HC from Misfired Combustion

Engine may misfire under engine idling and low load operation as the residual gas dilution is high.  Presence of high residual gas content retards combustion and more fuel burns during expansion stroke. However, during expansion as the cylinder pressure falls the temperature of the unburned mixture ahead of the flame  also decreases which  may result in extinction of flame and consequently in partially misfired combustion. Such bulk flame quenching in spark-ignition engines leads to very high HC emissions. Use of excessively lean mixtures also decreases burning rates and increases incidence of bulk gas flame quenching. The propensity to partial misfiring increases under transient engine operation. Use of large amounts of exhaust gas recirculation (EGR) or high residual gas dilution though reduces NO formation but results in high HC emissions.