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Turbulent combustion in internal engines is known to cover a wide range of regimes and flame topology. Engine turbulent combustion modeling should account for these various regimes observed, as auto-ignition, premixed, partially premixed or diffusion flames. The corresponding reaction zones are controlled by spray evaporation, and vapor fuel turbulent mixing with air that is coupled with combustion. This paper discusses improved modeling for the non-premixed (or diffusion) combustion phase. A new closure is proposed that is called ECFM-CLEH, for Extended Coherent Flame Model (ECFM) with Combustion Limited by Equilibrium Enthalpy (CLEH). It simulates the different phases of Diesel combustion, auto-ignition, premixed and diffusion flame burning. In the premixed phase, combustion is mainly controlled by flame propagation, while fuel and air mixing plays a crucial role in diffusion flames. In ECFM-CLEH, auto-ignition is modeled from tabulated fully detailed chemistry of n-heptane.