Life-Cycle Greenhouse Gas Emission Benefits of Natural Gas Vehicles

Abundant supply, low prices, and low carbon content may result in an increased use of natural gas (NG) as an alternative transportation fuel. Assessments of the associated climate benefits are sensitive to the climate metric and time horizon selected to equate methane to equivalent CO2 emissions and to the assumptions of methane emissions during NG production, distribution, and use. We report life-cycle greenhouse gas (GHG) emissions from cars, light-duty trucks (LDTs), and heavy-duty trucks (HDTs) that are powered directly or indirectly using NG as a function of methane emission rates. We show that whether internal combustion engine vehicles (ICEVs) powered by fossil compressed natural gas (CNG) have GHG benefits over their gasoline and diesel counterparts depends on a value judgment of using either an integrated or end-point climate metric (e.g., global warming potential [GWP] or global temperature change potential [GTP]) and a short or long time horizon (e.g., 20 or 100 years). Conversion of NG into electricity or hydrogen for use in battery electric vehicles (BEVs) or fuel-cell vehicles (FCVs) has clear GHG benefits for cars and LDTs. Benefits are less clear for HDTs where heavy batteries and CNG tanks lead to a relatively poor GHG performance of BEVs and CNG_ICEVs compared to the incumbent diesel technology.