There is an inherent difficulty in trying to distinguish between the thermodynamic and the chemical effects in a homogeneous charge compression ignition (HCCI) engine. This article attempts to isolate the chemical kinetics effects in the framework of a zero-dimensional, thermo-kinetic model combined with a detailed chemical mechanism for iso-octane and an auto-ignition correlation. The study focuses on the behavior of single-stage ignition fuels near top dead center (TDC), as a means of relating conditions at TDC to predicted ignition timings. It is found that a unique relationship exists between combustion phasing, and the constant volume ignition delay at TDC conditions expressed in crank angle degrees ( CAD). This relationship holds for given engine parameters and composition over a wide range of RPM. For ignition near TDC, the ignition delay at TDC conditions expressed as a crank angle interval must be between 8 and 12 CAD, depending on engine design parameters.