Different Properties of Biodiesel in Comparison with Standard Diesel Fuel and their Impact on EURO VI Exhaust Aftertreatment Systems

摘要

The use of fatty acid methyl esters (FAME), often referred to as biodiesel, instead of fossil diesel fuel is under consideration in order to increase the share of fuels from renewable sources and to reduce greenhouse gas emissions. In Europe, commercial diesel fuels already contain up to 7% biodiesel. Higher biodiesel blends or the use of pure biodiesel are probable measures to further increase the share of fuels from renewable sources. Due to its different feedstock and refining process, the specification of biodiesel reveals some important distinctions in comparison with standard diesel fuel. The current work aims to discuss the possible implications of biodiesel utilization on the aftertreatment systems of recent heavy-duty diesel (HDD) vehicles compliant with EURO VI legislation. In particular, the effect of biodiesel on heat-up operation, i.e., the increase of the exhaust gas temperature by catalytic combustion of fuel within a diesel oxidation catalyst (DOC), is investigated. This operation is often used to facilitate the soot regeneration of a diesel particulate filter (DPF) mounted downstream of the DOC. Engine bench testing has been performed, using two different 7-liter HDD engines. Secondary fuel injection upstream of the DOC was conducted at different temperatures and exhaust mass flows, comparing 100% biodiesel (B100) and a 30% biodiesel blend (B30) with standard diesel fuel (B0). The results indicate that biodiesel requests a higher minimum exhaust gas temperature for catalytic combustion of the fuel within the DOC. While under the current test conditions 625°C downstream DOC can be achieved at DOC inlet temperature of 270°C, operation with B30 requires approximately 320°C inlet temperature to achieve the same outlet temperature. When B100 is used for secondary fuel injection, even 34 mDC inlet temperature is required. In order to explain the limitations of secondary fuel injection with biodiesel and biodiesel blends at lower exhaust gas temperatures, the evaporation characteristics of the utilized fuels are discussed. The higher evaporation temperature level of fatty acid methyl esters in comparison to diesel fuel is suggested to be the main reason for the different behavior.