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A hybrid vehicle driving system has a transmission means (22) for selectively transmitting the output torque of an internal-combustion engine (10), the output torque of an electric motor (19) or a composite torque produced by combining the respective output torques of the internal-combustion engine

A hybrid vehicle driving system has a transmission means (22) for selectively transmitting the output torque of an internal-combustion engine (10), the output torque of an electric motor (19) or a composite torque produced by combining the respective output torques of the internal-combustion engine (10) and the electric motor (19) to driving wheels. The hybrid vehicle driving system includes a turbogenerator (7) capable of converting energy of the exhaust gas discharged from the internal-combustion engine (10) into electric energy, and an electrical connecting means (1) for electrically connecting the turbogenerator (7) to the electric motor (19). The hybrid vehicle driving system is controlled so that the engine (10) operates on a lean mixture for the largest possible part of the total operating time of the engine (10).

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1. 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

1. 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. 2. A hybrid vehicle driving method comprising: operating an internal-combustion engine selectively in a first operating mode in which a mixture of 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 mixture of 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; combining respective output torques of the internal-combustion engine and an electric motor to generate a composite torque; and applying the combined torque to drive wheels of said vehicle; wherein the electric motor is driven and the air-fuel ratio of the mixture supplied to the internal-combustion engine is maintained at the first value when the air-fuel ratio decreases below the first value while the internal-combustion engine is operating in the first operating mode. 3. The hybrid vehicle driving method according to claim 2, further comprising: estimating a second air-fuel ratio engine torque that would be produced by the internal-combustion engine when a fuel mixture having the second air-fuel ratio is supplied thereto; calculating a torque difference between an engine torque produced by the internal-combustion engine when a fuel mixture having the first air-fuel ratio is supplied thereto and the estimated second air-fuel ratio engine torque; and changing the operating mode of the internal-combustion engine from the first operating mode to the second operating mode when an output torque of the electric motor becomes approximately equal to the calculated torque difference. 4. The hybrid vehicle driving method according to claim 2, further comprising: estimating a second air-fuel ratio engine torque produced by the internal-combustion engine when it is supplied with a mixture having the second air-fuel ratio; and changing the operating mode of the internal-combustion engine from the first operating mode to the second operating mode when the sum of an engine torque produced by the internal-combustion engine when a mixture of the first air-fuel ratio is supplied thereto and an output torque of the electric motor exceeds the estimated second air-fuel ratio engine torque. 5. The hybrid vehicle driving method according to claim 4, further comprising: estimating the second air-fuel ratio engine torque, based on a driving torque variation that occurs when the operating mode of the internal-combustion engine changes from the first operating mode to the second operating mode.