In addition, in embodiments of the present disclosure, whether to limit an engine torque may be determined by considering the type of kick-down shift. Here, the type of kick-down shift may include a sequential shift type in which the gearshift is sequentially lowered by one stage and a skip shift type in which the gearshift is shifted two or more stages lower than the current gear. The skip shift type may be applied when a higher acceleration (high torque required) is required compared to the sequential shift type. According to an embodiment, in the case of the skip shift type, the engine torque may not be limited, and in the case of the sequential shift type, determining whether to restrict the engine torque is determined based on the difference between the total available torque of the first motor 120 and the second motor 140 and the required torque in the target gear.

Further, determining the shift type may be performed in a form in which the transmission control unit 250 refers to a shift map according to an accelerator pedal position sensor (APS) value and a vehicle speed, and a determined result may be a form transmitted from the transmission control unit 250 to the HCU 240.

Hereinafter, a method for controlling a hybrid electric vehicle according to an embodiment of the present disclosure will be described with reference to FIGS. 3 and 4. In the following description, for convenience, a situation in which the kick-down shift in the EV mode and the switching to the HEV mode are required together will be referred to as “EV kick-down”.

As described above, in an embodiment of the present disclosure, whether to limit the engine torque may be determined based on the shift type, the total available motor torque, and the required torque. First, a form of controlling the hybrid electric vehicle in the EV kick-down situation is described when the engine torque is limited with reference to FIG. 3.

FIG. 3 is a view showing an example of a configuration of a type of EV kick-down control that limits an engine torque according to an embodiment of the present disclosure.

Referring to FIG. 3, firstly, in section S1, the HCU 240 may obtain information about a target gear, shift progress process (shift phase), and a shifting class (shift type) from the transmission control unit 250. In addition, the HCU 240 may determine whether to switch to the HEV mode according to the required torque during driving in the EV mode.

Accordingly, the HCU 240 determines the driving mode switching of the HEV and determines the EV kick-down, the situation in which the EV kick-down shift is notified from the transmission control unit 250 therewith.

In the EV kick-down situation, the HCU 240 may be on standby for information about the shift phase from the transmission control unit 250.

In addition, the HCU 240 may crank the engine 110 through the first motor 120 in section S2 according to determining the switching from the EV mode to the HEV mode.

When the transmission control unit 250 notifies (i.e., SS point detection) the shift control start (shift start, SS) at the start of section S3 while on standby for information on the shift phase, the HCU 240 may calculate the transmission input speed of a target gear to determine the respectively available torque of the first motor 120 and the second motor 140 from discharging limitation power, respectively. For example, available torque for the respective motor may be obtained by dividing the discharging limitation power for the respective motor by the transmission input speed of the target gear, but is not necessarily limited thereto.

The HCU 240 may determine whether to limit (use) the engine torque by comparing the calculated available torque of the respective motors with a value of ‘required torque—actual torque (e.g., engine ground torque—engine friction torque).’ Here, the engine ground torque may be a torque greater than or equal to the engine friction torque generated by mechanical friction during the engine rotation and may be defined as a torque value at a level that does not affect the state transition determination when the engine clutch 130 is engaged, and may be determined through an experiment for each vehicle, but is not necessarily limited thereto. In addition, a value obtained by subtracting the engine friction torque from the engine ground torque may be referred to as the ‘actual engine output torque’, which is an actual engine output torque of the engine 110 transmitted to the drive system (i.e., the transmission input terminal).

For example, when the required torque and the actual engine output torque are not more than a present torque threshold value, and the shift type is the sequential type, the HCU 240 may be controlled by the engine 110 to output only the ground torque to exclude the possibility of torque mismatch before a shift finish (SF) point (in FIG. 3, between sections S6 and S7). Such control is applied to the sequential shift because, considering that the driver requires relatively low acceleration, the sensation of acceleration may be lower than when the engine torque is used above the ground torque, but the robustness of the shift may be secured.

Returning to section S3, the engine clutch 130 may be joined as the engine start is completed.

In section S4, after the joining of the engine clutch 130 is completed, the torque of the first motor 120 and the engine 110 may be transmitted to the transmission input shaft, by which the input torque of the transmission may be increased at a high inclination using the torque of the first motor 120.

Accordingly, a high total torque may be obtained relatively early compared to a hybrid powertrain in which a starter-generator motor connected to the engine by a pulley is applied. Thus, the release solenoid of the transmission 150 may be released at a higher hydraulic pressure level.

Accordingly, the actual shifting point may be brought forward, and after the actual shifting begin (SB) point is detected in section S5 and before the actual shifting finish (SF) point is detected at the end of section S6, the HCU 240 may control the required torque dissatisfaction other than the actual engine output torque to be within the available torque in the order of the second motor 140 to the first motor 120.

In section S6, after detecting the SB point, as an intervention control for reducing the torque of the transmission input terminal is performed before the actual shift occurs, the reduction torque may be performed as the sum of the engine torque and the second motor torque. After the actual shift finish (SF) to which the target gear is engaged in the transmission 150 due to the reduction of the transmission input terminal torque by the reduction torque, the effect of the engine torque mismatch in terms of the sensation of gear-shifting sensation is not significant. Therefore, according to the default setting, the HCU 240 may determine the operating point of the respective drive source as a part-load or a full-load to satisfy the required torque.

Next, an EV kick-down control method in which the engine torque is not limited will be described.