How are hydro-carbons formed in an automobile engine

How are hydro-carbons formed in an automobile engine:

Hydro carbons are formed due to the unburnt portion of fuel,emitted from the engine exhaust. Hydro-carbons are the only major pollutants, which release from carburettor and the engine crank case flumes in addition to the engine exhaust.

Due to quenching the hydro carbons are formed in SI engines. And also by the extinguish generated in the combustion chamber. We can see the hydro-carbons can burn and eliminated during the expansion and exhaust stroke in an engine if the temperature in the exhaust and concentration of oxygen are available in suitable proportions.

Due to the too rich mixture in an engine the unburnt hydro carbons are emitted from the exhaust line in CI engines.

Due to high boiling and condensation temperature present in CI engine at there more soot is formed. When the combustion process continues the solid soot is used to burn due to high temperature generated because of continuous air fuel mixture in combustion chamber. By this the amount of soot is used to decrease. By this less amount of hydro carbon is produced.