# Combustion question

## Main Question or Discussion Point

If i am trying to burn a fuel in air am i right in saying if i have a higher temerature compared to a lower temperature of air, the higher temperature air has more moles of air to burn than the lower temperature air for complete combustion to occur?

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brewnog
Gold Member
How much air? A constant mass? Constant volume?

At a given pressure, air is less dense at higher temperature (less mass per unit volume).

LURCH
In general, yes. Cold air is denser (all else being equal), and therefore has more O2 molecules per volume, and so less air (by volume) is required to accomplish the same combustion. This of course does not hold true if the "all else" is not equal. Ari pressure has a significant effect and humidity can make a huge difference.

brewnog
Gold Member
In general, yes.
The OP asked whether having a higher temperature provided more air for combustion. That's clearly not true.

LURCH
Ah yes; I read it as saying higher temperature requires more air, but upon re-read, I see that was almost the oposite of the poster's intent.

Engineerign, higher temperature air is thinner and contains less O2 than colder air (again, all else being equal), and therefore provides a better comustion reaction.

FredGarvin
It's not that a higher temperature air contains less 02, it is simply less dense. Since you are dealing with a constant volume device, then the lower density means less air (coincidentally less O2).

Hi there:

Here is a free MS Excel spreadsheet on combustion -- http://engware.i-dentity.com/pf/combustion.xls.

For any fuel and oxidant combustion scenario whwre stoichiometry is => 1, one can calculate the combustion flame temperature, combustion products composition on both weight and mole basis, stoichiometry and stoichiometric ratio.

Thanks,

Gordan