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A flexible fuel engine under development that does not require batteries or hydrogen

Issuing time:2021-09-28 10:37

The California Air Resources Board has passed a regulation requiring truck and engine manufacturers to reduce nitrogen oxide (NOx) emissions from new heavy-duty trucks by 90% starting from 2027. The nitrogen oxides emitted by heavy trucks are one of the main sources of air pollution, producing smoke and threatening respiratory health. This regulation requires California to achieve significant air pollution reduction in over a decade. How can manufacturers effectively and economically achieve this ambitious goal?

Daniel Cohn is a research scientist at MIT's Energy Program, and Leslie Bromberg is a research scientist at MIT's Center for Plasma Science and Fusion. They have been researching a gasoline ethanol engine that is cleaner and more cost-effective than existing diesel engine technologies.

Here, Cohen explains the approach of flexible fuel engines and why it may be a realistic solution - to help California achieve its strict vehicle emission reduction targets in the short term. This study was sponsored by the Arthur Samberg MIT Energy Innovation Fund.

Q: How does your flexible fuel gasoline engine technology operate?

A: Our goal is to provide a cost-effective solution for heavy-duty vehicle (HDV) engines that emit low levels of nitrogen oxides, meet California's nitrogen oxide emission regulations, and rapidly reduce gasoline consumption in HDV fleets.

At present, large trucks and other heavy trucks generally use diesel engines. The main reason is their efficiency, which reduces fuel costs - a key factor for commercial trucks (especially long-distance trucks) as they need to travel large distances.

However, the nitrogen oxide emissions of these diesel powered vehicles are approximately 10 times higher than those of spark ignition engines powered by gasoline or ethanol.

Spark ignition gasoline engines are mainly used in cars and light trucks, which adopt a three-way catalytic exhaust treatment system (usually referred to as a catalytic converter) to reduce the vehicle's nitrogen oxide emissions by more than 98% at a moderate cost.

The use of this exhaust treatment system is due to the fact that spark ignition engines can operate at a stoichiometric air/fuel ratio (the total amount of air matches the amount of air required for fuel combustion).

Diesel engines do not operate according to stoichiometric air/fuel ratios, making it more difficult to reduce nitrogen oxide emissions. Their advanced exhaust treatment systems are much more complex and expensive than catalytic converters. Even with such advanced exhaust treatment systems, diesel engine vehicles produce about 10 times higher nitrogen oxide emissions than spark ignition engine vehicles.

Therefore, it is very challenging for diesel engines to further reduce nitrogen oxide emissions to meet the requirements of California's new regulations.

Our method is to use a spark ignition engine, which can be powered by gasoline, ethanol, or a mixture of gasoline and ethanol, as an alternative to diesel engines in HDVs.

Gasoline has a very attractive feature, which is that it is easy to obtain and its price is comparable to or lower than diesel. In addition, the existing ethanol in the United States emits 40% less greenhouse gases (GHG) than diesel or gasoline, and has a widely available distribution system.

In order to make gasoline and/or ethanol driven spark ignition engines attractive for the widespread application of HDVs, we have developed methods to make spark ignition engines more cost-effective for heavy-duty truck owners.

Our approach is to prevent engine detonation in spark ignition gasoline engines (unwanted self ignition can damage the engine) by using various methods, thereby providing diesel like efficiency and high power in gasoline engines. This results in a higher level of turbocharging and the use of higher engine compression ratios.

These characteristics provide a rate similar to that provided by diesel engines. In addition, when the engine is powered by ethanol, the required detonation resistance is provided by the inherent high detonation resistance of the fuel itself.

Q: What are the main challenges faced in implementing your technology in California?

A: California has always been a pioneer in air pollution control, and states such as Washington, Oregon, and New York often follow suit. As a populous state, California has a great influence - it is a trendsetter. What happens in California will affect other regions of the United States.

The main challenge in implementing this technology is that some believe there is no need for better internal combustion engine technology, as by 2035, battery powered HDVs, especially long-distance trucks, can play the necessary role in reducing nitrogen oxide and greenhouse gas emissions.

We believe that the market penetration of pure electric vehicles in the automotive industry will take a considerable amount of time. Compared to light vehicles, battery power has almost no penetration in HDV fleets, especially in users with larger diesel volumes - long-distance trucks.

One of the reasons is that long-distance trucks powered by batteries face the challenge of reducing their freight capacity due to excessive battery weight. Another challenge is that the charging time of pure electric vehicles is much longer than the refueling time of most HDVs currently available.

Hydrogen powered trucks using fuel cells have also been proposed as alternatives to pure electric trucks, which may limit people's interest in adopting improved internal combustion engines.

However, hydrogen powered trucks face the daunting challenge of producing zero greenhouse gas hydrogen at an affordable cost, as well as significant challenges posed by the storage and transportation costs of hydrogen. At present, high-purity hydrogen required for fuel cells is generally expensive.

Q: Overall, how do your ideas compare to battery driven and hydrogen driven HDVs? How will you convince people that this is an attractive path?

A: Our design utilizes existing power systems and can operate on existing liquid fuels. Due to these reasons, it will be economically attractive to long-distance truck operators in the short term.

In fact, it could be a lower cost option than diesel power because its exhaust treatment costs are much lower, and the engine size is smaller at the same power and torque. This economic attractiveness can make large-scale market penetration possible, which will inevitably have a significant impact on reducing air pollution.

In addition, we believe that it will take 20 years for pure electric or hydrogen powered vehicles to achieve the same level of market penetration.

Our method also utilizes existing corn based ethanol, which can provide greater greenhouse gas emission reduction benefits in the short term compared to batteries or hydrogen powered long-distance trucks.

Although using existing ethanol can reduce greenhouse gas emissions by 20% to 40%, in the short term, this market penetration may be much larger than pure electric or hydrogen powered vehicle technologies. This may have a much greater overall impact on reducing greenhouse gases.

In addition, we also see the migration path after 2030, which is to process the carbon dioxide generated during ethanol production through carbon capture and storage, which can further reduce greenhouse gas emissions from corn ethanol. In this case, the total emissions of carbon dioxide may decrease by 80% or more.

The technology of producing ethanol (and another alcohol fuel methanol) from waste at an attractive cost is emerging, which can provide fuel with zero or negative greenhouse gas emissions.

One way to address the negative greenhouse gas impact is to seek alternative landfill methods, as these methods can lead to severe methane greenhouse gas emissions. Converting biomass waste into clean fuels can also generate negative greenhouse gas impacts, as biomass waste can be carbon neutral and the carbon dioxide produced from producing clean fuels can be captured and stored.

In addition, our flexible fuel engine technology can also be used in conjunction as a range extender for plug-in hybrid HDVs, which use limited battery capacity to avoid the problems of reduced cargo capacity and fuel shortage in pure electric long-distance trucks.

With the increasing threat of air pollution and global warming, HDV solutions are becoming an increasingly important choice for reducing air pollution in the near future and providing a faster starting point for reducing greenhouse gas emissions from heavy vehicle fleets. It also provides an attractive path for the HDV field to reduce greenhouse gas emissions in the long term and to a greater extent.


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