Layperson here in re: physics and marginally more than a layperson when it comes to engines (I rebuilt one once, so I have a decent understanding of how they work). Please frame your answers to address the physics of nitrous.
If nitrous oxide is used to deliver additional oxygen to an engine, why not just use pure oxygen instead? What is special about nitrous oxide that it trumps regular oxygen in the combustion cycle? And please don’t point me to the Wikipedia article on nitrous. I’ve read it. I understand nitrous delivers more oxygen than atmospheric air, but I don’t get how it would outperform pure oxygen. Or would it, and there are other considerations that prevent the use of pure oxygen to boost engine performance?
All of which is even more true of liquid oxygen, so it goes no way at all to answering the question asked.
That’s part of the problem.
Put simply, using liquid oxygen in an internal combustion engine is a pain in the arse.
Nitrous is nice and simple. It can be injected directly into the cylinders all by itself, as a liquid. It’s that simple. You replace the air intake with a nitrous intake and Bob’s you uncle. Even in more complex systems this is a major feature, since a failure of some component isn’t going to immediately result in catastrophic failure.
Liquid oxygen OTOH does not play well with others. It tends to react with anything that is remotely oxidisable. If you mix liquid oxygen with liquid hydrocarbon there’s at least a chance it’s going to spontaneously combust in the manifold. In contrast, nitrous doesn’t release oxygen until combustion is already underway in the cylinder, so no chance of accidental ignition.
You are also not going to be able to inject pure liquid oxygen directly into any production engine. The combustion would be way to fast and too hot for the cylinder/piston to take to take. You’d need to blend the oxygen with an inert gas like nitrogen to produce a tolerable level of oxygen. This adds another layer of complication to the system, since you are now trying to blend oxygen fuel and inert gas in the perfect ratio, and that ratio is rarely going to be above the 30% oxygen that pure nitrous gives. And of course if the system fails, such as the nitrogen line becoming blocked, you will be injecting pure oxygen into the cylinder, with all the problems that causes.
Oxygen is also just generally a PITA to handle. You have to take pains not to mix it with anything organic such as grease or most plastics, which in an engine is annoying. It’s more expensive than nitrous, it’s much more dangerous in the event that the cylinder ruptures, especially in a fire and so forth.
In principle you could do it, but it’s much more complicated, expensive and dangerous to both the driver and the engine. When you consider that most production engines are at risk from the extra power generated by a decent nitrous system, the tiny advantage that could be gained by using LOX to produce a 40% ration rather than 30% just isn’t worth it.
Here’s something I’ve wondered, since it’s on the topic of nitrous.
Do they add something to automotive nitrous to keep people from abusing it? I realize that doing “whippets” isn’t really a thing anymore…at least I don’t think…but I have to imagine that when easy to install nitrous kits came out, that there was a fear people would buy the tanks to get high off of, yes?
O2injection is prone to detonation. Nitrous has to disassociate before combining with the fuel. This is a endothermic reaction that moderates the burn rate . O2 injection works with diesels, because the burn rate is controlled by the fuel injection rate.
Ok, that makes a lot of sense. However, it also assumes using LOX. What about plain old oxygen gas like you’d use with an oxyacetylene torch? Why can’t I swap out my tank of nitrous for a tank of oxygen gas and send a 50% (or whatever) oxygen blend into the combustion chamber?
Most molecules’ lowest energy state are “singlets”, all their electrons are paired up.
Oxygen isn’t, it’s a double radical, a “triplet”. Two of its electrons are unpaired. This does not mean that there are no singlet molecules in a bottle of oxygen, but there’s a lot less singlets than triplets and the singlets are more energetic.
Singlets react with singlets. (this is a rule of thumb, but there’s a reason it’s a rule of thumb: it works)
A reaction between O2 and a normal (singlet as lowest state) molecule is harder to start up than one between a different O-donor which is a singlet because of concentration: if you have 25%O2+Molecule and 25%OtherDonor+Molecule, the concentration of reactive singletO2 will be much lower than 25%, whereas the concentration of reactive singletOtherDonor will be almost 25%. So, at concentrations low enough for it to be safe you need a bigger spark… such as the one provided to start up an acetylene torch (and acetylene burns more easily than gasoline, too; acetylene has a triple bond which is much more reactive than the single bonds in gasoline). Once you have enough concentration it becomes so reactive that it’s a PITA to handle: we want the explosions in motors to be controlled.
There’e two problems with that.
The first is that you have removed one of the largest benefits of nitrous: the vaporisation. It is the vaporisation that cools the fuel increases pressure within the cylinder. Using gaseous oxygen you don’t get that benefits.
I suppose you could rig up a system whereby you injected liquid nitrogen along with gaseous oxygen to retain the vaporisation effects, but that just puts you back to the same problems as injecting liquid oxygen and liquid nitrogen.
The second problem is that you now have quite a complicated system, with a gaseous oxygen line, an air intake line and a fuel line. You are probably also going to need some sort of air compression mechanism like a turbo to compensate for weather effects and produce predictable results.
And then, if supercharger or the air line fails, you are injecting pure oxygen into the cylinder. That means either the whole gas volume is made of oxygen gas, and you get really severe detonation and blow the big end off the cam, or you only get the 40% oxygen load and no air, leading to partial detonation and incomplete combustion. No idea what the effects of that is going to be at high revs, but you can be sure it won’t be good.
I’m not a hot rodder, so this is something I don’t get…
If an engine is running with fuel and air in correct proportions and suddenly nitrous oxide (or some other oxidizer) is injected, wouldn’t that cause a “lean” condition? Seems like the amount of fuel would also have to be increased at the same time. How is that accomplished in Nitrous systems?
With a regular engine, you keep adding more fuel to make it produce more energy, but eventually you run into a limit because it can only suck in so much air to provide oxygen to burn with the fuel. When you get to that point, you can then add nitrous oxide into the mix to allow you to burn more fuel.
Correct. A container of liquid N[sub]2[/sub]O at 700 psi holds about 40% more oxygen than the same volume of pure O[sub]2[/sub] gas pressurized to 2000 psi.
As discussed earlier, using a tank of gaseous O2 would also eliminate the charge-cooling effect provided by the introduction of liquid N[sub]2[/sub]O into the intake manifold.
Also as discussed earlier, the introduction of large amounts of pure O[sub]2[/sub] into the intake tract and combustion chamber may destroy the engine due to uncontrollable detonation (very different from “normal” combustion) or rapid oxidation of various engine parts. Pure O[sub]2[/sub] is crazy shit, as aptly demonstrated by Apollo 1; things that would not burn in a normal atmosphere will combust with incredible vigor (or even detonate) in a pure O[sub]2[/sub] environment.
I have a related question.
As stated above there is a charge cooling effect with nitros oxide due to the vaporisation. Is there also a cylinder cooling effect from the inert nitrogen picking up heat and carrying it out the exhaust?
In other words does running N[sub]2[/sub]O result in lower cylinder temps than running an equivalent O[sub]2[/sub] to fuel ratio would?
[NITPICK=] I’ve never seen a cam with a big end on it, much less one that was blown off.
Big ends are found on connecting rods, the little end is attached to the piston by the gudgeon pin, and the big end bolts together around the crankshaft. [/NITPICK]
Yes, if you replaced the N[sub]2[/sub]O with an equivalent mass of O[sub]2[/sub], you would see higher peak combustion temperatures, with correspondingly greater heat transfer to the combustion chamber walls. This is a well-understood effect, and is the reason behind the use of exhaust gas recirculation (EGR) for emissions control in production car engines. By diluting the incoming charge with inert exhaust gas, the peak combustion temperatures are lowered, resulting in less NO[sub]x[/sub] being produced. At the same time, the lower combustion temperatures result in less heat rejection to the cylinder walls; combined with a wider throttle opening (for a given load), this tends to result in improved efficiency along with the NO[sub]x[/sub] reduction.
That’s part of it; the other part is that lower peak in-cylinder temps mean less heat is lost to the combustion chamber walls, leaving more of it available for conversion to mechanical work. Those two factors are also at work in diesel engines, where there is no throttle plate, and lots of excess air.
You can’t run WOT with pure O[sub]2[/sub], for reasons explained upthread. Rather than using pure O[sub]2[/sub] and running at part-throttle, you might as well add diluent nitrogen (either by using atmospheric air or running on N[sub]2[/sub]O) and enjoy the benefits of reduced throttle restriction and reduced heat loss to combustion chamber walls.
An engine running at WOT will be more efficient with EGR than without - but it will also make less power. That won’t be popular, except in situations where you don’t need max power and are trying to conserve fuel. In those situations, racers already are able to conserve fuel by leaning out the mixture; in that case, the excess air (rather than exhaust gas) is the inert diluent. So EGR would be redundant, with the ducting and control valve just adding weight to the car.