Burning Pure Oxygen vs. "Air"

[jeff88]

We all know fire loves oxygen, so I was wondering what would happen to efficiency and emissions if one was to specifically burn pure oxygen rather than ambient air. Would the pureness of the oxygen allow for a more complete burn, resulting in higher efficiency and less emissions? Would the lack of other molecules (e.g. nitrogen) make for a less polluting engine (i.e. no NOx)?

[RedDevil]

Hmm. Pure oxygen, atmospheric pressure, iron and heat equals Thermal lance.
It would allow lean burn without nitrous oxide, but the unused oxygen would be a risk for the engine.
If you reduce the pressure to near-vacuum then it could work, but you'd have massive pumping loss. And you will have to store the oxygen at very low temperature or very high pressure, turning your car into a potentially explosive device.

It is pressure, not heat, that makes your engine work. 80% of the air entering is inert, but will raise in volume when heated up. Without it the mixture would need to be 5 times as hot to provide the same pressure.
Even with air as it is there are heat issues, that's why water injection, adding inert matter to trade heat for volume, can be effective.

[jamesqf]

There are engines that use pure oxygen: http://ecomodder.com/forum/showthrea...cle-25557.html Don't think anyone has ever done a smog check on one, though: https://encrypted-tbn0.gstatic.com/i...dfnNn3QXC7e-4Q

[RustyLugNut]

Originally Posted by RedDevil

Hmm. Pure oxygen, atmospheric pressure, iron and heat equals Thermal lance.
It would allow lean burn without nitrous oxide, but the unused oxygen would be a risk for the engine.
If you reduce the pressure to near-vacuum then it could work, but you'd have massive pumping loss. And you will have to store the oxygen at very low temperature or very high pressure, turning your car into a potentially explosive device.

It is pressure, not heat, that makes your engine work. 80% of the air entering is inert, but will raise in volume when heated up. Without it the mixture would need to be 5 times as hot to provide the same pressure.
Even with air as it is there are heat issues, that's why water injection, adding inert matter to trade heat for volume, can be effective.

. . . needed to make this work. One of the Xprize teams was looking into this avenue of thinking to make a high power, low emissions engine. Pure bottled oxygen was direct injected along with a stiochiometric mix of a hydrocarbon fuel along with a measured amount of water for the expansion/working fluid.

High strength/temperature materials would have been needed for such an engine, but it would have been plausible. Low emissions would have been possible as no NOx would have been formed and only fuel/lubricant slip would have to be accounted for.

The oxygen/water replenishment would have been a critical issue. The Xprize requirement to provide a plausible way to provide widespread infrastructure pretty much killed the idea.

[RedDevil]

I'd go for a very small engine, like a model aircraft engine. Then pumping loss is less of an issue. And those are designed to run on rather explosive fuel anyway.
Maybe use alcohol or a a gasohol blend as a fuel, mixed with water to keep the temperature down (instead of injecting it). Water is readily available (the X-prize concern) and should not count as fuel(*).

Then again, instead of pure oxygen you could use a hydrogen peroxide mixture just like the Messerschmitt Me 163. That though lost it wheels as soon as it took off, a feature you'd probably want to avoid for your daily driver.

(*) The X-prize requirement does not seem particularly stern. They could simply claim that anything you'd have to replenish should be counted as fuel.
Once nuclear fusion plants get scaled down from star to car engine size, plain old water would definitely be seen as fuel.

[sheepdog 44]

I started a similar thread a while ago. My crazy idea was to get the kind of oxygen tank that some people drag around with them, and pipe it in upstream of the intake. Just enough to get a 25-50% increase in the oxygen content. Heck, millions of years ago there was much larger percentage of oxygen in the atmosphere!

Edit: It seems the problem is oxygen storage. As a scuba tank full of pure oxygen could only give a 10% increase in oxygen over the course of one gallon of gasoline. In the case of a prius, since it's fuel efficient you get more bang for your oxygen buck. You can get a 125% increase in oxygen for 16 miles, or a 150% increase for 8 miles. Which means if there were no oxygen in the atmosphere at all (only nitrogen), you'd have enough oxygen in your scuba tank to drive your car for just 4 miles!

You wouldn't need a super strong engine per se to keep it from melting. You'd just need to run your engine 5X as lean as it normally runs. But if your'e only compressing and expanding oxygen, then there might be issues as RedDevil says.

[RedDevil]

Originally Posted by sheepdog 44

I started a similar thread a while ago. My crazy idea was to get the kind of oxygen tank that some people drag around with them, and pipe it in upstream of the intake. Just enough to get a 25-50% increase in the oxygen content. Heck, millions of years ago there was much larger percentage of oxygen in the atmosphere!

When that percentage suddenly dropped the dinosaurs became extinct.
Who knows, one day we might find that at that time they had developed like us now and had a transportation network spanning the globe and powered by fossil fuel.
When the oxygen dropped to 20% they all got stuck on their motorways because their engines couldn't cope with that...

[Xist]

Originally Posted by RedDevil

When that percentage suddenly dropped the dinosaurs became extinct.
Who knows, one day we might find that at that time they had developed like us now and had a transportation network spanning the globe and powered by fossil fuel.
When the oxygen dropped to 20% they all got stuck on their motorways because their engines couldn't cope with that...

Then they died because they couldn't get home, where their wives had cooked dinner--or maybe the women went to work every day and the men stayed home! :P

[elhigh]

One wonders if you could do a sort of super-Atkinson cycle, like a 2:1 ratio instead of the 1.3:1 ratio of current modified Atkinson cycles. If you're injecting only a tiny amount of pure oxygen and a small amount of water to do the expanding, how much air you compress during the compression cycle wouldn't matter so much, would it?

Compression stroke = 40mm, expansion stroke = 89.4 (for instance).

If you're using water injection for expansion mass, that'll absorb a tremendous amount of heat. Could you do away with a radiator (and all its associated parasitic losses)?

Hmm, maybe not all. You'd probably want to reuse that water, or else have to carry around a large/heavy tank full of water. So a condenser is needed, aka a radiator. Dang it.

[RustyLugNut]

Originally Posted by elhigh

One wonders if you could do a sort of super-Atkinson cycle, like a 2:1 ratio instead of the 1.3:1 ratio of current modified Atkinson cycles. If you're injecting only a tiny amount of pure oxygen and a small amount of water to do the expanding, how much air you compress during the compression cycle wouldn't matter so much, would it?

Compression stroke = 40mm, expansion stroke = 89.4 (for instance).

If you're using water injection for expansion mass, that'll absorb a tremendous amount of heat. Could you do away with a radiator (and all its associated parasitic losses)?

Hmm, maybe not all. You'd probably want to reuse that water, or else have to carry around a large/heavy tank full of water. So a condenser is needed, aka a radiator. Dang it.

The direct injection of pure oxygen and fuel/water means you do not need an intake valve. You could run the engine as a two stroke. Yes, under full load, you can inject your oxy/fuel/water to maximize temperature and pressure. But, you then have the option under light load to inject the minimum oxy/fuel/water and derive as much energy from it by over expansion. By using over expansion, it makes it easier to condense the water and reuse it. The ability to carry oxygen is the main limitation to this idea.

ReDevil's idea of a small single piston engine is very doable. Originally, ceramic pistons, cylinders and heads were specified. However, using water as our expansion fluid means we can control peak temperatures. Ceramic coated parts would probably work. With no nitrogen present in the combustion chamber, we can run hotter than the 2300 degrees C temperature of formation for NOx to achieve higher theoretical efficiency while tempering maximum heat load via our water flow.