Optimizing regenerative braking efficiency in hybrid vehicles

Abstract

The present disclosure relates to optimizing regenerative braking efficiency in a hybrid vehicle. A vehicle includes a motor and a controller. The controller is configured to, in response to an autonomous braking request and a magnitude of a predicted average braking torque associated with the autonomous braking request being less than a magnitude of a powertrain regenerative torque limit, brake the vehicle using only a torque curve of the motor adjusted from a most efficient torque curve of the motor and having an average value within a specified range of the average braking torque.

Description

Technical Field

[0001] The present disclosure relates to optimizing regenerative braking efficiency in a vehicle equipped with regenerative braking. More specifically, the present disclosure relates to fully autonomous vehicles or semi-autonomous vehicles that automatically activate braking events in a manner that attempts to optimize the amount of energy recovered via regenerative braking during the braking event. Background Art

[0002] A hybrid electric powertrain includes an engine and an electric motor. The torque (or power) generated by the engine and / or electric motor can be transferred through a transmission to the drive wheels to propel the vehicle. A traction battery supplies energy to the electric motor. Hybrid powertrains are also capable of regenerative braking, in which the electric motor brakes the vehicle by converting mechanical power into electricity to recharge the battery. Summary of the Invention

[0003] According to one embodiment, a vehicle includes a motor and a controller. The controller is configured to, in response to an autonomous braking request and a magnitude of a predicted average braking torque associated with the autonomous braking request being less than a magnitude of a powertrain regenerative torque limit, brake the vehicle using only a torque curve of the motor adjusted from a most efficient torque curve of the motor having an average value within a specified range of the average braking torque.

[0004] According to another embodiment, a vehicle includes a traction battery, an engine, and a stepped-ratio transmission having a plurality of discrete gear ratios. A motor is selectively connectable to the engine and / or the transmission and configured to perform regenerative braking. A controller is configured to, in response to a requested autonomous braking event, brake the vehicle using only the motor according to a torque curve having a first value for one of the plurality of discrete gear ratios and a second value for another of the plurality of discrete gear ratios, wherein the first value is derived by setting the torque of the motor to the most efficient motor torque for the one of the plurality of gear ratios and the second value is derived by setting the torque of the motor to the most efficient motor torque for the other of the plurality of gear ratios.

[0005] According to an embodiment of the present invention, the controller is further configured to generate a predicted deceleration of the vehicle for the autonomous braking event and a corresponding predicted average braking torque to achieve the deceleration.

[0006] According to an embodiment of the present invention, the first value is further obtained by comparing the most efficient motor torque for the one gear ratio with the average brake torque.

[0007] According to an embodiment of the present invention, the second value is further derived by comparing the most efficient motor torque for another gear ratio of the plurality of gear ratios with the average brake torque.

[0008] According to an embodiment of the present invention, the first value and the second value have different magnitudes.

[0009] According to an embodiment of the invention, the torque curve is in the area of the wheels of the vehicle.

[0010] According to an embodiment of the invention, one of the plurality of gear ratios is a higher gear ratio than another of the plurality of gear ratios, and a magnitude of the first value is smaller than a magnitude of the second value.

[0011] According to one embodiment of the invention, the transmission comprises a torque converter coupled to the motor.

[0012] According to yet another embodiment, a method for regenerative braking of an autonomous vehicle is provided. The method includes, in response to a predicted autonomous braking event being requested and a magnitude of a predicted average braking torque of the autonomous braking event being less than a magnitude of a powertrain regenerative torque limit, braking the vehicle using only an electric machine according to a torque curve adjusted from a most efficient torque curve of the electric machine and having an average value within a specified range of the average braking torque. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic diagram of a hybrid electric vehicle according to one embodiment.

[0014] Figure 2A is a schematic diagram of a vehicle equipped with sensors configured to detect the distance to another vehicle in front of the vehicle.

[0015] Figure 2B is a graph of the expected vehicle deceleration over the distance to safely slow or stop the vehicle.

[0016] Figure 3 A flow chart for constructing a vehicle speed profile during a braking event of a vehicle is shown.

[0017] Figure 4A and Figure 4B A flow chart illustrating an algorithm for generating a brake torque curve to be used during a braking event of a vehicle.

[0018] Figure 5 is a graph illustrating a braking event and includes a plurality of traces representing different parameters of the braking event.

[0019] Figure 6is an efficiency distribution map for a motor according to one embodiment. DETAILED DESCRIPTION

[0020] Embodiments of the present disclosure are described herein. However, it should be understood that the disclosed embodiments are merely examples and that other embodiments may take various and alternative forms. The drawings are not necessarily drawn to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, the specific structural and functional details disclosed herein should not be interpreted as limiting, but merely as a representative basis for teaching those skilled in the art to utilize the embodiments in various forms. As will be understood by those of ordinary skill in the art, the various features shown and described with reference to any one of the figures may be combined with features shown in one or more other figures to produce embodiments that are not explicitly shown or described. The combinations of features shown provide representative embodiments for typical applications. However, various combinations and variations of features consistent with the teachings of the present disclosure may be desired for specific applications or implementations.

[0021] Reference Figure 1 , a schematic diagram of a hybrid electric vehicle (HEV) 10 is shown according to an embodiment of the present disclosure. Figure 1 1 and 2 show representative relationships between components. The physical layout and orientation of the components within the vehicle may vary. The HEV 10 includes a powertrain 12. The powertrain 12 includes an engine 14 that drives a transmission 16, which may be referred to as a modular hybrid transmission (MHT). As will be described in further detail below, the transmission 16 includes an electric machine (such as an electric motor / generator (M / G) 18), an associated traction battery 20, a torque converter 22, and a multi-ratio automatic transmission or gearbox 24. Figure 1 As shown, the engine 14, M / G 18, torque converter 22, and automatic transmission 24 are connected in series. For simplicity, the M / G 18 may be referred to as a motor.

[0022] Both the engine 14 and the motor 18 are drive sources for the HEV 10. The engine 14 generally represents a power source that can include an internal combustion engine (such as a gasoline, diesel, or natural gas powered engine) or a fuel cell. When the disconnect clutch 26 between the engine 14 and the motor 18 is at least partially engaged, the engine 14 generates engine power and corresponding engine torque that is supplied to the motor 18. The motor 18 can be implemented by any of a variety of types of electric machines. For example, the motor 18 can be a permanent magnet synchronous motor. As will be described below, the power electronics regulate the direct current (DC) power provided by the battery 20 to meet the requirements of the motor 18. For example, the power electronics can provide three-phase alternating current (AC) to the motor 18.

[0023] When the disconnect clutch 26 is at least partially engaged, power can flow from the engine 14 to the M / G 18 or from the M / G 18 to the engine 14. For example, the disconnect clutch 26 can be engaged and the M / G 18 can operate as a generator to convert rotational energy provided by the crankshaft 28 and the M / G shaft 30 into electrical energy for storage in the battery 20. The disconnect clutch 26 can also be disengaged to isolate the engine 14 from the rest of the powertrain 12, allowing the M / G 18 to serve as the sole drive source for the HEV 10. The shaft 30 extends through the M / G 18. The M / G 18 is continuously drivably connected to the shaft 30, while the engine 14 is drivably connected to the shaft 30 only when the disconnect clutch 26 is at least partially engaged.

[0024] A separate starter motor 31 can selectively engage the engine 14 to rotate the engine, thereby allowing combustion to begin. Once the engine is started, the starter motor 31 can be disconnected from the engine, for example, by a clutch (not shown) between the starter motor 31 and the engine 14. In one embodiment, the starter motor 31 is an integrated belt starter generator (BISG). In one embodiment, the starter motor 31 starts the engine 14 while the disconnect clutch 26 is disengaged, disconnecting the engine from the M / G 18. Once the engine has started and is brought up to the same speed as the M / G 18, the disconnect clutch 26 can couple the engine to the M / G, allowing the engine to provide drive torque.

[0025] In another embodiment, the starter motor 31 is not provided, and instead the engine 14 is started by the M / G 18. To do this, the disconnect clutch 26 is partially engaged to transfer torque from the M / G 18 to the engine 14. The torque ramp of the M / G 18 may be required to meet the driver's demand while also starting the engine 14. Subsequently, once the engine speed is brought up to the M / G speed, the disconnect clutch 26 may be fully engaged.

[0026] The M / G 18 is connected to the torque converter 22 via shaft 30. Thus, when the disconnect clutch 26 is at least partially engaged, the torque converter 22 is connected to the engine 14. The torque converter 22 includes an impeller fixed to the M / G shaft 30 and a turbine fixed to the transmission input shaft 32. The torque converter 22 provides a fluid coupling between the shaft 30 and the transmission input shaft 32. When the impeller rotates faster than the turbine, the torque converter 22 transfers power from the impeller to the turbine. The magnitude of the turbine torque and the impeller torque generally depends on the relative speeds. When the ratio of the impeller speed to the turbine speed is sufficiently high, the turbine torque is several times the impeller torque. The torque converter bypass clutch 34 can be configured so that when engaged, it frictionally or mechanically couples the impeller and turbine of the torque converter 22, allowing for more efficient power transfer. The torque converter bypass clutch 34 can also operate as a launch clutch to provide a smooth vehicle launch. Alternatively, or in combination, for applications that do not include a torque converter 22 or a torque converter bypass clutch 34, a launch clutch similar to the disconnect clutch 26 may be provided between the M / G 18 and the gearbox 24. In some applications, the disconnect clutch 26 is generally referred to as the upstream clutch, and the launch clutch 34 (which may be a torque converter bypass clutch) is generally referred to as the downstream clutch.

[0027] The gearbox 24 may include a gear set (such as a planetary gear set) that is selectively placed in different gear ratios by selectively engaging friction elements such as clutches and brakes to establish a desired plurality of discrete gear ratios or stepped gear ratios. For simplicity, the gear ratios may be referred to as gears, i.e., first gear, second gear, etc. The friction elements may be controlled by a shift schedule that connects and disconnects certain elements of the gear set to control the gear ratio between the transmission output shaft 36 and the transmission input shaft 32. The gearbox 24 is automatically shifted from one gear ratio to another based on various vehicle and environmental operating conditions by an associated controller (such as a powertrain control unit (PCU)). The gearbox 24 then provides the powertrain output torque to the output shaft 36.

[0028] It should be understood that the hydraulically controlled gearbox 24 used with the torque converter 22 is merely one example of a gearbox or transmission arrangement; any multi-ratio gearbox that receives input torque from an engine and / or motor and then provides torque to an output shaft at different gear ratios is acceptable for use with embodiments of the present disclosure. For example, the gearbox 24 may be implemented as an automated mechanical (or manual) transmission (AMT) that includes one or more servo motors that translate / rotate a shift fork along a shift rail to select the desired gear ratio. As is generally understood by those skilled in the art, an AMT may be used, for example, in applications with higher torque requirements.

[0029] like Figure 1As shown in the representative embodiment of FIG, the output shaft 36 is connected to a differential 40. The differential 40 drives a pair of wheels 42 via respective axles 44 connected to the differential 40. The differential transmits approximately equal torque to each wheel 42 while allowing for slight speed differences (such as when the vehicle is turning). Different types of differentials or similar devices can be used to distribute torque from the drivetrain to one or more wheels. For example, in some applications, the torque distribution can be changed according to a specific operating mode or condition.

[0030] The powertrain 12 also includes an associated controller 50, such as a powertrain control unit (PCU). Although shown as a single controller, the controller 50 may be part of a larger control system and may be controlled by multiple other controllers throughout the vehicle 10, such as a vehicle system controller (VSC). It should be understood that the powertrain control unit 50 and one or more other controllers may be collectively referred to as a "controller," which controls multiple actuators in response to signals from multiple sensors to control various functions, such as starting / stopping, operating the M / G 18 to provide wheel torque or charge the battery 20, selecting or scheduling transmission shifts, etc. The controller 50 may include a microprocessor or central processing unit (CPU) that communicates with various types of computer-readable storage devices or media. For example, the computer-readable storage devices or media may include volatile and non-volatile memory such as read-only memory (ROM), random access memory (RAM), and keep-alive memory (KAM). KAM is persistent or non-volatile memory that can be used to store various operating variables when the CPU loses power. The computer-readable storage device or medium may be implemented using any of a number of known storage devices, such as PROM (programmable read-only memory), EPROM (electrically programmable read-only memory), EEPROM (electrically erasable programmable read-only memory), flash memory, or any other electrical, magnetic, optical, or combination thereof storage device capable of storing data, some of which represents executable instructions used by the controller to control the engine or vehicle.

[0031] The controller communicates with multiple engine / vehicle sensors and actuators via an input / output (I / O) interface, which may be implemented as a single integrated interface that provides various raw data or signal conditioning, processing and / or conversion, short circuit protection, etc. Alternatively, one or more dedicated hardware or firmware chips may be used to condition and process specific signals before providing them to the CPU. Figure 1As generally shown in the representative embodiment of FIG, the controller 50 can send and / or receive signals to and from the engine 14, the disconnect clutch 26, the M / G 18, the launch clutch 34, the transmission gearbox 24, and the power electronics 56. Although not explicitly described, one of ordinary skill in the art will recognize the various functions or components within each of the above-mentioned subsystems that can be controlled by the controller 50. Representative examples of parameters, systems, and / or components that can be actuated directly or indirectly using control logic executed by the controller include fuel injection timing, rate, and duration, throttle position, spark plug firing timing (for spark-ignition engines), intake / exhaust valve timing and duration, front end accessory drive (FEAD) components such as the alternator, the air conditioning compressor, battery charging, regenerative braking, M / G operation, clutch pressures for the disconnect clutch 26, the launch clutch 34, and the transmission gearbox 24, etc. Sensors transmitting inputs through the I / O interface can be used to indicate, for example, turbocharger boost pressure, crankshaft position (PIP), engine speed (RPM), wheel speed (WS1, WS2), vehicle speed (VSS), coolant temperature (ECT), intake manifold pressure (MAP), accelerator pedal position (PPS), ignition switch position (IGN), throttle position (TP), air temperature (TMP), exhaust gas oxygen (EGO) or other exhaust gas constituent concentration or presence, intake air flow (MAF), transmission gear, gear ratio or mode, transmission oil temperature (TOT), transmission turbine speed (TS), torque converter bypass clutch 34 state (TCC), deceleration or shift mode (MDE).

[0032] The control logic or functions executed by the controller 50 may be represented by flowcharts or similar diagrams in one or more of the accompanying figures. These figures provide representative control strategies and / or logic that may be implemented using one or more processing strategies (such as event-driven, interrupt-driven, multi-tasking, multi-threading, etc.). Therefore, the multiple steps or functions shown may be performed in the sequence shown, in parallel, or omitted in some cases. Although not always explicitly stated, one of ordinary skill in the art will recognize that one or more of the steps or functions described may be repeated depending on the specific processing strategy used. Similarly, the processing order is not necessary to achieve the features and advantages described herein, but is provided for ease of illustration and description. The control logic may be primarily implemented in software executed by a microprocessor-based vehicle, engine, and / or powertrain controller (such as the controller 50). Of course, depending on the specific application, the control logic may be implemented in software, hardware, or a combination of software and hardware in one or more controllers. When implemented in software, the control logic may be provided in one or more computer-readable storage devices or media having stored data representing code or instructions executed by a computer to control the vehicle or its subsystems. Computer-readable storage devices or media may include one or more of a number of known physical devices that utilize electrical, magnetic, and / or optical memory to retain executable instructions and associated calibration information, operating variables, and the like.

[0033] The vehicle operator uses accelerator pedal 52 to provide a desired torque, power, or drive command to propel the vehicle. Typically, depressing and releasing pedal 52 generates an accelerator pedal position signal, which is interpreted by controller 50 as a request for increased or decreased power, respectively. Based at least on the input from the pedal, controller 50 commands torque from engine 14 and / or M / G 18. Controller 50 also controls shift timing within gearbox 24 and the engagement or disengagement of disconnect clutch 26 and torque converter bypass clutch 34. Similar to disconnect clutch 26, torque converter bypass clutch 34 is adjustable within a range between engaged and disengaged positions. This creates variable slip in torque converter 22, in addition to the variable slip generated by the fluid coupling between the impeller and turbine. Alternatively, torque converter bypass clutch 34 can be operated in a locked or disengaged mode of operation, depending on the specific application.

[0034] To propel the vehicle using the engine 14, the disconnect clutch 26 is at least partially engaged to transfer at least a portion of the engine torque through the disconnect clutch 26 to the M / G 18 and then from the M / G 18 through the torque converter 22 and the gearbox 24. When the engine 14 alone provides the necessary torque to propel the vehicle, this operating mode may be referred to as an "engine mode," "engine-only mode," or "mechanical mode."

[0035] The M / G 18 can assist the engine 14 by providing additional power to rotate the shaft 30. This operating mode may be referred to as a "hybrid mode," an "engine-motor mode," or an "electric assist mode."

[0036] To drive the vehicle using the M / G 18 as the sole power source, power flow remains unchanged, except that the disconnect clutch 26 isolates the engine 14 from the rest of the drivetrain 12. During this time, combustion in the engine 14 may be disabled or otherwise shut down to save fuel. The traction battery 20 transmits stored electrical energy via line 54 to power electronics 56, which may include, for example, an inverter. The power electronics 56 converts the DC voltage from the battery 20 to an AC voltage for use by the M / G 18. The controller 50 commands the power electronics 56 to convert the voltage from the battery 20 to an AC voltage that is provided to the M / G 18 to provide positive torque (drive torque) or negative torque (regenerative braking) to the axle 30. This operating mode may be referred to as "electric-only mode," "EV (electric vehicle) mode," or "motor mode."

[0037] In any operating mode, the M / G 18 can function as a motor and provide motive force to the powertrain 12. Alternatively, the M / G 18 can function as a generator and convert kinetic energy from the powertrain 12 into electrical energy for storage in the battery 20. For example, the M / G 18 can function as a generator when the engine 14 is providing propulsion power to the vehicle 10. The M / G 18 can also function as a generator during regenerative braking, in which rotational energy from the rotating wheels 42 is transferred back through the gearbox 24 and converted into electrical energy for storage in the battery 20. The M / G 18 can be said to provide negative torque when functioning as a generator.

[0038] It should be understood that Figure 1 The schematic diagram shown in FIG. 1 is merely exemplary and is not intended to be limiting. Other configurations are contemplated that utilize the selective engagement of both the engine and the motor for transmission. For example, the M / G 18 may be offset relative to the crankshaft 28, and / or the M / G 18 may be positioned between the torque converter 22 and the gearbox 24. Other configurations are contemplated without departing from the scope of the present disclosure.

[0039] In addition to the regenerative braking capability, the vehicle 10 is also provided with conventional friction brakes 53 at the wheels, which, similar to regenerative braking, can be activated autonomously or by depressing the brake pedal. These brakes can be controlled by, for example, a hydraulic braking system. The friction brakes can selectively provide braking force to the vehicle depending on the availability of the regenerative braking system. The amount of friction braking can vary. For example, if the state of charge of the battery 20 is relatively high (e.g., above a high threshold), regenerative braking can be disabled for at least a portion of the braking event to prevent overcharging the battery. Alternatively, the friction brakes 53 can be activated to slow the vehicle. In certain braking situations, friction braking can be used to supplement regenerative braking, or vice versa, to provide the total braking force necessary to brake the vehicle while also maximizing the amount of regenerative braking.

[0040] Autonomous and semi-autonomous vehicles have the ability to automatically command and control the vehicle's brakes without driver initiation. The vehicle may be equipped with sensors configured to detect the distance to an external object (stop sign, another vehicle, stop light, etc.) that the vehicle is approaching.

[0041] Figure 2A and Figure 2B An exemplary driving scenario is shown, in which vehicle 10 is provided with sensors 80 coupled to a controller. Vehicle 10 is following a preceding vehicle 90, which is located in front of vehicle 10. Based on information collected from sensor 80, the controller determines that the vehicle needs to be decelerated to maintain a safe following distance from preceding vehicle 90. The controller, associated with sensor 80, can define a deceleration rate 82 required to maintain a safe following distance. The time required to safely brake vehicle 10 depends on the speed of vehicle 10, the speed of vehicle 90, and the distance between vehicle 10 and vehicle 90. The controller can configure the vehicle's deceleration rate to occur within the necessary time (i.e., between t_start and t_end).

[0042] The present disclosure provides a control strategy for constructing a vehicle speed profile or a braking torque profile for use during a regenerative braking event to maximize the amount of electrical power captured during the regenerative braking event, according to an embodiment.

[0043] During normal driving operation, the amount of regenerative braking that can be performed is limited by and can be predicted by the characteristics of the powertrain. This is referred to herein as the powertrain regenerative torque limit (PT limit). As used herein, PT limit refers to the torque at the wheels, rather than the torque in some other area of the powertrain. Characteristics that affect PT limit may include the state of charge of the battery 20, the regenerative torque limit of the M / G 18, the braking stability limit, and the ability of the transmission to support regenerative braking. For a vehicle having the structure of vehicle 10, the ability of the transmission to support regenerative braking depends on the gear, and the ability of the transmission to support regenerative braking in high gear may be the main factor. Typically, due to the torque multiplication in the gearbox, the PT limit in a higher gear (e.g., 5th gear) is smaller than the PT limit in a lower gear (e.g., 2nd gear).

[0044] It should be noted that in describing these figures and the relationship between these values, torque values are shown as negative values because they are braking torques ("negative torques"). All relative terms describing torque (e.g., higher, lower, etc.) refer to absolute values. For example, when a first torque value or magnitude is "higher" than a second torque value or magnitude, it means that the first value or magnitude is more negative than the second value or magnitude.

[0045] The present disclosure provides various embodiments to accomplish a braking event while recovering as much regenerative energy as possible and still braking the vehicle appropriately and at the generally constant deceleration that the driver experience desires.

[0046] A vehicle speed profile and / or associated brake torque profile may be constructed by the controller to satisfy an average deceleration that takes into account regenerative braking efficiency. Figure 3A flowchart 150 is shown, which provides a general overview of the process for constructing a speed profile for vehicle 10 during an exemplary braking event. Similar to other processes described herein, flowchart 150 may be executed by a controller. At operation 152, the controller may use data received from sensor 80 and other sources to determine an initial vehicle speed prediction within a time window (t_start, t_end) to safely brake the vehicle. If a braking event is not necessary at operation 154, control returns to the start and continues to monitor vehicle speed and distance to objects in front of the vehicle. If a braking event is necessary at operation 154, the controller determines a desired deceleration for the time window to safely stop or slow the vehicle at operation 156. At operation 158, the controller constructs a vehicle speed profile within the time window for regenerative braking efficiency while meeting the desired deceleration. In other words, the controller may construct multiple speed profiles, all with the same average deceleration, understanding that some speed profiles can produce more efficient regenerative braking than others while sacrificing less delineation for a constant deceleration. For example, it may be desirable to brake the vehicle using a first braking torque, and then later in the braking event brake the vehicle using a larger second braking torque, thereby recovering more electrical energy from regenerative braking during the braking event.

[0047] Using the teachings provided above, a controller can (1) establish that a braking event should automatically occur, (2) determine the PT limits for each gear ratio of the transmission during an upcoming braking event, (3) determine the necessary deceleration required to successfully stop or slow the vehicle within a time window before contacting a leading vehicle or other object, and (4) determine the average braking torque desired or necessary to successfully stop the vehicle during the braking event.

[0048] Figure 4A and Figure 4B An algorithm 200 is shown for constructing a vehicle torque curve to be used during an autonomous braking event. Figure 5 The curve graph and Figure 6 The steps of the algorithm are explained using a motor efficiency profile. Algorithm 200 begins in response to a request for an autonomous braking event for the vehicle. At operation 202, the controller determines a desired deceleration 250 of the vehicle during the braking event, which in the illustrated embodiment is an autonomous or semi-autonomous vehicle automatically braking for a stop sign. Of course, other types of braking events are also contemplated. At operation 204, the controller determines a desired brake torque profile at the wheels for the desired deceleration 250, and the controller calculates an average brake torque profile. des_avg) (as shown by trace 252 ). As used herein, braking torque refers to torque at the wheels (ie, in the wheel region), while regenerative torque refers to torque at the M / G (ie, the motor region).

[0049] At operation 206, the controller estimates the PT limit at the wheels 254. As explained above, the PT limit may be based on the state of charge of the battery 20, the regenerative torque limit of the M / G 18, the braking stability limit, and the transmission capability.

[0050] At operation 208 , the controller determines the brake torque des_avg 252 is greater than the PT limit 254. If so, then exit the control strategy and use other control strategies. If not, then control proceeds to operation 210, and the controller will brake torque des_avg 252 is converted into motor torque des_avg ) 256. This can be done for each gear ratio of the transmission, or only for the gear ratios expected to be used during braking events. The controller can use Equation 1 to convert brake torque to motor torque and vice versa. In other words, Equation 1 converts torque between the motor domain and the wheel domain.

[0051]

[0052] As can be seen from trace 256, higher gears require much greater motor torque than lower gears to achieve similar braking torque at the wheels. Therefore, if the vehicle 10 is commanded to perform regenerative braking in all gears according to the average braking torque 252, the M / G 18 will need to operate at very high torque for higher gears and very low torque for lower gears.

[0053] Electric machines, such as the M / G 18 , vary in efficiency depending on the torque and speed of the electric machine. Figure 6 An exemplary efficiency profile for the M / G 18 is shown. The M / G 18 is generally more efficient when the regenerative torque is in a mid-range, rather than at minimum and maximum regenerative torque. For a given range of motor speeds, the efficiency profile can be used to select a more efficient regenerative torque to recover more power during regenerative braking. As will be discussed in more detail below, regenerative braking efficiency can be improved by commanding different motor torques for different gears of the transmission so that the commanded motor torque is within a more efficient range of the M / G 18.

[0054] At operation 212, the controller calculates the regenerative torque limit 260 of the M / G 18, i.e., the maximum regenerative torque in the motor zone. At operation 214, the controller calculates the minimum braking torque in the wheel zone required to maintain a safe following distance between the vehicle 10 and the preceding object (e.g., the preceding vehicle). safe_dis ). This torque is shown as trace 251. For each gear ratio or a selected number of gear ratios, the wheel torque from the wheel region safe_dis Converted into safe distance torque in the motor area (motor safe_dis ) to determine the minimum torque required at the M / G 18 for each gear ratio. This torque is shown as trace 257.

[0055] At operation 216, the controller calculates the most efficient motor torque for regenerative braking. This can be calculated for each gear ratio of the transmission, or only for the gear ratio expected to be used during a braking event. The motor has an efficiency profile, such as Figure 6 As shown in , the efficiency distribution diagram indicates the motor efficiency based on the motor speed and motor torque. Figure 6 As can be seen in FIG, the M / G 18 is most efficient for medium torque and speed, and in the corners the efficiency of the M / G 18 is lower. Figure 6 This is merely one example of an efficiency profile; the exact efficiency of the motor will vary depending on the specific design of the motor. The efficiency profile can be stored in a lookup table and used by the controller in operation 216 to calculate the most efficient motor torque. Based on the transmission shift schedule and vehicle speed, the rotational speed of the M / G 18 during a braking event is known, so the controller can optimize the torque value commanded to the M / G 18 during a regenerative braking event. The most efficient motor torque for an exemplary regenerative braking event is shown as trace 258. Comparing the most efficient torque 258 with the torque curve 256 shows that commanding the vehicle to brake at the average braking torque 252 is inefficient, particularly at higher and lower gear ratios. Regenerative braking efficiency can be improved by reducing the magnitude of the motor torque for higher gear ratios and increasing the magnitude of the motor torque for lower gear ratios. However, changing the commanded motor torque affects the braking torque at the wheels, so changes should only be made if they are safe (i.e., the vehicle will maintain a safe following distance from a preceding object (such as a vehicle) or the vehicle will stop at a desired location).

[0056] At operation 218 , the controller determines whether the most efficient torque 258 is greater than the motor torque limit 260 , or whether the most efficient torque 258 is less than the motor torque limit 260 . safe_dis257. This is done for each of the gear ratios, or only for the gear ratio predicted to be used during the braking event. If so, then the most efficient torque cannot be used for at least that gear ratio 258, and the controller sets the motor torque for that gear ratio at operation 220 to (i) the motor regeneration limit 260, (ii) the motor torque des_avg 256 or (iii) motor safe_dis The controller performs this operation for each gear ratio to generate a torque curve for the motor region. If no at operation 218, control proceeds to operation 222, and the controller sets the motor torque to the most efficient motor torque 258 for each gear ratio to generate a motor torque curve. Control then proceeds to operation 224.

[0057] At operation 224, the controller converts the motor torque profile from operation 220 or operation 222 into a brake torque profile in the wheel area. The controller then calculates the average brake torque profile (profile avg ), and set the profile avg and brake torque des_avg 252. If they match or their averages are within a predetermined tolerance (i.e., profile avg In brake torque des_avg 252), then the final brake torque curve 262 is output at operation 226. The predetermined tolerance is a calibratable value that may vary from vehicle to vehicle due to different vehicles having different deceleration differences for a given brake torque difference.

[0058] If profile avg and brake torque des_avg 252 is greater than a predetermined tolerance, control passes to operation 228, and the controller adjusts the profile and sends the modified profile back to operation 224. This adjustment process is repeated for the necessary number of iterations until the profile avg and brake torque des_avg The difference of 252 is within the predetermined tolerance.

[0059] The adjustment process is based on a transmission shift schedule that is set independently of regenerative braking. The adjustment process can include a constant offset relative to the most efficient torque 258 in different gears, i.e., increasing or decreasing the torque. Alternatively, the offset can be gear-dependent, with each gear having its own offset. For example, the magnitude of the offset used in higher gears can be greater than the magnitude of the offset used in lower gears to provide more consistent deceleration.

[0060] The final brake torque curve 262 can be used by a low-level controller responsible for sending torque commands to the M / G 18. The low-level controller can command the M / G 18 to apply regenerative torque according to the curve 262 to achieve vehicle braking. The low-level controller or another controller can convert the final brake torque curve 262 in the wheel area into a motor torque commanded to the M / G 18.

[0061] In an alternative embodiment, an algorithm similar to algorithm 200 may output a speed profile for the vehicle during a braking event. The speed profile may be used by a low-level controller to send commands to the motor to apply regenerative torque according to the profile, thereby braking the vehicle. The speed profile may first be converted into a braking torque in the wheel area, and then into a motor torque that may be commanded to the M / G 18.

[0062] The processes, methods or algorithms disclosed herein may be transmitted to / implemented by a processing device, a controller or a computer, wherein the processing device, the controller or the computer may include any existing programmable electronic control unit or a dedicated electronic control unit. Similarly, the processes, methods or algorithms may be stored in a variety of forms as data and instructions that can be executed by a controller or a computer, including but not limited to: information permanently stored on a non-writable storage medium (such as a ROM device) and information variably stored on a writable storage medium (such as a floppy disk, a magnetic tape, a CD, a RAM device and other magnetic and optical media). The processes, methods or algorithms may also be implemented as software executable objects. Alternatively, the processes, methods or algorithms may be implemented in whole or in part using suitable hardware components (such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a state machine, a controller or other hardware components or devices) or a combination of hardware, software and firmware components.

[0063] Although exemplary embodiments have been described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The terms used in the specification are descriptive rather than restrictive, and it should be understood that various changes may be made without departing from the spirit and scope of the present disclosure. As previously mentioned, the features of the various embodiments may be combined to form further embodiments of the present invention that may not be explicitly described or illustrated. Although multiple embodiments may have been described as providing advantages or being superior to other embodiments or prior art implementations with respect to one or more desired characteristics, those skilled in the art will recognize that, depending on the specific application and implementation, one or more features or characteristics may be compromised to achieve the desired overall system properties. These properties may include, but are not limited to, cost, strength, durability, lifecycle cost, marketability, appearance, packaging, size, maintainability, weight, manufacturability, ease of assembly, and the like. Therefore, to the extent that any embodiment is described as being less desirable than other embodiments or prior art implementations in terms of one or more characteristics, these embodiments are not outside the scope of the present disclosure and may be expected to be used in specific applications.

Claims

1. A vehicle comprising: motor; A controller is configured to, in response to an autonomous braking request and a magnitude of a predicted average braking torque associated with the autonomous braking request being less than a magnitude of a powertrain regenerative torque limit, perform the following operations: Based on the motor efficiency distribution map, the most efficient motor torque curve is generated for the braking event; converting the most efficient motor torque curve into a brake torque curve; and In response to the most efficient motor torque curve being less than a motor torque limit and an average value of the brake torque curve being within a specified range of the predicted average brake torque, braking the vehicle using only the motor according to the most efficient motor torque curve.

2. The vehicle of claim 1 further comprising a stepped ratio transmission having a plurality of discrete gear ratios.

3. The vehicle according to claim 2, wherein: The most efficient motor torque curve has a first magnitude that is commanded to the motor when the transmission is in one of the plurality of discrete gear ratios and a second magnitude that is used when the transmission is in another of the plurality of discrete gear ratios.

4. The vehicle according to claim 2, wherein: The transmission includes a torque converter coupled to the motor.

5. The vehicle of claim 1 further comprising an engine and a disconnect clutch, wherein The disconnect clutch is configured to selectively couple the motor to the engine.

6. The vehicle according to claim 1, wherein The controller is further configured to, in response to the most efficient motor torque curve being less than a torque required to maintain a desired following distance to a leading object, brake the vehicle using only the motor according to a torque curve having at least one magnitude equal to the torque required to maintain the desired following distance.

7. The vehicle according to claim 1, wherein The braking torque curve is in the wheel area.

8. A method of regenerative braking of an autonomous vehicle, comprising: generating a vehicle speed profile for a braking event; determining an average braking torque corresponding to the vehicle speed profile; Generate the most efficient motor torque curve for braking events based on the motor efficiency distribution map; converting the most efficient motor torque curve into a braking torque curve; and In response to the most efficient motor torque curve being less than a motor torque limit and an average value of the brake torque curve being within a specified range of the average brake torque, braking the vehicle using only the motor according to the most efficient motor torque curve.

9. The method according to claim 8, wherein The most efficient motor torque curve has a first magnitude and a second magnitude, the first magnitude being commanded to the motor when the transmission of the vehicle is in a first gear ratio and the second magnitude being commanded to the motor when the transmission is in a second gear ratio.

10. The method according to claim 8, wherein The braking torque curve is in the wheel area.

11. The method according to claim 8, further comprising: In response to a predicted autonomous braking event being requested and a magnitude of a predicted average braking torque for the autonomous braking event being greater than a magnitude of a regenerative torque limit for the vehicle, braking the vehicle using only the electric machine according to a torque profile having at least one magnitude set as the regenerative torque limit.

12. The method according to claim 8, further comprising: In response to the most efficient electric machine torque curve being less than a torque required to maintain a desired following distance to a leading object, braking the vehicle using only the electric machine according to a torque curve having at least one magnitude equal to the torque required to maintain the desired following distance.

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