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Old 02-17-2021, 01:00 PM   #1 (permalink)
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Oxygen generator on cooled EGR only inake?

I have no idea if this would ever work nor can I seem to find anyone else that's even so much as thought of the idea, much less tried it.

Why
I think I'll explain the why first. If you can take all the nitrogen out of the intake charge and replace it with carbon dioxide but still have it mixed with about 20% oxygen then the formation of nitrogen oxides should be theoretically eliminated. With no NOx then you can run as lean as you want, beneficial to both diesel and gasoline engine efficiencies. You also can run as high of a combustion temperature as the engine and fuel can handle, which can also increase efficiency.

A side benefit is that we will be getting free water with this system which could be used for water injection, perhaps so that you could run higher compression ratios without detonation or so that you could run stoichiometric AFR's at high load.

A possible side benefit is that there would be the possibility of running higher concentrations of oxygen than 20%. I'm not sure if that would be practical, but from a theoretical standpoint ICE's should be more efficient with higher concentrations of oxygen since the combustion temperatures would be higher.

To sum it up, theoretically you should get an ultra-low emissions engine that has the potential of getting better fuel mileage.

How it's supposed to work
Take an engine and loop the exhaust through a cooler and right back into the intake. Then take a high purity oxygen generator (probably of the Vacuum Pressure Swing Adsorption variety) and fit that on the vehicle somehow and feed the oxygen into the intake. We may also want to prefill the exhaust/intake loop with CO2 before starting just to remove any nitrogen from the system.

How this theoretical engine works is that we start off with CO2 and some water vapor in the system headed towards the intake. The oxygen generator separates the NOx out of the air and introduces the O2 into that CO2 in the intake. Fuel is added any way seen fit (port injection, TBI, carburetor, direct injection, etc.).

During the combustion process we should get about everything we normally get emissionswise except NOx. Since the engine can run leaner with no emissions dependency for EGR we should get less CO, HC and PM emissions even on diesels. Not only that, but a lot of the exhaust (75-80%?) will be cooled and funneled right back into the engine to give it another chance to burn. The remaining exhaust can be run through a catalytic converter that should have an easy time burning up anything left over for a super-ultra-very-very-low emissions ICE. The catalytic converter could also be made much cheaper or perhaps even eliminated all together and still achieve these ultra-low emissions.

Of course cooling the exhaust means we'll get a lot of liquid water which may be used in water injection. Since water has an octane effect this could lead to some very high compression rations.

Problems
Well of course cost is a major one. Complexity another.

There's also the question of how efficient the oxygen concentrator will need to be since there exists the potential for it to sap way too much power from the engine just to provide enough oxygen to run. Whether or not the efficiency benefits from not having to deal with NOx emissions would be enough to overcome the power needed to run the oxygen generator would be key to whether this would make for better fuel mileage or not.

Also it will need a lot more cooling since we'll have to be cooling a large amount of the exhaust back down to near ambient. Which also means there can be the problem of not being able to cool the exhaust enough which could cause detonation problems. We could could try to compensate by holding back on the amount of oxygen being used but that would defeat the purpose of this setup.

Possible application
Since the oxygen generator will take up a lot of space this might work best on a very small engine so that the oxygen generator can be made as small as possible too. I'm thinking that this might be good as a range-extending engine on a BEVx type vehicle, or at least some sort of hybrid that has enough umph! from the electric (or hydraulic) drive train so that the engine can be made quite small.

Thanks!
Anyhow, thanks for reading my crazy idea.

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Old 02-18-2021, 01:52 AM   #2 (permalink)
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Your idea is not that crazy.

A team for the Auto X-Prize was planning to run an oxygen only combustion engine to eliminate pretty much all the emissions and to increase combustion efficiency dramatically.

It's one thing to test with bottled oxygen. It's another to run a trunk full of adsorption oxygen concentrators. Also, the use of steam injection to moderate temperatures resulted in a very large condenser otherwise miles per gallon of . . . water became a thing to watch.
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Old 02-18-2021, 10:00 PM   #3 (permalink)
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I guess you have seen something about HHO systems before. That's too energy-intensive for little to no actual benefit.
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Old 02-18-2021, 10:46 PM   #4 (permalink)
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In a parallel to this discussion.

Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
I guess you have seen something about HHO systems before. That's too energy-intensive for little to no actual benefit.
HHO (oxygen and hydrogen from electrolysis) does have benefit to fuel efficiency in the order of 5%. That sounds familiar? The paper quoted in the original post of the ozone question gave a similar gain in it's result. What is assumed in the electrolysis production of the HHO being pure oxygen is incorrect. It is not all O2. A percentage of the oxygen is in the form of O3. If specific pollutants are introduced into the electrolyte, even more ozone is produced during electrolysis. It is this ozone that has the greatest affect on combustion reactivity. Hydrogen does form radicals of its own but it is greatly aided by the presence of the ozone.
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Old 02-18-2021, 11:11 PM   #5 (permalink)
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Umm, how do you propose to remove all nitrogen from the air going into the engine? N2 is roughly 70% of what goes into the engine to start with. Even after adding hydrocarbons and then combusting them, you'll still have around 70% of it is nitrogen of one form or another, mostly N2.

Just routing exhaust gas (cooled or not) into the intake won't substantially change the amount of N2 going through the system, since you'll be displacing 70% N2 air with 70% N2 exhaust.

If you could eliminate NOx, that would enable lean mixtures for good fuel economy. But it's hard to see how this would accomplish it.

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Old 02-18-2021, 11:49 PM   #6 (permalink)
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Medical oxygen concentrators can remove the bulk of the nitrogen.

Quote:
Originally Posted by some_other_dave View Post
Umm, how do you propose to remove all nitrogen from the air going into the engine? N2 is roughly 70% of what goes into the engine to start with. Even after adding hydrocarbons and then combusting them, you'll still have around 70% of it is nitrogen of one form or another, mostly N2.

Just routing exhaust gas (cooled or not) into the intake won't substantially change the amount of N2 going through the system, since you'll be displacing 70% N2 air with 70% N2 exhaust.

If you could eliminate NOx, that would enable lean mixtures for good fuel economy. But it's hard to see how this would accomplish it.

-soD
Once you have reduced the nitrogen concentration down to the low single digits, the formation of NOx becomes proportionally minuscule. The heat becomes a problem as all hydrocarbon fuels burn rapidly in near pure oxygen. Brake specific fuel consumption (BSFC) is reduced via ignition near the theoretical optimum of top-dead-center (TDC).

The problem is removing the nitrogen gas. Even a small engine needs a volume of air in the order of hundreds of liters per minute at cruising speeds. The use of adsorption oxygen concentrators limits the volume through-put of working gas.

The elevated heat of combustion also becomes a problem. Ceramic coatings or parts do help to a degree, but the temperatures can approach that of an acetylene torch!

One solution is to provide a replacement to the N2 gas that makes up 79% of our air. This gas provides a buffering for the flame reaction as well as providing mass to create pressure. However, at 2300 degrees centigrade, nitrogen becomes reactive though much less so than hydrocarbons - it does steal some oxygen to form NOx. But what if you could use a far less reactive gas than nitrogen?

That is where water injection comes in. A small volume of water, directly injected into the combustion chamber pre-ignition point will flash into a large volume of steam providing the buffering gas and the pressure producing gas.

Then you have to design for an on-board condenser to try to re-use some of the water.

And now you have complexity heaped on complexity.

It is far easier to run the engine on outside air, limit combustion temperatures by running in the ultra-lean regime and feeding a small amount of ozone into the fuel/air mixture to reduce the co-efficient of variability for combustion to reduce or eliminate your un-burned HCs.
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Old 02-18-2021, 11:52 PM   #7 (permalink)
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Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
I guess you have seen something about HHO systems before. That's too energy-intensive for little to no actual benefit.
Electrolysis is way too energy-intensive using way more energy than could be produced from it. Oxygen absorption from air should require less energy, but can still be quite energy-intensive.

Of course oxygen could be produced from electrolysis or absorption or some other means off the car from some energy neutral source (solar, wind, hydro) and bottled and then put on the car. But it would be better to use that energy in a BEV instead of bottling oxygen.

Quote:
Originally Posted by some_other_dave View Post
Umm, how do you propose to remove all nitrogen from the air going into the engine? N2 is roughly 70% of what goes into the engine to start with. Even after adding hydrocarbons and then combusting them, you'll still have around 70% of it is nitrogen of one form or another, mostly N2.

Just routing exhaust gas (cooled or not) into the intake won't substantially change the amount of N2 going through the system, since you'll be displacing 70% N2 air with 70% N2 exhaust.

If you could eliminate NOx, that would enable lean mixtures for good fuel economy. But it's hard to see how this would accomplish it.

-soD
If you first charge the intake/exhaust circuit with pure CO2 to start with then only add oxygen and fuel it should theoretically not have any nitrogen in it ever again. Even if you don't with every cycle it should add more and more CO2 which will eventually displace all the nitrogen until there isn't any.
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Old 02-18-2021, 11:57 PM   #8 (permalink)
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Quote:
Originally Posted by RustyLugNut View Post
The elevated heat of combustion also becomes a problem. Ceramic coatings or parts do help to a degree, but the temperatures can approach that of an acetylene torch!

One solution is to provide a replacement to the N2 gas that makes up 79% of our air...

That is where water injection comes in. A small volume of water, directly injected into the combustion chamber pre-ignition point will flash into a large volume of steam providing the buffering gas and the pressure producing gas.
I didn't think about using purely water injection, but that might work.

My original idea is to cool nearly all the exhaust and reroute that back into the intake and use some of the condensated water from it for some water injection.
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Old 02-19-2021, 06:05 PM   #9 (permalink)
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I am usually more favorable to water injection anyway.
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Old 02-28-2021, 08:07 PM   #10 (permalink)
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I think I just debunked my own myth.

The specific heat rato for air hovers around 1.4, whereas CO2 is around 1.28. doing some simple math points to needing a much higher compression ratio to get the same efficiency. For an example, to get the same thermodynamic efficiency as an air fed ICE with an 11:1 CR we'd need over a 30:1 CR to get the same efficiency in a CO2 fed ICE. Water vapor also doesn't help much with it's 1.3 specific heat ratio. The only thing I haven't factored in is if there'd be an efficiency benefit to running nearly as much liquid water injection as we want since lots of water condensation would be available in this theoretical engine.

Helium would be a great working fluid in such an engine with it's 1.5 specific heat ratio. The problem is now we'd have to very efficiently separate the water and CO2 out of the helium in order to make it work as an ICE.

So as it stands now, such an engine would be very inefficiency and cause a lot of CO2 emissions in an effort to remove NOx emissions.

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