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• Natural gas vehicles demonstrated lower NOx emissions than diesel vehicles. • Strong dependence of NOx between drive cycles, with higher NOx under lower loads. • HVO showed NOx reductions for non-SCR vehicles, but not for SCR vehicles. • Natural gas vehicles showed higher ammonia emissions. • N 2 O emissions showed no strong trends, with many vehicles near detection limits. The current study characterized nitrogen oxide (NOx), nitrous oxide (N 2 O), and ammonia (NH 3) emissions from 14 heavy-duty vehicles from different vocations (school bus, transit bus, refuse hauler, delivery vehicle, and goods movement vehicle) over different drive cycles using a chassis dynamometer. Test vehicles with engines ranging in model year from 2009 to 2018 and including diesel and compressed natural gas (CNG) vehicles. The diesel vehicles included both vehicles without selective catalytic reduction (no-SCR) and with SCR certified to the 0.2 g per brake horsepower-hour (g/bhp-hr) [0.27 g/kilowatt-hour (kW-hr)] NOx emissions standard that were operated with diesel fuel, with a subset tested on hydrotreated vegetable oil (HVO). The CNG vehicles were all three-way catalyst (TWC) equipped and certified to either the 0.2 or 0.02 g/bhp-hr NOx standard [0.27 or 0.027 g/kW-hr]. This study is part of a larger project that included over 200 in-use heavy-duty vehicles, which is one of the most extensive studies of emissions from modern heavy-duty vehicles to date. NOx, NH 3 , and N 2 O emissions varied depending on the vocation and the technology. Average NOx emissions for the Urban Dynamometer Driving Schedule (UDDS) cycle over all vehicles ranged from 0.003 to 6.16 g/bhp-hr [0.004 to 8.25 g/kW-hr] and from 0.02 to 17.2 g/mile [0.012 to 10.7 g/km], with the diesel vehicles showing the highest emissions, the 0.2 g/bhp-hr certified CNG vehicles showing lower emissions, and the 0.02 g/bhp-hr certified CNG vehicles showing very low emissions. NOx emissions over other different cycles were generally in the same range as those for the hot start UDDS, with the highway cycles generally showing the lowest emissions. CNG vehicles had relatively high NH 3 emissions ranging from 0.13 to 0.34 g/bhp-hr [0.17 to 0.46 g/kW-hr] and from 0.44 to 0.97 g/mile [0.27 to 0.6 g/km] over the hot start UDDS compared to the SCR-equipped diesel vehicles due to NH 3 formation across the TWC. N 2 O emissions did not show consistent trends and were near/at the detection limits for a number of the vehicles. [ABSTRACT FROM AUTHOR]

This study assessed the real-world emissions from two heavy-duty natural gas vehicles certified to the optional 0.02 g/bhp-hr nitrogen oxides (NOx) standard and one diesel vehicle equipped with diesel particulate filter (DPF) to control particulate matter (PM) mass emissions and selective catalytic reduction (SCR) to control NOx emissions. Testing was performed on four different routes typical of goods movement in the greater South Coast Air Basin using a mobile emissions laboratory equipped with a constant volume sampling (CVS) system. This study emphasized the characterization of solid particle number (SPN) and total particle number (TPN) emissions, carbonyl compounds, as well as the greenhouse gas (GHG) emissions of carbon dioxide (CO 2), methane (CH 4), and nitrous oxide (N 2 O). Results showed that SPN and TPN emissions were several orders of magnitude higher for the natural gas vehicles compared to those of the DPF-equipped diesel vehicle. Natural gas vehicles under motoring conditions were found to have a strong nucleation mode of ultrafine particles below 23 nm. PM mass chemical speciation of metals and trace elements indicated that the source of PM mass from natural gas vehicles was lubricant oil combustion. The low molecular weight formaldehyde, acetaldehyde, and acrolein were the predominant aldehydes in the exhaust of all vehicles, with the formaldehyde emissions below the emissions limits for both diesel vehicles and natural gas vehicles. Emissions of CO 2 were lower for the natural gas vehicles, as a result of the lower carbon-to-hydrogen (C/H) molar ratio of natural gas compared to diesel fuel. Similarly, the CO 2 equivalent emissions for the natural gas vehicles were about 20% lower than those of the diesel vehicle. CH 4 emissions were the dominant fraction of total hydrocarbons for the natural gas vehicles, while for the diesel vehicle CH 4 concentrations were negligible. N 2 O emissions were very low for the natural gas vehicles, whereas N 2 O formation was favored in the SCR-equipped diesel vehicle. [Display omitted] • Natural gas vehicles showed more than an order of magnitude higher SPN and PM emissions than diesel. • About 56% of TPN emissions reside below the 23 nm size threshold under high engine load conditions. • Strong nucleation mode particles below 23 nm observed during motoring conditions. • PM formation from natural gas vehicles was related to lubricant oil combustion. • Formaldehyde, acetaldehyde, and acrolein were the dominant aldehydes in vehicle exhausts. [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]