Fresh and Aged SCRT Systems Retrofitted on a MY 1998 Class-8 Tractor: Investigation on In-use Emissions 2011-24-0175

In order to comply with stringent 2010 US-Environmental Protection Agency (EPA) on-road, Heavy-Duty Diesel (HDD) emissions regulations, the Selective Catalytic Reduction (SCR) aftertreatment system has been judged by a multitude of engine manufacturers as the primary technology for mitigating emissions of oxides of nitrogen (NO_x). As virtually stand-alone aftertreatment systems, SCR technology further represents a very flexible and efficient solution for retrofitting legacy diesel engines as the most straightforward means of cost-effective compliance attainment. However, the addition of a reducing agent injection system as well as the inherent operation limitations of the SCR system due to required catalyst bed temperatures introduce new, unique problems, most notably that of ammonia (NH₃) slip. Even the most refined systems, while performing flawlessly during standardized certification tests, may encounter excursions during real-world operation, thereby leading to possible formation of secondary emissions and emit unacceptable high NO_x.

The following study, funded by the South Coast Air Quality Measurement District (AQMD) and supported by Johnson Matthey, was initiated to provide a better understanding of the performance and especially durability of retrofit exhaust aftertreatment systems comprising of Diesel Particulate Filter (DPF) and SCR devices. Specifically, two SCRT® systems retrofitted to a Class-8 Heavy-Duty Diesel (HDD) truck, whereof one was new and another been in on-road operation for the duration of 15 months, were evaluated on a chassis dynamometer with regard to overall system performance, secondary emissions formation and NO_x conversion efficiency deteriorations due to catalyst aging. SCR conversion efficiencies ranged between 67% to 71% and Particulate Matter (PM) filtration efficiencies above 90%, depending on vehicle test cycle. No statistical evidence for a reduction in SCR® efficiency over the course of a 15 months on-road operation period could be found at the 5% significance level.