Heavy-duty diesel vehicles are a major contributor to diesel emissions in the South Coast Air Basin. While emission measurements of these vehicles in engine dynamometer certification laboratories are showing nitrogen oxides (NOx) and particulate matter (PM) emissions meeting the U.S. Environmental Protection Agency’s (EPA’s) and California Air Resources Board’s (CARB’s) emissions standards, some values from in-use conditions are showing increased emissions of ammonia from liquefied natural gas (LNG) trucks and of NOx from diesel trucks. As such, additional studies are required to assess the impact of technology on emissions from heavy-duty engines used in variety of heavy-duty applications. The objective of this study was to carry out chassis dynamometer testing of heavy-duty natural gas and diesel vehicles using near-certification and in-use driving cycles while measuring: 1) regulated emissions; 2) unregulated emissions such as ammonia and formaldehyde; 3) greenhouse gas levels of carbon dioxide (CO2) and nitrous oxide (N2O); and 4) ultrafine PM emissions.
In December 2010 and October 2011, the SCAQMD Board awarded contracts to University of California, Riverside (UCR) and West Virginia University (WVU) to conduct chassis dynamometer testing of twenty-four model year (MY) 2007-2012 heavy-duty vehicles from different vocations and fueling technologies, and if necessary, to evaluate emission-reduction potential of retrofit technology for ammonia emissions from a natural gas heavy-duty engine. The test vehicle vocations included goods movement, refuse, transit and school bus applications, and the test cycles used for the specific vocations were port drayage truck cycles for goods movement, SCAQMD refuse truck cycles for the refuse applications, and Orange County Transportation Authority (OCTA) and Central Business District (CBD) cycles for transit applications. The Heavy Duty Urban Dynamometer Driving Schedule (HD-UDDS) was a common cycle for all vocations. The test matrix involved five natural gas and four dual-fuel vehicles to be tested on a chassis dynamometer by WVU, eight diesel and two propane vehicles tested by UCR, and five diesel vehicles tested by both WVU and UCR for inter-laboratory comparison. The heavy-duty natural gas engines were both stoichiometric fueled and three-way catalytic converter (TWC) equipped; lean burn high-pressure direct injection (HPDI) engines were equipped with diesel particulate filters DPFs and selective catalytic reduction (SCR) technology. Diesel engines tested in were either U.S. EPA 2007 emissions compliant or U.S. EPA 2010 emissions compliant. The U.S. EPA 2007 emissions compliant engines were equipped with exhaust gas recirculation (EGR) technology and DPFs, while the U.S. EPA 2010 emissions compliant engines were of two types: a) with EGR and DPF only b) with DPF and SCR.
The emission results for PM and NOx are summarized below:
· PM emissions from the diesel test vehicles were below 0.01 grams per brake horsepower-hour (g/bhp-h) measured over port drayage, CBD, and UDDS drive cycles. Cold start PM emissions were relatively high for two diesel vehicles; one was a port SCR equipped vehicle and the other was a refuse SCR equipped vehicle. The port vehicle was 17 times higher (22.9 mg/mi vs 1.33 mg/mi) and the refuse vehicle was 8 times higher (18.4 mg/mi vs 2.75 mg/mi). In both cases the high cold start emission factors were below the certification standard. PM emissions were well below the certification for all diesel tests, thus suggesting DPF-based solutions are robust and reliable in meeting targeted standards. In addition, PM emissions from a liquefied petroleum gas (LPG) test vehicle was approximately 0.14 g/bhp-hr measured over the UDDS cycle, which is above the certification standard.
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