Six principle mechanisms that are believed to be responsible for hydrocarbon emissions are crevices, oil layers, deposits, liquid fuel, flame quench and exhaust valve leakage. When liquid fuel did not find sufficient oxygen to burn prior to the end of combustion, hydrocarbon is formed in the exhaust. Since oxygen enrichment ensures additional oxygen inside the combustion chamber more complete combustion is possible and it lowers the hydrocarbon level in the exhaust. Oxygen enrichment can decrease the quenching distance of the mixture [21] as it was found to be a decreasing function of flame temperature. It is well known that the flame temperature increases in the case of oxygen enrichment [7] and this allows the flame to propagate much closer to the cylinder wall and reduce HC emissions. The hydrocarbon emissions were reduced to a minimum of 10% at 23% oxygen to a maximum of 40% at 27% oxygen enrichment levels as seen in Fig. 6.