Transportation is one of the major causes of greenhouse gas emissions in the US, and medium- and heavy-duty trucks account for about a quarter of all transportation-related emissions. At present, semis and other long-haul trucks are mostly diesel-powered, so they emit nitrogen oxides and particulates that aren't just bad for the climate; they're bad for human health as well.

Tesla made a splash in 2017 when it introduced its all-electric semi truck , and announcements from other trucking companies followed. Daimler sold small electric delivery trucks and has an electric Cascadia in development, Nikola announced a hydrogen-powered fuel cell truck, and Siemens debuted a catenary system for freight. Yet two years later, trucking in the US is still driven by diesel-fueled, compression-ignition (CI), internal combustion engines.

Daniel Cohn and Leslie Bromberg, a pair of researchers from the Massachusetts Institute of Technology (MIT), published a paper with the Society of Automotive Engineers, suggesting that the best way forward is not to wait for all-electric or hydrogen-powered semis, but to build a plug-in hybrid electric (PHEV) truck with an internal combustion engine/generator that can burn either gasoline or renewable ethanol or methanol.

Such a setup preserves the range and affordability that's expected of diesel long-haul trucks while significantly reducing the emissions associated with diesel. To boot, it's a near-term solution; no waiting for battery weight to fall or hydrogen refueling stations to be installed.

Why find an alternative approach

A hybrid heavy-duty system isn't a completely novel idea, though a PHEV system has yet to be widely applied and tested in long-haul heavy-duty trucking. A company called Hyliion introduced a hybrid electric-diesel truck in 2017, and San Diego uses a hybrid electric-compressed natural gas bus on its transit system, though the former still grapples with diesel emissions and the latter is not for long-haul use.

But there are some distinct problems with all-electric and all-diesel trucks that a hybrid flex-fuel truck could solve. First, freight companies are looking for the cheapest way to transport goods from point A to point B, so expensive electric vehicles don't make short-term economic sense, especially if you're competing with other freight companies using cheaper diesel engines.

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But cheap diesel engines aren't a magic bullet either. As electric vehicle advocates will point out, internal combustion engines are usually cheaper upfront, but volatile fuel costs can drive up the total cost of ownership of a fossil fuel-powered car. In addition, states with strict air quality standards like California impose a Low Carbon Fuel Standard, which increases the price of diesel. In ports and highly populated cities, a PHEV truck could rely on battery power for short distances to comply with metropolitan air quality standards.

With a truck that can run on electricity, gas, and renewably sourced ethanol or methanol, trucking companies can make the best of several worlds. Although gas-powered spark ignition (SI) vehicles are usually less efficient than diesel-powered compression ignition vehicles, the researchers say that SI inefficiencies are mostly found during light load operations where the truck's battery could take over.

Additional modifications to a hybrid truck drivetrain could further reduce the gap in efficiency between the flex-fuel engine and a comparable diesel engine (some of these modifications include increasing the compression ratio of the engine or decreasing the size of a gas engine). The researchers used computer modeling to show that, with specific modifications, such an engine setup would have efficiency comparable to that of diesel engines.

Getting to low emissions quickly

Using flex-fuel gasoline-alcohol engines has also been shown to reduce nitrogen oxide emissions by 90 percent, the MIT researchers wrote, if the emissions reduction system on the truck uses a three-way catalyst (TWC) instead of the diesel-focused selective catalytic reduction (SCR). (The paper notes that this isn't theoretical. A 90-percent reduction in tailpipe NO x from diesel has already been achieved in light-duty gas vehicles and in the heavy-duty Cummins Westport 9 liter natural gas engine.)

A flex-fuel gasoline-alcohol engine could also help freight companies achieve "both the lowest air pollution and lowest greenhouse gas emissions when the internal combustion engine operates," the paper notes.


In addition, "the relative use of battery power, gasoline power, and alcohol power can be optimized for meeting varying prices and availability of these energy sources as a long-haul truck travels through various regions."

What about practical applications?

The researchers note that they only did the computer modeling to show that such a system would be as efficient and clean as a diesel setup, but "more detailed engine modeling" is necessary to bring an electric flex-fuel hybrid truck to market.

However, the private sector does seem to have dipped its toes in hybrid long-haul truck powertrains. In 2017, truck engine maker Cummins announced that it would be developing a 100-mile-range electric semi with a diesel generator on board that could extend the battery range to 300 miles. Last September, the company debuted a hybrid electric and diesel or natural gas powertrain called the PowerDrive. "The PowerDrive replaces the conventional transmission and switches in real time between two hybrid and two pure electric modes, optimizing the powertrain for the best fuel economics in any driving situation," a Cummins press release stated.


The crucial point, though, is that a hybrid electric truck is more accessible to freight companies right now, and, at a time where carbon emissions need to be curtailed as quickly as possible, waiting for an all-electric or hydrogen fuel cell truck may simply make perfect the enemy of the good.