Method of estimating residual exhaust gas concentration in a variable cam phase engine

A method of estimating a concentration of residual exhaust gas in an internal combustion engine, where the engine includes variable phase control of one or more valves that control a flow of gases through said engine, the method comprising the steps of:

• determining a volumetric efficiency of said engine based on known and measured engine parameters for various values of said phase control;
• simulating the volumetric efficiency and the residual exhaust gas concentration for the various phase control values using an engine simulator;
• comparing the determined volumetric efficiency with the simulated volumetric efficiency for the various phase control values, and adjusting at least one parameter of said engine simulator until the simulated volumetric efficiency substantially matches the determined volumetric efficiency;
• modeling the residual exhaust gas concentration of said engine as a function of the various phase control values based on the simulated residual exhaust gas concentration;
• and
• estimating the concentration of residual exhaust gas in the engine during operation of the engine based on said modeled residual exhaust gas concentration.