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Viable exhaust-driven on-board ethanol reforming for improvements in fuel economy and emissions

Schematic diagram of the successful “shoebox” reformer design and a picture of the core, after insertion of the catalyst. Credit: ACS, Sall et al. Click to enlarge.

A team at Monsanto and colleagues at AVL Powertrain have successfully designed and demonstrated an onboard low-temperature ethanol reformer that can be driven by exhaust heat. A paper on their work is published in the ACS journal Energy & Fuels.

The low-temperature ethanol-reforming pathway, catalyzed by copper-nickel powder catalysts, transforms ethanol into a mixture of H2, CO, and CH4 at temperatures between 300 and 350 °C. Blending 25-50% of this low-temperature ethanol reformate with ethanol or E85 fuels enables dilute engine operation, resulting in substantial improvements in fuel economy and emissions.

Many papers have described high-temperature steam reforming of ethanol at around 600 °C but...this temperature is too high to be driven by engine exhaust. Coking is also an issue because of the dehydration of ethanol at acid sites on the catalyst at high temperature, generating ethylene, which then carbonizes.

...A 2005 paper in this journal reported that ethanol could be reformed by a different pathway at about 300 °C, enabling the reaction to be sustained by exhaust heat. Unlike high-temperature reforming, supplementary water is not required. Thus, the reaction is compatible with anhydrous ethanol fuels, such as E85, without requiring a secondary water tank.

...Despite its much lower hydrogen content, research in single-cylinder engines over the last four years has demonstrated that the H2/CO/CH4 mixture, “ethanol reformate”, enables engine efficiency improvements comparable to those seen in reformed methanol engines. Further work revealed that, because of the enhanced flammability range of hydrogen, only 25-50% of the fuel needed to be reformed to obtain the efficiency benefits of highly dilute engine operation. This finding, along with the observation that reformate was not required at high engine load, implies that a reformer of modest size could be developed for automotive applications. Unlike reformers for fuel cell vehicles, low-temperature ethanol reformers need only provide a fraction of the fuel over the low and midload portions of the drivecycle to achieve significant fuel economy and emission benefits.

—Sall et al.

High-temperature reformers need to burn a portion of the fuel to maintain their reforming temperatures, thereby generating a fuel-economy penalty. Because exhaust-driven reforming uses only waste engine heat, it avoids this efficiency hit, the authors note. However, they add, this benefit comes at the cost of a substantial design complication: the reformer must serve as both a reactor and a heat exchanger between the fuel and the exhaust stream. Other design considerations for automotive applications include:

  • The device must exhibit low thermal mass to reach operating temperatures early in the drive cycle—a challenge when there must be enough surface area to deliver adequate heat exchange while also ensuring a pressure rating sufficient to withstand the fuel-side backpressure created by the catalyst bed.

  • The device must minimize exhaust backpressure to negligible values (inches of water) because exhaust backpressure places a load on the engine and reduces efficiency.

  • A low-cost, low-weight, compact design capable of being packaged in a vehicle exhaust train.

The team developed three low-temperature ethanol reformer architectures (representing a design evolution) and tested them at automotive scale with exhaust from a V8 engine: shell-and-tube; finned tube; and “shoebox” reformer.

The linear finned tube reformers ultimately failed because of backpressure resulting from slow migration of the powder catalyst and the inexorable rise in backpressure that resulted.

Only the shoebox unit—consisting of transverse shell-and-tube design in which banks of parallel, vertical catalyst tubes extended through a transverse stack of exhaust-side heat-exchange plates—exhibited sustained high conversion with low and stable fuel-side pressure throughout a 500 h test period. This design has relatively low thermal mass and can be readily packaged on a vehicle.

The ethanol reformate produced by the copper/nickel sponge catalyst has been shown in earlier studies to enable very low emissions at cold start along with substantial improvements in fuel economy. At steady state, the shoebox reformer provides sufficient reformate to realize these benefits in a passenger vehicle. E85 and hydrous ethanol motor fuels are now widely available in the U.S. and Brazil, respectively, making this approach practical in these two countries, at least for vehicles with longer drive cycles, such as taxicabs or delivery vehicles, for which reformer heating time is less of a concern.

For an ordinary passenger vehicle with relatively short drivecycles, reduced heating time is clearly necessary. For experimental convenience, relatively long and massive exhaust lines connected the engine to the reformers in this study, resulting in heating times approaching 20 min. Even with substantial reductions in exhaust line thermal mass, downsizing of the reformer is likely to be required to achieve heating times significantly below 5 min. The excellent heat transfer and conversion observed in the shoebox reformers, particularly reformer H with reduced catalyst volume, make us confident that this can be achieved, although closer coupling to the engine and catalytic converters will likely be required. Experimental work in this direction is ongoing.

—Sall et al.


  • Erik D. Sall, David A. Morgenstern, James P. Fornango, James W. Taylor, Nichilos Chomic, and Jennifer Wheeler (2013) Reforming of Ethanol with Exhaust Heat at Automotive Scale Energy & Fuels doi: 10.1021/ef4011274



I have not read the ACS paper but the instant considerations would be: The laws of thermochemistry set limits for efficiency of a particular fuel. For example, reforming of methanol is more energy efficient than ethanol reforming. It is also simpler. Blending gasoline in alcohols only makes things worse. Why not go for the simple and efficient option? Well, perhaps because E85 is available on the market and methanol is not preferred by anybody.


The USA did have an M-85 program at one time, but it fell by the wayside (likely because it gave nothing to the agricultural lobby, which loves ethanol because it sops up grain surpluses).

Trevor Carlson

Good idea to use the otherwise wasted exhaust heat to enrich the fuel for low-duty cycle operations. For long trips this could boost highway mileage for hybrids especially. Obviously the engine would need to be tuned to utilize the enriched fuel as efficiently as possible which hopefully wouldn't require any more expensive hardware.

I'm doubtful this system would make economic sense though against the alternatives unless the "shoebox" reformer could be packaged by integrating it into the resonator and mufflers. Also, the fire hazard seems quite high towards the end of the vehicles service life.

I'm curious how this system would compare cost-wise to an exhaust system cooled by thermo-electric generators sized to give the same fuel economy.


The point is that the engine wouldn't need enriched fuel; the hydrogen content increases the lean-combustion limit so that low-temperature combustion regimes become feasible without misfires.


An engine running with lean-burn combustion will produce very high NOx emissions under certain operating conditions. Conventional three-way catalyst to reduce NOx can only be utilized at stoichiometric conditions (and during fuel enrichment, if used). Thus, the NOx problem would have to be solved if lean combustion is used. Potentially, NOx storage catalysts and SCR could be used but these are more complicated and do not reduce NOx as much as conventional TWC. If the potential to increase EGR is used for an engine running on reformate fuel but with stoichiometric combustion, the potential for reduced fuel consumption is significantly smaller. Then I doubt if it is worth the effort.


What does hydrogen content do for combustion efficiency under high-EGR conditions?


Hydrogen addition extends the combustible limit and counteracts the declining flame speed of dilute and low-oxygen concentration (i.e. via EGR) combustion.


THis is very interesting, but it seems like a fairly complicated solution compared to the Ethanol enabled Direct Injection engine designed by Cohn, Bromberg and Heywood, MIT.

This engine achieves 30% to 35% better gas mileage than the standard ICE on gasoline only using turbo-charging to boost combustion chamber pressures and downsizing of the engine - possible because of the increase in power ouput. The ethanol, directly injected, comes to about 5% of the total fuel usage and the marginal cost is approximately $1,000 - $1,600, making it much more readily adopted by consumers as it would be about one third to one fourth the cost of a hybrid.



To put this into some context, if all the cars and light trucks on the road were equipped with this engine, we would achieve a 30% reduction in gasoline consumption using a volume of ethanol that is 5% of the fuel burned!

Rapid adoption is critical as we are soon to be out of time to prevent GW from going into 'automatic' (i.e. irretrievable) mode.

Also, this engine can use Methanol or any blend of Methanol, Ethanol and gasoline. With methanol, we could replace a much greater part of the gasoline we currently consume.

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