A comparison between phase diagrams of HCCI engines operations and explosion diagrams of n-heptane at lean conditions is here presented. Because of the autoignition behavior of the air-fuel mixture in HCCI engines, this analysis can improve the understanding of combustion phenomena in this system. A detailed kinetic scheme coupled with a non adiabatic stirred reactor model allows to investigate the effect of time and composition on the explosion diagrams. Results show that the low residence time in HCCI engine allows complete combustion only after a critical compression ratio, when the system is able to overcome the explosion limits.