Patent Publication Number: US-9409566-B2

Title: Hybrid vehicle and associated control method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 13/539,945, now U.S. Pat. No. 8,834,317, filed Jul. 2, 2012, the disclosure of which is incorporated in its entirety by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to controlling the engine speed and combined output torque of a hybrid vehicle in response to driver inputs. 
     BACKGROUND 
     In a vehicle having a discrete ratio transmission, the speed of the transmission input shaft is constrained to be proportional to the vehicle speed with a finite set of ratios, except during the brief interval while the transmission is shifting from one ratio to another ratio. When the torque converter is locked, the engine speed is also constrained to be proportional to vehicle speed. In a hybrid electric vehicle having a power-split architecture, on the other hand, the transmission does not mechanically impose a strict relationship between the engine speed and the vehicle speed. 
     Even in vehicles with automatic transmissions, in which selection of the gear ratio or engine speed is ordinarily determined by a controller, some drivers prefer to occasionally over-ride the controller to provide operation similar to a manual transmission. Some vehicles are equipped with shift paddles or other driver interface features which permit the driver to signal a desire for a higher or a lower gear ratio relative to the gear ratio automatically selected by the vehicle controller, with an associated change in engine speed and vehicle torque. In a discrete ratio transmission, the controller responds to such a command by shifting to a different one of the discrete gear ratios, which adjusts engine speed accordingly and provides associated torque multiplication at the vehicle wheels. However, in a vehicle with a continuously variable transmission or similar gearbox, such as a power-split hybrid, the response is more complicated because the transmission does not inherently provide discrete gear ratios with associated different torque multiplication. 
     SUMMARY 
     The disclosed hybrid vehicle control strategy implements four different operating modes. The driver determines which operating mode is utilized at any given time via various driver interface elements including a shift lever, a downshift selector, and an upshift selector. In two of the modes, the controller simulates the operation of a discrete ratio transmission, both with regard to the engine speed and with regard to the combined output torque of the engine and the traction motors. The controller utilizes different logic for shutting the engine off and driving solely with electric power depending on which operating mode is active. 
     In one embodiment, a hybrid electric vehicle includes an internal combustion engine; a continuously variable transmission having an input shaft drivably connected to the internal combustion engine, the transmission including at least one traction motor; a driver operated range selector having first and second forward driving positions; driver operated upshift and downshift selectors; a driver operated accelerator pedal; and a controller. The controller operates the vehicle in multiple operating modes including i) a first mode (Normal), corresponding to the first position (D) of the range selector, ii) a second mode (Sport), corresponding to the second position (S) of the range selector, wherein the controller operates the engine and motor to provide an output torque higher than in the first mode at substantially equal accelerator pedal position and vehicle speed, iii) a third mode (LID), activated from the first mode by operation of the downshift selector, wherein the controller controls engine speed to be proportional to output speed based on a selected virtual gear number, and iv) a fourth mode (SST), activated from the second mode by operation of the upshift or downshift selectors, wherein the engine speed is controlled to be proportional to the transmission output speed, the constant of proportionality determined by the selected virtual gear number. In some embodiments, the controller, when operating in the third or fourth mode, selects a different virtual gear number based on signals from the upshift and downshift selectors or in response to changes in output speed. In some embodiments, selection of a lower virtual gear number in response to a change in output speed causes a transition from the third mode to the first mode. In some embodiments, activation of a cruise control feature or a decrease in accelerator pedal position causes a transition from the third mode to the first mode. In some embodiments, however, transitions from the third mode to the first mode are inhibited when driver workload is high. 
     In one embodiment, a method of controlling a hybrid electric vehicle having a traction motor and an engine includes, when operating in a first mode (Normal), controlling the engine and traction motor to provide an output torque than varies based on vehicle speed and accelerator pedal position, when operating in a second mode (Sport) activated from first mode by movement of a range selector, controlling the engine and traction motor to provide higher output torque than in the first mode for corresponding vehicle speed and accelerator pedal position, and when operating in a third mode (LID) activated from the first mode by operation of a downshift selector, controlling the engine and traction motor such that the engine speed is proportional to vehicle speed. In some embodiments, operating in the first mode includes automatically starting and stopping the engine such that the engine is not running for a significant portion of the time that the vehicle is in motion. In some embodiments, operating in the second mode includes automatically starting and stopping the engine such that the engine is stopped only when the vehicle is stopped and is restarted as soon as the brake pedal is released and controlling the engine and traction motor such that, when the engine is running, the engine speed is higher than it would be in the first mode for substantially identical vehicle speed and accelerator pedal position. In some embodiments, operating in the third mode includes computing an initial value of a virtual gear number upon entering the third mode, decrementing the virtual gear number in response to operation of the downshift selector, incrementing the virtual gear number in response to operation of an upshift selector, and controlling the engine and traction motor such that the engine speed is proportional to the vehicle speed with a constant of proportionality determined by the virtual gear number, the engine speed lower for higher values of the virtual gear number. Operating in the third mode can also include, in response to activation of a cruise control feature or decrease in accelerator pedal position that exceeds an associated threshold, computing a metric representative of driver workload and, if the metric is below an associated workload threshold, transitioning to the first mode. In some embodiments, a transition from the third mode to the first mode occurs when the vehicle speed decreases below a threshold value. In some embodiments, operating in the second mode includes transitioning to a fourth mode in response to operation of the downshift selector or an upshift selector. Operating in the fourth mode can include computing an initial value of a virtual gear number upon entering the fourth mode, decrementing the virtual gear number in response to operation of the downshift selector, incrementing the virtual gear number in response to operation of an upshift selector, and controlling the engine and traction motor such that the output torque approximates a function of vehicle speed, accelerator pedal position, and virtual gear number, the output torque lower for higher values of virtual gear number at substantially equal vehicle speed and accelerator pedal position and the engine speed is proportional to the vehicle speed with a constant of proportionality determined by the virtual gear number, the engine speed lower for higher values of the virtual gear number. Operating in the fourth mode can include transitioning to the second mode when the upshift selector is held for a period longer than a threshold value. 
     Embodiments according to the present disclosure can also include a controller for a hybrid powertrain, wherein the controller sends control signals to an engine and at least one traction motor; the controller responds to signals from a range selector with at least two forward driving positions, an upshift selector, a downshift selector, and an accelerator pedal; the controller operates in a first mode (Normal) when the range selector is moved to a first forward driving position; the controller transitions from the first mode to a second mode (Sport) when the range selector is moved to a second forward driving position, the controller operating the engine and the at least one traction motor to deliver a higher output torque in the second mode than in the first mode; and the controller transitions from the first mode to a third mode (LID) when the downshift selector is operated, the controller operating the engine and the at least one traction motor in the third mode such that the engine speed simulates the operation of a discrete ratio transmission. In some embodiments, the controller transitions from the second mode to a fourth mode (SST) when the upshift or downshift selectors are operated, the controller operating the engine and the at least one fraction motor in the fourth mode such that the engine speed and output torque simulates the operation of a discrete ratio transmission. 
     Various embodiments according to the present disclosure can provide one or more advantages. For example, systems and methods for controlling a hybrid vehicle according to the present disclosure mimic or emulate a manual or select shift mode of an automatic step-ratio transmission in a hybrid vehicle having a continuously variable transmission or similar gearbox. In addition, various strategies of the present disclosure provide drivers of hybrid vehicles more interactive controls to manually command powertrain speed and acceleration to provide enhanced luxury features and a sporty feel. 
     The above advantages and other advantages and features will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a vehicle powertrain, controller, and user interface features of a representative embodiment of a hybrid vehicle according to the present disclosure; 
         FIG. 2  is a state transition chart illustrating operation of a system or method of an embodiment of the present disclosure; 
         FIG. 3  is a flow chart illustrating operation of a system or method according to various embodiments when in a Normal operating mode; 
         FIG. 4  is a graph illustrating a relationship between vehicle speed, accelerator pedal position, and wheel torque command of a representative embodiment according to the disclosure; 
         FIG. 5  is a flow chart illustrating operation of a system or method according to various embodiments when in the Live-In-Drive (LID) operating mode; 
         FIG. 6  is a graph illustrating a relationship between vehicle speed, virtual gear number, and engine speed of a representative embodiment according to the disclosure; 
         FIG. 7  is a graph illustrating a relationship between actual accelerator pedal position, virtual gear number or operating mode, and modified pedal position of a representative embodiment according to the disclosure; 
         FIG. 8  is flow chart illustrating the selection of an initial virtual gear number when transitioning into LID or Select Shift Transmission (SST) operating modes of one embodiment of the disclosure; 
         FIG. 9  is flow chart illustrating operation of a system or method according to embodiments of the disclosure when in the Sport operating mode; 
         FIG. 10  is a flow chart illustrating operation of a strategy for shutting off and restarting the engine in certain operating modes of various embodiments of the disclosure; and 
         FIG. 11  is a flow chart illustrating operation of a system or method when in the SST operating mode according to various embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     A powertrain for a hybrid electric vehicle is illustrated schematically in  FIG. 1 . The powertrain includes an internal combustion engine  20  driveably connected to a planet carrier  22 , a generator  24  driveably connected to a sun gear  26 , and an output shaft  28  driveably connected to a ring gear  30 . Elements are driveably connected when there is a mechanical power flow path between them such that the speeds of the elements are constrained to be substantially proportional. Planet carrier  22  supports a set of planet gears  32  such that each planet gear is in continuous meshing engagement with sun gear  26  and ring gear  30 . Output shaft  28  drives the vehicle wheels directly or indirectly, such as via a differential assembly, for example. 
     Traction motor  34  is driveably connected to the output shaft  28 . Both the generator  24  and the traction motor  34  are reversible electrical machines that are capable of converting electrical power into rotational mechanical power or converting rotational mechanical power into electrical power. The terms generator and motor should be regarded merely as labels for ease of description and does not limit the function or operation of either electrical machine. Generator  24  and fraction motor  34  are both electrically connected to battery  36 . 
     The rotational speed of sun gear  26 , carrier  22 , and ring gear  30  are linearly related such that speed of carrier  22  is a weighted average of the speed of sun gear  26  and ring gear  30 . Consequently, the speed of the engine  20  is not constrained to be proportional to the speed of the output shaft  28  in this arrangement. Instead, the engine speed can be selected or controlled independently of the vehicle speed by setting the generator speed accordingly. Power flows from the engine to the output shaft through a combination of mechanical power transfer and electrical power transfer. During some operating conditions, the engine  20  can generate more power than what is delivered to the output shaft  28  with the difference, neglecting efficiency losses, delivered to battery  36 . Under other operating conditions, the battery  36  in combination with generator  24  and/or traction motor  34  can supplement the power delivered by the engine  20  such that more power is delivered to the output shaft  28 . 
     The engine  20 , generator  24 , and traction motor  34 , all respond to control signals from controller  38 . These control signals determine the amount of torque generated. The controller also receives speed signals from the engine  20 , generator  24 , and traction motor  34  and a state of charge signal from battery  36 . The controller accepts input signals indicating driver intention from a brake pedal  40 , an accelerator pedal  42 , a shift lever  44 , a steering wheel  46 , a downshift selector  48 , an upshift selector  50 , and a cruise control button  51 . Shift lever  44  allows the driver to select Park, Reverse, Neutral, Drive, and Sport driving modes. 
     The top level control states are illustrated in  FIG. 2 . The controller starts in state  60  and transitions to Normal mode  62  as soon as the driver selects the Drive (D) position using shift lever  44 . Operation in Normal mode is illustrated by the flow diagram of  FIG. 3 . In Normal mode, the controller repeatedly performs the operations of setting the output torque  66 , setting the engine mode  68 , and setting the engine speed  70 . In Normal mode, the target output torque is calculated at step  66  based on accelerator pedal position and vehicle speed using a table such as that illustrated in  FIG. 4 . Vehicle speed can be calculated from traction motor speed or wheel speed sensors. Engine mode is set to either running or stopped at step  68  using a variety of input signals including battery state of charge, output power command, accelerator pedal position, and vehicle speed. If the engine mode is running, a target engine speed is calculated to minimize fuel consumption while delivering the desired output torque and maintaining the battery at a desired state of charge. In this Continuously Variable Transmission (CVT) mode, the engine speed varies continuously, as opposed to varying in discrete steps, in response to changes in accelerator pedal position and vehicle speed. Finally, operating parameters of the engine, generator, and traction motor are adjusted such that the actual output torque and engine speed tend toward the selected targets. 
     Referring again to  FIG. 2 , the controller transitions from Normal mode  62  to Live-In-Drive (LID) mode  72  whenever the driver activates the downshift selector  48 . LID mode simulates the experience of driving a vehicle with a discrete ratio transmission. Operation in LID mode is illustrated by the flow diagram of  FIG. 5 . Upon entering LID mode, the controller selects an initial virtual gear ratio at step  74  and then repeatedly performs the operations of setting the output torque at steps  76  and  66 ′, setting the engine speed at step  78 , and updating the virtual gear ratio in steps  80  and  82 . Each of these operations is discussed in additional detail below. As shown in  FIG. 2 , a number of conditions cause the controller to transition back to Normal mode  62  including vehicle speed dropping below a low threshold value or an automatically selected downshift. Additionally, a transition can be triggered when the controller detects a cruising condition, as indicated by activation of the cruise control  51 , or a tip-out condition, indicated by a reduction in accelerator pedal position, and the condition persists for some predetermined amount of time. This latter type of condition will not result in a transition, however, if the controller detects a high driver workload at step  84 , such as might be indicated by large displacements of steering wheel  46 , large yaw, pitch, or roll rates, or high longitudinal or lateral accelerations, for example. 
     As also shown in  FIG. 5 , in LID mode  72 , the engine speed is calculated in step  78  based on the vehicle speed and the virtual gear number as illustrated in  FIG. 6 . For a particular virtual gear number (1 st  through 8 th  in the representative embodiment illustrated), the engine speed (We) is directly proportional to vehicle speed (V), as it would be with a step ratio transmission. However, if that fixed ratio would result in an engine speed (We) less than a minimum engine speed (We min ), the engine speed (We) is set to the minimum engine speed (We min ). Similarly, the engine speed (We) is not set higher than a maximum engine speed (We max ). Both minimum and maximum engine speeds can be a function of vehicle speed (V). 
     In step  76 , a modified accelerator pedal position is calculated from the measured accelerator pedal position using a table such as illustrated in  FIG. 7 . This modified accelerator pedal position is used in place of the actual pedal position in step  66 ′ to calculate the target output torque. The curves in  FIG. 7  are selected to simulate the output torque capability of a powertrain with a discrete ratio transmission. Specifically, as the virtual gear number (1 st  through 8 th  in this example) increases, the resulting target output torque is lower for any given non-zero accelerator pedal position. The combined effect of steps  76  and  66 ′ is operation of the engine and at least one traction motor such that combined output torque corresponds to one of a plurality of output torque functions, each output torque function having a distinct output torque at a maximum value of accelerator pedal position for an associated vehicle speed. 
     As also shown in  FIG. 5 , in step  80 , the controller checks for activations of either the upshift selector or the downshift selector and adjusts the virtual gear number accordingly. In step  82 , the controller determines if there is a need to automatically adjust the virtual gear number. In particular, an upshift can be triggered by an increase in vehicle speed. Similarly, a downshift can be indicated when vehicle speed decreases. However, as mentioned previously, the controller transitions back to Normal mode  62  when an automatic downshift is indicated. The automatic shift criteria are calibrated such that automatic changes in virtual gear number are less common than shifts in a traditional discrete ratio automatic transmission. 
     The algorithms for calculating target engine speed and target output torque both utilize the virtual gear number. Therefore, an initial virtual gear number is determined upon transitioning into LID mode. At step  74 , the controller selects an initial virtual gear number that will result in an increase in engine speed. The procedure for setting the initial virtual gear number is further illustrated in the flow chart of  FIG. 8 . In step  84 , the controller calculates We max , the maximum engine speed at the current vehicle speed, using a formula or a lookup table, for example. Next, in step  86 , the controller computes Gear min , the lowest virtual gear number for which the target engine speed would be less than We max  at the current vehicle speed. This step can be done with either an iterative algorithm or using a lookup table. Next, the controller computes the target engine speed corresponding to Gear min  at the current vehicle speed, W(V, Gear min ). In step  88 , this is compared to the current engine speed, We current . If We current  is greater than W(V, Gear min ), then the target engine speed will be restricted by maximum engine speed. Consequently, in step  90 , the target gear is set to Gear min -1 and the target engine speed is set to We max . However, in the more typical case where We current  is less than W(V, Gear min ), step  92  selects the highest virtual gear number that will result in an increase in engine speed relative to the current engine speed. 
     Referring once again to  FIG. 2 , the controller transitions from Normal mode  62  to Sport mode  94  whenever the driver moves the shift lever  44  to the Sport (S) position. Operation in Sport mode is illustrated by the flow diagram of  FIG. 9 . The controller repeatedly performs the operations of setting the output torque  96  and  66 ″, setting the engine speed  99 , and setting the engine mode  98 . To provide a more sporty reaction to accelerator pedal movements, the target output torque is computed based on a modified accelerator pedal position as illustrated by the upper heavy line  238  in  FIG. 7 . The mapping between actual accelerator pedal position and modified accelerator pedal position is selected such that the value is equal at the minimum  237  and maximum  239  values, but the modified value is higher for all intermediate levels. 
     As also shown in  FIG. 9 , target engine speed is set in step  99  using a similar algorithm to that used in Normal mode. However, the target engine speed is scaled up by a designated amount, such as 10-20% for example, relative to the value that would be used in Normal mode. Unlike the algorithm for setting engine mode used in Normal mode, the algorithm used in Sport mode as indicated at step  98  only stops the engine when the vehicle is stationary and the brake pedal is depressed. The modified engine mode setting algorithm is illustrated in  FIG. 10 . If the engine is currently stopped  100 , then the engine is restarted at step  102  if the vehicle is moving  104  or the brake pedal is released  106 . Similarly, if the engine is currently running, then the engine is stopped at step  108  only if the vehicle is stationary  110  and the brake pedal is pressed  112 . 
     If the driver activates either the upshift of downshift selector while in Sport mode  94 , the controller transitions to Select Shift Transmission (SST) mode  114 , as shown in  FIG. 2 . In SST mode, the target engine torque and target engine speed are set to simulate a discrete ratio transmission, as described with respect to LID mode. However, the controller will remain in SST mode until the driver indicates a desire to leave this mode by either holding a shift selector  48  or  50  for several seconds of by moving shift lever  44  back to the Drive (D) position. Operation in SST mode is illustrated by the flow diagram of  FIG. 11 . In SST mode, the virtual gear number is adjusted at step  80 ′ in response to activation of downshift selector  48  and upshift selector  50  in the same manner as in LID mode. In addition, the controller can automatically adjust the virtual gear number, either up or down, in response to changes in vehicle speed or accelerator pedal position. This automatic feature sets the virtual gear number to 1st gear as the vehicle comes to a stop. However, the driver can override this selection by manipulating the shift selectors while the vehicle is stationary in step  118 . In SST mode, the engine mode depends on the virtual gear number, vehicle speed, and accelerator pedal position. In step  120 , the controller calculates an engine shutdown limit, which is an accelerator pedal position below which electric drive is enabled. The shutdown limit is a function of output power demand, virtual gear number, and vehicle speed. The shutdown limits for several gear ratios at a particular vehicle speed and output power demand are illustrated by black circles in  FIG. 7 . When one of the higher virtual gear numbers, i.e. 5th-8th, is active and the accelerator pedal position is less than the shutdown limit, the normal engine mode algorithm  68 ′ of Normal mode is used. If a lower virtual gear number, i.e. 1st-4th, is active, or if the accelerator position is above the engine shutdown limit, then the more restrictive algorithm  98 ′ of Sport and LID modes is used. 
     As illustrated by the representative embodiments described above, various embodiments according to the present disclosure can provide one or more advantages, such as emulating a manual or select shift mode of an automatic step-ratio transmission in a hybrid vehicle having a continuously variable transmission or similar gearbox. In addition, various strategies of the present disclosure provide drivers of hybrid vehicles more interactive controls to manually command powertrain speed and acceleration to provide enhanced luxury features and a sporty feel. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. The embodiments described herein that are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.