In-Use NOx Emissions from Diesel and Liquefied Natural Gas Refuse Trucks Equipped with SCR and TWC, Respectively

The California Air Resources Board (ARB) and the City of Sacramento undertook this study to characterize the in-use emissions from model year (MY) 2010 or newer diesel, liquefied natural gas (LNG), and hydraulic hybrid diesel engines during real-world refuse truck operation. Emissions from five trucks, two diesels equipped with selective catalytic reduction (SCR), two LNG's equipped with three-way catalyst (TWC), and one hydraulic hybrid diesel equipped with SCR, were measured using a portable emissions measurement system (PEMS) in the Sacramento area. Results showed that the brake-specific NOx emissions for the LNG trucks equipped with the TWC catalyst were lowest of all the technologies tested. Results also showed that the brake specific NOx emissions from the conventional diesel engines were significantly higher despite the exhaust temperature being high enough for proper SCR function. Like diesel engines, the brake specific NOx emissions from the hydraulic hybrid diesel also exceeded certification although this can be explained on the basis of the temperature profile. Future studies are warranted to establish whether the below average SCR performance observed in this study is a systemic issue or is it a problem specifically observed during this work.