Evaluation of the Performance and Air Pollutant Emissions of Vehicles Operating on Various Natural Gas Blends – Heavy-Duty Vehicle Testing

Published: April 2014

Client: The California Air Resources Board and California Environmental Protection Agency

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Executive Summary

Natural gas vehicles (NGVs) have been implemented in a variety of applications as part of efforts to improve urban air quality, particularly within California. In California, the use of natural gas has been increasing for a number of years, due predominantly to expanded power and home heating needs. The availability of natural gas (NG) within the State from a wider range of sources is also expanding, with the rapid development of NG production via horizontal drilling and hydraulic fracturing as well as the potential of liquefied natural gas (LNG) from the Costa Azul LNG terminal in Baja California, Mexico. The expansion of these new sources coupled with changes in the extent of NG processing to meet markets for natural gas liquids (NGLs) could contribute to a wider more varied composition of NG being used throughout the State that could impact the emissions and performance of NGVs. The California Air Resources Board (CARB) is currently revisiting the compressed natural gas (CNG) fuel standards for motor vehicles. Previous studies of interchangeability, or the impacts of changing NG composition, have been conducted on small stationary source engines, such as compressors, heavy-duty engines, and light-duty NGVs. Some of the previous studies have shown that NG composition can have an impact on emissions, including studies that have shown increases in oxides of nitrogen (NOx) emissions with increasing Wobbe number. Wobbe Number is defined as the higher heating value (HHV) of a gas divided by the square root of the specific gravity of the gas with respect to air. The higher the WN of the gas, the greater the heating value per volume of gas that will flow through a hole of a given size in a given amount of time. The objective of the present study is to evaluate the impact of NG composition on the performance and emissions of heavy-duty vehicles. For this study, three NG buses were tested over the Central Business District (CBD) cycle and a NG waste hauler was tested over the Refuse Truck Cycle on a heavy-duty chassis dynamometer on a range of five to seven different test gases. The test vehicles included a bus with a 2009 8.9L stoichiometric combustion spark ignited Cummins Westport ISLG engine with cooled exhaust gas recirculation (EGR) and a three-way catalyst (TWC), a bus with a 2004 8.1L 6081H John Deere lean burn spark ignition engine with an oxidation catalyst (OC), and a bus with a 2003 C-Gas Plus lean burn spark ignition engine with an OC, and a waste hauler with a 2002 Cummins Westport 8.3L C-Gas Plus lean burn engine with an OC. The certification values for these engines are provided in Appendix A. The test gases included three gases representative of historical baseline fuels for Southern California (labeled H1, H2, and H7) and four gases representing low methane gases (labeled LM3, LM4, LM5, and LM6). The historical test gases were representative of Texas Pipeline Gas (H1) and Rocky Mountain Pipeline Gas (H2), which is also representative of that found in the Kern/Mohave Pipeline between 2000 and 2010. The third historical gas was an L-CNG fuel, which is a natural gas blend produced from liquefied natural gas, identified as H7. This is also a base gas. Since NG-fueled waste haulers come equipped for dedicated fueling on either LNG or CNG, an L-CNG fuel was included to capture the LNG fueled base line. Note that LNG refers to North American supplies that have been processed to take out most components heavier than methane. The four low methane gases included a Peruvian LNG with nitrogen added to achieve a Wobbe Number of 1385 (LM3), a Middle East LNG (WN above 1400 labeled LM4) and two gases with high WN and low MN, one with a high ethane content and the other with a high propane content, identified as LM5 and LM6. The WN and MN are the same for both LM5 and LM6. The gases were designed to determine whether there are differences due to composition. Comparisons between test gases were made for regulated exhaust emissions, fuel economy, PM mass, particle number (PN) and particle size distributions, ammonia emissions, carbonyl compounds emissions, and power maps.

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