The results, published in an open access paper in the ACS journal Environmental Science & Technology , suggest that policymakers wishing to address climate change should use caution before promoting fuel switching to natural gas, the authors concluded.

A study by a team from the Environmental Defense Fund, in collaboration with a colleague from the Lenfest Center for Sustainable Energy at Columbia University has found that while switching a heavy-duty truck fleet from diesel to natural gas has the potential to produce climate benefits, realizing that potential would require a combination of significant reductions well-to-wheels methane emissions (i.e., addressing leakage) and efficiency improvements in the natural gas vehicles themselves. Otherwise, fuel switching can produce net climate damages (more radiative forcing) for decades.

… since natural gas has relatively low carbon intensity, releasing less carbon dioxide (CO 2 ) per unit of usable energy than other fossil fuels, it is often assumed that switching to natural gas is comparatively beneficial for the climate. As recent literature suggests, the latter statement deserves a closer look. While it is true that natural gas emits less CO 2 than other fossil fuels during combustion, potential climate benefits could be reduced or even delayed for decades or centuries depending on the magnitude of methane (CH 4 ) loss from the natural gas supply chain—an area of active research.

Although CH 4 decays more rapidly than CO 2 in the atmosphere, it is a more powerful greenhouse gas (GHG), and its influence on the climate is significant on decadal time frames. Even small amounts of CH 4 can potentially overwhelm large CO 2 reductions to increase radiative forcing in the short run. Taking CH 4 emissions into consideration is critical: short-term radiative forcing will determine the rate at which climatic changes occur, and it is crucial to address both short and long-term net radiative impacts in order to minimize social and ecological disruptions from climate change. —Camuzeaux et al.

For the study, the EDF team used the Technology Warming Potentials (TWP) methodology, which considers the radiative efficiency of both CO 2 and CH 4 and their atmospheric fate as a function of time, thereby providing a view of climate impacts from fuel switching across both short and long time frames.

The researchers modified the TWP methodology to differentiate upstream and in-use methane emissions, and examined several engine technologies (spark ignition (SI) and high-pressure direct injection (HPDI)) and fuel types (LNG and CNG).

For engines, they mainly used two commonly used configuration in the heavy-duty sector: 11.9L, both as diesel compression and natural gas SI types; and 15L, available both as diesel and HPDI engine. They also examined an 8.9 L heavy-duty engine, also available both as diesel CI and natural gas SI type, included for completeness and to enable direct comparison to other studies.

Based on EPA data, they assumed the 11.9 L SI natural gas engine to be on average 13% less efficient than its diesel counterpart, and the 15.L HPDI to be on average 5.5% less efficient than the 15 L diesel CI engine.

They conducted sensitivity analyses to better understand climate implications under a range of assumptions for key parameters: well-to-pump (upstream) CH 4 emissions; efficiency differences between natural gas and diesel engines (efficiency penalty); and pump-to-wheels (in-use) CH 4 emissions.

The results show which combinations of these input parameters produce climate benefits on all time frames when switching diesel truck fleets to natural gas.



TWP results for diesel to natural gas HDT fleet conversions. Technology Warming Potential (TWP) for three diesel to natural gas heavy-duty fleet conversion cases (rows from top to bottom: 11.9 L diesel to 11.9 L SI CNG; 11.9 L diesel to 11.9 L SI LNG; 15 L diesel to 15 L HPDI LNG), with each column showing the sensitivity to alternative ranges of upstream CH 4 emissions; relative vehicle efficiency; and the combination of the two.



The “Sensitivity to WTP CH 4 Emissions” case assumes a range of upstream emissions between 0 and 4% of natural gas throughput, with vehicle efficiency fixed at reference case levels.



The “Sensitivity to Relative Vehicle Efficiency” case assumes a range of diesel to natural gas vehicle efficiency penalty values between 0% and 20% (or equal efficiency) for the SI fleets and between 0% and 10% for the HPDI fleets, with upstream emissions fixed at reference case levels.



The “Combined” cases show the sensitivity of the TWP results to both the upstream CH 4 emissions and the assumed vehicle efficiency penalty, with the most optimistic scenario assuming zero upstream emissions and equal vehicle efficiency and the most pessimistic scenario assuming 4% upstream loss and upper bound vehicle efficiency penalty for the natural gas trucks.



Pump-to-wheels CH 4 emissions are held constant at 2.6 g/mile for the SI fleet conversion cases and 4.2 g/mile for the HPDI case, which equals approximately 0.6% and 1% of natural gas fuel consumption respectively. Credit: ACS, Camuzeaux et al. Click to enlarge.

Broadly, using reference case assumptions reflecting currently available data, they found that converting heavy-duty truck fleets leads to damages to the climate for several decades: around 70–90 years for the SI cases, and 50 years for the more efficient HPDI.

Our results show that under our reference case assumptions, reductions in CH 4 losses to the atmosphere are needed to ensure net climate benefits on all time frames when switching from diesel to natural gas fuel in the heavy-duty sector. By combining such reductions with engine efficiency improvements for natural gas HDTs, it may be possible to realize substantial environmental benefits. However, until better data is available on the magnitude of CH 4 loss, especially for in-use emissions, the precise climate impacts of a switch remain uncertain in this sector. —Camuzeaux et al.

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