Hybrid vehicle driven by composit torque generated by an internal-combustion engine and an electric motor, and method of operating same

A hybrid vehicle driving system comprising:

• an internal-combustion engine capable of selectively operating in a first operating mode in which a fuel mixture having an air-fuel ratio not lower than a first predetermined value is supplied to the internal-combustion engine, and a second operating mode in which a fuel mixture having an air-fuel ratio not higher than a second predetermined value, which is smaller than the first predetermined value is supplied to the internal-combustion engine;
• an electric motor;
• and
• a transmission for producing a composite torque by combining an output torque of the internal-combustion engine and an output torque of the electric motor and transmitting the composite torque to driving wheels;
• wherein the electric motor produces a torque that is applied to reduce the size of step increases or decreases in the output torque of the internal-combustion engine which occurs when the internal-combustion engine changes between the first operating mode and the second operating mode.