Torque assist based on battery state of charge allocation

A hybrid vehicle includes a traction battery, an internal combustion engine, an electric machine configured to provide torque assistance to the engine, and a controller or a powertrain control system having a controller. The controller is programmed to respond to a percentage of state of charge (SOC) allocated for torque assistance. An allocation for torque assistance is a change in SOC of the battery attributed to a current flow to the electric machine for torque assistance. When the change in SOC attributed to the current flow is greater than a predetermined change, the controller will halt the current flow to the electric machine to cease torque assistance.

TECHNICAL FIELD

This application generally relates to energy management for hybrid vehicles.

BACKGROUND

A hybrid-electric vehicle includes a fraction battery, an internal combustion engine and an electric machine. The engine may be operated to provide power for vehicle propulsion and accessory features. During operation, the traction battery may be charged or discharged based on the operating conditions including a battery state of charge (SOC), driver demand and regenerative braking

SUMMARY

A powertrain control system includes a fraction battery, an electric machine, and a controller. The electric machine is electrically coupled to the battery and configured to provide engine torque assist. The controller is programmed to respond to a change in state of charge (SOC) of the battery resulting from a current flow to the electric machine during the engine torque assist being greater than a predetermined change. The response of the controller is to halt the current flow to cease the engine torque assist.

A method of operating a vehicle having a traction battery and an electric machine includes halting a torque assist current flow to the electric machine. The halting is in response to a change in state of charge (SOC) of the battery attributed to the current flow being equal to a predetermined change of SOC apportioned for torque assistance to an engine.

A vehicle powertrain control system includes a traction battery, an electric machine, and a controller. The electric machine is coupled to the battery and configured to provide torque assistance to an engine. The controller is programmed to respond to a current flow to the electric machine greater than a predetermined electric charge. The response of the controller is to halt the current flow to the electric machine to cease torque assistance.

DETAILED DESCRIPTION

FIG. 1depicts a typical plug-in hybrid-electric vehicle (PHEV) having a powertrain or powerplant that includes the main components that generate power and deliver power to the road surface for propulsion. A typical plug-in hybrid-electric vehicle12may comprise one or more electric machines14mechanically connected to a hybrid transmission16. The electric machines14may be capable of operating as a motor or a generator. In addition, the hybrid transmission16is mechanically connected to an internal combustion engine18also referred to as an ICE or engine. The hybrid transmission16is also mechanically connected to a drive shaft20that is mechanically connected to the wheels22. The electric machines14can provide propulsion and deceleration capability when the engine18is turned on or off. The electric machines14also act as generators and can provide fuel economy benefits by recovering energy that would normally be lost as heat in the friction braking system. The electric machines14may also reduce vehicle emissions by allowing the engine18to operate at more efficient speeds and allowing the hybrid-electric vehicle12to be operated in electric mode with the engine18off under certain conditions. A powertrain has losses that may include transmission losses, engine losses, electric conversion losses, electric machine losses, electrical component losses and road losses. These losses may be attributed to multiple aspects including fluid viscosity, electrical impedance, vehicle rolling resistance, ambient temperature, temperature of a component, and duration of operation.

A fraction battery or battery pack24stores energy that can be used by the electric machines14. A vehicle battery pack24typically provides a high voltage DC output. The traction battery24is electrically connected to one or more power electronics modules26. One or more contactors42may isolate the traction battery24from other components when opened and connect the traction battery24to other components when closed. The power electronics module26is also electrically connected to the electric machines14and provides the ability to bi-directionally transfer energy between the traction battery24and the electric machines14. For example, a typical traction battery24may provide a DC voltage while the electric machines14may operate using a three-phase AC current. The power electronics module26may convert the DC voltage to a three-phase AC current for use by the electric machines14. In a regenerative mode, the power electronics module26may convert the three-phase AC current from the electric machines14acting as generators to the DC voltage compatible with the traction battery24. The description herein is equally applicable to a pure electric vehicle. For a pure electric vehicle, the hybrid transmission16may be a gear box connected to an electric machine14and the engine18may not be present.

In addition to providing energy for propulsion, the traction battery24may provide energy for other vehicle electrical systems. A typical system may include a DC/DC converter module28that converts the high voltage DC output of the traction battery24to a low voltage DC supply that is compatible with other vehicle loads. Other high-voltage loads46, such as compressors and electric heaters, may be connected directly to the high-voltage without the use of a DC/DC converter module28. The low-voltage systems may be electrically connected to an auxiliary battery30(e.g., 12V battery).

The vehicle12may be an electric vehicle or a plug-in hybrid vehicle in which the traction battery24may be recharged by an external power source36. The external power source36may be a connection to an electrical outlet that receives utility power. The external power source36may be electrically connected to electric vehicle supply equipment (EVSE)38. The EVSE38may provide circuitry and controls to regulate and manage the transfer of energy between the power source36and the vehicle12. The external power source36may provide DC or AC electric power to the EVSE38. The EVSE38may have a charge connector40for plugging into a charge port34of the vehicle12. The charge port34may be any type of port configured to transfer power from the EVSE38to the vehicle12. The charge port34may be electrically connected to a charger or on-board power conversion module32. The power conversion module32may condition the power supplied from the EVSE38to provide the proper voltage and current levels to the traction battery24. The power conversion module32may interface with the EVSE38to coordinate the delivery of power to the vehicle12. The EVSE connector40may have pins that mate with corresponding recesses of the charge port34. Alternatively, various components described as being electrically connected may transfer power using a wireless inductive coupling.

One or more wheel brakes44may be provided for decelerating the vehicle12and preventing motion of the vehicle12. The wheel brakes44may be hydraulically actuated, electrically actuated, or some combination thereof. The wheel brakes44may be a part of a brake system50. The brake system50may include other components to operate the wheel brakes44. For simplicity, the figure depicts a single connection between the brake system50and one of the wheel brakes44. A connection between the brake system50and the other wheel brakes44is implied. The brake system50may include a controller to monitor and coordinate the brake system50. The brake system50may monitor the brake components and control the wheel brakes44for vehicle deceleration. The brake system50may respond to driver commands and may also operate autonomously to implement features such as stability control. The controller of the brake system50may implement a method of applying a requested brake force when requested by another controller or sub-function.

One or more electrical loads46or auxiliary electric loads may be connected to the high-voltage bus. The electrical loads46may have an associated controller that operates and controls the electrical loads46when appropriate. Examples of auxiliary electric loads or electrical loads46include a battery cooling fan, an electric air conditioning unit, a battery chiller, an electric heater, a cooling pump, a cooling fan, a window defrosting unit, an electric power steering system, an AC power inverter, and an internal combustion engine water pump.

The various components discussed may have one or more associated controllers to control and monitor the operation of the components. The controllers may communicate via a serial bus (e.g., Controller Area Network (CAN), Ethernet, Flexray) or via discrete conductors. A system controller48may be present to coordinate the operation of the various components.

A traction battery24may be constructed from a variety of chemical formulations. Typical battery pack chemistries may be lead acid, nickel-metal hydride (NIMH) or Lithium-Ion.FIG. 2shows a typical traction battery pack24in a series configuration of N battery cells72. Other battery packs24, however, may be composed of any number of individual battery cells connected in series or parallel or some combination thereof. A battery management system may have a one or more controllers, such as a Battery Energy Control Module (BECM)76that monitors and controls the performance of the traction battery24. The BECM76may include sensors and circuitry to monitor several battery pack level characteristics such as pack current78, pack voltage80and pack temperature82. The BECM76may have non-volatile memory such that data may be retained when the BECM76is in an off condition. Retained data may be available upon the next key cycle.

In addition to the pack level characteristics, there may be battery cell level characteristics that are measured and monitored. For example, the terminal voltage, current, and temperature of each cell72may be measured. The battery management system may use a sensor module74to measure the battery cell characteristics. Depending on the capabilities, the sensor module74may include sensors and circuitry to measure the characteristics of one or multiple of the battery cells72. The battery management system may utilize up to Ncsensor modules or Battery Monitor Integrated Circuits (BMIC)74to measure the characteristics of all the battery cells72. Each sensor module74may transfer the measurements to the BECM76for further processing and coordination. The sensor module74may transfer signals in analog or digital form to the BECM76. In some embodiments, the sensor module74functionality may be incorporated internally to the BECM76. That is, the sensor module hardware may be integrated as part of the circuitry in the BECM76and the BECM76may handle the processing of raw signals.

The BECM76may include circuitry to interface with the one or more contactors42. The positive and negative terminals of the traction battery24may be protected by contactors42.

Battery pack state of charge (SOC) gives an indication of how much charge remains in the battery cells72or the battery pack24. The battery pack SOC may be output to inform the driver of how much charge remains in the battery pack24, similar to a fuel gauge. The battery pack SOC may also be used to control the operation of an electric or hybrid-electric vehicle12. Calculation of battery pack SOC can be accomplished by a variety of methods. One possible method of calculating battery SOC is to perform an integration of the battery pack current over time. This is well-known in the art as ampere-hour integration.

Battery SOC may also be derived from a model-based estimation. The model-based estimation may utilize cell voltage measurements, the pack current measurement, and the cell and pack temperature measurements to provide the SOC estimate.

The BECM76may have power available at all times. The BECM76may include a wake-up timer so that a wake-up may be scheduled at any time. The wake-up timer may wake up the BECM76so that predetermined functions may be executed. The BECM76may include non-volatile memory so that data may be stored when the BECM76is powered off or loses power. The non-volatile memory may include Electrical Eraseable Programmable Read Only Memory (EEPROM) or Non-Volatile Random Access Memory (NVRAM). The non-volatile memory may include FLASH memory of a microcontroller.

When operating the vehicle, actively modifying the way battery SOC is managed can yield higher fuel economy or longer EV-mode (electric propulsion) operation, or both. The vehicle controller must conduct these modifications at both high SOC and low SOC. At low SOC, the controller can examine recent operating data and decide to increase SOC via opportunistic engine-charging (opportunistic means to do this if the engine is already running) This is done to provide longer EV-mode operation when the engine turns off. Conversely, at high SOC, the controller can examine recent operating data and other data (location, temperature, etc.) to reduce SOC via EV-mode propulsion, reduced engine output, or auxiliary electrical loads. This is done to provide higher battery capacity to maximize energy capture during an anticipated regenerative braking event, such as a high-speed deceleration or hill descent.

FIG. 3is an exemplary graph300that illustrates battery state of charge and torque assistance allocation in relation to time. The graph shows a traction battery state of charge (SOC)302along with battery charge and discharge cycles304with respect to time306. The traction battery SOC is represented by the line308and the traction battery SOC level of an allocation or ration of battery SOC is represented by line310. Here, the level of the allocation310is specifically for torque assistance. When a vehicle is propelled by an internal combustion engine and a driver power demand or driver demand exceeds the power capability of the engine, an electric machine coupled to the drive wheels may be used to provide a torque to assist the torque applied by the engine. The torque assistance in this exemplary graph is shown to be 10% of the total battery SOC. However, the percentage may vary based on vehicular characteristics including battery capacity, vehicle mass, desired performance profile. Also, a vehicle may be equipped with a powertrain operation mode switch. The powertrain operation mode switch may be used to select a mode of operation out of a plurality of operating modes including economy, normal, towing, pursuit, sport or performance. The mode of operation may have a corresponding percentage, for example, an allocation percentage for economy mode operation may be less than 10%, a percentage for normal mode may be 10 to 20%, and a percentage for sport or performance mode may be as much as 40%. And, a percentage for pursuit mode for an emergency vehicle may be 80%.

The SOC allocation310has an associated torque assistance state or variable which has two states, enable and disable. The state may be implemented in hardware such as flip-flops or combinational logic, or it may be implemented in software in which a controller or processor is programmed to toggle the state and based on the state, the controller may activate features. In this example, the state toggles from disable to enable upon the SOC allocation310equal to 10% SOC due to a recharge current. And, the state toggles from enable to disable upon the SOC allocation310equal to 0% SOC due to a current flow to the electric machine to provide torque assistance to the engine. This hysteresis may be adjusted so that the enable and disable toggle points are independent of the total SOC allocation of 10%. For example, while maintaining the SOC allocation of 10%, the torque assistance enable may be set at 5%, so that once the SOC allocation equals 5%, the state will toggle to enable and torque assistance will be available based on a pedal demand event. A pedal demand event includes a request from a driver during a drive cycle for a change in power, such as propulsion power. For example, a driver moving a foot pedal may constitute a request for a change in power. A benefit of this is that it may allow torque assistance during certain stop and go traffic in which it was disabled when the hysteresis required the full allocation.

The charging and discharging of the battery312is based on a direction of current flow from an electric machine. A current generated by the electric machine is represented by a high indication equal to the Chg level, and a current flowing into the electric machine is represented by a low indication equal to the Dch level. Here, the vehicle is first propelled using torque assistance, which depletes the SOC308, which is reduced from approximately 80% to 70%. This reduction of SOC has a corresponding reduction of the SOC allocated to torque assist. In general, torque assistance from the electric machine may require a pedal demand event in which the power request associated with the pedal demand exceeds the available power from the internal combustion engine. Also, torque assistance may require a battery SOC greater than a threshold such as a minimum SOC, or that an allocation of battery SOC is greater than a SOC threshold. In this exemplary graph, the SOC allocation310to torque assistance is reduced from 10% to 0% at which point the allocation equals a lower SOC allocation and the torque assistance is disabled at time314by toggling TA_disable. As the battery is recharged from point314to point316, the SOC allocation310to torque assistance is increased by the same percentage. During the SOC discharge between point316and318, the SOC allocation310to torque assistance is held constant as the decrease in SOC is not attributable to torque assistance. The SOC allocation310associated with torque assistance continues to increase based on charging of the battery until the SOC allocation310is equal to a threshold. In this example, the threshold is 10% which is equal to the total SOC allocated to torque assistance. When the SOC allocation310equals 10% at point320, torque assistance is again enabled, however, as there is no power request from a pedal demand event greater than the engine power, current does not flow to the electric machine to provide torque assistance. However, at point322when the power request from a pedal demand event is greater than the engine power and the SOC allocation state is enabled, a current will flow to the electric machine to provide torque assistance. The current to the electric machine continues until the torque assistance is disabled at point324as a result of the SOC allocation310being reduced to 0%.