US Patent 7980342 Plug-in hybrid electric vehicle

A method for operating a vehicle may include operating a motor and an engine to propel the vehicle, and increasing consumption of fuel as a residence time of the fuel stored on-board the vehicle increases. The residence time of the fuel stored on-board the vehicle can provide an indication of fuel degradation, enabling the duty cycle of the engine to be selectively increased to enforce fuel use conditions, particularly in the context of a plug-in hybrid electric vehicle where fuel use may be less frequent.