Pump-to-wheels (PTW) methane emissions from the heavy-duty (HD) transportation sector, which have climate change implications, are poorly documented. In this study, methane emissions from HD natural gas fueled vehicles and the compressed natural gas (CNG) and liquefied natural gas (LNG) fueling stations that serve them were characterized. A novel measurement system was developed to quantify methane leaks and losses. Engine related emissions were characterized from twenty-two natural gas fueled transit buses, refuse trucks, and over-the-road (OTR) tractors. Losses from six LNG and eight CNG stations were characterized during compression, fuel delivery, storage, and from leaks. Cryogenic boil-off pressure rise and pressure control venting from LNG storage tanks were characterized using theoretical and empirical modeling. Field and laboratory observations of LNG storage tanks were used for model development and evaluation. PTW emissions were combined with a specific scenario to view emissions as a percent of throughput. Vehicle tailpipe and crankcase emissions were the highest sources of methane. Data from this research are being applied by the authors to develop models to forecast methane emissions from the future HD transportation sector.

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The Advanced Collaborative Emissions Study (ACES) has the objective of characterizing the emissions and assessing the possible health impacts of the 2007-2010 heavy-duty diesel engines and their control systems. The program seeks to examine emissions from engines operated in a realistic duty cycle and requires the development of an engine test schedule described in this paper. Field data on engine operation were available from Engine Control Unit (ECU) broadcasts from seven heavy heavy-duty trucks (HHDDT) tested during the Coordinating Research Council (CRC) E-55/59 study. These trucks were exercised at three weights (30,000 lb [13,610 kg], 56,000 lb [25,400 kg], and 66,000 lb [29,940 kg]) through four different active modes of a chassis test schedule that were developed from the data of in-use HHDDT operation in the state of California. The trucks were equipped with heavy-duty engines made by three major U.S. engine manufacturers with a range of model years from 1998 to 2003. This paper reports on the development of four engine test modes, termed creep, transient, cruise, and high-speed cruise (HHDDT_S), which correspond to the E-55/59 HHDDT chassis test modes. The creep and transient modes represent urban travel, and the cruise and HHDDT_S modes represent freeway operation. The test mode creation used the method of joining selected truck trips together while ensuring that they offered a reasonable statistical representation of the whole database by using a least-square errors method. Least-square errors between test modes and the database are less than 5%. The four test modes are presented in normalized engine

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Newly collected pump-to-wheels methane emissions data for current natural gas technologies were combined with future market growth scenarios, estimated technology advancements, and best practices to examine the climate benefit of future fuel switching. The analysis indicates the necessary targets of efficiency, methane emissions, market penetration, and best practices necessary to enable a pathway for natural gas to reduce the carbon intensity of the heavy-duty transportation sector.