A reduction approach for coupling complex kinetics with engine computational fluid dynamics (CFD) code has been developed. An on-the-fly reduction scheme was used to reduce the reaction mechanism dynamically during the reactive flow calculation in order to couple comprehensive chemistry with flow simulations in each computational cell. KIVA-3V code is used as the CFD framework and CHEMKIN is employed to formulate chemistry, hydrodynamics and transport. Mechanism reduction was achieved by applying element flux analysis on-the-fly in the context of the multidimensional CFD calculation. The results show that incorporating the on-the-fly reduction approach in CFD code enables the simulation of ignition and combustion process accurately compared with detailed simulations. Both species and time-dependant information can be provided by the current model with significantly reduced CPU time. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 1305-1314, 2010