Future Challenges and Incoming Solutions in Emission Control for Heavy Duty Diesel Vehicles

This paper provides a review of the incoming regulations in the heavy duty diesel (HDD) area and discusses potential new catalyst technologies to enable HDD engines to meet such legislation. A major challenge is to develop combined engine + aftertreatment systems to enable optimum fuel consumption (and therefore low CO₂) while meeting the legislated levels of criteria pollutants (CO, HC, NOx and particulate matter). Improved fuel consumption generally leads to higher engine-out NOx levels and lower exhaust temperatures, both of which present challenges for catalyst systems. Future emission control systems will need to enable increased NOx control under all conditions, but particularly at lower temperatures. Current HDD aftertreatment systems used in the developed markets employ the diesel oxidation catalyst  + catalysed soot filter + selective catalytic reduction (SCR) + ammonia slip catalyst (ASC) architecture, and are very successful at controlling pollutant emissions. Nevertheless, as outlined above, further improvements are necessary in future. Potential approaches to meet these incoming challenges include the use of new, improved SCR catalysts, potentially extruded or combined with high porosity substrates (to enable increased catalyst loading, and therefore activity, per unit volume). Incorporating the SCR catalyst onto the diesel particulate filter to make an SCRF^® component also leads to improved low temperature performance since this allows the SCR catalyst to be located closer to the engine. The cold start performance can be further enhanced by using a Diesel Cold Start Concept catalyst, which stores NOx at very low temperatures and then releases it at higher temperatures when the downstream SCR system is hot enough to convert it. This exciting new technology, and the improvement in cold start performance that it enables, is discussed in detail in this article. The final component of the system is the ASC, whose main function is to prevent the release of ammonia, slipped from the upstream SCR system, to the atmosphere. While the removal of ammonia by oxidation is relatively easy, there is a critical need to maximise the selectivity to N₂ and minimise the generation of both N₂O (which is a potent Greenhouse gas) and NOx (since the SCR system is used to remove NOx)—second generation ASCs are quite effective at doing this since they use a component with SCR activity as an overlayer above the principal ammonia oxidation catalyst. New catalysts under development show a further increase in the desired selectivity to N₂. This article discusses the current state-of-the-art in these incoming technologies, which will enable future HDD engines to meet the dual aims of improved fuel consumption and lower pollutant emissions.