There has been recent interest in a new engine type, Homogeneous Charge Compression Ignition (HCCI), to combine the advantages of SI and CI engines. In this paper, a locally deterministic approach is employed to consider spatial inhomogeneities using the KIVA-CHEMKIN package. Validation is performed for two experimental HCCI engines fueled, respectively, by hydrogen and n-heptane. The full mechanism for hydrogen and a skeletal mechanism for n-heptane are used for combustion chemistry. Differences in the reaction flow paths are shown at ignition and the heat release reaction stages of the two fuels. Results show good agreement between measured and calculated pressures for different initial mixture temperatures with estimated residual fractions. A parametric study is performed in both engines to consider the influences of the physical parameters wall temperature, swirl ratio and global equivalence ratio. The ignition time of n-heptane is shown to be relatively insensitive to variations in these parametric due to its two-stage ignition behavior.