Cruise control method for a hybrid electric vehicle

Abstract

Cruise control method for a hybrid electric vehicle, comprising: Switching on a cruise control mode in the hybrid electric vehicle, the hybrid electric vehicle using an internal combustion engine (31) and a drive motor (32) as vehicle drive sources; Selecting a selected mode in a pulse and slide (PnG) mode in dependence on vehicle condition information, wherein the selected mode is selected between a first PnG mode and a second PnG mode; and Executing control of the hybrid electric vehicle with the selected mode, where: in the first PnG mode, driving of the hybrid electric vehicle is carried out in a sliding phase by inertia of the hybrid electric vehicle, wherein a pulse phase and the sliding phase are alternately repeated between a predetermined upper threshold value of a vehicle speed and a predetermined lower threshold value of the vehicle speed; and in the second PnG mode, acceleration of the hybrid electric vehicle is carried out in the pulse phase by the internal combustion engine (31) or both the internal combustion engine (31) and the drive motor (32), and deceleration of the hybrid electric vehicle is carried out in the slip phase by inertia of the hybrid electric vehicle and torque assistance of the drive motor, wherein the pulse phase and the slip phase are alternately repeated between the predetermined upper threshold value of the vehicle speed and the predetermined lower threshold value of the vehicle speed, wherein the vehicle state information is an absolute value of a vehicle acceleration, and, when the absolute value of the vehicle acceleration is greater than a predetermined threshold value in the first PnG mode, the first PnG mode switches to the second PnG mode, wherein the vehicle acceleration is an acceleration or deceleration degree of the hybrid electric vehicle.

Description

Technical FieldThe present disclosure relates to an automatic cruise control method for hybrid electric vehicles. More specifically, it relates to an automatic cruise control method for improving fuel efficiency and also driving characteristics.BackgroundThe statements in this section merely represent background information related to the present disclosure and may not form prior art.In general, an automatic speed control device for a vehicle performs automatic driving of the vehicle at a predetermined vehicle speed without operation of an accelerator pedal by a driver, and is thus referred to as a constant speed driving system.When a target vehicle speed is set by simple operation of a driver, the automatic cruise control device controls a vehicle so as to maintain the set target vehicle speed, and thus significantly reduces operation of an accelerator pedal by the driver, thus improving riding comfort.In the case of an internal combustion engine motor vehicle such as a gasoline or diesel vehicle, when a required torque (cruise torque) for maintaining a target vehicle speed is detected, a conventional automatic cruise control apparatus controls driving of an engine so that the required torque can be output through cooperative control between control units and performs automatic driving (auto cruise) to thereby maintain the target vehicle speed.Further, in the case of an electric vehicle driven using an engine, the conventional automatic cruise control device controls an engine torque depending on a required torque for maintaining a target vehicle speed, and in the case of a hybrid electric vehicle driven by an engine and an internal combustion engine, the conventional automatic cruise control device distributes conduction to the engine and the internal combustion engine so as to output the required torque.When automatic running is performed at a constant speed in an internal combustion engine vehicle, the operating point of an internal combustion engine is determined by a vehicle speed and a transmission gear shift position, regardless of an optimum engine operating line (hereinafter referred to as an "OOL") as exemplarily shown in FIG. 1.Accordingly, automatic driving of the vehicle with the internal combustion engine is disadvantageous in terms of fuel efficiency, and therefore, a cruise control technology capable of improving fuel efficiency is proposed.For example, the usefulness of a pulse and glow driving pattern (hereinafter referred to as a "PnG" driving pattern) has been proven in various fields in which acceleration and deceleration of a vehicle are repeated in a determined cycle to improve fuel efficiency under real surrounding driving conditions.However, when using the known PnG cruise control, there is a trade-off relationship between a variation in vehicle speed (which relates to driving characteristics) and a fuel saving amount, and therefore an optimum control technology capable of satisfying both driving characteristics and improvement in fuel efficiency is required.In US 2014 / 0 195 116 A1, a method is described that includes modulating the vehicle speed around a target speed by operating a vehicle with a high power engine and then operating the vehicle with the engine off, and adjusting operation of a suspension system based on vehicle operation with the high power engine and the engine off to control vehicle pitch during modulating the vehicle speed around the target speed.US 2014 / 0 277 989 A1 describes a cruise control module, an engine control module and a brake control module. The cruise control module determines a cruise control torque request based on at least one of the distance of the vehicle from the preceding vehicle and the speed at which the vehicle approaches an object. The engine controller detects a negative torque capacity of a powertrain. The powertrain includes an internal combustion engine and an electric motor. The brake control module applies a friction brake when the cruise torque request is less than the negative torque capacity of the powertrain.DE 10 2016 226 128 A1 describes a method and a system for automatic speed control for hybrid electric vehicles.Furthermore, US 2014 / 0 195 135 A1 is known, which shows a vehicle drive control device.The Invention Subject MatterThe present disclosure provides an automatic cruise control method in which a PnG driving pattern is adopted in consideration of characteristics of hybrid electric vehicles so as to improve fuel efficiency.The present disclosure also provides an optimum automatic cruise control method that can satisfy both driving characteristics and improvement in fuel efficiency.According to the present invention, there is provided a cruise control method for a hybrid electric vehicle having the features of claim 1.Further areas of applicability will become apparent from the description provided herein. It is to be understood that the description and specific examples are provided for purposes of illustration only and are not intended to limit the scope of the present disclosure.FIGS. FiguresIn order that the disclosure may be readily understood, various embodiments of the same exemplary nature will now be described, with reference to the accompanying figures, and wherein: FIG. 1 is a graph showing the operating point of an internal combustion engine during auto-cruise driving of an internal combustion engine vehicle; FIG. 2 is a graph showing a PnG cruise control running state of a conventional general vehicle with an internal combustion engine; FIG. 3 is a graph showing the operating point of an internal combustion engine during automatic cruise control of a general hybrid electric vehicle; FIG. 4 is a graph showing cruise control driving conditions in respective PnG modes of a hybrid electric vehicle; FIG. 5 is a block diagram showing a configuration of an automatic cruise control system of a hybrid electric vehicle; FIG. 6 is a flow chart showing an automatic cruise control processor of a hybrid electric vehicle; FIGS. 7( a) and 7( b) are graphs exemplarily showing a real vehicle running state according to an automatic cruise control method of a hybrid electric vehicle; FIGS. 8 and 9 are graphs illustrating vehicle speed variations exemplarily depending on loads during control in a trade-off PnG mode; and FIG. 10 is a graph showing comparison of respective modes.The figures described herein are for illustrative purposes only and are not intended to limit the scope in any way of the present disclosure.Detailed DescriptionThe following description is merely exemplary in nature and is not intended to limit the present disclosure, use, or application. It should be understood that corresponding reference numerals designate like or corresponding parts and characteristics throughout the figures.As prior art documents related to the present disclosure, there are a publication having publication number US 2013 / 0 226 420 A1 (Patent Document 1) and a publication having publication number US 2013 / 0 103 238 A1 (Patent Document 2). According to technologies disclosed in Patent Documents 1 and 2, an operating point is tracked ("tracked") on an engine brake specific fuel consumption (BSFC) map with high efficiency.Patent Document 1 discloses a control device and a method implementing a PnG function in a general vehicle with an internal combustion engine, and more specifically, a technology in which control is executed to monitor upper and lower threshold target vehicle speeds set based on a reference vehicle speed during control of a vehicle speed, and in which the target vehicle speeds are monitored by an increase and a decrease in a fuel amount of a combustion chamber.Further, Patent Document 2 discloses an apparatus and a method that improve fuel efficiency by minimizing vehicle speed fluctuations and accurately controlling a throttle value by a PnG control, and more specifically, a technology in which a pulse is applied in a fast cycle to a throttle value without vehicle speed fluctuation, and an engine operating point moves to an operating point with high efficiency on a BSFC map so as to improve fuel efficiency.The present disclosure relates to a method that implements a PnG function in a hybrid electric vehicle (HEV) that uses an internal combustion engine and an engine as drive sources, and the object of the present disclosure is to improve fuel efficiency and to satisfy both improvement of driving characteristics and fuel efficiency using a PnG drive pattern in consideration of hybrid electric vehicles. Generally, a hybrid electric vehicle is configured to operate at the optimal operating point, i.e., on an optimal operating line (OOL), by a hybrid power optimization strategy between an internal combustion engine and an engine.That is, during automatic cruise control of a hybrid electric vehicle at a constant speed, as exemplarily shown in FIG. 3, an operating point is set so that the OOL is tracked ("tracked") to exert optimum efficiency, and then the engine is operated. When a required torque is less than an engine torque that coincides with the optimum operating point of the OOL, an amount of the engine torque corresponding to the required torque is used for operating the vehicle, the rest of the engine torque is used as a reverse torque (regenerative torque) in a motor operated as a generator, and is therefore used for charging a battery (motor regeneration and charging).On the other hand, when the required torque is larger than the engine torque, the required torque is satisfied by engine output (motor drive torque) (motor assist and discharge).In FIG. 3, an "operating point "during general constant speed running" may denote an operating point at which a constant speed can be maintained regardless of the OOL, as in a general vehicle with an internal combustion engine and torque at such an operating point, may denote the above-described required torque for maintaining a constant speed.However, the constant speed temperature drive strategy of the hybrid electric vehicle described above results in a reduction in efficiency due to charging / discharging in an electrically powered system.Therefore, when an engine operating point is determined as the optimum operating point depending on a vehicle state, simultaneously with the reduction in the use of the electrically driven system, fuel efficiency can be improved.Based on the above aspect, according to the present disclosure, during automatic cruise control of a hybrid electric vehicle, vehicle acceleration (a pulse phase) and vehicle deceleration (slip phase) are alternately periodically repeated while maintaining an average target speed, thereby improving fuel efficiency under real surrounding driving conditions.The present disclosure may be applied to a transmission mounted electric device (TMED) type hybrid electric vehicle in which a drive motor for driving the vehicle is disposed at the side of a transmission.In a general TMED type hybrid electric vehicle, two drive sources for driving the vehicle, that is, an internal combustion engine and a drive motor are arranged in series, an engine clutch is arranged between the internal combustion engine and the drive motor, and a transmission is arranged at the output side of the drive motor.The engine clutch serves to connect the internal combustion engine and the engine to each other so as to selectively transmit power therebetween, or cut off a connection between the internal combustion engine and the engine so as to inhibit power transmission therebetween. In a closed state of the engine clutch, the engine and the motor are connected so that power can be transmitted to drive shafts and drive wheels through the transmission.That is, the engine clutch is arranged to selectively transmit power or inhibit power transmission between the engine and the drive motor, and as is known, during a travel of the vehicle in the electric vehicle (EV) mode, the engine clutch is opened, and therefore, the vehicle is driven only by power of the drive motor, and during a travel of the vehicle in the hybrid electric vehicle (HEV) mode, the engine clutch is closed, and therefore, the vehicle is driven by powers of the engine and powers of the drive motor.Further, during braking of the vehicle or during inertial running of the vehicle, an energy regeneration mode is executed in which the drive motor is operated as a power generator for charging a battery.Further, a separate motor generator directly connected to the internal combustion engine so as to transmit power to the internal combustion engine, that is, a hybrid starter generator (HSG), is provided, and the HSG is operated using power of the battery and therefore transmits power to the internal combustion engine during the starting of the internal combustion engine and is operated as a power generator by rotational force transmitted from the internal combustion engine and therefore charges the battery during the power generation.In a general hybrid electric vehicle, various control units are provided for controlling respective devices in the vehicle.That is, a hybrid control unit (HCU), an engine intron unit (ECU) for controlling an operation of an internal combustion engine, an engine control unit (MCU) for controlling an operation of a drive motor, a transmission control unit (TCU) for controlling an operation of a transmission and an engine clutch, a battery management system (BMS) for controlling and managing a battery, etc. are provided, and control of the respective devices is performed by cooperative control between the control units under the control of the HCU serving as a highest-level control unit.For example, the TCU may actuate the clutch and control the hydraulic pressure in response to a control command from the HCU, and thus the engine clutch may close or open.In the present disclosure, such cooperative control between the control units may be performed during vehicle speed control processes in the respective modes during automatic cruise control, and operations of the engine, the drive motor, the transmission, and the engine clutch are controlled by the respective control units.Although the above description mentions a plurality of control units for controlling respective devices in the vehicle, an integrated control unit may be used instead of the control units, and in the description, both the control units and the integrated control element are commonly referred to as control units.First, a cruise control automatic mode according to the present disclosure includes a PnG mode that is executed by turning on the PnG mode under the condition that a driver turns on the cruise control automatic mode by setting a target vehicle speed, and the PnG mode has a plurality of sub-divided driving modes that can be selected based on vehicle state information such as a state of charge (SoC) of a battery, a vehicle acceleration, etc.That is, the PnG mode according to the present disclosure may include a plurality of sub-divided color modes, that is, a PnG constant speed cruise mode (PnG-constant), a PnG swing mode (PnG-swing), and a trade-off PnG mode (compressed PnG).Here, the PnG swing mode (PnG swing) may be divided into a first PnG swing mode (PnG swing ideal) corresponding to an ideal driving mode in which vehicle dynamic characteristics and a transient state (transient state) are not reflected and are considered, and a second PnG swing mode (PnG swing real) corresponding to a real driving mode in which the vehicle dynamic characteristics and the transient state (transient state) are reflected and are considered.For example, the PnG mode may be divided into four modes, i.e., the constant speed tempomat PnG mode (PnG-constant), the first PnG swing mode (PnG-swing ideal), the second PnG swing mode (PnG-swing real), and the compromised PnG mode (compressed PnG).Since the first PnG swing mode (PnG swing ideal) is an ideal driving mode in which the vehicle dynamic characteristics and the transient state are not reflected and considered, the first PnG swing mode (PnG swing ideal) is not actually adopted as the PnG mode in the present disclosure. Hereinafter, the PnG swing mode (PnG swing) denotes the second PnG swing mode (PnG swing real).In summary, the PnG mode of the present disclosure may include three driving modes, that is, the PnG constant speed template mode (PnG-constant) in which the vehicle is driven while a target vehicle speed set by the driver is constantly maintained, the PnG swing mode (PnG-swing) in which vehicle acceleration (the pulse phase) and deceleration (the slip phase) are alternately periodically repeated, and, in the slip phase, the transmission is in the neutral position with the engine clutch opened and idling of the vehicle is performed in the fuel cut state of the engine (driving of the vehicle by rotating the vehicle), and the compromised PnG mode (compromised PnG) in which vehicle acceleration (the pulse phase) and deceleration (the slide phase) are alternately repeated periodically, and, in the slide phase, deceleration of the vehicle is performed along a speed profile set by inertia of the vehicle and power of the drive motor.Hereinafter, the PnG swing mode is referred to as a first PnG mode, the trade-off PnG mode is referred to as a second PnG mode, and the constant speed template PnG mode is referred to as a third PnG mode.FIG. 4 is a graph showing cruise driving states ("cruise driving states") in the respective PnG modes of a hybrid electric vehicle according to the present disclosure.In the third PnG mode (PnG-constant), a constant speed general travel of the hybrid electric vehicle is performed, and a target vehicle speed set by a driver is constantly maintained.Since a constant vehicle speed is maintained in the third PnG mode (PnG-const), the third PnG mode (PnG-const) is a drive mode having the highest drivability, and in order to maintain a constant vehicle speed, a general constant speed travel control of the hybrid electric vehicle is executed as described with reference to FIG. 3.In the third PnG mode (PnG-constant), hybrid power of the engine and the drive motor is used under the condition that the engine clutch is closed, and driving control tracking (OOL) of the OOL is executed (the OOL driving strategy is maintained).While maintaining a constant speed during a constant speed running drive of an internal combustion engine vehicle, an operating point at which a required torque may be satisfied is determined as an engine operating point regardless of the OOL, during a general constant speed running drive of a hybrid electric vehicle, an operating point on the OOL is determined as an engine operating point, and an electrically driven system including a drive motor is partially employed.Therefore, in the third PnG mode (PnG-constant), a decrease in efficiency due to a loss in the electrically driven system and charging / discharging occurs, but a desired load may be satisfied in a wide speed range.Next, in the first PnG mode (PnG swing) and the second PnG mode (compressed PnG), a driving pattern is set to alternately repeat vehicle acceleration (pulse phase) and deceleration (slip phase). The first PnG mode (PnG swing) and the second PnG mode (compressed PnG) are different in terms of control of the pulse phase and the slip phase.In more detail, the first PnG mode (PnG swing) and the second PnG mode (compressed PnG) are the same in that a desired power of the pulse phase is increased so as to perform vehicle acceleration.Further, in the pulse phase of the first PnG mode (PnG swing), only power of the internal combustion engine is used for accelerating the vehicle and driving the motor, assist (discharge) and regeneration of the motor are not performed.Therefore, in the pulse phase of the first PnG mode (PnG swing), the electrically driven system is not used, and therefore, a loss due to the electrically driven system does not occur during charging / discharging.Further, in the first PnG mode (PnG-swing) pulse phase, an operating point on the OOL is determined as an engine operating point, but in the second PnG mode (compressed PnG), an optimum operating point on a brake specific fuel consumption (BSFC) map, that is, a sweet spot (hereinafter referred to as a "SWS") is determined as an engine operating point.Here, in the pulse phase of the first PnG mode (PnG swing), an engine operating point is determined so as to monitor the OOL, and an engine output and the operating point vary due to the non-use state of the electrically driven system (PE). However, in the pulse phase of the second PnG mode (compressed PnG), when the SWS is determined as an engine operating point, engine drive control is executed using the SWS as the engine operating point, and therefore the engine operating point and an engine output are fixed.In the pulse phase of the second PnG mode (compressed PnG), a part of surplus power of the internal combustion engine can be absorbed by regenerative operation of the electrically driven system using the drive motor.The SWS is an operating point having the minimum fuel consumption rate on the BSFC map expressing fuel consumption rate information in outlines, and since the BSFC has a reverse proportionality to the engine efficiency, the SWS is a point having the maximum engine efficiency of the hybrid electric vehicle.In a case of the above-described first PnG swing mode (PnG swing ideal), the SWS is determined as an engine operating point in the pulse phase, and coasting ("coasting") is performed in the sliding state under the condition that the engine is stopped and the engine clutch is opened, and therefore, the hybrid electric vehicle can be driven at an operating point with the theoretically highest efficiency.Such a first PnG swing mode (PnG swing ideal) corresponds to an ideal driving state in which vehicle dynamic characteristics and a transient state are not taken into account, and a vehicle speed variation is relatively increased in a direction toward a lower power range and has a negative influence on driving characteristics ("drivability").In a case of the second PnG swing mode (PnG swing real) actually adopted as the PnG swing mode (i.e., the first PnG mode) in the present disclosure, a SWS monitoring limit due to a fixed gear shift ratio that takes into account vehicle dynamic characteristics and the transient state (transient state), and therefore, an efficiency is lowered.Since the SWS is an operating point having the minimum fuel consumption rate and the maximum engine efficiency, operating point loss (engine efficiency loss) may possibly occur in the first PnG mode (PnG-swing) in which an operating point on the OOL is determined, but optimum efficiency can be maintained within a wide range as compared with the second PnG mode (compressed PnG) in which the SWS is determined as the operating point in the pulse phase.Further, in the pulse phase of the second PnG mode (compressed PnG), the SWS having the minimum fuel consumption rate is determined as an engine operating point (the engine operating point and engine output are fixed as the SWS), and therefore, the hybrid electric vehicle is in a slight acceleration state in the pulse phase, that is, is relatively smoothly accelerated, and thus, with a relatively small degree of acceleration, as compared with in the first PnG mode (PnG-swing) in which an engine operating point is determined, so as to simulate the OOL (the operating point varies along the OOL and an engine output varies).The above state is the same in the sliding phase, which will be described later, in the second PnG mode (compressed PnG), wherein the hybrid electric vehicle is in a light deceleration state, that is, is relatively smoothly decelerated, and has a relatively small degree of deceleration compared to that in the first PnG mode (PnG-swing).The slip phases of the first PnG mode (PnG-swing) and the second PnG mode (compressed PnG) are the same in that the engine is stopped in the fuel cut state and the engine clutch is opened to decelerate the vehicle.In more detail, in the sliding phase of the first PnG mode (PnG-swing), the vehicle drive source does not generate power (the internal combustion engine is stopped in the fuel cut state), coasting of the vehicle is executed only by inertia, so that the vehicle is decelerated with the drive motor not generating power, and therefore, electric power is not consumed for driving the vehicle.Here, since the engine clutch is opened, the transmission is in the neutral position, regeneration is not performed, and the electrically driven system is not used.In both the pulse phase and the sliding phase of the first PnG mode (PnG-swing), the electrically driven system with the drive motor is not used, and therefore, a loss due to the electrically driven system does not occur.On the other hand, in the sliding phase of the second PnG mode (compressed PnG), torque assist of the drive motor is performed, so that the travel range of the vehicle during deceleration can be increased by consuming a small amount of energy in the vehicle, unlike in the sliding phase of the first PnG mode (PnG-swing).Specifically, during deceleration of the second PnG mode (compressed PnG), power of the drive motor is transmitted to the drive shafts and is transmitted to the drive wheels through the transmission (the transmission is controlled in the in-gear state ("in-gear state")), and therefore, the vehicle is decelerated with a gentle deceleration gradient (i.e., a lower deceleration rate) compared to during deceleration of the vehicle in the first PnG mode (PnG-swing).For example, unlike decelerating the first PnG mode (PnG-swing) in which the vehicle is driven only by inertia during deceleration of the second PnG mode (compressed PnG), a specified amount of required torque is generated so as to control the vehicle speed during deceleration, and the engine performs torque assist identical to the amount of desired torque, thus increasing a range (driving range).An engine torque assist in which the engine generates and outputs driving force corresponding to a torque assist amount by the engine and the vehicle is decelerated by a force obtained by adding the driving force of the engine (i.e., torque assist force) to the inertial force of the vehicle, and therefore the vehicle is decelerated at a slow deceleration rate by the torque assist force by the engine employed in the deceleration state as compared to during deceleration of the vehicle in the first PnG mode (PnG-swing).Torque assist in the slip phase means not accelerating the vehicle by torque assist, but using the engine output so as to decelerate the vehicle using a speed profile with a smoother deceleration gradient, as compared with the slip phase in which vehicle deceleration is performed only by inertia.This results in vehicle deceleration in the second PnG mode (compressed PnG) to an energy consuming vehicle as compared to vehicle deceleration in the first PnG mode (PnG-swing), but has an increased driving range and excellent driving characteristics.Therefore, the second PnG mode (PnG compressed PnG) can be referred to as a mode in which a trade-off is made between driving power of the first PnG mode (PnG swing) and driving power of the third PnG mode (PnG-constant), and in the second PnG mode (PnG compressed PnG), both high efficiency of the first PnG mode (PnG swing) and excellent driving characteristics of the third PnG mode (PnG-constant) can be partially achieved.Accordingly, in the sliding phase of the second PnG mode (compressed PnG), the vehicle does not maintain a vehicle speed that is as high as in the third PnG mode (PnG-constant), but is not decelerated as much as in the first PnG mode (PnGswing).Further, even in the pulse phase of the second PnG mode (compressed PnG), a part of an engine output is converted into electric energy by engine regeneration and stored in the battery, and thereby the vehicle does not maintain a vehicle speed as high as in the third PnG mode (PnG-constant), but is not accelerated as much as in the first PnG mode (PnG-swing).As for running characteristics, the third PnG mode (PnG-constant) in which the vehicle maintains a constant vehicle speed has the highest running characteristics ("highest drivability"), and the second PnG mode (compressed PnG) in which the vehicle is accelerated and decelerated at a relatively smooth rate in the pulse phase and slide phase, has a higher running characteristic than the first PnG mode (PnG-swing) in which the vehicle is rapidly accelerated and decelerated in the pulse phase and slide phase.In the present disclosure, autotempomat travel control according to any one of the above three modes, that is, the third PnG mode (PnG-constant), the first PnG mode (PnG-swing), and the second PnG mode (compressed PnG) is performed by mode selection by a driver, and a control unit 20 performs predetermined control of respective devices in the vehicle depending on each mode.FIG. 5 is a block diagram showing the configuration of an automatic cruise control system of a hybrid electric vehicle according to the present disclosure, and FIG. 6 is a flowchart showing an auto cruise control process of a hybrid electric vehicle according to the present disclosure.Referring to FIGS. 5 and 6, an auto cruise control process (auto cruise control process) will be described. When a driver sets a target vehicle speed through a user interface (UBI) device 10 and then turns on the PnG mode (steps S 11 and S 12) to execute any of the above-described modes divided from the PnG mode, the control unit 20 performs control of an internal combustion engine 31, a drive motor 32, an internal combustion clutch 33, a transmission 34, etc., performs control of fuel supply to the internal combustion engine 31 (including fuel cut), control of closing or opening of the internal combustion clutch 33, control of gear position of the transmission 34 (including the neutral position), etc., for example.Basically, running of the vehicle in the PnG mode is performed on the condition that the driver turns on both the automatic cruise control mode and the PnG mode. The auto cruise control mode may be turned on by setting a target vehicle speed by the driver (cruise control). That is, an operation of an automatic cruise control by the driver is selected, and the control unit 20 recognizes a signal from the UI device 10 depending on an operation by a driver, and thus recognizes that the automatic cruise control function is turned on by the driver.Further, the PnG mode may also be turned on by the driver operating a user interface (UI) 10 in the vehicle, such as a switch (PnG "On"). That is, an operation of the PnG mode control by the driver is selected, and the control unit 20 receives a signal from the UI device 10 depending on an operation by the driver, and thus recognizes that the PnG function has been turned on by the driver.Of course, in the vehicle, the UI device 10 or operations for turning on / off the automatic cruise control function should be distinguished from the UI device 10 or operation for turning on / off the PnG function.As described above, when the driver sets a target vehicle speed, the control unit 20 determines a threshold target upper vehicle speed ("target vehicle speed+a" in FIG. 4 ) and a threshold target lower vehicle speed ("target vehicle speed-a" in FIG. 4 ) and controls the vehicle to be accelerated and decelerated between the threshold target upper vehicle speed and the threshold target lower vehicle speed in the first PnG mode (PnG-swing) and the second PnG mode (compressed PnG) described later (with reference to FIG. 4 ).Here, "a)" for determining the upper threshold target vehicle speed and the lower threshold target vehicle speed from the target vehicle speed set by the driver takes a predetermined value.Further, when the PnG mode is not turned on or completion conditions of the PnG mode are maintained under the condition that the automatic cruise control mode is turned on, a known general constant speed cruise control mode of hybrid electric vehicles, that is, a general constant speed drive control in which the vehicle maintains a target vehicle speed set by the driver is executed (step S21).When the termination conditions (terminating conditions) of the PnG mode are released under the condition that the automatic cruise control mode is turned on and the PnG mode is turned on, the control unit 20 confirms whether the current SOC of the battery is within a set range (S 13), and when the current SOC of the battery deviates from the set range, travel of the vehicle is controlled in the third PnG mode (step S 21).The third PnG mode under the condition that the PnG mode is turned on is the same as the general constant speed cruise control mode of hybrid electric vehicles in that a general constant speed drive control is executed in which the vehicle keeps a target vehicle speed set by the driver.When the current SOC of the battery is within the set range in step S 13, the control unit 20 selects the first PnG mode (step S 14), and the drive of the vehicle is controlled in the first PnG mode.When certain PnG ending conditions (including a driver turning off the PnG mode) are satisfied during the travel of the vehicle in the first PnG mode, the vehicle switches to the general constant speed cruise control mode (steps S 15 and S 21).Further, during traveling of the vehicle in the first PnG mode, the control unit 20 continues checking whether or not the vehicle needs to shift in the second PnG mode based on the current vehicle acceleration |v̇ x| ( step S 16).Here, the acceleration includes a degree of deceleration of the vehicle in the slide phase, an acceleration of the vehicle during deceleration, that is, an acceleration of the vehicle in the slide phase is defined to have a negative value, the value of |vdv x|, which is expressed by the absolute value, includes a degree of deceleration of the vehicle, and the degree of deceleration of the vehicle increases as the absolute value increases.Here, the control unit 20 compares the current vehicle acceleration |vdv x| with a predetermined threshold (step S 16). When the current vehicle acceleration |vdc x| is greater than the threshold value, the control unit 20 switches the vehicle to the second PnG mode on condition that the SOC of the battery is within a predetermined range (steps S 17 and S 18), and then controls the vehicle to be driven to the second PnG mode.Further, when the specified PnG termination conditions (including the driver turning off the PnG mode) are satisfied during traveling of the vehicle in the second PnG mode, the vehicle switches to the general constant speed cruise mode (steps S 19 and S 21).Further, during traveling of the vehicle in the second PnG mode, the control unit 20 continues checking whether or not the vehicle needs to be switched to the first PnG mode based on the current vehicle acceleration |v̇ x| ( step S 20).That is, the control unit 20 compares the current vehicle acceleration |vdv x| with a predetermined threshold value (step S 20). When the current vehicle acceleration |vdc x| is less than the threshold value, the control unit 20 switches the vehicle to the first PnG mode on condition that the SOC of the battery is within a predetermined range (steps S 13 and S 14), and then controls the vehicle to be driven in the first PnG mode.In the above-described control process according to the present disclosure, the vehicle acceleration may be obtained from wheel speed information detected by a sensor.In the control process of FIG. 6, |vdv denotes x| the current vehicle acceleration, the threshold is predetermined, and the threshold in the pulse phase and the threshold in the slide phase may be set identically or differently.Further, the threshold for switching from the first PnG mode to the second PnG mode and the threshold for switching from the second PnG mode to the first PnG mode may be set identically or differently.Further, the threshold value may be variably set depending on a vehicle speed.As such, the acceleration threshold for mode switching between the first PnG mode and the second PnG mode is predetermined.Further, according to the present disclosure, even when a fuel efficiency optimization strategy is employed, driving characteristics depending on loads can be satisfied. Therefore, the first PnG mode is preferably executed before the second PnG mode, although the driver gives preference to the driving characteristics.Further, in each mode, the SOC state, the PnG termination conditions, and the acceleration value are continuously monitored, and when the current acceleration value reaches each threshold value set by mode switching, the mode switching between the first PnG mode and the second PnG mode is executed.Further, in any mode, when the battery SOC deviates from a normal range or when the PnG termination conditions are satisfied, mode switching is performed in the constant speed cruise mode.FIGS. 7( a) and 7( b) are graphs exemplifying a real vehicle running state depending on an auto cruise control method of a hybrid electric vehicle according to the present disclosure, that is, showing a vehicle running state when mode switching is executed based on vehicle acceleration in the process of FIG. 6.FIG. 7( a) is a graph exemplarily showing mode switching between the first PnG mode and the second PnG mode based on acceleration as in the control process shown in FIG. 6, and FIG. 7( b) is a graph exemplarily showing driving of the vehicle using only the first PnG motor without mode switching.Referring to FIGS. 7( a) and 7( b), when the first PnG mode and the second PnG mode are properly employed together to execute mode switching based on acceleration according to the present disclosure, correct vehicle acceleration can be maintained despite disturbance such as a road surface gradient as exemplarily shown in FIG. 7( a), thus contributing to securing driving characteristics.On the other hand, while only the first PnG mode is employed, vehicle acceleration is greatly varied depending on disturbances such as road surface engraving as exemplarily shown in FIG. 7( b), and hence driving characteristics are lowered.FIGS. 8 and 9 are graphs exemplifying vehicle speed variations depending on loads during control in the second PnG mode according to the present disclosure. The ultimate reason why the PnG mode is employed is to achieve an improvement in fuel efficiency even if some driving characteristics are sacrificed.Here, a decrease in driving characteristics means that although a driver wants to drive a vehicle at a constant speed, the vehicle is accelerated or decelerated.On the other hand, it is understood that excellent drivability is achieved when the vehicle is driven at a constant speed and therefore the acceleration of the vehicle is maintained at zero.Therefore, a drivability ("drivability") of the vehicle can be determined from how much the degree of the absolute value of the vehicle acceleration deviates from zero. As the absolute value of the acceleration increases, a drivability of the vehicle is decreased, and when the acceleration is maintained at zero, a drivability of the vehicle is improved.When, instead of traveling in the first PnG mode for improving fuel efficiency, traveling of a PnG mode is requested with a second strategy for traveling property, control for preventing vehicle acceleration from deviating from a specified range is desired, and such control is referred to as an acceleration-based PnG strategy.Referring to FIG. 8, under low speed conditions in which the running load of the vehicle is low, relatively high acceleration will occur in the pulse phase rather than the slide phase, and thus running characteristic is reduced. Therefore, the acceleration is limited by lowering the output in the pulse phase by regenerative braking, and hence a drivability is secured.On the other hand, under high-speed conditions in which the driving load of the vehicle is high, relatively large deceleration occurs in the slide phase instead of the pulse phase, and thus a drivability is reduced. Therefore, deceleration is reduced by compensating for output in the slide phase by motor assist, and thus driving performance is secured.FIG. 10 is a graph illustrating a comparison between the respective modes according to the present disclosure. In FIG. 10, the X axis denotes power, and the Y axis denotes efficiency.In the hybrid electric vehicle, a point having the maximum engine efficiency is referred to as a sweet spot SWS, and such sweet spot SWS represents the optimum operating point on the BSFC map.Around the first PnG swing mode (PnG swing ideal) that is an ideal driving mode, an engine operating point is located at the sweet spot SWS in the pulse phase, and the engine is stopped in the slip phase, and therefore, the vehicle can be theoretically driven with the improved efficiency.Here, since the vehicle dynamic characteristics and the transient state are not taken into account, a variation width of the vehicle speed is relatively rapidly increased in the direction toward a lower power range, and thus a drivability of the vehicle is adversely affected.On the other hand, in the second PnG swing mode (PnG swing ideal) representing a real drive mode, there is a sweet spot tracking limit due to a fixed gear ratio, and the vehicle dynamic characteristics and the transition state are considered, and therefore, efficiency is lowered.In the PnG constant speed cruise mode (i.e., the third PnG mode) (PnG-constant), an operating point is located on the OOL depending on the HEV propulsion strategy. Here, a power transmission efficiency is determined depending on a power distribution to the internal combustion engine and the drive motor, and a power used for performing charge / discharge results in efficiency reduction.The trade-off PnG mode (i.e., the second PnG mode) (compressed PnG) is a mode in which trade-off is made between the driving strategies of the PnG swing mode (i.e., the first PnG mode) (PnG swing) and the PnG constant speed template mode (PnG-constant), optimum acceleration and driving characteristic can be obtained using engine regeneration and engine assist depending on vehicle loads or vehicle speed conditions in the pulse phase and the slide phase, and particularly in the slide phase, a part of the engine assist torque (assist torque corresponding to the required torque) is generated, and therefore a driving range is extended.That is, a part of driving electric power that can be completely stored during coasting can be directly used in the sliding phase, and thus disadvantages generated by lowering a circulation efficiency of driving electric power can be supplemented.Therefore, in the compromised PnG mode (compressed PnG), a vehicle speed is not maintained as high as in the PnG constant speed cruise mode (PnG-constant), but acceleration and deceleration are not performed as much as in the PnG swing mode (PnG-swing).Accordingly, by such a trade-off strategy, both a high efficiency corresponding to the benefit of the PnG swing mode (PnG swing) and a high drivability corresponding to the benefit of the PnG constant speed template mode (PnG-constant) can be partially realized.As will be apparent from the above description, an autotempomat control method according to the present disclosure substitutes for a PnG drive pattern in which consideration of characteristics of hybrid electric vehicles takes place, and thus may improve fuel efficiency.Further, in the auto cruise control method according to the present disclosure, the PnG mode may be divided into a PnG constant speed template mode, a PnG swing mode, and a trade-off PnG mode, so that a vehicle can be driven in a selected mode more advantageous in terms of fuel efficiency and driving characteristics according to vehicle conditions such as a battery SOC, acceleration, etc., and driving of the vehicle in the trade-off PnG mode is enabled so as to satisfy both driving characteristics and improvement in fuel efficiency.Moreover, proper mode switching between the PnG swing mode and the trade-off PnG mode is performed depending on vehicle acceleration, thereby providing both improved driving characteristics and improvement in fuel efficiency.The description of the disclosure is merely exemplary in nature and, therefore, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.

Claims

A cruise control method for a hybrid electric vehicle, comprising: turning on a cruise control mode in the hybrid electric vehicle, the hybrid electric vehicle using an internal combustion engine (31) and a drive motor (32) as vehicle driving sources; selecting a selected mode in a pulse and slide (PnG) mode depending on vehicle state information, the selected mode being selected between a first PnG mode and a second PnG mode; and executing control of the hybrid electric vehicle with the selected mode, wherein: in the first PnG mode, driving of the hybrid electric vehicle is executed in a slide phase by inertia of the hybrid electric vehicle, wherein a pulse phase and the slide phase are alternately repeated between a predetermined and upper threshold of a vehicle speed and a predetermined lower threshold of the vehicle speed; and in the second PnG mode, acceleration of the hybrid electric vehicle is performed in the pulse phase by the engine (31) or both the engine (31) and the drive motor (32), and deceleration of the hybrid electric vehicle is performed in the slip phase by inertia of the hybrid electric vehicle and torque assist of the drive motor, wherein the pulse phase and the slip phase are alternately repeated between the predetermined upper threshold of the vehicle speed and the predetermined lower threshold of the vehicle speed, wherein the vehicle state information is an absolute value of vehicle acceleration, and when the absolute value of the vehicle acceleration is greater than a predetermined threshold in the first PnG mode, the first PnG mode switches to the second PnG mode, wherein vehicle acceleration is an acceleration or deceleration level of the hybrid electric vehicle.The cruise control method for the hybrid electric vehicle according to claim 1, wherein the predetermined upper threshold value of the vehicle speed is set to an upper value obtained by adding a value "a)" to a target vehicle speed, and the predetermined lower threshold value of the vehicle speed is set to a lower value obtained by subtracting the value "a" from the target vehicle speed, the value "a)" being predetermined.The cruise control method for the hybrid electric vehicle according to claim 1 or 2, wherein the PnG mode further comprises a third PnG mode for constantly maintaining the target vehicle speed using the vehicle driving sources, and wherein a selected PnG mode is selected depending on the vehicle state information, and the control of the hybrid electric vehicle is executed with the selected PnG mode, wherein the selected PnG mode is selected from the first PnG mode, the second PnG mode, and the third PnG mode.The cruise control method for the hybrid electric vehicle according to claim 3, wherein in the third PnG mode, an engine operating point is determined to follow an engine optimum operating line (OOL), and an operation or a regeneration of the drive motor is controlled, wherein the hybrid electric vehicle maintains the target vehicle speed while the engine (31) is operated at an optimum operating point of the OOL.The cruise control method for the hybrid electric vehicle according to claim 3 or 4, wherein when the state of charge (SOC) of a battery as the vehicle state information deviates from a predetermined range, the third PnG mode is selected, and the control of the hybrid electric vehicle is executed to constantly maintain the target vehicle speed.The cruise control method for the hybrid electric vehicle according to any one of claims 3 to 5, wherein, when the PnG mode is not turned on after the cruise control mode is turned on by the driver setting the target vehicle speed, the third PnG mode is selected, and the control of the hybrid electric vehicle is executed to constantly maintain the target vehicle speed.The cruise control method for the hybrid electric vehicle according to any one of claims 3 to 6, wherein a predetermined PnG completion condition including a PnG off mode is satisfied while the PnG mode is turned on and the cruise control mode is turned on by setting the target vehicle speed by the driver, selecting the third PnG mode, and executing the control of the hybrid electric vehicle to constantly maintain the target vehicle speed.The cruise control method for the hybrid electric vehicle according to any one of the preceding claims, wherein, in the sliding phase of the second PnG mode, generation of power from the drive motor is controlled to decelerate the hybrid electric vehicle with a gentle deceleration gradient compared to the sliding phase of the first PnG mode.The cruise control method for the hybrid electric vehicle according to any preceding claim, wherein, in the sliding phase of the first PnG mode, an engine clutch (33) is decoupled, a transmission (34) is in a neutral position, and a fuel cut state of the engine is maintained, the engine clutch (33) being disposed between the engine (31) and the drive motor (32).The cruise control method for the hybrid electric vehicle according to any preceding claim, wherein, in the slip phase of the second PnG mode, the engine clutch (33) is disengaged, a transmission (34) is in an in-gear state, and the engine fuel cut state is maintained.The cruise control method for the hybrid electric vehicle according to any one of the preceding claims, wherein, in the pulse phase of the second PnG mode, the internal combustion engine is controlled or both the internal combustion engine (31) and the drive motor are controlled to accelerate the hybrid electric vehicle with a gentle acceleration gradient as compared to the pulse phase of the first PnG mode.The cruise control method for the hybrid electric vehicle according to any preceding claim, wherein, in the pulse phase of the first PnG mode, the engine operating point is determined to simulate an engine optimum operating line (OOL), the engine (31) being controlled to be operated at an optimum operating point of the OOL.The cruise control method for the hybrid electric vehicle according to any preceding claim, wherein, in the pulse phase of the first PnG mode, the hybrid electric vehicle is accelerated only by power of the engine (31) without using the drive motor, the engine clutch (33) is engaged, and a transmission (34) is in the gear-engaged state.The cruise control method for the hybrid electric vehicle according to any one of the preceding claims, wherein, in the pulse phase of the second PnG mode, a sweet spot is determined as the engine operating point, and an operation of the engine (31) is controlled accordingly, the sweet spot being an operating point with a minimum fuel consumption rate on a brake specific fuel consumption (BSFC) map.The cruise control method for the hybrid electric vehicle according to claim 14, wherein, in the pulse phase of the second PnG mode, simultaneously controlling the operation of the engine (31) and the operation or regeneration of the drive motor (32) for maintaining the target vehicle speed are performed, the operation of the engine (31) being controlled by determining the sweet spot as the engine operating point.The cruise control method for the hybrid electric vehicle according to any preceding claim, wherein the vehicle state information is an absolute value of vehicle acceleration, and when the absolute value of the vehicle acceleration is less than a predetermined threshold in the second PnG mode, the second PnG mode switches to the first PnG mode.The cruise control method for the hybrid electric vehicle according to claim 15 or 16, wherein the predetermined threshold is set in advance depending on the vehicle speed, and the predetermined threshold is updated depending on the current vehicle speed.

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