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MIT team proposes flex-fuel gasoline-alcohol engine PHEV long-haul trucks

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 NOx 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 NOx 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 (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 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.

Comments

mahonj

Seems like a good idea to me. A truck is big enough to support a complex serial hybrid engine. By varying the battery size, you can alter its characteristics to suit your situation. It should be possible to run them on batteries in city centres and chemical fuels on the motorway sections.
I still think PHEVs are a good idea for cars and SUVs as you can size the battery to an average day's driving, and use the gas engine for occasional long runs.

Engineer-Poet

Looks like some people at MIT have finally come to the conclusions I arrived at more than 10 years ago.  What took them so long?

gryf

This study has some interesting information.
First, the basic requirement that "The ideal engine for a range extender or other plug-in hybrid power train should be powered by a readily available, affordable room temperature liquid fuel" is an important basis for all transportation.
Today, that is generally only diesel or spark ignition gasoline engines.
Thanks to Formula One we now know that the thermal efficiency of turbocharged pre-chamber jet ignition technology (referenced in this study) exceeds 50% which in a hybrid configuration is as good as fuel cell efficiency (Mahle has demonstrated this technology in passenger autos.).
This technology is also applicable to aviation and not only the General Aviation class which uses 100LL gasoline. Series hybrid aircraft using flex-fuel spark ignition engines could replace even aircraft engines in the Zunum Aero 6 to 12 seat aircraft that currently is designed with a Safran 1500 kW turboshaft engine.

Lad

As concluded; this makes sense to do on an interim basis as we work toward battery electric solutions.

SJC

This could have been done twenty years ago.
The PNGV cars were done and results known.

Peter_XX

This makes sense regarding air pollution if an HD diesel engine could not ever improve beyond the 2010 status. But, surprise, surprise... technology development is a continuous process and a couple of things have happened under a period of almost 10 years. Today, a state-of-the-art diesel engine can also emit one-tenth of NOx compared to EPA 2010 limit. This has been shown in published literature. Engine manufacturers will, of course, start to certify at this level as soon as the new regulation is enforced. Once again, this illustrates the problem of aiming at a moving target.

Peter_XX

@SJC
Your comment is a little off target... PNGV was a program for light-duty vehicles; not HD vehicles. Nevertheless, it might be interesting in this context to note that all three US car manufacturers worked on downsized 3-cylinder diesel engines with a parallel hybrid drive system for their PNGV cars. Some of the projects actually came pretty close to the FE target of 80 mpg with their prototypes. Moreover, one should note that the prototypes were not SUVs, but instead, more like Porsche Panamera or Tesla Model S in their body shape. Some of us would love to drive such a car today. Technically, it would be feasible. If it would be cost-effective, we could discuss further...

gryf

@Peter_XX
While the MIT study does focus on SI vs CI engines, it does mention turbocharged pre-chamber jet ignition technology which have some characteristics of CI engines.
The real focus is a "flex-fuel gasoline-alcohol engine approach for a series-hybrid powertrain for long-haul Class 8 trucks".
In this case I would include the Achates opposed piston engine and the Mazda Skyactiv-X SPCCI (Spark Plug Controlled Compression Ignition) engine.
You may recall that Rolf Reitz at University of Wisconsin and Bengt Johnasson, at Lund University in Sweden both looked into gasoline and ethanol fuel CI engines.
The key issue is really that plug-in hybrid Class 8 trucks will become required in some form in urban areas and jet ignition, SPCCI, gasoline opposed piston engines may do it in a more economical way than standard diesel designs.

Carl

Focusing exclusively on NOx emissions is an incomplete way of assessing exhaust emissions impact on air quality. Lower NOx emissions at the expense of higher emissions of other criteria pollutants is potentially disbeneficial.

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