A hybrid vehicle propulsion system and method of operation have been provided. As one example, the system comprises an internal combustion engine including at least a combustion chamber configured to propel the vehicle via at least a drive wheel, a motor configured to propel the vehicle via at least a drive wheel, an energy storage device configured to store energy that is usable by the motor to propel the vehicle, a fuel system configured to deliver gasoline and alcohol to the combustion chamber in varying relative amounts, a control system configured to operate the motor to propel the vehicle and to vary the relative amounts of the gasoline and alcohol provided to the combustion chamber in response to an output of the motor.