-treatment systems have been considered, studies have mainly concentrated on diesel particulate filters and nitrogen oxide reduction systems. To date, no detailed studies in the open literature have addressed the performance of the diesel oxidation catalyst and, subsequently, this work presents the first thorough examination in this area. This study investigated the relative impacts of thermal and chemical factors, when using rapeseed-based biodiesel, on the performance of a diesel oxidation catalyst. The oxidation reaction intensity inside the catalyst brick was examined and used to identify any possible effects caused by different hydrocarbon speciation when using biodiesel compared with that when using baseline diesel fuel. It was found that the carbon monoxide catalyst conversion efficiency over a legislative driving cycle decreased as the biodiesel percentage increased, with conversion reduced by 10% and 16% for 25% biodiesel fuel blend and 50% biodiesel fuel blend respectively compared with baseline diesel. The reduction in the spatial and temporal average catalyst brick temperature between baseline diesel and 50% biodiesel fuel blend over the New European Drive Cycle was found to be up to 15.5 °C. Catalyst light-off curves showed very similar responses when the engine-out emissions of carbon monoxide and hydrocarbons were closely matched to those of baseline diesel fuel. No statistically significant difference between the light-off temperatures of 50% biodiesel fuel blend and baseline diesel was found, indicating that exhaust gas hydrocarbon speciation did not have a significant impact on the catalyst performance. The results show that the exhaust gas temperature and the energy released during the exothermic reactions within the catalyst are the most significant causes of the variations in the catalyst performance when using biodiesel blends. These findings indicate that the increased use of biodiesel could require after-treatment design and control optimisation to negate an adverse impact on the catalyst light-off time and the catalyst performance.