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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.

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.

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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