Method of determining engine output power in a hybrid electric vehicle

This novel silent operating mode for a hybrid electric vehicle (HEV) reduces noise and emissions compared to traditional HEV operating modes. It is a complementary series of software control functions that allows the vehicle to operate with reduced noise and emissions where specifically needed, while phasing-in engine power where allowed. The method utilizes an energy storage system budget associated with a modal quantity of energy allocated for the mode, and is adapted to automatically adjust the operation of the vehicle to accommodate deviations from the budgeted energy amount. The method which implements the mode also adjusts the vehicle operation in conjunction with changes in the parametric conditions of the ESS, including the selective use of the engine output power. In particular, the silent operating mode comprises a method of selecting an engine output power from a speed-based engine output power and a grade-based engine output power. The grade-based engine output power may be determined from a plurality of vehicle state parameters.