Why does incomplete combustion of hydrocarbons reduce energy efficiency?
Less energy is released per mole of fuel
In complete combustion, a hydrocarbon burns in plenty of oxygen and all its carbon is oxidised to carbon dioxide and all its hydrogen to water, releasing the maximum heat. For example, CH4 + 2O2 → CO2 + 2H2O.
When the oxygen supply is insufficient, combustion is incomplete. Part of the carbon is converted only to carbon monoxide (CO) or left as unburnt carbon (soot), which gives a sooty, yellow flame.
Since this carbon is not fully oxidised to CO2, the chemical energy stored in it is not fully released. So less energy is released per mole of fuel, and the fuel is used less efficiently.
Incomplete combustion forms less, not more, carbon dioxide, along with carbon monoxide and soot, so the products are never just water, and the energy released is lower, not higher, than in complete combustion.
Hence, Less energy is released per mole of fuel.
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