Question

Why does incomplete combustion occur?

Answer 1

Because while carbon can be incompletely oxidized, the oxidation is still thermodynamically downhill.

Explanation:

Insufficient oxygen, an improperly stacked fire, a poorly designed furnace; all of which can limit egress of oxygen to the centre of the pile of fuel. Note that sometimes you want incomplete combustion, and wood is burnt under conditions that promote formation of charcoal. (Why do want charcoal? Well, you need it for your barbecue; also (and unfacetiously) blacksmiths and metallurgists would have needed it to make and work iron and steel.)

A typical diesel fuel, `C_16H_34`, is so long that complete combustion is quite difficult (sometimes the fuel is unsaturated, but the stuff is a mixture of many hydrocarbons). Engineers try to maximize the energy output with respect to oxygen gas intake, and size of the cylinder or furnace. Diesel engines (as well as petrol engines) still produce quantities of carbon monoxide, and carbon (as particulate soot).

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