A hybrid propulsion system for a vehicle and method of operation are provided. As one example, the system comprises an engine including at least one combustion chamber, a motor configured to selectively propel the vehicle via the drive wheel, a fuel system configured to deliver a first substance and a second substance to the combustion chamber in varying relative amounts, wherein the first substance includes a fuel and the second substance includes a greater concentration of a knock suppressing substance than the first substance; and a control system configured to operate the fuel system to vary the relative amounts of the first substance and the second substance delivered to the combustion chamber in response to an operating condition while operating the motor to propel the vehicle.