Abstract:
Some vehicle maneuvers in a hybrid electric vehicle require a high discharge current from the battery. The capacity of a battery to supply a large discharge current depends on, among other things, the recent charge and discharge history. When the demand can be predicted in advance, the vehicle is operated to aggressively charge the battery during a time period close in time to the predicted event. As a result of this aggressive charging, the battery delivers more current to satisfy the power demand without allowing the terminal voltage to decrease below a minimum level than the battery would be capable of delivering at the same state of charge following a period of rest.

Description:
TECHNICAL FIELD 
     This disclosure relates to the field of hybrid electric vehicles. More particularly, the disclosure pertains to a method of operating a hybrid electric vehicle to improve the capability of a battery to satisfy short term power demands. 
     BACKGROUND 
     Hybrid electric vehicles improve fuel economy by storing energy in a battery during some driving conditions and utilizing that energy to supplement the power of an internal combustion engine in other driving conditions. Furthermore, hybrid electric vehicles can use a smaller internal combustion engine than a comparably sized conventional vehicle because battery power can supplement the power from the internal combustion engine to satisfy short term power requirements such as when accelerating to enter a freeway. Using a smaller engine improves fuel economy because internal combustion engines are typically more efficient when operated at a higher percentage of their maximum power capability. 
     However, the battery voltage decreases when power is withdrawn. In order to ensure battery with designed operational life, vehicle control strategies typically limit the maximum power withdrawn from the battery to ensure that the battery voltage stays above a predetermined minimum voltage. This maximum battery power limits vehicle performance with a particular engine and limits the opportunity to use a smaller engine. 
     SUMMARY OF THE DISCLOSURE 
     A method of operating a hybrid electric vehicle utilizes a prediction of future power demands to aggressively charge the battery in anticipation of high power demand events such as accelerating on a highway entrance ramp. The prediction may be generated by a GPS system using a database of road segments associated with speed limits or average speeds. The method may wait until close to the time of the event to perform the aggressive charging. The appropriate time to initiate the aggressive charging may be determined by the battery time constant. The method may adapt to changes in state of charge, temperature, and battery age which influence the battery time constant. By charging aggressively just before the event, the battery may deliver a greater current or deliver it for a longer duration while the battery voltage stays above the minimum voltage. 
     A hybrid electric vehicle includes a battery having a time constant, at least one motor configured to draw current from the battery, an internal combustion engine, and a controller. The motor and the engine are both configured to deliver torque to vehicle wheels. The controller is programmed to respond to a prediction of a future torque demand event by waiting until within two time constants of the event and then operating the vehicle to supply a charging current to the battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a hybrid vehicle powertrain. 
         FIG. 2  is a schematic of a circuit used to model the dynamic behavior of a battery. 
         FIG. 3  is a graph illustrating the dynamic behavior of a battery following a charging event. 
         FIG. 4  is a graph illustrating the dynamic behavior of a battery during a discharge event. 
         FIG. 5  is a flow chart for a method to operate a hybrid electric vehicle. 
         FIG. 6  is a graph illustrating the dynamic behavior of a battery in a vehicle following the method of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could 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. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
       FIG. 1  is a schematic representation of a power-split type hybrid vehicle. Solid lines represent mechanical connections among components. Lines with long dashes represent electrical power connections among components. Lines with short dashes represent signal connections. This configuration is called a power-split because planetary gear set  20  splits the power flowing from the engine to the wheels into a mechanical power flow path and an electrical power flow path. Planetary gear set  20  includes sun gear  22 , ring gear  24 , and carrier  26  which are rotate about a common axis. A number of planet gears  28  are supported for rotation with respect to carrier  26  and mesh with both sun gear  22  and ring gear  24 . 
     Internal combustion engine  30  is drivably connected to carrier  26 . Sun gear  22  is drivably connected to generator  32 . Ring gear  24  is drivably connected to output shaft  34 . A driveable connection is established between two components if rotation of one component causes the other component to rotate at a proportional speed. In  FIG. 1 , these connections are shown as direct connections, but the connections may include gearing. Output shaft  34  is also drivably connected to traction motor  36  and differential  38 . Differential  38  transmits power to a left wheel  40  and a right wheel  42  while permitting slight variations in speed, such as when the vehicle turns a corner. 
     Generator  32  and traction motor  36  are both reversible electrical machines capable of converting electrical energy into rotational mechanical energy and converting rotational mechanical energy into electrical energy. For example, generator  32  and traction motor  36  may each be DC motors or AC motors, such as synchronous motors or induction motors, in combination with inverters. Generator  32  and traction motor  36  are both electrically connected to battery  44 . Battery  44  converts electrical energy into chemical energy for storage and converts the chemical energy back into electrical energy. 
     The level of torque produced by the internal combustion engine, generator, and traction motor, respectively, are controlled by commands from controller  46 . The controller determines the desired torque levels based on sensors associated with accelerator pedal  48 , engine  30 , generator  32 , traction motor  36 , and battery  44 . Additionally, the controller may receive information, such as current location and anticipated future driver demands, from global positioning system  50 . Controller  46  may be implemented as a single microprocessor, as multiple communicating microprocessors, or other means. Controller  46  may be programmed by means of software, hardware, or some combination thereof. 
       FIG. 2  shows a Randles circuit model that may be used to model the dynamic behavior of a battery such as battery  44 . The battery has negative terminal  52  and positive terminal  54 . The battery provides electrical power by forcing electrical current to flow out the positive terminal, through a load such as an electric motor, and back into the negative terminal. To charge the battery, electrical current is forced by a power source, such as a generator, to flow into the positive terminal and out the negative terminal. Voltage source  56  represents the voltage resulting from the state of the chemicals. This voltage can vary slightly depending upon state of charge of the battery and the battery temperature. To improve battery life, the state of charge may is maintained between a minimum state of charge and a maximum state of charge. Resistors  58  and  60  represent the ohmic and charge transfer resistances of the battery. Due to these resistances, the net electrical power that can be withdrawn from the battery is less than the net electrical power used to charge the battery. These resistances typically vary with temperature and state of charge. Finally, capacitor  62  represents the fact that the conversion between electrical energy and chemical energy may not proceed at the same rate that electrical energy is provided or withdrawn by the attached circuit. A battery tends to lose its power capability (power fade) and capacity (capacity fade) as it is used over time. Both phenomena are attributed to battery aging. As battery ages, its dynamics change as well, which is represented by changes in its model parameters, such as R 1 , R 2 , and C, at the same temperature and state of charge. There are known techniques for a controller to adaptively adjust these parameters during use. 
     The dynamic behavior of a typical battery following a charge event is illustrated in  FIG. 3 . The thick line represents the voltage between the positive and negative terminals. Prior to point  64 , a charging current is supplied, causing the positive voltage across each resistor  58  (R 1 ) and  60  (R 2 ). At point  64 , charging is terminated and no current flows into or out of the battery. At point  64 , the voltage across resistor  58  (R 1 ) decreases to zero immediately. However, the voltage across resistor  60  (R 2 ) and capacitor  62  (C) begin a gradual decay which asymptotically approaches the resting voltage  56  V(SOC). This decay is characterized by a time constant equal to R 2 *C. Since a Randles circuit such as that shown in  FIG. 2  is merely a way of modeling dynamic behavior of a battery, one cannot directly measure R 2  and C. However, the time constant for a given battery can be determined experimentally by measuring the time required for the voltage to decay 63.2% of the way to the resting voltage. The time constant for a given battery can also be adaptively learned during battery operation. For the types of batteries commonly used in hybrid electric vehicles, the time constant is typically around 5-100 seconds. 
     The dynamic behavior of a typical battery during a discharge event is illustrated in  FIG. 4 . If the battery has been in a resting state for a significant time prior to point  66 , then the voltage between the terminals is equal to V(SOC). Starting at  66 , the vehicle draws a discharging current I. The voltage between the terminals drops immediately due to the resistor  58  (R 1 ). Initially, the current flows through capacitor  62  (C) such that resistor  60  (R 2 ) does not cause a voltage drop. Over time, the current through resistor  60  (R 2 ) increases toward I and the voltage drop across resistor  60  asymptotically approaches I*R 2 . At point  68 , the voltage across the terminals decreases below the minimum voltage  70  (V min ). The minimum voltage is a calibratable value selected to balance performance, which favors a lower value, and battery life, which favors a higher value. 
       FIG. 5  illustrates a method that takes advantage of the transient characteristics of a battery to enable the vehicle to better respond to short term high power demand events. At  72 , the vehicle controller projects the next segment of the most likely route for the vehicle. This may involve interfacing with an onboard navigation system that determines the current vehicle location and contains a database of roads. The database may include various information about the roads, such as the speed limit. The driver may enter a destination into the navigation system and request route guidance. Additionally, the vehicle controller may interface with turn signals or may utilize historical information about the driver&#39;s habits to project the most likely route. The extent of the upcoming route that should be forecast is related to the battery time constant. It is not necessary to project further than what the vehicle will traverse in about ten time constants. At  74 , the controller projects the vehicle speed and acceleration as a function of progress along the projected route. The controller may utilize stored data about typical speeds and accelerations on various road segments. The controller may also utilize other information, such as real time traffic information, if it is available. Based on this information, at  76  the controller projects the demand for battery power along the route. The demand for battery power is positive whenever the vehicle power demand exceeds the power that will be available from the engine and is negative when the engine is capable of producing more power than the vehicle requires. 
     At  78 , the controller attempts to identify a high power demand event based on the battery power projection. A high power demand event is an event that would cause the battery voltage to drop below Vmin if the event is initiated with the battery voltage at V(SOC) as illustrated at  68  in  FIG. 4 . High power demand events typically occur when the expected speed on one road segment is significantly higher than the expected speed on the previous road segment. For example, freeway entrance ramps are known to be associated with rapid acceleration to highway speed. If no such event is identified, the controller returns to  72 . If a high power demand event is identified, the controller estimates the event location at  80  and estimates the latest charging opportunity at  82 . The latest charging opportunity is the region with projected negative battery power demand that is closest to the event location. At  84 , the controller checks whether the vehicle has reached the beginning of the latest charging opportunity. If not, then it refines the estimated speed and acceleration projections at  86  and returns to  82 . In this circumstance, waiting to start charging the battery is advantageous. Once the vehicle has entered the region identified as the latest charging opportunity, the controller commands the vehicle to aggressively charge the battery at  88 . Once the event is reached, as determined at  90 , the controller commands the vehicle to aggressively discharge the battery. 
       FIG. 6  shows the battery voltage when the method of  FIG. 5  is used. The battery is initially at V(SOC). Between  94  and  96 , the battery is charged with a charging current of I′. Consequently, the battery voltage is V(SOC)+I′*R 2  when the charging is terminated at  96 . It may be impractical to increase the voltage this much by increasing the state of charge because V(SOC) only increases slightly with state of charge. Even at the maximum state of charge, V(SOC) is only modestly increased relative to V(SOC) at the minimum state of charge. This voltage begins to decline as soon as charging is terminated, so it is beneficial to delay the charging phase until very close to the beginning of the power demand event. The power demand event begins at  98 . As in  FIG. 4 , the voltage would eventually decrease to the minimum voltage. However, due to the higher voltage at the beginning of the high power demand event, it takes longer for the battery voltage to reach V min  for a given discharge current. Alternatively, a larger discharge current may be drawn for the same time interval as  FIG. 4 . 
     The processes, methods, or algorithms disclosed herein can be deliverable to/implemented by a processing device, controller, or computer, which can include any existing programmable electronic control unit or dedicated electronic control unit. Similarly, the processes, methods, or algorithms can be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The processes, methods, or algorithms can also be implemented in a software executable object. Alternatively, the processes, methods, or algorithms can be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software and firmware components. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments 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 can be desirable for particular applications.