Control system for a hybrid vehicle in which output from the engine is assisted by the motor based on air-fuel ratio determination

1. Field of the Invention A control system provided in a hybrid vehicle with a combustion engine for outputting a driving force, an electric motor for generating a force for assisting the output from the engine, depending on driving conditions, a power storage unit for storing electric energy generated by the motor acting as a generator using the output from the engine and electric energy regenerated by the motor when the vehicle decelerates. The control system includes an output assist determination means for determining, based on a determination threshold value as the standard, whether to assist the output from the engine by the motor, depending on the driving conditions of the vehicle. An air-fuel controller is provided for changing the air-fuel ratio of the mixture, which is to be supplied to the engine, to a condition leaner or richer than the stoichiometric air-fuel ratio. A determination threshold value changing means is provided for changing the determination threshold value, depending on whether the air-fuel ratio of the mixture is leaner or richer than the stoichiometric air-fuel ratio. A determination threshold value change prohibiting means is provided for prohibiting the operation of the determination threshold value changer when the air-fuel controller changes the air-fuel ratio of the mixture from a condition leaner than the stoichiometric air-fuel ratio to a condition richer than the stoichiometric air-fuel ratio.