1. Method to control an internal combustion engine comprising a fuel injector and a corresponding cylinder and operable in a homogenous charge compression ignition combustion mode, the method comprising: operating the engine in the homogenous charge compression ignition combustion mode comprising mixed mode operation; operating a learning cycle to approximate characteristics of the fuel injector, comprising: monitoring operation of the engine; approximating the characteristics of the fuel injector by utilizing a recursive least squares approximation based upon the monitored operation; evaluating persistent excitation of the engine based upon the monitored operation; and completing the learning cycle based upon the persistent excitation indicating a predetermined rich excitation indicating that the operation of the engine has varied enough to complete the learning cycle; and once the learning cycle is complete, operating the engine based upon said approximated characteristics of the fuel injector.