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ORNL Researchers Find Thermochemical Exhaust Heat Recuperation In Internal Combustion Engines Could Provide Substantial Boosts in Second-Law Efficiency

Schematic of the conceptual ideal piston engine used for the TCR study. The outer control volume (CV0) encloses the entire system. The upper inner control volume (CV1) contains the combustion chamber and piston. The lower inner control volume (CV2) contains the reformer and intercooler. When reforming is not used, the fuel and exhaust pass through CV2 unchanged. Credit: ACS, Chakravarthy et al. Click to enlarge.

Thermochemical exhaust heat recuperation (TCR) in an internal combustion engine could result in substantial boosts in second-law efficiency (as measured in terms of single-stage work output from an ideal IC engine) for a range of fuels, according to a new study by researchers at Oak Ridge National Laboratory (ORNL). A paper on their work was published online 5 February in the ACS journal Energy & Fuels.

The basic concept of TCR involves using exhaust heat to promote on-board reforming of hydrocarbon fuels into syngas (a mixture of carbon monoxide and hydrogen). The syngas is then burned in the engine in place of some or all of the original fuel. Because the reforming reactions are endothermic, the researchers note, they provide a means for recycling exhaust energy in a chemical form.

Methanol can be converted directly to syngas by adding heat alone in the presence of a catalyst; other fuels, such as ethanol and isooctane, require additional oxygen or water for complete conversion to syngas.

One important factor that we include in this study is combustion irreversibility. For most fuels, the entropy generated by unconstrained combustion destroys up to a third of the original fuel exergy, making that portion of the fuel energy unavailable for generating work. As we explain below, carbon monoxide (CO) and hydrogen (H2) fuels have the potential to significantly reduce this exergy loss because of their unique thermodynamic properties. The extremely low flammability limit of hydrogen can also be used to extend the lean combustion limit, which can increase expansion work and reduce emissions. We therefore considered it important to include the expected changes in combustion exergy losses as part of our analysis.

TCR is particularly relevant to alcohol-based engine fuels, because combustion exergy losses for direct combustion of alcohols are typically higher than for alkanes and alkenes. In addition, ethanol produced by fermentation typically contains considerable water. Much of the cost associated with removing the water during ethanol production might be avoided if hydrous ethanol could be directly used by engines. Onboard, precombustion reforming could potentially help make use of hydrous ethanol more practical.

—Chakravarthy et al.

To reduce mechanical complexity while considering the fundamental thermodynamics, the team confined its analysis to a frictionless, single-stage IC engine operating over an ideal cycle with the following features: (i) constant pressure or constant volume mixing of gaseous fuel and air in the combustion chamber, (ii) isentropic compression of the fuel and air mixture, (iii) adiabatic constant volume combustion of the mixture at the point of maximum compression, (iv) isentropic expansion of the combustion gases to atmospheric pressure, and (v) operation at steady state, so that the engine state repeats precisely at each point in the cycle.

They also limited the study to work generation by a single-stage expansion of the combustion gases; they did not consider other ways to extract work from the exhaust gases, such as Rankine bottoming cycles.

For an ideal stoichiometric engine fueled with methanol, the researchers found that TCR can increase the estimated second law efficiency by about 3% for constant pressure reforming and over 5% for constant volume reforming. The improvement of constant volume reforming over constant pressure reforming results from the pressure boost caused by themolar expansion. When the engine is operated lean (e.g., at a fuel/air equivalence ratio of 0.4), the expected second law efficiency benefits for methanol could be raised an additional 2%. The estimated second law efficiency increases for constant volume TCR of ethanol and isooctane are 9 and 11%, respectively.

The second law efficiency benefits from TCR in the present study are mainly due to the higher cylinder input exergy for reformate and the pressure boost in the case of constant volume reformation. We note, however, that it will be important in future studies to consider the possibility for using combined cycle work extraction. When additional work can be extracted from the exhaust, the benefits of the reduced combustion irreversibility are likely to be more evident.

In the ideal engine system used here, there is significant potential exergy loss associated with the reformer, where we have made no attempt to minimize the temperature gradient or generate work from the heat transferred between the post-expansion exhaust and reformer. If the proposed engine concept is modified to include a bottoming cycle that uses this heat, one would expect considerable increases in the potential work. Still, even for the relatively simple system considered here, TCR could yield substantial efficiency gains.

—Chakravarthy et al.


  • V. Kalyana Chakravarthy, C. Stuart Daw, Josh A. Pihl and James C. Conklin (2010) Study of the Theoretical Potential of Thermochemical Exhaust Heat Recuperation for Internal Combustion Engines. Energy Fuels, Article ASAP doi: 10.1021/ef901113b



"ethanol produced by fermentation typically contains considerable water. Much of the cost associated with removing the water during ethanol production might be avoided if hydrous ethanol could be directly used by engines. Onboard, precombustion reforming could potentially help make use of hydrous ethanol more practical."

WOW..run straight wet E100 and get even more. I knew there had to be a way to use the waste heat, I just never gave this much thought. kewl


About 65% of the energy used by ICE is going into waste heat (mostly engine cooling + exhaust). It should not be that dificult to recuperate a major portion and redirect it to the wheels or other on-board systems.

The overall tank to wheels efficiency could perhaps be raised from 20/25% to 40/50% ???

That could reduce oil imports without major changes in acquired way of life.


They're talking about improvements of 3-5% here. Compare that with; http://www.cpowert.com/
Or the 15% improvement from BMW's turbosteamer; http://www.diseno-art.com/encyclopedia/archive/bmw_turbosteamer.html

Or howabout Bruce Crower's 6 stroke engine; http://en.wikipedia.org/wiki/Crower_six_stroke
Crower claims a 40% reduction in fuel consumption.

Roger Pham

I like it! Quite ingenious.
Essentially a H2 combustion engine optimized for such gaseous fuel. It has been estimated that H2 combustion engine can get as much as 50% thermal efficiency due to the rapid combustion of H2 and the much higher lean limit of combustion, thereby lower combustion temperatures and lower heat loss. Therefore, for higher efficiency, the CO fraction may be additionally turned into H2 via water-gas shift reaction, if possible, but, having the H2 there to jump start the combustion process will speed up the combustion significantly even in the presence of CO.

Having an onboard reformer using exhaust heat to reform hydrocarbon or alcohol fuels into H2 will definitely jump start the H2 economy, since the resultant vehicle can easily be made as a duel-fueled vehicle.


Heck, I shouldn't even have to use google to find better waste heat recovery ideas, Green Car Congress is keeping its own list; http://www.greencarcongress.com/waste_heat_recovery/


I still like the TurboSteamer idea. It requires extra hardware, but so does this. Heat recovery with Rankine could be used with the air/electric hybrid idea as well. Get every last BTU out of the radiator and exhaust that you can.


Doesn't sound much different than this:


This is nothing new. If it is many others have talked about this including me when I was 10. I wrote about it in Diesel Power magazine last month and in blogs for over a year. I think exhaust heat to mechanical power from steam cause steam engines good low rpm diesel good high rpm. You won't need a starter. Or worry about idling in cities. Electrolysis with platinium plates not stainless steel that make hexavelent chromium. Why has this taken so long?

Aureon Kwolek

One early fuel reactor was by French inventor Jean Chambrin – That ran a car on 40% - 60% ethanol-water (patent number 75/06619) – Built around 1973-1974 in response to the Arab Oil Embargo. Worked, but wasn’t as efficient as the fuel processors that came later:

WO8203249 (1983) - It explains what actually happens in the transmutation process – what they call the thermonuclear plasma theory. They don’t fully understand exactly what’s happening. Matter is being transformed in a high speed vortex of hot gases heated by the exhaust – produces mostly hydrogen.

The next one:

This apparently was the inspiration for the GEET fuel processor, which runs on the same principles, using hot engine exhaust. Others showed up too. Lanny Schmidt, a professor of chemical engineering and materials science at the University of Minnesota, built one in 2004. Another one popped up at MIT they called a plasma fuel processor. HyTron Tecnologia, Brazil with a focus on ethanol and natural gas is another. And more recently, the “InnovaGen” multi-fuel processor that you can hold in your hand or put together like modules. With a slew of others.

They’re getting good results from 50-50 ethanol-water. With this combination in solution, the ethanol works as a surfactant, and reduces the surface tension of the water. Then it’s easier to split. All the hydrogen is stripped out of the ethanol, and half the hydrogen is stripped out of the water.

This is why hydrogen fuel cells are not dead, because a cheap fuel like 50-50 ethanol-water can be carried safely and reformed on board, without making hydrogen from scratch and then compressing it into high pressure tanks and hoses.
SEE: “Amazing Locomotion and Energy Systems -- Super Technology and Carburetors” by John Freeman (Plasma Fuel Reformers)


These guys claim 92 mpg with "fuel vapor".




I think light weight and aerodynamics has more to do with that 92mpg.


OTOH Fuel Vapor Technologies does claim fuel efficiency is increased 10% -20% while CO2 emissions are decreased 30%.


VERY interesting. This is probably the same effect that the E-Fuel GridBuster genset is using. It is burning 50-50 water ethanol to drive the generator. They say their blend is 220 octane - not clear they're reforming to H2 in the process - but something is making this work.


It would be interesting to see if the excess heat from the ICE could be used to heat water - a poor man's CHP unit.


BMW uses two circuits, one in the radiator using ethanol as I recall and one in the exhaust using water. The expander is a direct connect that does not require a hybrid drive.


15% less fuel means that 30 mpg can become 40 mpg, which might be enough to justify the cost.



Wouldn't the result be 34.5 mpg?


If I use 10 gallons to go 300 miles and now I use 8.5 gallons to go 300 miles I have 300/8.5 for 35.3 mpg. I had used 300/7.5 for 40 mph, which was wrong. Thanks for the correction.

But the point is that you can get more miles per gallon with the same or better performance with this method. Some would say diesels have that gain over gasoline engines, but you can do it with diesel engines or even diesel hybrids for that matter.

What I really like about the heat recovery and energy generation is that it works on the highway. Hybrids are good around town, but this is good around town AND on the highway. There are lots of commuters that would like to use less fuel.

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