vacuum and pressure

[Gab250]

Hey, got a couple of basic science questions, about how certain things would behave in a vacuum and under pressure, mostly related to engines, but other things too.

1) If you put a 4 stroke petrol engine into a vacuum, and fed it a supply of oxygen, or maybe even nitrous oxide (which I understand is an oxidizing agent) would the engine run? What would happen with the exhaust? Would it begin to pressurize the vacuum chamber? Would it make any difference as to weather the engine was turbo charged or supercharged? Would a Diesel engine behave in the same way?

2) If you put that same engine into a chamber that was pressurized to say, 50psi, Would it run? Would the engine have positive boost as though it was turbo/supercharged? Could the exhaust gases escape? Would having a turbo balance out the effect of trying to force the exhaust out under pressure? I think maybe you'd need a supercharger now that I think about it, due to the fact that turbos are exhaust driven.

3) If you put an inflated balloon or ball into a vacuum, would it immediately pop? what about if you put it into a pressurized enviroment?

4) What effects does pressure and vacuum have on the human body? ie; if you went into a pressurized enviroment, would you die immediately? I know you would in a vacuum

I think that concludes my questions about pressure and vacuum for the time being.

Cheers, Gab

[lsemmens]

The human body can withstand enormous pressures. Scuba diving subjects the divers to increasing pressures as they go deeper. You can also survive reduced pressures on the body, provided, of course, you have an adequate air supply. With the balloon, if there is enough room in it to cope with the reduced external pressure then all it would do is expand. Increased pressure would just compress the balloon and contents. Internal combustion engines are designed for a "controlled explosion" condition, provided the fuel/air supply is adequate to maintain the explosion (combustion), they should still perform under either scenario.

[Gab250]

The human body can withstand enormous pressures. Scuba diving subjects the divers to increasing pressures as they go deeper. You can also survive reduced pressures on the body, provided, of course, you have an adequate air supply. With the balloon, if there is enough room in it to cope with the reduced external pressure then all it would do is expand. Increased pressure would just compress the balloon and contents. Internal combustion engines are designed for a "controlled explosion" condition, provided the fuel/air supply is adequate to maintain the explosion (combustion), they should still perform under either scenario.

Ahh k, cool so what happens to the exhaust gases?

[gordon_s1942]

Ahh k, cool so what happens to the exhaust gases?

For Divers there is a device called a 'Rebreather' that scrubs the used air and recirculates it, I believe this system is also used in a space suit but like everything, it can only work for so long before the filters have to be cleaned.
This method was used by Frogmen during WW2 particularity on the manned torpedo's as it doesnt leave a trail of air bubbles behind to give away your position as a normal SCUBA system does.

[trash]

OK.... these are quite easy questions to answer.

The first thing to clarify is that the engine is in a vacuum but it's carburettor is being fed with AIR at 14psi.
The rest of the environment is 0psi. So... the first thing is that a piston engine could pretty much run by itself, the same way that a CO2 motor works. Just by pressure difference.
Otherwise the vaccum would just slightly improve the performance of the engine by allowing the exhaust gas to be evacuated from the engine quicker.
Think if a 2 stroke with a tuned chambered exhaust. The vacuum behind the exhaust shock wave improves engine performance.

The engine is designed to run on a specific range of oxygen pressure. Air is about 2.8pppsiO2 (partial pressure pounds per square inch Oxygen).
So... If you reduce the pressure in the room to 2.8psi and use 100% oxygen instead of air, the engine will run just fine in this partial vacuum.

If the ppO2 is low, then the engine will have reduced power. (running rich) if the engine is running a high ppO2 then the fuel mixture will be lean and again power will be reduced.
The point where the fuel doesn't ignite, the engine stalls. Petrol is rather difficult to burn.

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Now... putting the engine in a hyperbaric chamber, it will still run, but as you increase the pressure, the ppO2 goes up, the fuel leans out and it stalls.
IF you make the gas hypoxic (less oxygen) then you can still run the engine. If you use a gas which is about 94% nitrogen and 6% oxygen, the engine will run just fine.

The other option is to increase the fuel mixture. MORE FUEL, combined with more oxygen means MORE POWER !! (said like Tim Allen uuurrrrrgggh !)
And a third option is to use a different fuel that requires a different (more) oxygen ratio and again, the engine will run fine if the fuel/oxidiser mixture can be detonated.

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Nitrous Oxide produces more power. Why? Not because it burns better than oxygen, but because it produces more waste.
Lets just assume out engine runs on Butane (LPG) for simplicity. (C4H10)
C4H10+9O2 -> 4CO2 + 5H2O The equation for burning our fuel.

For each litre of butane (gas) we need nine litres of Oxygen (or ~45 litres of Air)
When burnt this produces four litres of CO2 and five litres of Water vapour.
This is a pressure loss, but because we put 10 litres in and got nine out.
But the heat that the reaction produced greatly increased the pressure give that the gas entering the engine was at 25degC.

If we substitute Oxygen/Air for Nitrous oxide.....
C4H10+18N2O -> 4CO2 + 5H2O + 9N2 We need 18 litres of Nitrous Oxide (as opposed to 9 of Oxygen or 45 litres of Air)
But the but clincher is that now the exhaust gas is 18 litres. The exhaust gas pressure is basically doubled because of the extra nitrogen being being forced out the exhaust.
Lets hope your engine can withstand the extra pressure.

It's important to realise that the nitrous is displacing air, not oxygen.
The same result can be achieved running higher oxygen and fuel pressures, though this uses more fuel.

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3> If you put a balloon in a vacuum, it will pop. If you put an astronaut in space, he's basically just a balloon with more stuff inside, yet he does not pop.
Try this logic. If I put a hot water bottle in a vacuum, does it pop. (actually yes) If I put a 330mL coke bottle in a vacuum, it doesn't.
The reason comes down to nothing but the strength of the container.

If you can pump a balloon up to 30psi internal pressure, then it will easily not pop when taking into space with nothing but sea level air pressure it in.
The balloon will withstand the pressure of the gas inside pushing on it's walls. A vacuum is 0psi and sea level pressure about 14psi. So 15psi container should be able to withstand the vacuum of space.
If you think of the Apollo lunar modules. They had very thin walls. About the same thickness as an aluminium can. They were able to contain an reduced pressure hyperoxic environment.

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4> Welcome to the world of Dive medicine.
Everything is based on oxygen pressure. Strange things happen to humans.
The first is Central Nervous system oxygen toxicity. If you breath more than 160% oxygen this will cause you to convulse and become unconscious.
Oxygen is Toxic at pressure. If the pressure is lowered below 1.6ppO2 (160%) then the person will regain consciousness and go back to normal.
This is bad for scuba divers because if they convulse under water, the reg falls out of their mouth and they drown.
If you dive below 5 metres using pure oxygen, you're as good as dead.
There's also an accumulative effect for how long you're exposed to oxygen above 50%. It's a bit like a freight train trying to stop. You can over run the toxicity limit on long duration dive. (but this is rare).

There is also Pulmonary Oxygen Toxicity, where your lungs produce fluid if you're exposed to ~>50% oxygen for long periods of time. You drown in your own body fluid.
It's something hospitals keep a careful eye on patients on long term oxygen.

Then there is Nitrogen Narcosis or Inert Gas Narcosis. This happens when the pressure is increased too fast. The diver becomes intoxicated, it's a bit like being drunk.
Naturally, this is a danger to have an idiot under water because he can do stupid shit. The effect is almost instantly relieved if the diver just comes back up a few metres.
Nitrogen is the main cause. But Oxygen can do it too and Argon is exceptionally good at turning people into idiots under pressure.

And the story doesn't end there. Hypercapnia (too much Carbon Dioxide) can also kill you very quickly. It's not a problem for open circuit divers, but for rebreather divers, if the scrubber stops working,
things can go from bad to extremely worse very quickly. Other gases like carbon monoxide under pressure just kill you so much quicker. This is one of the reasons dive shops have air intakes placed up high outside of a building and away from any exhaust gases.

The maximum depth for an air diver is about 60 metres, where air > 1.6ppO2. So divers use different gases.
At lesser depths they can use Nitrox, displacing nitrogen for oxygen. But below 60 metres, they need to displace oxygen.
They can't use nitrogen because the body absorbs and off gases it slowly, increasing decompression times.
So they use hypoxic gases like heliox or trimix.

The body cannot decompress too fast either. This causes the bends (and other decompression sicknesses) where bubbles form in the blood (and muscles and bones) nasty shit !
So decompression is very slow on air. To reduce decompression times the nitrogen is replaced with Oxygen, and gases like nitrox are used to speed up ascent times.
The longest decompression times are spent at 5 metres. So a diver can greatly reduce his decompression time by breathing pure oxygen at 4 metres allowing the nitrogen to off gas much faster.

Each gas used has it's own problems and Argon is avoided because of it's narcotic effect. The three gases used in the breathing loop are Oxygen, Nitrogen and Helium.
Oxygen is toxic (but essential) Nitrogen is a buffer, but it's slow to de/absorb and helium is expensive and hypothermic, a lot of body heat is lost breathing it, especially at pressure.

That is some of the other effects of exposing people to pressure. Gas becomes more efficient at thermal transfer, so a person can more quickly chill or over heat.

[Dave001]

Nitrous Oxide produces more power. Why? Not because it burns better than oxygen, but because it produces more waste.
Lets just assume out engine runs on Butane (LPG) for simplicity. (C4H10)
C4H10+9O2 -> 4CO2 + 5H2O The equation for burning our fuel.

For each litre of butane (gas) we need nine litres of Oxygen (or ~45 litres of Air)
When burnt this produces four litres of CO2 and five litres of Water vapour.
This is a pressure loss, but because we put 10 litres in and got nine out.
But the heat that the reaction produced greatly increased the pressure give that the gas entering the engine was at 25degC.

If we substitute Oxygen/Air for Nitrous oxide.....
C4H10+18N2O -> 4CO2 + 5H2O + 9N2 We need 18 litres of Nitrous Oxide (as opposed to 9 of Oxygen or 45 litres of Air)
But the but clincher is that now the exhaust gas is 18 litres. The exhaust gas pressure is basically doubled because of the extra nitrogen being being forced out the exhaust.
Lets hope your engine can withstand the extra pressure.

It's important to realise that the nitrous is displacing air, not oxygen.
The same result can be achieved running higher oxygen and fuel pressures, though this uses more fuel.

You seem to have completely misunderstood the use of nitrous oxide in a combustion engine Trash.
Nitrous Oxide produces more power by injecting it into the engine in it's liquid state, it then heats up in the combustion chamber, and converts to it gas state, which gives you more oxygen allowing you to burn more fuel and hence make more power.

[trash]

I've kept it simple by trying to keep everything as gases, the reason why I didn't use octane.

I'm not a mechanic Dave,
How do you burn more fuel ?
If the fuel/gas mixture is roughly constant, how is more fuel in fuel introduced to the system ?

Your explanation doesn't sound right Dave. Just converting it from a liquid to a gas does not give the engine more power.
If that were the case, injecting water or liquid nitrogen would do the same thing.

You're correct that I've misunderstood where the extra power comes from. Not from the extra gas in the exhaust, but rather because (ref wiki)
the system increases the internal combustion engine's power output by allowing fuel to be burned at a higher rate than would normally be the case, because of the higher partial pressure of oxygen injected into the fuel mixture.

Basically, the power comes from more fuel being burned.


Just to be funny in my mistake;
Converting N2O from a liquid to a gas does not give you more oxygen. It doesn't give you any. It gives you nitrous oxide gas.
And displacing N2O for Oxygen gives you LESS oxygen to burn by half.
Displacing N2O for air gives you more oxygen to burn because air is 21% oxygen. (29% increase)
But if it were all about oxygen, then wouldn't be better just to inject oxygen instead of nitrous oxide ?
Just using oxygen gas at atmospheric pressure allows you to burn five times as much fuel. What am I missing here? Why use nitrous over oxygen ?

[Dave001]

I've kept it simple by trying to keep everything as gases, the reason why I didn't use octane.

I'm not a mechanic Dave,
How do you burn more fuel ?
If the fuel/gas mixture is roughly constant, how is more fuel in fuel introduced to the system ?

Depending on the nitrous kit being used, Fuel will either be injected into the engine at the same point as the nitrous (wet system, plate or fogger), or extra will be added via the injectors (dry system).

Your explanation doesn't sound right Dave. Just converting it from a liquid to a gas does not give the engine more power.
If that were the case, injecting water or liquid nitrogen would do the same thing.

You're correct that I've misunderstood where the extra power comes from. Not from the extra gas in the exhaust, but rather because (ref wiki)
the system increases the internal combustion engine's power output by allowing fuel to be burned at a higher rate than would normally be the case, because of the higher partial pressure of oxygen injected into the fuel mixture.

Basically, the power comes from more fuel being burned.


Just to be funny in my mistake;
Converting N2O from a liquid to a gas does not give you more oxygen. It doesn't give you any. It gives you nitrous oxide gas.
And displacing N2O for Oxygen gives you LESS oxygen to burn by half.

Displacing N2O for air gives you more oxygen to burn because air is 21% oxygen. (29% increase)
But if it were all about oxygen, then wouldn't be better just to inject oxygen instead of nitrous oxide ?
Just using oxygen gas at atmospheric pressure allows you to burn five times as much fuel. What am I missing here? Why use nitrous over oxygen ?

Nitrous compresses into a liquid, you can then inject that liquid into the cylinder (you can't do this with O2), where it will convert to gas, heat up (above 300C) and give you two parts Nitrogen one part Oxygen, the nitrogen has the added benefit of lowering combustion temps, the O2 will allow you to burn more fuel.

The way I explain it to customers is, you physically inject a thumb size amount of liquid nitrous into a cylinder, it converts to gas, heats and gives you a fist size amount of Oxygen.
Nitrous makes it's power the same way as a turbo or supercharged engine does, you pump in more O2, so you can burn more fuel, Nitrous just does it chemically, instead of mechanically.

Anyway, no need for me to reinvent the wheel.

This video explains how nitrous works in a combustion engine.


And this guy has already taken the time to write out why we don't use Oxygen.