# What defines incomplete combustion?

I know that complete combustion is a process where a fuel reacts with $$\ce{O2}$$ to become $$\ce{CO2}$$ and $$\ce{H2O}$$. But what is incomplete combustion?

According to the Internet, $$\ce{CH4 + O2}$$ and $$\ce{C6H12O6 + O2}$$ are examples of incomplete combustion. What makes them so?

In your average Bunsen burner, you can choose between complete and incomplete combustion by regulating the amount of air (meaning: oxygen) that reaches the flame. Complete combustion would follow the equation $$\ce{CH4 + 2 O2 -> CO2 + 2 H2O}$$. There is no unique equation for incomplete combustion, but one possibility would be $$\ce{CH4 + O2 -> C + 2 H2O}$$.
By the way, the way living beings burn carbohydrates in cellular respiration is, by definition, a complete combustion since every carbon atom ends up in $$\ce{CO2}$$ and a set of two hydrogen atoms always ends up as $$\ce{H2O}$$. The overall equation is $$\ce{C6H12O2 + 6O2 -> 6CO2 + 6H2O}$$. If glucose is combusted with less than 6 equivalents of $$\ce{O2}$$, then incomplete combustion results.