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Because of the potential for low NOx emissions with high efficiency, HCCI engines could develop a significant niche in the engine world. However, HCCI engines suffer from a narrow operating range between knock and misfire boundaries because the ignition timing is only controlled by mixture chemistry and compression conditions. Varying combinations of operating parameters are required to obtain good combustion under different conditions and chemical kinetic models are widely used as an engine research tool. The performance of such models depends critically on the accuracy of the chemical mechanisms which are still under development and require some optimization, particularly for larger hydrocarbon molecules. This study starts with a Chalmers University mechanism [1] which is well-proven for pure n-heptane but works less well for mixtures blended with significant amounts of reformer gas containing high fractions of H2 and CO [2].