In a paper being presented at WCX SAE World Congress Experience in Detroit this week, a team from MIT is proposing the use of a flex-fuel gasoline-alcohol engine approach for a series-hybrid powertrain for long-haul Class 8 trucks.

In this approach the engine would provide comparable (or possibly greater) efficiency than a diesel engine while also providing around 90% lower NO_x emissions than present cleanest diesel engine vehicles. Ethanol or methanol would be employed to increase knock resistance. Engines that could be deployed in the relatively near term could also use high rpm operation and /or water injection, to allow operation with a very small amount of alcohol in addition to a low concentration mixture such as E10 (or possibly with no additional alcohol). Further NO_x reduction (by use of higher levels of EGR) and increased efficiency (by use of alcohol enhance heat recovery) could potentially be obtained over a longer term.

—Cohn and Bromberg (2019)

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, 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 diesels], because it’s intrinsically much cleaner.

—Daniel Cohn

Compared to a diesel engine vehicle, a gasoline-powered vehicle produces only a tenth as much nitrogen oxide (NO_x) 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 say.

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.

An all-electric heavy-duty truck will be very challenging, Cohn says, 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, would require somewhere between 10 and 15 tons of batteries—a significant fraction of the payload such a truck could otherwise carry.

We think that the way to enable the use of electricity in these vehicles is with a plug-in hybrid.

—Daniel Cohn

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 the economics of what would be needed to develop such an engine to meet the needs of existing truck operators. In order 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 an artificial intelligence system, yielded clear indications of the most promising pathways and showed that such substitutions are indeed practically and financially feasible.

Bromberg says 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 diesels for heavy trucking no longer prevail.

Over time, gasoline engines have become more and more efficient, and they have an inherent advantage in producing less air pollution, Bromberg says. In a series hybrid system, the engine can always operate at its optimum speed, maximizing its efficiency.

The research was supported by the MIT Arthur Samberg Energy Innovation Fund.

Resources

• Cohn, D. and Bromberg, L. (2019) “Flex Fuel Gasoline-Alcohol Engine for Near Zero Emissions Plug-In Hybrid Long-Haul Trucks,” SAE Technical Paper 2019-01-0565, doi: 10.4271/2019-01-0565.