MIT Report On Plug-in Hybrid Heavy Duty Trucks: Don’t Let Perfect Be The Enemy Of Good

April 12th, 2019 by Steve Hanley

Researchers at MIT have some advice for electric vehicle advocates — don’t ignore plug-in hybrid powertrains just because battery electric drivetrains are the ideal. Significant reductions in diesel pollution could be achieved tomorrow — well, OK, in the very near future — with the right combination of flex-fuel range extender engines and electric motors. The motors would do the work while the engines would keep the battery charged. Think of it as a tractor trailer with the heart of a Chevy Volt or a Chrysler Pacific Hybrid.

Researchers Daniel Cohn and Leslie Bromberg point out that the news of coming electric trucks is all very welcome, but in the meantime virtually all heavy duty trucks are continuing to belch out diesel emissions, the kind that lead to serious health issues, 24 hours a day all across America and the rest of the world. The alternative, they suggest, is to build plug-in hybrid electric trucks with onboard range extender internal combustion engines. The combination of a hybrid drive and a flex-fuel engine is “a way to enable the introduction of electric drive into the heavy truck sector, by making it possible to meet range and cost requirements, and doing it in a way that’s clean,” Cohn says.

Such trucks would be far cheaper to produce than all electric alternatives, making them that much more attractive to fleet operators. They would emit far fewer pollutants and the best part is they could be on the road shortly — no waiting for the cost of batteries to go down or a network of hydrogen refueling stations to be constructed. The latest IPCC 6 Climate Assessment warns humans must act quickly to reduce carbon emissions. PHEV trucks are not the perfect solution but they could reduce diesel emissions by as much as 90% and do it within months, not years or decades. That’s a pretty powerful argument.

Most CleanTechnica readers pooh pooh PHEV technology, thinking of it as a stopgap measure only, and there is merit to that argument. But there is also merit to ridding the skies of diesel contaminants as soon as possible. Sometimes the good is a reasonable alternative when the perfect is not obtainable.

Cohn and Bromberg acknowledge that gasoline engines are not as efficient as diesel engines, but believe most of that reduced efficiency occurs at light loads. A range extender engine would be working hard to keep batteries charged up and could therefore be configured for maximum efficiency. Adjustments to compression ratios and ignition timing can also boost efficiency. Using computer modeling, the researchers suggest a range extender gasoline engine could be nearly as efficient as a diesel.

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 researchers say. The crucial point, though, is that a hybrid electric truck is more accessible to freight companies right now at a time where carbon emissions need to be curtailed as quickly as possible.

“We don’t know which is going to be stronger, the desire to reduce greenhouse gases, or the desire to reduce air pollution.” In the U.S., climate change may be the bigger push, while in India and China air pollution may be more urgent, but “this technology has value for both challenges,” Cohn says.

Whichever desire is predominant, plug-in hybrid heavy trucks could be the immediate answer we need right now, today, not 5, 10, or 20 years down the road. Because by then it will be far too late — for all of us.









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