Cold air or hot air? Which one provides better combustion?

Question

You might need to bare with the explaination but I'll do my best! As you might know alot of engines work better in the cold then they do it the warmth. This is because of the temperature of the air being feed into the engine.

Why is this? Surely hot air holds more energy, which in my mind would make it provide better combustion in the engine, or does this energy prevent from a better combustion by the fuel once ignitied.

Somebody must know why this is, please answer in a scientific way, thanks!

Answer 1

Cold air is denser than warm air and holds slightly more O2.

An interesting side fact is humid air is less dense than dry air. The more water vapor, the less dense air becomes. Sounds strange, right?

Air contains 78% nitrogen molecules (N2), 21% oxygen (O2), and trace elements. The molecular weight of N2 is 28 (2 atoms of atomic weight 14), and that of O2 is 32 (2 atoms of atomic weight 16).

H2O molecules have a molecular weight of 18 (one O atom with atomic weight of 16, and two H atoms each with atomic weight of 1). This is lighter than both the N2 and the O2 molecules.

As Avogadro showed, the number of molecules in a given volume of gas at the same temp and pressure stays the same. Thus, in humid air, the water vapor molecules replace some of the N2 and O2 molecules, decreasing the density of the air.

Note that the H2O molecules that make air humid isn't liquid. It's water vapor, which is a gas.

This is from memory. It took a LONG time to find the following reference to verify my memory:

http://www.usatoday.com/weather/wdensity...

Edit: I was in the LONG process of writing this when you asked your follow up Q. Assuming PV = nRT, I believe my description also answers your 2nd Q.

Edit 2: Why the thumbs down? This is the explanation. Thanks for nothing.

Answer 2

All large external combustion engines like boilers use air preheaters for better combustion.

Answer 3

Hot air,because it can expand more an spread more heat where it's needed an hot air rises more above cold air because it's weight are light (if we assume.Also hot air reduces heat loss an retains it also,so things will burn or combust easilly.

Answer 4

A lot of strange answers.

Yes, hot air has more energy, but only a little bit.

That's offset by the fact that colder air contains more molecules per unit volume and thus more oxygen per unit volume. So in a combustion chamber, you will get more oxygen per "charge" and if the engine is smart enough to inject more gasoline, more energy when combustion occurs.

A lot of automobile engine strategy is to get more combustion per charge. The size of the combustion chamber is limited. You can push in more gasoline, but without enough oxygen to fully combust all the gasoline, extra gasoline is just wasted.

From there is it a simple step to turbos, which compress the air so that you get more oxygen into the chamber. Then the computer can inject more gasoline and you get more power.

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Answer 5

it has to do with air density.  everything except h2o contracts the colder it gets, so any given volume of air has a greater oxygen concentration at colder temperatures than it does at hotter temperatures.

Answer 6

Cold air is denser and therefore has more O2