The unsteady motion of chemical species in the intake and exhaust ducts of spark-ignition internal combustion (I.C.) engines is studied numerically by employing a finite-difference-based engine simulation code. The time-dependent mass fractions of six dominant products (CO2, H2O, CO, H-2, O-2, N-2) combined with five additional minor species (H, NO, O, OH, N) are computed in the combustion chamber and tracked throughout the breathing system for wide-open-throttle as well as part-load operating conditions. The property calculations employ the NASA database to determine the composition. The effect of the number of product species on the engine performance and dynamic quantities, including pressure and temperature in the breathing system, is also investigated. (C) 2003 Elsevier Ltd. All rights reserved.