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Hybrid Trucks Could Cut Emissions

May 21, 2019
Long-haul trucks with electric motors combined with gas-alcohol engines could slash pollution levels.

Heavy-duty trucks, such as the 18-wheelers that transport many of the world’s goods from farm, factory, or port to market, are virtually all powered by diesel engines. They account for a significant portion of worldwide greenhouse gas emissions, but little has been done so far to curb their exhaust. Now, researchers at MIT have devised a new way of powering these trucks that could drastically curb pollution, increase efficiency, and reduce or even eliminate their net greenhouse gas emissions.

The concept involves using a plug-in hybrid engine in which the truck would be primarily powered by batteries. That engine, which would let trucks conveniently travel the same distances as today’s conventional diesel trucks, would be flex-fuel models that could run on pure gasoline, pure alcohol, or blends of these fuels.

While the ultimate goal would be to power trucks entirely with batteries, the researchers say this flex-fuel hybrid option could provide a way for such trucks to gain early entry into the marketplace by overcoming concerns about limited range, cost, recharging times, battery disposal, or the need for heavy batteries to get a long range.

The new concept was developed by Daniel Cohn, a research scientist at the MIT Energy Initiative and Plasma Fusion and Science Center, and principal research engineer Leslie Bromberg.

“We’ve been working for a number of years on ways to make engines for cars and trucks cleaner and more efficient, and we’ve been particularly interested in what can be done with spark ignition as opposed to the compression ignition used in diesels because it’s intrinsically much cleaner,” Cohn says. Compared to a diesel engine, a gasoline-engine produces only a tenth as much nitrogen oxide (NOx) pollution, a major component of air pollution.

In addition, by using a flex-fuel configuration that lets them run on gasoline, ethanol, methanol, or blends of these, such engines could far less greenhouse gas than pure gasoline engines do, and the incremental cost for the fuel flexibility is small, according to Cohn and Bromberg. If run on pure methanol or ethanol derived from renewable sources such as agricultural waste or municipal trash, the net greenhouse gas emissions could even be zero. “It’s a way of using a low-greenhouse-gas fuel when it’s available, but always having the option of running it with gasoline to ensure maximum flexibility,” Cohn says.

“Although Tesla Motors has announced it will be producing an all-electric heavy-duty truck,” Cohn says, “we think that’s going to be very challenging, because of the cost and weight of the batteries needed to provide sufficient range.”

To meet the expected driving range of conventional diesel trucks, Cohn and Bromberg estimate it would require somewhere between 10 and 15 tons of batteries “That’s a significant fraction of the payload such trucks would otherwise carry,” Cohn says.

To get around that, Cohn and Bromberg will use though a plug-in hybrid. The engine they propose for such a hybrid is a version of one the two researchers have been working on for years, developing a highly efficient, flexible-fuel gasoline engine that would weigh far less, be more fuel-efficient, and produce a tenth as much air pollution as the best of today’s diesel-powered vehicles.

Cohn and Bromberg did a detailed analysis of both the engineering and economics of what would be needed to develop such an engine to meet the needs of existing truck operators. To match the efficiency of diesels, a mix of alcohol with the gasoline, or even pure alcohol, can be used, and this can be processed using renewable energy sources, they found. Detailed computer modeling of a whole range of desired engine characteristics, combined with screening of the results using artificial intelligence, yielded clear indications of the most promising pathways and showed that such substitutions are indeed practically and financially feasible.

In both current diesels and proposed flex-fuel vehicles, emissions are measured at the tailpipe after a variety of emissions-control devices have done their work, so the comparison is a realistic measure of real-world emissions. “Combining a hybrid drive and flex-fuel engine introduces electric drives 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.

Bromberg says that gasoline engines have become much more efficient and cleaner over the years, and the relative cost of diesel fuel has gone up, so that the cost advantages that led to the near-universal adoption of diesels for heavy trucking no longer prevail. “Over time, gas engines have become more and more efficient, and they have an inherent advantage in producing less air pollution,” he says. And by using the engine in a hybrid truck, it can always operate at its optimum speed, maximizing its efficiency.

“Methane is an extremely potent greenhouse gas, so if it can be diverted to make a useful fuel by converting it to methanol through a simple chemical process, that’s one of the most attractive ways to make a clean fuel,” Bromberg says. “Alcohol fuels overall have a lot of promise.”

Already, he points out, California has plans for new regulations on truck emissions that are difficult to meet with diesel engine vehicles. “We think there’s a significant rationale for trucking companies to go to gasoline or flexible fuel,” Cohn says. “The engines are cheaper, exhaust treatments are cheaper, and ensures they can meet expected regulations. And combining that and electric propulsion in a hybrid truck, given an ever-cleaner electric grid, can should reduce the trucking sector’s emissions and pollution.”

“Pure electric propulsion for trucks is the ultimate goal, but today’s batteries don’t make that a realistic option yet,” Cohn says. “Batteries are great, but let’s be realistic about what they can provide.”

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