Evaluation of Small Off-Road Diesel Engine Emissions and Aftertreatment Systems

Off-road engines represent one of the largest categories for mobile source emissions in the United States. Emissions standards for small off-road diesel engines (SORDEs), which have not been updated since 2004, can be met without the use of aftertreatment on engines below 75 horsepower (hp) for nitrogen oxides (NOx) or below 25 hp for particulate matter (PM). It has been well established that aftertreatment systems can significantly decrease mobile source emissions, and improvements in these technologies since 2004 could warrant consideration for adopting more stringent standards for the SORDE category. This study evaluated the feasibility, efficiency, durability, and cost benefit of implementing regulations strict enough to require the use of selective catalytic reduction (SCR) and diesel particulate filter (DPF) controls on off-road engines under 75 hp. Durability testing was performed and emissions assessed on four stock engines equipped with retrofitted aftertreatment systems. The results suggest adding a DPF can provide >98% PM reductions. SCR systems provided NOx reductions ranging from 26 to 91% largely dependent on the cycle tested and engine exhaust temperature profile. Adoption of new SORDE standards could provide a PM reduction of 3.8% and a NOx reduction of 8.8–13.7% for the total off-road equipment inventory in California.