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The California Air Resources Board (ARB) undertook this study to characterize the in-use emissions of model year (MY) 2010 or newer diesel engines. Emissions from four trucks: one equipped with an exhaust gas recirculation (EGR) and three equipped with EGR and a selective catalytic reduction (SCR) device were measured on two different routes with three different payloads using a portable emissions measurement system (PEMS) in the Sacramento area. Results indicated that brake-specific NO x emissions for the truck equipped only with an EGR were independent of the driving conditions. Results also showed that for typical highway driving conditions, the SCR technology is proving to be effective in controlling NO x emissions. However, under operations where the SCR’s do not reach minimum operating temperature, like cold starts and some low load/slow speed driving conditions, NO x emissions are still elevated. The study indicated that strategies used to maintain exhaust temperature above a certain threshold, which are used in some of the newer SCRs, have the potential to control NO x emissions during certain low-load/slow speed driving conditions.

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