An aerodynamic model of the entrainment of the head wall quench layer during blowdown and exhaust of an internal combustion engine has been developed. The model may be used to calculate the time resolved concentration and mass flowrate of hydrocarbons (HC) in the exhaust, from a knowledge of engine geometry and operating conditions. It predicts that the area As from which HC are swept will be proportional to the cube root of the ratio of the quench layer thickness δq to the thickness of the viscous boundary layer δv. Since the mass of HC emitted is proportional to the product of the HC density ρHC, the area As and the thickness δq, the HC emissions will be proportional to the product ρHC δq4/3 and this is the most important factor determining the emissions.The model also predicts that the time dependence of the HC mass flowrate will depend relatively strongly on the pressure ratio p4/pe across the exhaust valve when it opens: in an unthrottled engine virtually all the head wall HC exit during blowdown; in a heavily throttled engine the HC are emitted more or less uniformily during the exhaust stroke.It is expected that the model will be useful both for interpreting experimental measurements of HC emissions and for predicting HC emissions from practical engines. Comparisons of the results obtained to date with the available experimental data shows good agreement.