Engine supercharger for maintaining a battery charge

A supercharging system, comprising:

• a first supercharger including a first compressor for compressing air to be supplied to an engine;
• a second supercharger including a second compressor for further compressing air compressed by the first compressor;
• and
• a controller for controlling the first supercharger and the second supercharger,
• wherein one of the first supercharger and the second supercharger further includes an electric motor for driving corresponding one of the first compressor or the second compressor,
• wherein at least one of the first supercharger or the second supercharger further includes a turbine configured to be rotary driven by exhaust gas from the engine, and a nozzle vane configured to adjust a flow-path area of the exhaust gas flowing into the turbine, and
• wherein the controller includes:
• an electric motor control part configured to, if an amount of charge of the battery for supplying electric power to the electric motor is less than a first threshold, set an upper limit value of an output command value for the electric motor to be lower than when the amount of charge of the battery is not less than the first threshold, or switch operation of the electric motor to regenerative operation, and if the amount of charge of the battery is less than the first threshold and not less than a second threshold which is smaller than the first threshold, set the upper limit value of the output command value for the electric motor to a value which is greater than zero and smaller than when the amount of charge of the battery is not less than the first threshold;
• and
• a vane control part configured to, if the amount of charge of the battery is less than the first threshold, control an opening degree of the nozzle vane so that the flow-path area decreases with a control of the electric motor by the electric motor control part,
• wherein the controller further includes a threshold determination part configured to determine the first threshold and the second threshold on the basis of a load of an alternator configured to be driven by the engine to generate electric power to be supplied to the electric motor;
• and
• if the amount of charge of the battery is less than the first threshold and the opening degree of the nozzle vane is not less than a third threshold, switch the operation of the electric motor to the regenerative operation.