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By using a four-stroke gasoline engine equipped with a fully variable valve operation system, combustion performance was investigated from the aspect of a gas exchanging difference at various internal exhaust gas recirculation conditions due to the negative valve overlap variations. The in-cylinder gas temperature throughout the cycle process was analyzed thermodynamically. The experimental data revealed that in-cylinder gas temperature at the end of compression stroke (TAI) dominates the onset of autoignition and ΔT, which is an index that represents the heat capacity of the working gas, dominates the heat release of auto-ignition. This paper intends to evolve the experimental knowledge to an engineering tool, which could predict possibilities and limits of auto-ignition. As a result, a controlling mechanism of auto-ignition is proposed. According to this mechanism, a possible maximum load of auto-ignition operation is estimated and also demonstrated in the engine experiments.