The major challenge in Homogeneous Charge Compression Ignition (HCCI) combustion is the difficulty to control auto-ignition timing. It has been shown that HCCI combustion is governed by chemical kinetics; therefore it is highly dependent on fuel/air mixture properties. Auto-ignition timing or Start of Combustion (SOC) is usually characterized by the crank angle where 5% of fuel is consumed. Understanding the effect of charge properties on SOC is essential to establish a model with the capability of predicting the auto-ignition timing. Despite the extensive work done on control-oriented modeling of HCCI ignition timing, there is still a need to improved real-time models with easily measured inputs considering engine working conditions. The purpose of this paper is to introduce a real-time model for predicting HCCI auto-ignition timing. Thus a new correlation similar to Mean Value Method (MVM) equations has been introduced. This correlation named as Simple Algebraic Model (SAM) is based in the effects of inlet parameters on SOC. SAM has been employed to reduce runtime in control-oriented models such as Modified Knock Integral Model (MION). Results show the acceptable accuracy in prediction of SOC for control applications and noticeable increase in calculation speed in contrast with MKIM.