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

The term ‘biodiesel’ refers, in this paper, to the Fatty Acid Alkyl Esters (FAAE) 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 CO2 emissions and the volatile price of crude oil, are leading to alternative transport fuels such as biodiesel being produced, and consumed, in increasing quantities.
Most published research has focused on the impact of biodiesel on engine performance and emissions and, to a lesser degree, with the intention of understanding the impact on aftertreatment systems. Where the aftertreatment systems have been considered, studies have mainly concentrated on diesel particulate filters and NOX reduction systems. To date, no detailed studies in open literature have addressed diesel oxidation catalyst performance and, subsequently, this work presents the first thorough examination in this area.
This study investigated the relative impact 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 HC speciation when using biodiesel compared to baseline diesel fuel.
It was found that the CO catalyst conversion efficiency over a legislative drive cycle reduced as the biodiesel percentage increased, with conversion reduced by 10% and 16% for B25 and B50 respectively compared to baseline diesel. The reduction in spatial and temporal average catalyst brick temperature between B0 and B50 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 CO and HC’s were closely matched to the baseline diesel fuel. No statistically significant difference in the light-off temperatures between B50 and baseline diesel were found, indicating that exhaust gas HC speciation did not have a significant impact on catalyst performance.
The results show that exhaust gas temperature, and the energy released during the exothermic reactions within the catalyst, are the most significant cause of variations in catalyst performance when using biodiesel blends. These findings indicate that the increased use of biodiesel could require aftertreatment design and control optimisation to negate adverse impact on catalyst light-off times and performance.