Cars and the Height of the Atmosphere

[UncleFred]

I am almost embarrased to ask this question but here goes;

Is the volume of the exhaust coming out of a car larger than the volume of air that went into the engine? I mean even after it has had a chance to cool down to something closer to surrounding temperature.

If yes, does that mean the size of the atmosphere is getting larger?

Wiki ( Atmosphere of Earth - Wikipedia ) says 3/4 of the atmosphere is contained within about 36,000 feet of the earth’s surface.

With all the 100’s of millions of cars operated every day, is that number crawling upwards?

[Smeghead]

For the first question, I’d hazard a guess of “yes”, simply because your car’s job is to take a liquid (gas) and turn it into various gasses. Wow, that sentence turned out to be confusing.

As for the second question, that’s complicated enough that I’ll leave it for others to tackle.

[Fubaya]

@Smeghead

For the first question, I’d hazard a guess of “yes”, simply because your car’s job is to take a liquid (gas) and turn it into various gasses. Wow, that sentence turned out to be confusing.

But it also takes in a lot of air. If you put your hand over the air intake, it sucks like a vacuum cleaner. I’m not sure the exaust blows as hard. This is all just a guess too but I’m going to vote the other way.

[beowulff]

From somewhere on the web:

If heat weren’t produced, the
combustion would overall not result in any volume increase at all - in fact,
gas volume would decrease. You would be converting 280 liters oxygen + .16
liters isooctane to 180 liters carbon dioxide + 0.16 liters water, for an
overall volume LOSS of 100 liters per mole. In other words, burning 1 unit
volume of isooctane will cause a net reduction in gas volume of 1125 unit
volumes.

[Chronos]

I’d have to see the calculations behind that, beowulff, because it doesn’t look like it holds up, to me. Gases have the convenient property that, at a given temperature and pressure, the volume is directly proportional to the total number of molecules, without caring what kind of molecule they are.

OK, so the nitrogen (and other trace gases like argon) in the air will pretty much just go straight through unchanged, so we can ignore that. The oxygen will combine with carbon and hydrogen in the fuel to form carbon dioxide and water. Any oxygen that combines with carbon will go from O[sub]2[/sub] to CO[sub]2[/sub], so it’s the same number of molecules in and out there. Any oxygen that combines with hydrogen, meanwhile, will take a single molecule of O[sub]2[/sub] in, and with the addition of the hydrogen, convert it to two molecules of H[sub]2[/sub]O, so there’s an increase in the number of molecules there.

[beowulff]

@Chronos

I’d have to see the calculations behind that, beowulff, because it doesn’t look like it holds up, to me. Gases have the convenient property that, at a given temperature and pressure, the volume is directly proportional to the total number of molecules, without caring what kind of molecule they are.

OK, so the nitrogen (and other trace gases like argon) in the air will pretty much just go straight through unchanged, so we can ignore that. The oxygen will combine with carbon and hydrogen in the fuel to form carbon dioxide and water. Any oxygen that combines with carbon will go from O[sub]2[/sub] to CO[sub]2[/sub], so it’s the same number of molecules in and out there. Any oxygen that combines with hydrogen, meanwhile, will take a single molecule of O[sub]2[/sub] in, and with the addition of the hydrogen, convert it to two molecules of H[sub]2[/sub]O, so there’s an increase in the number of molecules there.

But, little of the water is going to stay as a gas.

[california_jobcase]

A bit of high school stoichiometry says:

0.302 L octane reacts with 470.4 L oxygen, yielding 224 L water vapor plus 358.4 L carbon dioxide. This means 470.7 L of reactants produce 582.4 L of products (exhaust).

I know gasoline isn’t all octane, but this should be representative. Since most heavily trafficked roads I’ve seen aren’t wet with water from exhaust, I think the water stays gaseous until it precipitates somewhere, sometime. Burning hydrogen-bearing fuels such as oil and natural gas creates water, which means there’s more water on Earth now than there was a few centuries ago- it would be interesting to know how much more.

[MikeS]

@beowulff

@Chronos

OK, so the nitrogen (and other trace gases like argon) in the air will pretty much just go straight through unchanged, so we can ignore that. The oxygen will combine with carbon and hydrogen in the fuel to form carbon dioxide and water. Any oxygen that combines with carbon will go from O[sub]2[/sub] to CO[sub]2[/sub], so it’s the same number of molecules in and out there. Any oxygen that combines with hydrogen, meanwhile, will take a single molecule of O[sub]2[/sub] in, and with the addition of the hydrogen, convert it to two molecules of H[sub]2[/sub]O, so there’s an increase in the number of molecules there.

But, little of the water is going to stay as a gas.

Right. Under complete combustion of a long-chain simple hydrocarbon (C[sub]n[/sub]H[sub]2n[/sub]), the process is pretty close to x moles of N[sub]2[/sub] and 3 moles O[sub]2[/sub] go to x parts N[sub]2[/sub], 2 moles CO[sub]2[/sub], and 2 moles H[sub]2[/sub]O. So you end up with more moles of gas in the inital exhaust (x+4 rather than x+3.) However, that water vapor will eventually precipitate out of the air, leaving you with x+2 moles of nitrogen and carbon dioxide — less than you started with.

[Quartz]

Plus the output vapour is hotter than the inputs, so it’s going to be less dense and thus an even greater volume.

[Chronos]

Plus the output vapour is hotter than the inputs, so it’s going to be less dense and thus an even greater volume.

Yes, but the OP already took that into account, by saying to wait until it cooled.

And come to think of it, the OP is also concerned about the long-term effects, which would give the water cycle time to do its thing, so most of the water is going to precipitate out eventually. Which puts me into agreement with the other posters saying it’s a small decrease in net volume (the volumes of the liquids involved can be neglected).

Of course, by the time you got far enough to make a significant difference in the total volume of the atmosphere, you’d also be well past enough carbon dioxide to turn the Earth into a greenhouse hell.

[Squink]

Video Here. Just think of wax as really high octane gasoline.
Of course, heat is an issue too.

[JWT_Kottekoe]

Now that we’ve resolved that the net effect is negative, we still have to decide whether an increase of decrease in the number of molecules changes the height below which 3/4 of the atmosphere resides.

If the composition of the atmosphere is unchanged the answer is no. The atmospheric pressure at sea level would go up or down, but not the height of the atmosphere. The scale height of the atmosphere is given by the height at which the gravitational potential energy is equal to the thermal energy. Stated in another way, it is the height a molecule would travel in a vacuum, if it were launched straight up from sea level at the average thermal velocity. There would be a minor change caused by a change in the average mass of an air molecule, since the thermal velocity goes inversely with the square root of the mass.