ORNL Research Project Seeks to Boost Combustion Engine Efficiency To 50-60% By Reducing Combustion Irreversibility; RAPTR
04 June 2009
| The RAPTR experimental device. RAPTR is being designed to investigate Regenerative Air Preheating and Thermochemical Recuperation concepts for reducing losses from combustion irreversibility. Click to enlarge. |
Researchers at Oak Ridge National Laboratory (ORNL) are investigating mechanisms for extending the efficiency of combustion engines closer to their theoretical potential. The work, which started in FY 2005, involves a complex optimization of materials, controls and ultimately, the thermodynamics, said C. Stuart Daw in a presentation at the recent DOE merit review meetings in Washington, DC.
Today’s engines can deliver a fuel efficiency of 40-42%, with corresponding loss of initially available fuel energy of 58-60%. The ORNL project is seeking essentially to flip that, to enable efficiencies of 50-60%, with losses of 40-50%. “This is long-term, high-risk [research], but obviously the payback is increasingly important,” Daw said.
ORNL is working with a number of university partners (Texas A&M University, University of Wisconsin, Illinois Institute of Technology, University of Alabama, University of Michigan, Dearborn) on the project, as well as Delphi on the catalyst side, a not-for-profit R&D institution, and an unnamed OEM.
Really we’re trying to push the boundaries in our thinking...at this stage, it’s very much a conceptual project as much as experimental...in particular we are trying to address the issues associated with energy efficiency limits for IC engines including advanced combustion modes.
—Stuart Daw
| Impact of combustion irreversibility revealed by second Law (exergy) analysis. Click to enlarge. |
The first phase of the project involved clarifying theoretical combustion engine efficiency limits based on literature and selected case studies. This review found that the most under-investigated issue in this area is combustion irreversibility—the destruction of the useful energy of the fuel during the combustion process itself—with a 20-25% loss of fuel energy right off the top.
The ORNL team is currently working to develop and demonstrate more thermodynamically efficient combustion, with the target of reducing the 20-25% combustion irreversibility loss by half. The following phase of the work will involve defining and demonstrating an engine implementation.
The source of combustion irreversibility is entropy generation occuring during conventional combustion.
Where there is a large gradient both in temperature and chemical potential that occurs in the flame, or even without a flame—even with HCCI you have this problem—we carry out the reactions in such a way that they spontaneously happen very rapidly, but that produces this entropy generation that occurs because of the heat transfer between molecules and chemical potential gradient. We are far from equilibrium, and when you carry out reactions far from equilibrium, you generate entropy.
—Stuart Daw
The first approach the team tried was a constant-pressure experiment applying concepts of managing the thermal transfer process to reduce the temperature difference, and also to apply chemical recuperation—catalytic reforming of the hydrocarbon fuel to syngas to break up the original fuel molecules to reduce to some extent the chemical potential gradient and thereby reduce the entropy generation. When syngas—CO and H2—burns, the irreversibility is much less.
The feedback during the 2008 merit review criticized the constant-pressure approach, suggesting that a constant volume approach was more relevant to combustion engines. Based on that input, the team moved to a different experimental design with the objective of cycling constant volume combustion—something that’s closer to the idea of a regular combustion engine.
The modified approach incorporates two main concepts— Regenerative Air Preheating (RAP) and Thermochemical Recuperation (TR)—resulting in the acronym RAPTR.
Regenerative Air Preheating (RAP) uses a counterflow heat exchange between exhaust and inlet air via solid media (e.g., as in Stirling engines) to minimize the temperature gradient.
Thermo-chemical recuperation uses recovered exhaust heat to drives endothermic reforming reactions. The reforming catalyst is included with thermal regenerator solids.
The RAPTR apparatus, which is under construction, is essentially a “tinker-toy”-like device with ceramic linings that allows separate and combined assessment of many factors under study without the complexity of a full engine.
The team is exploring the use of different regenerator materials; the ability to maintain large axial regenerator temperature gradients; air vs. fuel preheating; the location of fuel injection and reformer catalyst; and the effect of different fuels.
Key measurements will include transient temperatures and pressures in the combustion, inlet/exhaust, regenerator and catalyst; inlet and outlet flows; comparisons with baseline operation (without preheating or reforming); and the partitioning of preserved exergy between work potential and hot exhaust.
Looking ahead, Daw said that engine designs that utilize thermal and chemical recuperation might include elements of previously discussed concepts, including complex mechanisms such as dual pistons with flexible phasing, but including new combinations of materials, mechanisms and controls.
Planned near-term activities include:
Completing RAPTR construction and shakedown with hydrogen and methane;
Continue characterization of reformer catalyst for other fuels including methanol, ethanol (hydrous and anhydrous) and iso-octane. Hydrous ethanol is an area of particular interest here, Daw said, as if you reform the ethanol before burning it in the engine, the water is not necessarily a penalty, and may actually be a benefit. One of the issues concerning the potential use of ethanol as a fuel is the energy required to remove the water (i.e., transformation from hydrous to anhydrous).
Conduct experiments and analyses to quantify baseline availability (exergy) balances without thermal and chemical recuperation.
Longer term, the team is looking to evaluate fuel effects, the effects of major configuration changes, and to translate the results to proposed ICE concepts.
Asked whether there was a conflict between the thermal recuperation aspect of this work and the effort that the industry has put into chargers and EGR coolers, Daw said:
If you are looking at just extracting piston work alone, cooling the charge actually is a benefit. What you are doing, though, is destroying some of that exergy which could be extracted in other ways from the exhaust. You have to take a global systems perspective, and not look just at what you get out from the piston work by itself...
The other example of that now is where we have been going to lean engines. Going lean also destroys exergy because essentially you are diluting the energy released from flame. While in the end that is beneficial for producing piston work, the global effect is in the wrong direction.
Where all this is leading to, we need to relook at whole picture particularly with engines now where we may perhaps extract work in multiple ways.
Resources
Jerald A. Caton (2000) On the destruction of availability (exergy) due to combustion processes — with specific application to internal-combustion engines. Energy Volume 25, Issue 11, Pages 1097-1117 doi: 10.1016/S0360-5442(00)00034-7
The use of exhaust heat to reform the fuel to reduce entropy increase in combustion reminds me of an old article (SciAm, I bet) which suggested the use of methanol as motor fuel and reforming it to CO+H2 as an endothermic process using recycled exhaust heat. Not only would this have reduced combustion entropy, the gaseous fuel has greater energy than the methanol so the system would regenerate exhaust heat into additional fuel energy.
Posted by: Engineer-Poet | 04 June 2009 at 06:54 PM
"water is not necessarily a penalty, and may actually be a benefit."
Using wet ethanol more efficiently and effectively could be very good. Cellulose E100 could be a possible fuel for the future.
Posted by: SJC | 04 June 2009 at 09:05 PM
While I approve of the research I also have to note any 'long-term research' risks being overtaken. In this case by the time we see these improved ICEs on the road we'll most likely have switched to BEVs.
Posted by: ai_vin | 04 June 2009 at 09:08 PM
I don't understand this notion of "energy loss in the combustion process because of the irreversibility of combustion" I understand that if you recompress the CO2 and H2O formed in the combustion you only get Hot high pressure CO2 and H2O and not cool Gazoline ond O2. So the heat would escape in the environment generating entropy. But in the case of combustion you generates heat that's it, not entropy, you only generates entropy through heat loss, which could be avoided by using insulating material for example (like ceramics). I also understand that if you recycle these heat loss to transform the gasoline in a fuel with higer enthalpy you improve the efficiency, but that's a very complex way to do it compare to put insulation to avoid heat loss. Isn't it ?
Posted by: Treehugger | 04 June 2009 at 09:23 PM
What I got out of it was that the combustion process is so fast that not all the energy is being used.
"we carry out the reactions in such a way that they spontaneously happen very rapidly, but that produces this entropy generation that occurs because of the heat transfer between molecules and chemical potential gradient"
If you can use the heat to reform the fuel, you can do better.
"When syngas—CO and H2—burns, the irreversibility is much less."
I remember auto thermal reformers, where you use an exothermal reaction (in this case combustion) to create the heat for an endothermal reaction. (in this case fuel reforming)
Posted by: SJC | 04 June 2009 at 09:55 PM
I still don't get it,
well if the combustion is very fast there is strong gradient of temperature in the flamme and obviously there is dilution of the heat in the mixture and then the efficiency is less than if the mixture was perfectly homogeneous and burning everywhere at the same timing. hmmm sounds fishy. Or they refeer to the partial combustion of the gazoline because it's a fast flamme, well there is way to improve that using higher CR with lean mixture or trough massively turbulence like the Scuderi engine. And why CO H2 would burn better, because they are gas, but Vapor fuel engine exist too...I don't get it
Posted by: Treehugger | 04 June 2009 at 10:10 PM
"Today’s engines can deliver a fuel efficiency of 40-42%, with corresponding loss of initially available fuel energy of 58-60%. The ORNL project is seeking essentially to flip that, to enable efficiencies of 50-60%, with losses of 40-50%. “This is long-term, high-risk [research], but obviously the payback is increasingly important,” Daw said.
Yes, typical a heavy-duty diesel engine is currently capable of peak BTE of 42%, with 35% of initial heat energy in the exhaust. This exhaust heat is hot enough to run a bottoming cycle using a Cyclone power steam piston engine having 35% efficiency. So, 35% x 35% will result in an additional~12% of initial energy from fuel converted to mechanical energy, giving a total of 54% thermal efficiency from a given quantity of diesel fuel. This compares quite favorably with a large combined-cycle electrical power generating plant capable of 55-60, but much larger and more complicated. We can achieve up to ~54% thermal efficiency using relatively small-size piston engine technology of TODAY. A two-cycle humongous ship-born diesel engine can achieve 50% thermal efficiency, although it is as large as a two or three story house.
I simply cannot understand the relevance of the statement that 20-25% of combustion energy is loss to irreversibility. This irreversibility has root in the second law of thermodynamic itself, implying the irreversibility of entropy. That's something we have to accept.
Posted by: Roger Pham | 04 June 2009 at 11:08 PM
If you have a charcoal fueled car that runs with a gasifier, a scandinavian man invented the use of recirculated CO2 in the exhaust to cool the gasification bed by decomposing the CO2 to CO with additional Carbon to increase the efficiency. Other people have traditionally injected water or steam for this purpose.
Water injection is used in gas turbines to increase power and lower NOX production. It was once used in cars to reduce knocking with low octane gasoline.
Slow piston engines can now reach %50 efficiency for ships and stationary generators, but if combined with exhaust and cylinder heat recovery with a Rankine engine according to the Still invention they might get an additional %10.
The use of hydraulic hybrid drive systems can reduce fuel consumption to half anyway, but if combined with a smaller engine even better fuel economy is achieved. Lower speed limits will also reduce fuel use.
Regenerative fuel combustion is just now becoming popular in industry for reducing costs and NO2
..HG..
Posted by: Henry Gibson | 05 June 2009 at 01:40 PM
treehugger, the problem is entropy (disorder) increase in the combustion process. If you create 8 molecules of CO2 and 9 molecules of H2O from 1 molecule of octane (plus 12½ molecules of O2), the products are far more disordered than the inputs. And entropy, once created, cannot be destroyed; it has to be released as waste heat, meaning wasted energy.
I suggest taking an introductory thermodynamics course. It will clarify many things that are totally mysterious without it.
Posted by: Engineer-Poet | 05 June 2009 at 04:04 PM
Thanks you Engineer-Poet
But I have no problem with the notion of entropy itself, I know that entropy is irreversible (but I refuse to assimilate entropy to disorder, you can read this in every thermodynamic book but it is wrong, entropy has nothing to do with order or disorder, what does that mean that O2 and H2o is more disordeerde than CH4 or CO?) My problem that I don't understand where you generate this entropy in the combustion process as long as there is no heat loss or dilution of the heat. I think they just refer to the heat loss related to the fact that fast combustion reach instantaneous very high temperature that generate a lot of heat loss (which is then a lot of entropy, and it is not reversible yes I agree). if you had a slow combustion and the piston expands as the combustion goes on the temperature wouldn't go so high and the heat loss would be mimimized. But again CO + H2 won't help in that regards, it just a way to recycle that heat loss. But again using ceramic for the combustion chamber and piston and cylinder could help to reduce drastically these heat loss then just increase expansion and that's it.
Posted by: Treehugger | 05 June 2009 at 06:48 PM
Kyocera has been trying to develop the ceramic engine for decades. It is part of the Holy Grail of engines as I understand it.
Posted by: SJC | 05 June 2009 at 07:45 PM
The entropy comes from the inherent disorder of the larger number of molecules. You also have generation of disorder when a flame front moves through a mixture, because heat is transferred out of the flame to cooler gas and molecules diffuse across a zone where the fractions of different compounds and radicals varies widely. All of this is irreversible, creating entropy.
Posted by: Engineer-Poet | 06 June 2009 at 06:06 AM
Internal combustion is a chaotic event. It has worked well so far, but is less than optimally efficient. If these explorations can lead to reforming methanol, ethanol, gasoline or diesel into a form that can burn more efficiently with less pollution, then bring it on! If it takes ceramic engines, then let's make progress there. The I.C.E. will be with us for some time, no matter what detractors say.
Posted by: SJC | 06 June 2009 at 10:00 AM
Engineer-Poet
I totaly reject the idea that entropy is associated with order or disorder this is wrong. entropy only encompasses the fact that heat only goes from hot to cold (then it is irreversible) and then unovoidably leads to irreversible loss when the leaks goes to the environement where it becomes so diluted that it is litteraly destroyed. No more no less. Entropy has nothing to do with order or disorder. And if it was true that entropy discribes disorder then the bio-evolution of species would violate the thermodynamic law, as well as the fact that material are essentially cristal (then ordered) and not amorphous. Nature prefers order than disorder so it is certainly not that entropy is a good indicator of order or disorder. Entropy only describes the amount of heat loss (and associated energy destroyed) in the environement by over-dilution, no more no less. It is a flaw in thermodynamic to say that entropy measure order or disorder it is dead wrong
Posted by: Treehugger | 06 June 2009 at 10:17 AM
SJC
You are right, internal combustion is chaotic so difficult to control, basically a deflagration that is inherently lossy and bad combustion.
Ceramic introduction in the 80s was a failure due to the very brittle nature of these materials. Now if you consider the MUSIC engine where the combustion chamber is a small external cylinder, you might be able to use a ceramic material to contain heat, because you can cope with the fragile ceramics in that case, though the problem would be the connection between ceramic and metal.
Posted by: Treehugger | 06 June 2009 at 10:23 AM
What you accept or reject has no bearing on reality.
You are wrong. The entropy of a gas increases as it is allowed to occupy more volume. If you have two identical samples of an ideal gas at the same temperature, they have identical energy but the one with the greater volume has greater entropy. If you mix a concentrated salt solution with pure water, the diluted solution has greater entropy. Etc.<snort> That is actually a standard creationist fallacy. Further, you don't have to go anywhere near evolution to disprove that claim; all you have to do is show a green plant turning high-entropy CO2 and water into low-entropy sugars and cells using only sunlight and Earth as a heat sink. What the creationists don't bother to mention (and you obviously aren't quick enough to get) is that the Earth is not a closed system; it receives solar radiation at a blackbody temp of about 5700K, and radiates to space at a temp of about 250 K. There is a great deal of availability (ability to do work) between the income and the outgo, and that is exactly what plants have taken such good advantage of.
Posted by: Engineer-Poet | 06 June 2009 at 08:51 PM
system with less number of state is less ordered than a system with a greater number of state ? that is a strange definition of order indeed.
Your exemple on the gas doesn't contradict my statement
Entropy is Delta Enthalpy loss by the sytem to the exterior / temperature of the exterior. The entropy ony describe the dilutio of heat loss on the environment.
I don't care what creationist say. What is the enthalpy of an amorphous state and of crystalline state for a given material, why when you cool down a molten material it prefers to go to a crystaline phase that is more ordered than an amorphous state ? the enthalpy of the amorphous phase is higher than the crystaline phase you have to supply energy to do the transformation so you can not create entropy to go from crystaline to amorphous whch is less ordered.
Posted by: Treehugger | 07 June 2009 at 07:15 PM
Why don't they look at increased compression like transonic? It sounds like the lab is just pouring tax dollars down a rathole... http://www.greencarcongress.com/2009/05/transonic-cround-20090506.html
Posted by: ejj | 07 June 2009 at 07:19 PM
Engineer_poopoo
Physical chemist Peter Atkins, for example, who previously wrote of dispersal leading to a disordered state, now writes that "spontaneous changes are always accompanied by a dispersal of energy", and has discarded 'disorder' as a description.[12][26]
I am not the only to reject the idea that entropy doesn't measure order or disorder...
Posted by: Treehugger | 07 June 2009 at 07:51 PM
eji
I agree that they approach is awfully complicated and unlikely to yield anything practical. Re-introduction of ceramic using nanostructured material to reduce their brittleness would be more productive. You could slash the heat loss by a huge amount and reduce the generation of entropy...
Posted by: Treehugger | 07 June 2009 at 07:55 PM
Indeed, the enthalpy of the amorphous state is greater. This is why we have the concept of Gibbs Free Energy; G = H - TS. When a substance crystallizes, the heat discarded to the environment increases net entropy more than the lost entropy of the substance as it becomes more ordered.
And now that you have made it obvious that you have not even taken a college chemistry course (where G would be covered), I am done attempting to educate you. There are good schools out there which can reduce your ignorance for a nominal fee. I suggest you attend one instead of posting here and looking foolish.
Posted by: Engineer-Poet | 07 June 2009 at 08:12 PM
E-P,
There could be a problem with chemical recuperation as tempting as it might seem. All reforming reactions occur at high temperatures only. Without a catalyst the rate drops to almost zero below already 1000 deg C. If a catalyst is used (in this case transition metal catalyst which require very low or zero sulfur in the feed) the useful temperature might be lowered to 700 deg C. The exhaust gas leaves way below this temperature.
Heat of exhaust can be used to raise steam and then make it do work. This concept is already used in combined cycle systems.
Posted by: black ice | 07 June 2009 at 11:18 PM
I think I finally understand what they means by this entropy generated in the combustion, they mean that most of molecules or atoms around are brought in an excited state that will not necessarily come back to the ground state (through emission of photon or phonon) during the combustion or expansion, ok that's clearly entropy, then it is wasted in the exaust through phonon or photon emission. But there is an easy way to recycle this is to use an extra bigger cylinder were the exhaust will over expands and were the gas will have the time to return to equilibrium (of course wit hproper thermal insulation and zero piston slap to avoid heat and frcition losses) . They have a very confusing way of describing the problem and and an even more confusing way of solving it.
to Engineer-Poopoo
There is physics much more serious than you who reject the concept of order related to entropy, so cool down your arrogance mister corn-con-ethanol
Posted by: Treehugger | 08 June 2009 at 06:07 PM
@Treehugger
Don't be so hard on them, they are scientists ;)
Posted by: black ice | 08 June 2009 at 10:36 PM
@Treehugger,
Don't be so hard on Eng-Poet, either, because on this subjet of entropy, he explains it like a scientist. Peace!
Posted by: Roger Pham | 08 June 2009 at 11:07 PM