Main sources of hydrocarbon emissions in the four-stroke, homogeneous charge spark ignition engines are:

(i) Flame quenching on the cylinder walls

(ii) Flame quenching in crevices

(iii) Absorption and desorption in oil film on cylinder walls

(iv) Absorption and desorption in carbon deposits in the chamber

(v) Misfired combustion or bulk gas quenching

(vi) Liquid fuel in the cylinder

(vii) Exhaust valve leakage

(viii) Crankcase blow by gases.

Hydrocarbons: No HC should be going to exhaust if complete combustion of fuel takes place but practically this does not happens and HC is emitted through exhaust due to the following reasons.

Incorrect Air-fuel (A/F) Ratio: amount of HC increase with A/F mixture getting richer. This is caused by incomplete combustion due to inadequate oxygen. On the other hand, if A/F ratio is made to lean, HC in exhaust gases again increase, from slow flame propagation due to leak of fuel and misfiring of engine occurs.

Low compression: during deceleration, very little or no air is drawn to cylinders due to the .closure of throttle valve, although feel is drawn. This result in rich mixture and low compression in cylinder. Hence complete combustion does not take place and HC is emitted through exhaust.

Quenching: Temperature of flame inside cylinder is low in areas where flame reaches the last. Also, area near cylinder wall and valve are at lower temperature (as they give out heat to surrounding). These are called quenching zones. Therefore, fuel in these quenching zone is not able to born before it is exhausted.

Valve overlapping: for a very short time, both inlet and exhaust valve are open simultaneously. During this period HC escape to exhaust.

Surface/volume (s/v) ratio: A small engine of low displacement volume will have higher surface to volume ratio which is responsible for formation of hydrocarbon (HC). Experimental results have shown the low C.R., lower bore to stroke ratio and larger displacement lowers the s/v ratio, hence, reduce HC emission.