Unburned Hydrocarbons

The concentration of hydrocarbons in diesel exhaust varies from a few ppm to a several thousand ppm, depending on the load on the engine and its speed. The hydrocarbons in diesel exhaust are composed of a mixture of many individual hydrocarbons in the fuel supplied to the engine as well as partly burned hydrocarbons produced during combustion process.

During the normal operation of the engine, the relatively cold wall quenches the fuel-mixture and inhibits the combustion, leaving a thick layer of unburned fuel air mixture over the entire surface of the combustion chamber. The thickness of this layer depends on the combustion pressure, temperature, mixture ratio, turbulence, and residual gases in the engine at the end of the exhaust stroke. A greater surface to volume ratio of the combustion chamber leads to the formation of greater fraction of hydro-carbon from the quenched zone.