Real-World Emissions from Modern Heavy-Duty Diesel, Natural Gas, and Hybrid Diesel Trucks Operating Along Major California Freight Corridors

Emissions were measured from seven heavy-duty (HD) on-road vehicles that were operated along six common route types used for freight transport in California. All vehicles had engines that were certified to the 0.01 g/bhp-h particulate matter (PM) and either a 0.2, 0.3, or 2.3 g/bhp-h nitrogen oxide (NOx) standard. Diesel vehicles had low carbon monoxide (CO) and total hydrocarbon (THC) emissions below brake-specific standards, with route averages ranging from 0.24 to 3.35 g CO/mi and from 0.02 to 0.45 g THC/mi. Diesel vehicles equipped with selective catalytic reduction (SCR) had route average NOx emissions ranging from 0.58 to 3.99 g/mi (0.16 to 0.96 g/bhp-h). NOx emissions were less route-dependent for the one vehicle with a 12-L compressed natural gas (CNG) engine and three-way catalyst (TWC), with route averages ranging from 0.16 to 0.46 g/mi (0.06 to 0.13 g/bhp-h). The ranking of certification NOx emissions for the seven engines reported during engine-dynamometer-based certification was not maintained during real-world testing; for example, highway driving NOx emissions were lower than certification values for some engine families and higher than certification values for others. Route-average gravimetric particulate matter (PM) emissions ranged from 4 to 12 mg/mi, which on a brake-specific basis were at least two times below the 0.01 g/bhp-h standard. Ion speciation of PM emissions indicated that the most prevalent species were sulfate (SO₄ ²⁻) for the model year (MY) 2007 diesel vehicle equipped with a diesel particulate filter (DPF) and no SCR, nitrate (NO₃ ⁻) for conventional diesel vehicles with a DPF and SCR, and sodium (Na⁺) was the most abundant species for the CNG vehicle. NOx and PM emissions were compared to, and show generally good agreement with, the latest California mobile source model (EMFAC2014).