[JackJi]

Okay, I am really confused about the whole concept of complete and incomplete combustion. The websites seem to say different things. Here is a few questions i had in mind:
1. is it true that complete combustion would give a higher heat of combustion?
2. or is it vice versa?
3. is it true that any fuel is able to undergo complete combustion given enough oxygen?

[Arkcon]

Heat of combustion, those words, run together like that, describe a constant, for a fuel combustion reaction.  We usually just publish the particular constant for a complete combustion, for simplicity, but I suppose there is one for incomplete combustion.

How to guarantee compete combustion is kind of a tricky.  If there is "enough" oxygen, then probably.  But what is enough?  Generally, if you want complete combustion, say, because you want to experimentally determine heat of combustion, it's not enough to just add "plenty" of oxygen.  You'd use pure oxygen, and a very limited amount of fuel, to make sure it gets a change to mix well.

Dunno if you're trying to figure out the definitions for class work, or build a better automotive engine, so it's hard to be more clear than that.

[JackJi]

NO, i guess my biggest problem in mind is that : Is it a rule that complete combustion will always yield a higher heat of combustion than incomplete combustion does?

[Arkcon]

I would assume so, thermodynamically.  If you could get more energy, by incomplete combustion, wouldn't we all do that, and then, combust the rest of the way, and get even more?

[JackJi]

Yes, that is exactly what i thought, but in a practical lab, we compared candle wax and ethanol. we know candle wax is incomplete combustion (yellow light flame) and ethanol is complete (blue flame). But, upon calculation, the candle wax has a higher heat of combustion (in Joules of energy per gram of material consumerd). How should I explain this?

[nj_bartel]

You're comparing apples and oranges.  That's like saying the incomplete combustion of trinitromethane must have a lower heat of combustion than the complete combustion of coal.

[Borek]

It is like saying that riding at full speed on bike you are much slower than driving a car at half full speed

[JackJi]

Oh, I think I know what you guys mean. So, the heat of combustion varies depending on the material. It is not a rule that complete combustion would always yield a higher heat. But is it okay to say that, of the same material, (for example candle wax), complete combustion of candle wax would yield a higher heat than incomplete combustion of candle wax??

Is it okay to say that then?

[nj_bartel]

I would say so.