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.

Heavy-duty diesel vehicle idling consumes fuel and reduces atmospheric quality, but its restriction cannot simply be proscribed, because cab heat or air-conditioning provides essential driver comfort. A comprehensive tailpipe emissions database to describe idling impacts is not yet available. This paper presents a substantial data set that incorporates results from the West Virginia University transient engine test cell, the E-55/59 Study and the Gasoline/Diesel PM Split Study. It covered 75 heavy-duty diesel engines and trucks, which were divided into two groups: vehicles with mechanical fuel injection (MFI) and vehicles with electronic fuel injection (EFI). Idle emissions of CO, hydrocarbon (HC), oxides of nitrogen (NO x ), particulate matter (PM), and carbon dioxide (CO 2 ) have been reported. Idle CO 2 emissions allowed the projection of fuel consumption during idling. Test-to-test variations were observed for repeat idle tests on the same vehicle because of measurement variation, accessory loads, and ambient conditions. Vehicles fitted with EFI, on average, emitted ϳ20 g/hr of CO, 6 g/hr of HC, 86 g/hr of NO x , 1 g/hr of PM, and 4636 g/hr of CO 2 during idle. MFI equipped vehicles emitted ϳ35 g/hr of CO, 23 g/hr of HC, 48 g/hr of NO x , 4 g/hr of PM, and 4484 g/hr of CO 2 , on average, during idle. Vehicles with EFI emitted less idle CO, HC, and PM, which could be attributed to the efficient combustion and superior fuel atomization in EFI systems. Idle NO x , however, increased with EFI, which corresponds with the advancing of timing to improve idle combustion. Fuel injection management did not have any effect on CO 2 and, hence, fuel consumption. Use of air conditioning without increasing engine speed increased idle CO 2 , NO x , PM, HC, and fuel consumption by 25% on average. When the engine speed was elevated from 600 to 1100 revolutions per minute, CO 2 and NO x emissions and fuel consumption increased by Ͼ150%, whereas PM and HC emissions increased by ϳ100% and 70%, respectively. Six Detroit Diesel Corp. (DDC) Series 60 engines in engine test cell were found to emit less CO, NO x , and PM emissions and consumed fuel at only 75% of the level found in the chassis dynamometer data. This is because fan and compressor loads were absent in the engine test cell.

Alternative fuels and technologies offer potential for reducing emissions in public transportation. These potentials were explored by determining emissions levels and fuel consumption from the U.S. transit bus fleet and comparison of hypothetical scenarios in which implementation of specific alternative fuels and technologies is considered. Impacts from current transit bus procurements were also evaluated. Emissions benefits above and beyond the natural course of transit bus procurements were examined for new diesel buses running on ULSD fuel, diesel-electric hybrid buses, gasoline-electric hybrid buses, compressed natural gas and biodiesel. According to the analysis, reductions in emissions of CO, NMHC, NOx, PM and CO2, as well as fuel consumption, may be attained, and diesel hybrid buses yield the largest reductions in CO2 emissions and are the only technology to reduce fuel consumption relative to the present fleet. Introducing diesel-electric hybrid buses in 15% of the U.S. transit bus fleet would reduce annual end-use emissions by nearly 1,800 tons of CO, 400 tons of NMHC, 4,400 tons of NOx, 200 tons of PM, 491,400 tons of CO2, and fuel consumption by 50.66 millions of diesel gallons.

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