Diesel oxidation catalysts for commercial vehicle engines: Strategies on their application for controlling particulate emissions

A variety of diesel oxidation catalysts were tested on a Mercedes-Benz direct injection, turbocharged-intercooled commercial vehicle engine in order to identify the strategy for the selection of the optimal engine/catalyst combination for different engine applications and markets. The influence of the emissions certification test cycles on the catalyst choice was also investigated. The light-off temperatures for carbon monoxide (GO) and hydrocarbons (HC) were significantly lower for catalysts with a high platinum loading, as compared to catalysts with palladium and low platinum loadings. The particulate (PM) emission at light load operation was reduced by 75% to 85% with all catalysts due to the high conversion rate of the PM soluble organic fraction (SOF). The PM efficiency still increased by 5% at medium load operation, and then started to decrease at a temperature around 550 K due to increased sulfate formation. The sulfate formation could be minimized by a higher space velocity, an optimized washcoat, and a lower platinum or palladium loading, respectively. On the US transient cycle, PM was reduced by 20% to 35% with all catalysts tested. Among the two catalysts with 35% efficiency, the low Pt catalyst D which produced less sulfate at high temperatures was selected as first choice. Deterioration of catalyst D over 2200 h was 8% for PM, 13% for CO and 21% for HC. On the European 13-mode cycle, PM was reduced by 9% only with the optimized low Pt catalyst G, but increased dramatically with all other catalysts (including those selected for US) due to a considerable sulfate formation. Since the SOF fraction is lower on this cycle compared to the US transient cycle, the PM removal potential of an oxidation catalyst is generally lower under European certification conditions, and different catalyst technologies will have to be selected for US and European applications. Only if rape seed methyl ester (RSME) is used as an alternative to diesel fuel, an oxidation catalyst is an effective means not only for decreasing HC and CO, but also PM.