Search

Over the past decade, efforts to reduce emissions of particulate matter (PM) and oxides of nitrogen (NO + NO2, or NOx) from heavy-duty diesel vehicles (HDDVs) have led to the widespread adoption of both Diesel Particulate Filters (DPFs) to control PM and Selective Catalytic Reduction (SCR) to control NOx. We evaluated the performance of DPFs and SCR with 13,327 real-world fuel-based Black Carbon (BC) and NOx emission factors from 9,167 unique heavy-duty vehicles (primarily HDDVs) measured at four sites in California (two ports, two highways) from 2011 to 2018. BC emission factors have decreased by 90% during the past decade. At the same time, BC distributions have become increasingly skewed toward "high-emitters" – e.g., the portion of the HDDV fleet responsible for half of all BC emissions has decreased from ~16% to ~3%. NOx emission factors have also decreased over the past decade, but by only 31%. They remain roughly five times greater than in-use thresholds.We examined changes in BC and NOx emissions with engine age. BC emissions from DPF-only trucks decreased slightly but insignificantly, by 6 ± 15 mg/kg fuel per year, while for DPF+SCR trucks they increased by 5 ± 3. These changes are less than 5% of in-use thresholds. The annual increase in NOx emissions with age was much greater: 1.44 ± 0.28 g/kg for older SCR trucks without on-board diagnostic (OBD) capabilities and 0.48 ± 0.35 for newer trucks with OBD, roughly 20– 50% of in-use thresholds. Paired t-tests on the over 600 vehicles that were observed in multiple campaigns were consistent with these results. Observed changes in BC emissions with age were best fit with a "gross emitter" model assuming an annual DPF failure rate of 0.83 ± 0.01% for DPF-only trucks and 0.56 ± 0.01% for DPF+SCR trucks.Implications: These observations of real-world HDV emission factors have several major implications for regulatory efforts to reduce them. The increasing importance of a relatively small number of high BC emitters suggests that widespread sampling of the on-road fleet will be necessary to identify these vehicles. On the other hand, the much more ubiquitous deterioration in NOx control measures may be better addressed by incorporating on-board diagnostic systems, with telematic data transfer when possible, into inspection and maintenance programs. These NOx observations also highlight the need for strengthening heavy-duty SCR durability demonstration requirements. [ABSTRACT FROM AUTHOR]

This study characterized nitrogen oxides (NOx) and ammonia (NH 3) emissions from five in-use goods movement vehicles, including one diesel vehicle without selective catalytic reduction (SCR), two diesel vehicles with SCR, and two ultra-low NOx natural gas vehicles equipped with three-way catalysts (TWCs). Emissions testing was performed under real-world driving conditions in typical goods movement routes in the South Coast Air Basin (SCAB) of California. NOx emissions varied depending on the vehicle and the route. The no-SCR diesel vehicle showed the highest NOx emissions over all the routes. One of the SCR-equipped vehicles showed NOx emission rates that were two times higher than the other SCR-equipped vehicle with similar engine model year and mileage, which may be attributed to catalyst deterioration. The SCR-equipped diesel vehicles were within two to three times 0.2 g/bhp-hr certification standard over the different routes. The natural gas vehicles showed average NOx emissions around or below the optional Low NOx emissions standard of 0.02 g/bhp-hr. The three-bin moving average window (MAW) method was utilized to show the NOx emissions across various modes of operation, including idle, low load, and medium/high load. The highest fraction of vehicle operation was found in either the low load or medium/high load bins, depending on the route. NOx emissions for each of the bins varied between different vehicles, and are further analyzed in this paper. NH 3 emissions formation was favored for the TWC-equipped natural gas vehicles, which produced about 20 times more NH 3 emissions than the SCR-equipped diesel vehicles. [Display omitted] • Significant NOx reductions for the natural gas vehicles compared to the SCR-equipped diesel vehicles. • NH 3 formation favored in the TWC for the natural gas vehicles and produced during acceleration events. • Low load and medium/high load conditions dominated vehicle operation and NOx emissions. [ABSTRACT FROM AUTHOR]

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. • Regulations for small off-road diesel engines (SORDEs) limited aftertreatment use. • This study demonstrated the feasibility of aftertreatment for SORDEs. • Diesel particle filters provided 98% PM reductions for SORDEs. • Selective catalytic reduction provided 26–91% reductions for SORDEs. • Inventory reductions of 3.8% in PM and 8.8–13.7% in NOx were found for California. [ABSTRACT FROM AUTHOR]