Engineers devise new method of powering hybrid heavy-duty trucks

In new research, engineers have devised a new way of powering heavy-duty truck that could drastically curb pollution, increase efficiency, and reduce or even eliminate their net greenhouse gas emissions.

The study was presented at the annual SAE International conference meeting.

Heavy-duty trucks used for transportation across the world are virtually all powered by diesel engines accounting for a major portion of worldwide greenhouse gas emissions, but little has been done so far to curb their climate-change-inducing exhaust.

According to the research, the concept involves using a plug-in hybrid engine system, in which the truck would be primarily powered by batteries, but with a spark ignition engine (instead of a diesel engine).

That engine, which would allow the trucks to conveniently travel the same distances as today's conventional diesel trucks, would be a flex-fuel model 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 said, 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, or the need for excessive battery weight to achieve longer range.

The new concept was developed by MIT Energy Initiative and Plasma Fusion and Science Center research scientist Daniel Cohn 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 you can do with spark ignition (as opposed to the compression ignition used in diesel), because it's intrinsically much cleaner," Cohn said.

Compared to a diesel engine vehicle, a gasoline-powered vehicle 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 allows it to run on gasoline, ethanol, methanol, or blends of these, such engines have the potential to emit far less greenhouse gas than pure gasoline engines do, and the incremental cost for the fuel flexibility is very small, Cohn and Bromberg said.

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 making use of a low-greenhouse-gas fuel" when it's available, "but always having the option of running it with gasoline" to ensure maximum flexibility, Cohn said.

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.

In order to match the efficiency of diesel, 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 modelling of a whole range of desired engine characteristics, combined with a screening of the results using an artificial intelligence system, yielded clear indications of the most promising pathways and showed that such substitutions are indeed practically and financially feasible.

In both the present diesel and the proposed flex-fuel vehicles, the emissions are measured at the tailpipe, after a variety of emissions-control systems have done their work in both cases, so the comparison is a realistic measure of real-world emissions.

The combination of a hybrid drive and a flex-fuel engine is "a way to enable the introduction of an 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 said.

Bromberg said that gasoline engines have become much more efficient and clean 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 diesel 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 said. And by using the engine in a hybrid system, 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 produce 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. I think the alcohol fuels overall have a lot of promise," Bromberg said.

"The engines are cheaper, exhaust treatment systems are cheaper, and it's a way to ensure that they can meet the expected regulations. And combining that with electric propulsion in a hybrid system, given an ever-cleaner electric grid, can further reduce emissions and pollution from the trucking sector," said Cohn.

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

And the combination they propose can address two major challenges at once, they said, "We don't know which is going to be stronger, the desire to reduce greenhouse gases, or the desire to reduce air pollution," Cohn said.