The effect of fuel equivalence ratio on the autoignition and combustion processes in a Homogeneous Charge Compression Ignition (HCCI) engine has been studied using a primary reference fuel (PRF60). Experiments have been performed on a single-cylinder engine operated in an alternate-fire mode. The operating conditions of the engine used in this work were an intake air pressure and temperature of 1.5 bar and 75°C respectively, an engine speed of 1200 rpm, compression ratio of 12:1, with varying fuel equivalence ratio (φ). A single zone CHEMKIN model using reduced chemical kinetic model of 560 species and 2818 reactions based on a 5 components gasoline surrogate from Reaction Design's Model Fuels Consortium was used to predict the HCCI engine combustion behavior for different fuel equivalence ratios. The mechanism has been validated against ignition delay times measured in shock tube experiments for the octane number (ON) in the range of 0 ≺ ON ≺ 100, temperature (700 K ≤ T ≤ 1300 K), and pressure of 40 bar under stoichiometric condition. Steady state engine data were used to define the initial conditions for the model setup. The same initial conditions were kept to investigate a sweep of equivalence ratio in order to predict the behavior of the alternate-fired cycle. The simulated results show good agreement with the experimental data and replicate the trends for different fuel equivalence ratio. Sensitivity and reaction flow analyses have been performed to investigate the chemical influence of the equivalence ratio on combustion characteristics. The most sensitive reactions, reaction paths and radical species responsible for the behavior of cool flame and the main combustion have been identified.