Estimating internal residual during engine operation is essential to robust control during startup, steady state, and transient operation. Internal residual has a significant effect on combustion flame propagation, combustion stability and emissions. Accurate residual estimate also provides a better foundation for optimizing open loop fuel control during startup, while providing a basis for reducing emissions during closed loop control.In this paper we develop an improved model to estimate residual gas fraction by means of isolation and characterization of the physical processes in the gas exchange. Examining existing residuals model as the base, we address their deficiencies making changes to appropriate terms to the model. Existing models do not work well under wide angle dual independent cam phasing. The improved residual estimation model is not limited by the initial data set used for its calibration and does not need cylinder pressure data. The model can work with different valve lift profiles and compression ratios. The model is calibrated by using two datasets, a single cylinder engine simulation dataset comprising of a range of speeds from 180 to 6000 rpm and loads from an existing validated model; and simulation dataset from a validated multi-cylinder engine model. Simplified real-time ECU implementation is also discussed.