In general, a hybrid electric vehicle (HEV) is a vehicle that uses two types of power sources together, and the two types of power sources are typically an engine and an electric motor. Such a hybrid vehicle has excellent fuel efficiency and power performance and is advantageous in that the amount of exhaust emissions is reduced compared to a vehicle having only an internal combustion engine, and thus has been actively developed in recent years.
FIG. 1 illustrates an exemplary structure of a powertrain of a general hybrid vehicle.
Referring to FIG. 1, the powertrain of the hybrid vehicle adopts a parallel-type hybrid system, in which an electric motor (or a drive motor) 140 and an engine clutch (EC) 130 are mounted between an internal combustion engine (ICE) 110 and a transmission 150. In particular, since the electric motor 140 is mounted close to the transmission 150, the powertrain system of the hybrid vehicle may also be referred to as a transmission-mounted-electric-device-type (TMED-type) system.
Typically, when a driver depresses an accelerator pedal after starting the vehicle, the motor 140 is first driven using the electrical power of a high-voltage battery 160 in the state in which the engine clutch 130 is opened, and wheels are moved by the power transferred from the motor to a final drive (FD) (not shown) via the transmission 150 (i.e. an EV mode). When a greater driving force is required due to the gradual acceleration of the vehicle, the engine 110 may be driven by operating an auxiliary motor (or a starter/generator motor) 120.
Thus, when the number of revolutions per minute of the engine 110 and the number of revolutions per minute of the motor 140 are equal to each other, the engine clutch 130 is engaged so that the vehicle is driven by both the engine 110 and the motor 140 or by only the engine 110 (i.e. transition from the EV mode to an HEV mode). When a predetermined engine-off condition, such as the deceleration of the vehicle, is satisfied, the engine clutch 130 is opened and the engine 110 is stopped (i.e. transition from the HEV mode to the EV mode). In such a hybrid vehicle, a battery may be charged by converting the driving force of the wheels into electrical energy when a braking operation is performed, which is referred to as braking energy regeneration or regenerative braking.
The starter/generator motor 120 serves as a starter motor when the engine is started, and also serves as a generator when the rotational energy of the engine is recovered after starting or when starting off. Therefore, the starter/generator motor 120 may be referred to as a “hybrid starter generator (HSG)”, or may also be referred to as an “auxiliary motor” in some cases.
Describing the HSG 120 and the electric motor 140 in terms of current flow, the high-voltage battery 160 outputs a DC voltage, and an inverter 180 appropriately converts the DC voltage into an AC voltage in accordance with a torque command and a rotational speed of the electric motor 140. The converted AC voltage is supplied to the electric motor 140 and the HSG 120 so as to drive the vehicle or start the engine. The inverter 180 also converts the counter electromotive force of the electric motor 140 and the HSG 120 due to the driving force of the traveling vehicle or regenerative braking into a DC voltage so as to charge the high-voltage battery 160.
Hereinafter, the flow of high-voltage energy depending on whether the electric motor is performing a charge or discharge operation will be described with reference to FIG. 2.
FIG. 2 is a view schematically illustrating the flow of energy depending on the operation of the electric motor in a general hybrid vehicle.
Referring to FIG. 2, in the hybrid vehicle, when the electric motor 140 operates in a discharge mode, electrical energy for driving is transferred from the battery 160 to the electrical motor 140 via the inverter 180. When the electric motor 140 operates in a charge mode, charge energy, which is generated by the electrical motor 140, is transferred to the battery 160 via the inverter 180.
On the assumption that the electric motor 140 rotates at a constant speed in the positive (+) direction, the motor torque has a positive (+) value in the discharge mode for generating driving force, and has a negative (−) value in the charge mode.
However, at some operating points of the motor, the loss of energy may be larger than the charge energy generated by electricity generation due to loss in the energy transfer path, including copper loss of the motor. A driver expects that the battery will be charged whenever regenerative braking is performed, but in practice there is a problem in that the battery may be discharged depending on the operating point of the motor.