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Assessment of Heavy-Duty Natural Gas Vehicle Emissions: Implications and Policy Recommendations

Recent assessments indicate that natural gas vehicles could constitute up to 20 percent of the new 2025 heavy-duty vehicle market, potentially displacing a significant number of diesel vehicles. Considering the relative lack of regulatory guidance at this stage, there are many ways in which the U.S. heavy-duty vehicle “Phase 2” greenhouse gas rulemaking could influence the new fuel/engine technology and its deployment in the fleet. The possibilities range from provisions that promote natural gas use in trucks, with minimal protection against increased methane emissions, to those that promote only the lowest-carbon natural gas technology and can provide even greater carbon and oil-saving benefits.

Natural gas, which consists largely of methane, contains about a quarter less carbon per unit of energy than diesel; therefore, if the fuel supply chain is managed properly, natural gas could be instrumental in the shift to a lower-carbon heavy-duty vehicle fleet. However, recent studies on natural gas supply chain leakage and emissions indicate that even low levels of methane emissions could turn natural gas truck technology from an asset into a liability for the United States’ long-term climate mitigation planning.

This assessment analyzes the implications of a growing natural gas vehicle fleet for the emission benefits of Phase 2, synthesizing data on upstream emissions, vehicle emissions, and efficiency technology. It includes a stock turnover model analysis of the U.S. medium- and heavy-duty vehicle fleet. The analytical results are presented in terms of the total potential carbon dioxide equivalent (CO2e) emissions across scenarios with varying vehicle efficiency, methane emissions, and natural gas penetration among the fleet.

The study points to several high-level conclusions. First, minimizing overall well-to-wheels leakage is the key determinant in whether trucks using natural gas will offer long-term benefits as part of an overall shift to a more efficient and lower-carbon heavy-duty vehicle fleet. Keeping well-to-wheel natural gas leaks at or below 1 percent throughout the supply chain is critical to ensuring a climate benefit. Second, innovation will be needed for natural gas trucks to keep pace with diesel and gasoline engine efficiency improvements and thus to maintain their carbon reduction benefits, although the effect is less critical than reducing methane leakage. Finally, the decision as to which global warming potential (i.e., 100-year or 20-year) to use is approximately as important as an additional 1 percent of methane leakage, indicating the importance of considering both GHG time accounting metrics.

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