Why Engine Variables Affect Exhaust Hydrocarbon Emission

The relative importance of several internal environmental factors known to affect the hydrocarbons exhausted by an engine has been studied using an analytical model of the emission process.

The model included A: two sources of unburned fuel resulting from wall quenching of the flame, 1) the walls of the open part of the chamber, and 2) the crevices separated from the open part of the chamber by restrictive passages; and B: three subsequent effects, 1) the oxidation of unburned fuel in the chamber after flame propagation, 2) the preferential exhausting of the burned products from the chamber and 3) the oxidation of unburned fuel in the exhaust system.

Results obtained by empirically fitting the model to measured exhaust hydrocarbon concentrations for systematic changes in five operating variables indicated that different environmental factors predominated in causing the different variable effects.