Introduction The recent demand for natural gas (NG) in the State of California has increased, predominantly due to its use in commercial and residential power applications. The availability of natural gas from a wider range of sources is also expanding within the state, with the rapid development of natural gas production via horizontal drilling, hydraulic fracturing, and extracting liquefied natural gas from the Costa Azul gas terminal in Baja California, Mexico. The expansion of these new sources, in addition to changes in processing natural gas to meet markets, could contribute to a larger variety of natural gas compositions used throughout California. Since California has implemented the use of natural gas vehicles (NGVs) to improve urban air quality, the increase in variety of these natural gasses could influence the emissions and performance of NGVs. The California Air Resources Board is currently revisiting the compressed natural gas fuel standards for motor vehicles. Previous studies of interchangeability (the impact of changing natural gas composition) were conducted on small stationary source engines, such as compressors, heavy-duty engines, and light-duty natural gas vehicles. Some of these studies have shown that natural gas composition can have an impact on emissions, including increases in oxides of nitrogen emissions that affect the Wobbe number. The Wobbe number, otherwise known as the Wobbe Index, is the result of the higher heating value of a gas divided by the square root of the specific gravity of the gas with respect to air. The higher the Wobbe number, the greater the heating value per volume of gas that will flow through a hole of a given size within a given amount of time. The Wobbe number not only measures the energy content within the fuel, but it is also an indicator of the fuels interchangeability. Two fuels with the same Wobbe numbers are ideally interchangeable. This interchangeability typically occurs with gases containing hydrocarbon amounts with higher carbon numbers than methane. Project Purpose and Process The objective of this study was to evaluate the impact of natural gas composition on the emissions of heavy-duty vehicles weighing above 10,001 pounds. To determine impact values, researchers tested several different models of heavy-duty vehicles using a chassis dynamometer. The chassis dynamometer is a device that tests different cycles to measure the emissions and fuel economy output of vehicles; the test cycles simulate a range of driving conditions, such as highway or urban driving speeds. The tests were performed on several heavy-duty vehicles: a school bus with a 2005 8.1L lean burn combustion, spark ignited John Deere 6081H engine; a 2011 waste hauler with a 8.9L stoichiometric, spark ignited Cummins Westport ISL-G engine; a truck with a 2012 stoichiometric spark ignited Cummins Westport ISL-G 8.9L engine with EGR and a TWC; and a truck with a 2013 Cummins Westport ISX12G 11.9L stoichiometric spark ignited engine. The school bus was equipped with an oxidation catalyst – a device that remediates pollutants such as carbon monoxide and hydrocarbons in the exhaust. The waste hauler and both trucks used exhaust gas recirculation (EGR) – a technique used to reduce oxides of nitrogen emissions. The newer vehicles were also equipped with a three-way catalyst (TWC) – a device that remediates carbon monoxide and unburned hydrocarbons while simultaneously reducing oxides of nitrogen emissions. The NG school bus was tested using the Central Business District cycle, the NG waste hauler was tested with the Refuse Truck cycle, and two NG class 8 trucks were tested on the Near Dock duty cycle and the Local Haul duty cycle. The researchers tested seven fuels total—three historical baseline fuels available in Southern California (labeled H1, H2, and H7) and four low methane fuels (labeled LM3, LM4, LM5, and LM6). The first two historical test fuels were representative of Texas Pipeline gas (H1) and Rocky Mountain Pipeline gas (H2) between 2000 and 2010. The third historical fuel (H7) was a liquefied-compressed natural gas (L-CNG) fuel, which is a compressed natural gas blend produced from liquefied natural gas (LNG). The four low methane fuels included a Peruvian LNG with nitrogen added to achieve a Wobbe number of 1385 (LM3); a Middle East LNG with a Wobbe number above 1400 (LM4); a fuel with a high ethane content (LM5); and a fuel with a high propane content (LM6). Both LM5 and LM6 had the same high Wobbe number.
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