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CMU study compares lifecycle GHGs of natural gas pathways for MHDVs; MD BEVs can deliver large reductions, but diesel hard to beat for Class 8

A study by Carnegie Mellon University researchers comparing life cycle greenhouse gas (GHG) emissions from different natural gas pathways for medium and heavy-duty vehicles (MHDVs) found that the GHG reduction potentials of the pathways vary sharply between non-Class 8 MHDVs (e.g., pick-up trucks, parcel delivery trucks, and box trucks), Class 8 transit buses, and Class 8 MHDVs (e.g., refuse trucks and tractor-trailers).

Battery-electric (BEVs), LPG, and CNG pathways could reduce life cycle GHG emissions for non-Class 8 MHDVs compared to the baseline petroleum fuels. Similarly, BEVs achieve emission reductions for transit buses. On the other hand, none of natural gas pathways, CNG, LNG, and F-T liquids, achieve any emission reductions per unit of freight-distance moved for Class 8 trucks compared to conventional diesel. The study is published in the ACS journal Environmental Science & Technology.

Study system boundary of natural gas pathways. Different colored areas correspond to different life cycle stages: natural gas upstream (pink), fuel production (yellow), fuel transport (green), and vehicle operation (blue) (indicated by engine technologies). Both feedstock and energy carriers are marked along each pathway. LNG = liquefied natural gas; CNG = compressed natural gas; H2 = hydrogen; GH2 = gaseous hydrogen; LH2 = liquid hydrogen; F-T = Fischer−Tropsch; LPG = liquefied petroleum gas, or propane; ICEV = internal combustion engine vehicle; SI = sparking ignition; CI = compression ignition; BEV = battery electric vehicle; FCEV = fuel cell electric vehicle. Credit: ACS, Tong et al. Click to enlarge.

Specific findings included:

  • When compared to the petroleum-based fuels currently used in Class 8 trucks vehicles, CNG and centrally produced LNG increase emissions by 0–3% and 2–13%, respectively.

  • Battery electric vehicles (BEVs) powered with natural gas-produced electricity are the only fuel-technology combination that achieves emission reductions for Class 8 transit buses (31% reduction compared to the petroleum-fueled vehicles).

  • For non-Class 8 trucks (pick-up trucks, parcel delivery trucks, and box trucks), BEVs reduce emissions significantly (31–40%) compared to their diesel or gasoline counterparts.

  • CNG and propane achieve relatively smaller emissions reductions (0–6% and 19%, respectively, compared to the petroleum-based fuels), while other natural gas pathways increase emissions for non-Class 8 MHDVs.

A fundamental characteristic of the MHDV market is that MHDV fleets are extremely heterogeneous and their environmental performance is highly dependent on the use patterns (such as truck configurations, payloads, drive cycles, etc.). The complexity of modeling the MHDV market has posed serious barriers to understanding the magnitude of life cycle GHG emissions attributed to MHDVs.

… This paper aims to fill a specific knowledge gap in terms of GHG emissions estimates from MHDVs. More specifically, we evaluate the relative comparison of different ways of using natural gas for different types of MHDVs. To achieve this goal, we perform a LCA on a comprehensive set of natural gas-derived fuels, engine technologies, and vehicle types. The contribution of this paper is not methodological; instead we address an important gap in current policy discussions such as the Low Carbon Fuel Standard (LCFS) in California, US and the CAFE standards set to reduce fuel consumption and GHG emissions of MHDVs in the US. While the CAFE standards for MHDV only consider use phase emissions, it is of key relevance to identify whether the best strategies in terms of emissions reductions still hold when one accounts for the full life cycle emissions in order to avoid unintended negative consequences that may be derived from a use-phase-only policy design−as it becomes apparent in our results and analysis section.

—Tong et al.

The researchers focused on estimating emissions of three GHGs: CO2, methane (CH4), and N2O.

They modeled new vehicles available in the market rather than existing vehicles, and considered 7 types of MHDVs: Class 2b pick-up truck; Class 4 parcel delivery truck; Class 6 box truck (such as beverage delivery truck); Class 8 transit bus; Class 8 local-haul tractor-trailer; Class 8 long-haul tractor-trailer; and Class 8 refuse truck.

They included five vehicle engine technologies: spark ignition internal combustion engine vehicle (SI-ICEV); compression ignition internal combustion engine vehicle (CI- ICEV); hybrid electric vehicle (HEV); battery electric vehicle (BEV); and fuel cell electric vehicle (FCEV).

They used two functional units: vehicle distance traveled (gCO2-equiv/km) and freight-distance moved (gCO2-equiv/km-metric-ton). The first functional unit is simple but fails to reflect the functionality of MHDVs, they noted. Although heavier trucks have lower fuel economy, they are more efficient in moving the same weight of load than a lighter vehicle, thus getting lower load-normalized fuel economy (gallons per cargo-ton-mile).

Life Cycle GHG emissions of MHDVs (gCO2-equiv/km-metric-ton), with baseline methane emission estimate. In each panel, results 100-year GWP (left bar in each pair) and 20-year GWP (right bar in each pair) are shown side by side. Error bars are based on the 95% confidence interval of life cycle GHG emissions. Credit: ACS, Tong et al. Click to enlarge.

While using natural gas to fuel electric vehicles could achieve large emission reductions for medium-duty trucks, the results suggest there are no great opportunities to achieve large emission reductions for Class 8 trucks through natural gas pathways with current technologies.

… while this Policy Analysis focuses on GHG emissions, there are other environmental benefits from using natural gas for road transportation, such as health benefits from reduced air pollutants and lower operating noises, which could be significant. There are also other types of MHDVs beyond those included in this paper; for instance, we do not include school buses, port drayage trucks, and all off-highway MHDVs. We also exclude dual-fuel pathways (such as CNG and diesel, and plug-in hybrid electric vehicles) because of limited data, though these vehicles may serve as near-term options to meet the long-term goals of oil independence and emission reductions. While this Policy Analysis is the most up-to-date and comprehensive analysis of the potential environmental benefits of natural gas-based transportation fuels for the MHDV fleet, future analysis should be performed as data becomes available and analytical methods improve.

—Tong et al.


  • Fan Tong, Paulina Jaramillo, and Inês M. L. Azevedo (2015) “Comparison of Life Cycle Greenhouse Gases from Natural Gas Pathways for Medium and Heavy-Duty Vehicles” Environmental Science & Technology doi: 10.1021/es5052759



CNG may not be the ideal solution claimed to be.

Eventually, future larger more efficient FCs may be a much better solution for large trucks, buses, locomotives and ships?


I believe that the Class 8 Heavy Haul would be somewhat better using LNG with diesel pilot for ignition as being marketed by Westport Innovations. This allows the engine to run as a true compression ignition diesel while primarily running on natural gas. The article does note that this type engine was not considered as there was not sufficient data.



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