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Study: natural gas heavy-duty trucking fleet could benefit economy, but has mixed environmental effects

Switching from diesel fuel to natural gas may hold advantages for the US heavy-duty trucking fleet, but more needs to be done to reach the full environmental benefits, according to a new white paper released by the Institute of Transportation Studies at the University of California, Davis, and Rice University.

The recent shale-driven emergence of natural gas as an abundant, inexpensive fuel in the US has raised the possibility of a “momentous shift” in the level of natural gas used in transportation. The cost advantage of natural gas over diesel fuel is particularly appealing for vehicles with a high intensity of travel and thus fuel use. In the paper, the team investigated the possibility that natural gas could be utilized to provide fuel cost savings, geographic supply diversity and environmental benefits for the heavy-duty trucking sector—and whether it can enable a transition to lower carbon transport fuels.

On a resilience basis, an energy security basis, and on an economic basis, there can be advantages to switching to natural gas in key location. But to have an environmental advantage for reducing greenhouse gas emissions would take significant policy intervention.

—lead author Amy Myers Jaffe, executive director for Energy and Sustainability at UC Davis and an affiliate at ITS-Davis

For the study, the researchers used a proprietary spatial modeling program to investigate the possible advantages and disadvantages of natural gas as a transportation fuel and its potential role in enabling other alternative fuels. They analyzed vehicle applications where natural gas could potentially offer sustained fuel cost advantages in the commercial sector and investigated whether such fuel price savings would be sufficiently large to generate commercial drivers for construction of a national network of natural gas fueling infrastructure.

On the environmental side, the team considered what such a change in fuel type would mean both for air quality in terms of criteria pollutants and for cumulative greenhouse gas emissions.

Environmental considerations. The benefit of natural gas on a net carbon intensity basis in transportation is less clear than the economic benefit, according to the authors. Tailpipe CO2 emissions from burning natural gas in heavy-duty trucking applications are roughly ¼ to ⅓ those from burning gasoline or diesel.

However, for spark ignition LNG trucks to match high-efficiency diesel trucks in life-cycle carbon intensity, methane leakage from the natural gas production and distribution system must be negligible. If the more efficient and more expensive HPDI engine is used in LNG trucking, system methane leakage must be under 2.8% for natural gas to break even in carbon intensity. In addition, a large improvement in natural gas vehicle efficiency would be necessary for natural gas to compete effectively against future best-in-class diesel engines in life-cycle greenhouse emissions.

The authors also found that the effects of leakage can be more significant for CNG than LNG. CNG requires distribution via leaky natural gas local pipelines to the refueling stations where it is compressed, whereas LNG is transported as LNG from LNG plant to refueling station by truck. Because of this, they found no CO2 advantage is found compared to diesel in cases where NGV trucks are equipped with the less-efficient Si engine technology and the more methane leakage-prone CNG.

100-year carbon intensity (gCO2e/mile) of C8 diesel and natural gas under different leakages rate. Source: Myers Jaffe et al. Click to enlarge.

In general, natural gas-based fuels emit less particulate matter and sulfur components than diesel. Modeling research shows that a shift to LNG fuel can contribute a significant reduction in SOx tailpipe emissions as well as almost a full scale elimination in fine particulate matter in heavy-duty trucks.

Research suggests that aftertreatment technology is more important than the type of fuel used; this must also be taken into account to garner the optimum air quality benefits from a shift to natural gas in heavy-duty trucking.

Diesel engines with particulate filters could produce lower levels of particulates than natural gas engines not equipped with aftertreatment technologies, but when NGV engines are equipped with three-way catalyst technology they generally produce much lower particulate and SOx emissions than updated diesel engines. NGV engines without aftertreatment have less ability to control formaldehydes and NOx pollution but with appropriate technology they could produce similar or lower levels than diesel.

Economic and other findings

  • If recent, wide oil and gas price differentials hold, greater use of natural gas in the heavy-duty sector could potentially lower the cost of US freight supply chains, thereby enhancing global US competitiveness by lowering domestic fuel costs for long-distance trucking routes in certain regions.

  • The use of natural gas in the US freight system improves energy security through geographic supply diversification.

  • A concentrated regional focus in key markets for early investment is the least-cost strategy to initiate the development of natural gas transportation networks in the United States.

  • In the case of LNG heavy-duty trucking networks, California is uniquely positioned to launch a profitable natural gas network. The costs to provide dedicated coverage for LNG across California are estimated to be less than $100 million. The Great Lakes and mid-Atlantic areas are also well-positioned to incubate a natural gas transportation network.

  • Despite the fuel cost advantages that might result from some limited regional natural gas transportation network buildouts, the development of a US national natural gas transportation network will be encumbered by high initial investment costs for new cross country infrastructure relative to the fully discounted, incumbent oil-based network.

  • The level of profitability of natural gas fueling infrastructure is more highly correlated with access to a high volume of traffic flows of freight movements than with the locus of surplus supplies of natural gas. Thus, initiatives to introduce natural gas freight fueling businesses in regions with stranded or inexpensive gas resources (natural gas supplies that lack sufficient demand to be commercialized) run a greater risk of failure than efforts to introduce natural gas fueling infrastructure along major freight routes in California, the Great Lakes region and the US Mid-Atlantic.

  • The cost-benefit for natural gas as a direct fuel is most compelling for heavy truck fleets whose vehicles travel 120,000 miles (193,000 km) a year or more.

  • Current commercial economic drivers mean that conventional stations supported by mini-LNG facilities are likely to be the favored technology in the early stages of the market development. Additional options to supply CNG can be an enabling network feature.

  • The lessons for natural gas apply more broadly to the question of the barriers to the development of national networks for alternative fuels. Generally speaking, the lower cost of alternative fuel is an important element of commerciality but is not the only driver to a successful transition to low carbon fuels. The level of costs of new infrastructure is also a significant variable to developing new networks, potentially creating region specific economics.

Conclusions. Overall, the researchers concluded that a small, cost-effective intervention in markets could support a transition to a commercially sustainable natural gas heavy-duty fueling system in the state of California and that this could also advance some of the state’s air quality goals. Our research shows that an initial advanced natural gas fueling system in California could facilitate the expansion to other US states.

Such a network could:

  • Enable a faster transition to renewable natural gas, biogas and waste-to-energy pathways.

  • Improve energy security and weather-event resiliency by diversifying the geographic fuel supply.

  • Potentially lower the cost of national freight supply chains, which could enhance global US competitiveness by lowering domestic fuel costs for long-distance trucking in the United States.

However, stricter efficiency standards for natural gas heavy-duty trucks and stronger regulations of methane leakage along the natural gas supply chain are necessary for natural gas to advance California’s climate and air quality goals as a trucking fuel, according to the report.

The report, “Exploring the Role of Natural Gas in U.S. Trucking,” is from ITS-Davis’ NextSTEPS (Sustainable Transportation Energy Pathways) program. Research and modeling activities that contributed to the report were supported in part by funding from the California Energy Commission and GE Ecomagination.



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Forget diesel and natural gas. The future of heavy duty tractors is fully autonomous BEVs. You can start by building a network of supercharger stations for these autonomous heavy duty BEV tractors along the mayor highways. It will cost less than building gas stations for natural gas. This low cost is being proven by Tesla's fast expansion of their supercharging infrastructure. The idea is that an autonomous BEV tractor with a range of about 100 miles and a 200 kwh battery can move a standard container anywhere on land and that long-distance transport is done by changing tractors in seconds to new fully charged tractors at the tractor supercharging stations. This is done automatically without human involvement. I am certain the economics of a BEV heavy duty tractor is much better than that of a diesel or a natural gas because BEV tractors can be made for much longer mileage life and electricity is always a cheaper fuel than diesel and natural gas.

I will try to make the concrete business case for autonomous BEV tractors versus diesel tractors but before I do so I need a little help to get realistic assumptions about tractor economics as it is not my field. Specifically 1) how much does a new heavy duty diesel tractor that moves standard containers typically cost, 2) typical mpg, 3) life durability in miles, 4) insurance cost per 10,000 mile, 5) how much is a truck driver paid hourly and 6) maintenance cost per 10,000 miles. Any clue is welcome preferable with a source.

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The first Volvo XC90 SUV PHEV is in all likelihood going to be sold in 2020 with a fully autonomous highway capable system. Therefore, it should also be possible for a dedicated team at Volvo Trucks to get the world's first fully autonomous highway capable BEV truck in action on Swedish highways with the needed supercharger stations with automatic truck shifting capability. Trucks with human drivers can transport containers the last mile from customers to these supercharging stations and by 2025 human truck drivers may no longer be needed at all for land base container freight.


Driverless cargo trucks could go 24/24 with a few energy pick-up short stops to greatly reduce transport time without driver fatigue and/or rest or sleep stops.

Better use of machinery plus not havinf to pay for drivers would certainly reduce cost and accelerate delivery.


"...Diesel engines with particulate filters could produce lower levels of particulates than natural gas engines not equipped with aftertreatment technologies, but when NGV engines are equipped with three-way catalyst technology they generally produce much lower particulate and SOx emissions than updated diesel engines...."

I dispute this claim. Certified PM emissions from current-technology diesels are as low or lower than equivalent CNG engines even when they are equipped with a three-way catalyst. For example, the diesel version of the 2015 Cummins 8.9 liter urban bus engine has certified PM emissions of 0.000 grams/bhp-hr in both the FTP and SET (Supplemental Emissions test), while the CNG version of that engine (2015 Cummins 8.9 liter urban bus) has certified PM emissions of 0.002 g/bhp-hr and 0.001 g/bhp-hr, respectively. Not only that, but NMHC and CO emissions are much lower than the CNG version (NOx is the same at 0.13 g/bhp-hr in the FTP). The CNG version of this engine has a three-way catalyst according to the official cert data.

Furthermore, even the California Air Resources Board (CARB) found that diesel with DPF still had an emissions advantage over CNG even with a three-way catalyst ( CARB also found that CNG PM had much higher mutagenic activity (rough indicator of carcinogenicity) per unit mass than diesel, again even with a TWC (Norman Y. Kado, et al., California Air Resources Board, "Emissions of Toxic Pollutants from Compressed Natural Gas and Ultra-low Sulfur Diesel - Fueled Transit Buses.").

2010-compliant diesel truck engines have been thoroughly tested in the Advanced Collaborative Emissions Study (ACES) Phase 2 ( According to that comprehensive study, emissions overall are far below regulated levels, and even most of the hundreds of unregulated emissions tested have been effectively eliminated.

I'm skeptical of any environmental benefit of CNG over modern diesel.

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