A two-zone control-oriented charge mixing model is developed to describe charge mixing with thermodynamic interaction between fresh charge and residual gas during the intake stroke. This work enables operating range of homogeneous charge compression ignition (HCCI) to be extended without loss of stability. Cylinder volume is divided into two zones with a fictitious divider. In the mixed zone, fresh charge and residual gas expelled from the unmixed zone are mixed homogeneously. Otherwise, the unmixed zone contains rest of the residual gas burned in the previous cycle and trapped by early close of exhaust valves. Incoming fresh charge with high speed expels some of residual gas from the unmixed zone to the mixed zone by force. In this perspective, mass transfer represents charge mixing between two zones. For model validation, optical engine test is carried out. Optical access to the engines with an infrared (IR) camera allows graphical analysis to estimate thermodynamic states of two zones.