Auxiliary power supply for hybrid electric vehicle

A vehicle includes a first electric machine configured to generate low-voltage power. The vehicle includes a second electric machine configured to generate high-voltage power. The vehicle includes a power converter configured to convert high-voltage power generated by a second electric machine to low-voltage power. The vehicle includes a controller programmed to, responsive to a low-voltage power demand exceeding a limit of the power converter, operate the first electric machine and the power converter to satisfy the demand, and otherwise, responsive to the power converter being operational, operate only the power converter to satisfy the demand.

TECHNICAL FIELD

This application generally relates to a power supply in a hybrid electric vehicle configured to supplement power to a low-voltage power bus.

BACKGROUND

Hybrid-electric vehicles (HEV) utilize components that draw power from a high-voltage power bus and a low-voltage power bus. The source of power for the low-voltage power bus is derived from the high-voltage power bus. When sufficient power cannot be transferred from the high-voltage power bus to the low-voltage power bus, batteries connected to the low-voltage power bus may temporarily provide power but eventually become depleted. Once the batteries are depleted, components drawing power from the low-voltage power bus become inoperable.

SUMMARY

A vehicle includes a first electric machine configured to generate low-voltage power. The vehicle further includes a controller programmed to, (i) responsive to a low-voltage power demand exceeding a limit of a power converter configured to convert high-voltage power generated by a second electric machine to low-voltage power, operate the first electric machine and the power converter to satisfy the demand, and (ii) otherwise, responsive to the power converter being operational, operate only the power converter to satisfy the demand.

The controller may be further programmed to, operate the first electric machine to generate an amount of power that is a difference between the demand and the limit. The controller may be further programmed to, responsive to the power converter being unable to convert high-voltage power to low-voltage power, operate only the first electric machine to satisfy the demand. The controller may be further programmed to operate the first electric machine to generate an amount of power that is the lesser of the demand and a power limit of the first electric machine. The controller may be further programmed to, responsive to the demand exceeding the limit and an engine coupled to the first electric machine being stopped, operate the first electric machine to crank the engine. The controller may be further programmed to, operate an engine coupled to the first electric machine at a speed that is at least a predetermined speed configured to cause the first electric machine to generate an amount of power to satisfy the demand. The limit may be less than a maximum possible low-voltage power demand. A sum of the limit and a power capability of the first electric machine may be at least equal a maximum possible low-voltage power.

A method includes converting, by a power converter, high-voltage power generated by a first electric machine to low-voltage power. The method further includes operating the power converter and a second electric machine that is configured to generate low-voltage power responsive to a low-voltage power demand exceeding a limit of the power converter. The method further includes operating only the power converter to satisfy the demand otherwise, responsive to the power converter being operational.

The method may further include operating the second electric machine to crank an engine that is coupled to the second electric machine responsive to the demand exceeding the limit and the engine being stopped. The method may further include operating only the second electric machine to generate low-voltage power responsive to the power converter being unable to convert high-voltage power to low-voltage power. An amount of power generated by the second electric machine may be a lesser of the low-voltage power demand and a power limit of the second electric machine. The method may further include operating an engine coupled to the second electric machine at a speed that is at least a predetermined speed configured to cause the second electric machine to generate an amount of low-voltage power to satisfy the demand. The method may further include cranking an engine that is coupled to the second electric machine responsive to the engine being stopped. The limit may be less than a maximum possible low-voltage power demand.

A vehicle includes a first electric machine configured to generate low-voltage power and a power converter configured to convert high-voltage power generated by a second electric machine to low-voltage power. The vehicle further includes a controller programmed to, responsive to the power converter being unable to convert high-voltage power to low-voltage power, operate the first electric machine to generate an amount of power to satisfy a low-voltage power demand.

The controller may be further programmed to operate the first electric machine to generate the amount of power that is the lesser of the low-voltage power demand and a power limit of the first electric machine. The controller may be further programmed to, responsive to the power converter being able to convert high-voltage power to low-voltage power and the low-voltage power demand exceeding a limit of the power converter, operate the first electric machine and the power converter to satisfy the low-voltage power demand. The limit may be less than a maximum possible low-voltage power demand. Operating the first electric machine may include operating an engine configured to drive the first electric machine at a speed that is at least a predetermined speed configured to cause the first electric machine to generate the amount of power.

DETAILED DESCRIPTION

Referring toFIG. 1, a schematic diagram of a hybrid electric vehicle (HEV)110is illustrated according to an embodiment of the present disclosure.FIG. 1illustrates representative relationships among the components. Physical placement and orientation of the components within the vehicle may vary. The HEV110includes a powertrain112. The powertrain112includes an engine114that drives a transmission116, which may be referred to as a modular hybrid transmission (MHT). As will be described in further detail below, transmission116includes an electric machine such as an electric motor/generator (M/G)118, an associated traction battery120, a torque converter122, and a multiple step-ratio automatic transmission, or gearbox124.

The engine114and the M/G118are both drive sources for the HEV110. The engine114generally represents a power source that may include an internal combustion engine such as a gasoline, diesel, or natural gas powered engine, or a fuel cell. The engine114generates an engine power and corresponding engine torque that is supplied to the M/G118when a disconnect clutch126between the engine114and the M/G118is at least partially engaged. The M/G118may be implemented by any one of a plurality of types of electric machines. For example, M/G118may be a permanent magnet synchronous motor. Power electronics156condition direct current (DC) power provided by the traction battery120to the requirements of the M/G118, as will be described below. For example, power electronics may provide three phase alternating current (AC) to the M/G118.

When the disconnect clutch126is at least partially engaged, power flow from the engine114to the M/G118or from the M/G118to the engine114is possible. For example, the disconnect clutch126may be engaged and M/G118may operate as a generator to convert rotational energy provided by a crankshaft128and M/G shaft130into electrical energy to be stored in the traction battery120. The disconnect clutch126can also be disengaged to isolate the engine114from the remainder of the powertrain112such that the M/G118can act as the sole drive source for the HEV110. The M/G shaft130extends through the M/G118. The M/G118is continuously drivably connected to the M/G shaft130, whereas the engine114is drivably connected to the M/G shaft130only when the disconnect clutch126is at least partially engaged.

The M/G118is connected to the torque converter122via M/G shaft130. The torque converter122is therefore connected to the engine114when the disconnect clutch126is at least partially engaged. The torque converter122includes an impeller fixed to M/G shaft130and a turbine fixed to a transmission input shaft132. The torque converter122thus provides a hydraulic coupling between shaft130and transmission input shaft132. The torque converter122transmits power from the impeller to the turbine when the impeller rotates faster than the turbine. The magnitude of the turbine torque and impeller torque generally depend upon the relative speeds. When the ratio of impeller speed to turbine speed is sufficiently high, the turbine torque is a multiple of the impeller torque. A torque converter bypass clutch134may also be provided that, when engaged, frictionally or mechanically couples the impeller and the turbine of the torque converter122, permitting more efficient power transfer. The torque converter bypass clutch134may be operated as a launch clutch to provide smooth vehicle launch. Alternatively, or in combination, a launch clutch similar to disconnect clutch126may be provided between the M/G118and gearbox124for applications that do not include a torque converter122or a torque converter bypass clutch134. In some applications, disconnect clutch126is generally referred to as an upstream clutch and launch clutch134(which may be a torque converter bypass clutch) is generally referred to as a downstream clutch.

The gearbox124may include gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of friction elements such as clutches and brakes (not shown) to establish the desired multiple discrete or step drive ratios. The gearbox124may provide a predetermined number of gear ratios that may range from a low gear (e.g., first gear) to a highest gear (e.g., Nth gear). An upshift of the gearbox124is a transition to a higher gear. A downshift of the gearbox124is a transition to a lower gear. The friction elements may be controlled according to a shift schedule that sequences connecting and disconnecting certain elements of the gear sets to control the ratio between a transmission output shaft136and the transmission input shaft132. The gearbox124is automatically shifted from one ratio to another based on various vehicle and ambient operating conditions by an associated controller150, such as a powertrain control unit (PCU). The gearbox124then provides powertrain output torque to output shaft136.

It should be understood that the hydraulically controlled gearbox124used with a torque converter122is but one example of a gearbox or transmission arrangement; any multiple ratio gearbox that accepts input torque(s) from an engine and/or a motor and then provides torque to an output shaft at the different ratios is acceptable for use with embodiments of the present disclosure. For example, gearbox124may be implemented by an automated mechanical (or manual) transmission (AMT) that includes one or more servo motors to translate/rotate shift forks along a shift rail to select a desired gear ratio. As generally understood by those of ordinary skill in the art, an AMT may be used in applications with higher torque requirements, for example.

As shown in the representative embodiment ofFIG. 1, the output shaft136is connected to a differential140. The differential140drives a pair of wheels142via respective axles144connected to the differential140. The differential140transmits approximately equal torque to each wheel142while permitting slight speed differences such as when the vehicle turns a corner. Different types of differentials or similar devices may be used to distribute torque from the powertrain to one or more wheels. In some applications, torque distribution may vary depending on the particular operating mode or condition, for example.

The powertrain112may further include an associated powertrain control unit (PCU)150. While illustrated as one controller, the PCU may be part of a larger control system and may be controlled by various other controllers throughout the vehicle110, such as a vehicle system controller (VSC). It should therefore be understood that the powertrain control unit150and one or more other controllers can collectively be referred to as a “controller” that controls various actuators in response to signals from various sensors to control functions such as starting/stopping engine114, operating M/G118to provide wheel torque or charge the traction battery120, select or schedule transmission shifts, etc. Controller150may include a microprocessor or central processing unit (CPU) in communication with various types of computer readable storage devices or media. Computer readable storage devices or media may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. KAM is a persistent or non-volatile memory that may be used to store various operating variables while the CPU is powered down. Computer-readable storage devices or media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by the controller in controlling the engine or vehicle.

The PCU150communicates with various engine/vehicle sensors and actuators via an input/output (I/O) interface that may be implemented as a single integrated interface that provides various raw data or signal conditioning, processing, and/or conversion, short-circuit protection, and the like. Alternatively, one or more dedicated hardware or firmware chips may be used to condition and process particular signals before being supplied to the CPU. As generally illustrated in the representative embodiment ofFIG. 1, the PCU150may communicate signals to and/or from engine114, disconnect clutch126, M/G118, launch clutch134, transmission gearbox124, and power electronics156. Although not explicitly illustrated, those of ordinary skill in the art will recognize various functions or components that may be controlled by the PCU150within each of the subsystems identified above. Representative examples of parameters, systems, and/or components that may be directly or indirectly actuated using control logic executed by the controller include fuel injection timing, rate, and duration, throttle valve position, spark plug ignition timing (for spark-ignition engines), intake/exhaust valve timing and duration, front-end accessory drive (FEAD) components such as an air conditioning compressor, battery charging, regenerative braking, M/G operation, clutch pressures for disconnect clutch126, launch clutch134, and transmission gearbox124, and the like. Sensors communicating input through the I/O interface may be used to indicate turbocharger boost pressure, crankshaft position (PIP), engine rotational speed (RPM), wheel speeds (WS1, WS2), vehicle speed (VSS), coolant temperature (ECT), intake manifold pressure (MAP), accelerator pedal position (PPS), ignition switch position (IGN), throttle valve position (TP), air temperature (TMP), exhaust gas oxygen (EGO) or other exhaust gas component concentration or presence, intake air flow (MAF), transmission gear, ratio, or mode, transmission oil temperature (TOT), transmission turbine speed (TS), torque converter bypass clutch134status (TCC), deceleration or shift mode (MDE), for example.

An accelerator pedal152is used by the driver of the vehicle to provide a demanded torque, power, or drive command to propel the vehicle110. In general, depressing and releasing the accelerator pedal152generates an accelerator pedal position signal that may be interpreted by the PCU150as a demand for increased power or decreased power, respectively. Based at least upon input from the pedal, the PUC150commands torque from the engine114and/or the M/G118. The PCU150also controls the timing of gear shifts within the gearbox124, as well as engagement or disengagement of the disconnect clutch126and the torque converter bypass clutch134Like the disconnect clutch126, the torque converter bypass clutch134can be modulated across a range between the engaged and disengaged positions. This produces a variable slip in the torque converter122in addition to the variable slip produced by the hydrodynamic coupling between the impeller and the turbine. Alternatively, the torque converter bypass clutch134may be operated as locked or open without using a modulated operating mode depending on the particular application.

To drive the vehicle110with the engine114, the disconnect clutch126is at least partially engaged to transfer at least a portion of the engine torque through the disconnect clutch126to the M/G118, and then from the M/G118through the torque converter122and gearbox124. The M/G118may assist the engine114by providing additional power to turn the shaft130. This operation mode may be referred to as a “hybrid mode” or an “electric assist mode.”

To drive the vehicle110with the M/G118as the sole power source, the power flow remains the same except the disconnect clutch126is operated to isolate the engine114from the remainder of the powertrain112. Combustion in the engine114may be disabled or otherwise OFF during this time to conserve fuel. The traction battery120transmits stored electrical energy through a high-voltage (HV) bus154to a power electronics module156that may include an inverter, for example. The high-voltage bus154includes wiring and conductors for conducting current between modules and may include a positive-side conductor and a negative- or return-side conductor. The power electronics156convert DC voltage from the traction battery120into AC voltage to be used by the M/G118. The controller150commands the power electronics156to convert voltage from the traction battery120to an AC voltage provided to the M/G118to provide positive or negative torque to the shaft130. This operation mode may be referred to as an “electric only” operation mode.

In any mode of operation, the M/G118may act as a motor and provide a driving force for the powertrain112. Alternatively, the M/G118may act as a generator and convert kinetic energy from the powertrain112into electric energy to be supplied to the high-voltage bus154and/or stored in the traction battery120. The M/G118may act as a generator while the engine114is providing propulsion power for the vehicle110, for example. The M/G118may additionally act as a generator during times of regenerative braking in which rotational energy from wheels142, while rotating, is transferred back through the gearbox124and is converted into electrical energy for storage in the traction battery120.

It should be understood that the schematic illustrated inFIG. 1is merely exemplary and is not intended to be limiting. Other configurations are contemplated that utilize selective engagement of both an engine and a motor to transmit torque through the transmission. For example, the M/G118may be offset from the crankshaft128, an additional motor may be provided to start the engine114, and/or the M/G118may be provided between the torque converter122and the gearbox124. Other configurations are contemplated without deviating from the scope of the present disclosure. Other hybrid vehicle configurations are possible (e.g., power-split configuration) and the inventive aspects disclosed herein are applicable to these other configurations.

The vehicle110may utilize the M/G118to start the engine114. The controller150may command the disconnect clutch126to close and request torque from the M/G118via the power electronics156. The torque from the M/G118rotates the engine114so that the engine speed increases above a predetermined speed at which time the engine114may be commanded to provide fuel and spark to maintain continued engine rotation. The torque converter122may provide some torsional isolation during engine cranking and initial startup.

A low-voltage starter system168may also be coupled to the engine114to provide a secondary or backup means of starting the engine114. The low-voltage starter system168may be a belt-integrated starter/generator (BISG) system. An electric machine may be coupled to the engine114via a belt that is routed via pulleys. The belt may be configured to couple the engine114and the electric machine so that they rotate together. Further, a clutch may be present that is configured to engage and disengage rotation of the electric machine. For example, the electric machine may be disengaged when there is no demand for starting the engine114or generating power. The electric machine may be configured to generate electrical power when the belt is being driven by engine power. The electric machine may be configured to drive the belt to cause rotation of the engine crankshaft for starting the engine114. The low-voltage starter system168may include a control module that includes a power conversion system configured to transfer power between the electric machine and a vehicle power bus.

The vehicle110may further include a power converter module158and at least one auxiliary battery160. The auxiliary battery160may be low-voltage battery such as a 12 Volt battery that is commonly used in automobiles. Terminals of the auxiliary battery160may be electrically coupled to a low-voltage power network or bus166. The low-voltage power network166includes wiring and conductors for conducting current between connected modules. The power converter158may be electrically coupled between the high-voltage bus154and the low-voltage power bus166. The power converter158may be configured to transfer power from the high-voltage bus154to the low-voltage power bus166. The power converter158may be configured to convert high-voltage power from the M/G118to low-voltage power. For example, high-voltage power may be supplied at a voltage level that is compatible with the traction battery (e.g., 300 Volts). Low-voltage power may be supplied at a voltage level that is compatible with the auxiliary battery160(e.g., 12 Volts). The power converter module158may be a DC/DC converter that is configured to convert voltage from the high-voltage bus154to a voltage level compatible with the low-voltage power bus166(e.g., 12 Volts). The power converter158may be further configured to convert voltage from the low-voltage power bus166to voltage compatible with the high-voltage bus154. For example, the power converter158may be configured to provide a bi-directional flow of current between the high-voltage bus154and the low-voltage power bus166depending on the operating mode.

The vehicle110may include a display. For example, the display may be a part if an instrument panel. The display may include lamps, lights and/or other indicators for alerting the operator of conditions related to the vehicle. The display may be a liquid crystal display (LCD) module. The display may be in communication with controllers (e.g., PCU150) that are coupled to a communication bus.

FIG. 2depicts a possible power distribution system200for a vehicle. The power distribution system200may include the power converter158that is coupled between the high-voltage bus154and the low-voltage power network166. The low-voltage power network166may be comprised of a first low-voltage bus222and a second low-voltage bus210. An isolation switch202may be disposed between the first low-voltage bus222and the second low-voltage bus210.

The power converter158may be characterized by a power limit. The power limit may identify a maximum amount of power that may transferred by the power converter158. The power limit may be a function of a power rating of switching devices and/or a current capability of wiring within the power converter158. The power limit may vary during operation of the power converter158. For example, the power converter158may have a decreased power limit at higher temperatures. Reducing the power limit may prevent the power converter158from overheating during extreme operating conditions. The power converter158may increase and decrease the power limit based on the operating conditions.

A first auxiliary battery214may be electrically coupled to the first low-voltage bus222. A first set of electrical loads224may be electrically coupled to the first low-voltage bus222and receive power from the first low-voltage bus222when activated.

The isolation switch202may include a switching element206. The isolation switch202may include a diode (not shown) in parallel with the switching element206. The switching element206may be a transistor (e.g., metal oxide semiconductor field effect transistor (MOSFET)). The isolation switch202may be configured such that the switching element206is normally closed. In some configurations, the switching element206may be a relay. The switching element206may be configured such that, when closed, current may flow between the first low-voltage bus222to a second low-voltage bus210. The isolation switch202may be referred to as a vehicle power relay. The isolation switch202may be capable of isolating portions of the low-voltage power network166from one another.

A second auxiliary battery212may be electrically coupled to the second low-voltage bus210. An electric machine226of the low-voltage starter system168(e.g., BISG) may be electrically coupled to the second low-voltage bus210using a second power converter module204. The electric machine226may be configured to generate low-voltage power when operated as a generator. The second power converter module204may be configured to convert a voltage from the electric machine226to a voltage compatible with the second low-voltage bus210. For example, the electric machine226may provide an alternating current (AC) to the second power converter module204when driven by the engine114. The second power converter module204may be configured to convert the AC power to a direct current (DC) voltage/current that is compatible with the second low-voltage bus210(e.g., 12 Volt DC voltage). The second power converter module204may also be configured to provide a voltage/current to the electric machine226to operate as a motor for starting the engine114. The second power converter module204may include switching devices (e.g., solid-state transistors, relays) that are configured to selectively couple terminals of the electric machine226to terminals of the second low-voltage bus210. Power transfer between the second low-voltage bus210and the electric machine226may be achieved by operating the switching devices.

A second set of electrical loads208may be electrically coupled to the second low-voltage bus210. The second set of electrical loads208may include a cooling fan for the engine114.

The isolation switch202may be operated in a closed position such that the first low-voltage bus222and the second low-voltage bus210are electrically coupled together and effectively function as a single low-voltage bus or power network. In the description that follows, it may be assumed that the isolation switch202is in the closed position so that a single low-voltage network is present.

The power distribution system200may include a controller220. The controller220may be in communication with components within and external to the power distribution system200. The controller220may include a hardwired interface for communication and may include a serial communication interface for communicating via a vehicle communication network (e.g., Controller Area Network). The controller220may communicate with the power converter158and the second power converter204. The controller220may communicate with the PCU150that is associated with the engine114. The controller220may further be in communication with the first and second sets of electrical loads224,208that are coupled to the first low-voltage bus222and/or the second low-voltage bus210.

Operation of the electrical loads224,208creates a power demand on the low-voltage power network166(e.g., low-voltage power demand). The power demand of the low-voltage power network166may be characterized by a peak power demand or maximum total load power demand. The peak power demand may be a maximum possible power demand that may occur during vehicle operation. The peak power demand may occur during certain worst-case operating conditions.

The power demand on the low-voltage power network166may be satisfied with power from the auxiliary battery214and/or the second auxiliary battery212. The power demand may also be satisfied with power provided by the power converter158. In addition, the power demand may be satisfied with power from the second power converter204that is derived from the electric machine226driven by the engine114.

Under normal operating conditions, it may be desired to support the low-voltage power network166with power from the power converter158. Under these conditions, the power converter158may transfer energy from the high-voltage bus154to the low-voltage power network166. The power converter158may be controlled to output an amount of power to satisfy the power demand of the electrical loads224,208. In addition, the power converter158may be controlled to output power for charging the first auxiliary battery214and the second auxiliary battery212. During normal load conditions, the power limit of the power converter158may be sufficient to satisfy the power demand on the low-voltage power network166.

However, during some operating conditions, the power limit of the power converter158may be insufficient to satisfy the entire power demand on the low-voltage power network166. During conditions in which the engine114is operating for long periods at high loads, the power demands for electrical loads associated with engine cooling may be increased. For example, an engine cooling fan may be activated to cool the engine coolant. The additional power demand of the engine cooling fan may cause the power demand on the low-voltage power network166to exceed the power limit of the power converter158. Under this condition, the additional power demand may be satisfied with power from the first auxiliary battery214and/or the second auxiliary battery212. However, drawing power from the auxiliary batteries212,214may reduce the corresponding state of charge of the auxiliary batteries212,214. It may be desired to maintain the auxiliary batteries212,214at a high state of charge so that battery power is available between ignition cycles.

Note that the extreme power demand may only occur on rare occasions. It is possible to design the power converter158to satisfy the extreme power demand, but at greater cost and size. To reduce cost and size of the power converter158, it may be beneficial to design the power converter158for a power demand level that is less than the maximum possible low-voltage power demand. By reducing the maximum power capability of the power converter158, other means of satisfying the maximum possible power demands may be implemented. The presence of the low-voltage starter system168(including electric machine226) permits additional options for satisfying the power demand.

To prevent draining the auxiliary batteries214,212during high-load conditions, it may be desired to operate the electric machine226to generate power to the low-voltage power network166. Under a typical high-load condition, the engine114may be running at elevated speeds. The electric machine226may be controlled through the second power converter204to provide an amount of power to the low-voltage power network166. A sum of power provided by the power converter158operating at the power limit and a power generation capability of the electric machine226may be at least equal to the maximum total load power demand of the low-voltage power network166.

The controller220may be programmed to monitor the power demand of the low-voltage power network166. The power converter158may be configured to monitor an amount of power transferred from the high-voltage network154to the low-voltage power network166. For example, the power converter158may include a current sensor and a voltage sensor at the output of the power converter158. The power converter158may include a control module that monitors the voltage and current to derive a power output. The power value may be communicated to the controller220. In other configurations, signals from the current sensor and voltage sensor at the output of the power converter158may be routed to the controller220. As described above, the power converter158may be characterized by a power limit. If the amount of power being output by the power converter158is at or near the power limit, then the controller220may determine that the power limit has been reached. In some configurations, a power value that is within a predetermined range of the power limit may indicate that the power limit has been reached. For example, an amount of power delivered that exceeds 95% of the power limit may be determined to have reached the power limit. The predetermined range may allow for component tolerances of the power converter158.

The power demand of the low-voltage power network166may further be determined by monitoring power flow to and from the auxiliary batteries214,212. A first sensor unit216may be configured to measure a voltage and/or current of the first auxiliary battery214. A second sensor unit218may be configure measure a voltage and/or current of the second auxiliary battery212. The sensor units216,218may include a current sensor configured to measure the current flowing to and from the associated battery. For example, the current sensor may be a current shunt or a Hall effect sensor. The sensor units216,218may include a voltage sensor configured to measure a voltage across terminals of the associated battery. The voltage sensor may include a circuit configured to scale and filter the battery voltage to a level suitable for input to the controller220.

Power transferred to or from the auxiliary batteries212,214may be computed as the product of the associated voltage and current values. The power demand on the low-voltage power network166may exceed the power limit of the power converter158when it is determined that the power output of the power converter158has reached the power limit. The power demand may be determined to exceed the power limit when one or more of the auxiliary batteries212,214are delivering power to the low-voltage power network166while the power converter158is operating at the power limit.

If the power demand of the low-voltage power network166exceeds the power limit of the power converter158, the controller220may operate the electric machine226to generate power to the low-voltage power network166to satisfy the power demand. The power converter158may be operated at the power limit. The controller220may operate the electric machine226by controlling operation of the second power converter204and the engine114. The controller220may monitor an engine speed to determine if the electric machine226will have sufficient power generation capability. If the engine114is presently stopped, the controller220may request that the engine114be started. The controller220may operate the second power converter204to operate the electric machine226as a motor to crank the engine114. Once the engine114is running, the controller220may request the engine114to operate at an engine speed that is sufficient to generate a desired power output of the electric machine226and the second power converter204to satisfy the power demand.

If the power demand of the low-voltage network166is less than or equal to the power limit of the power converter158and the power converter158is operational, the controller220may operate only the power converter158to satisfy the power demand. The power converter158may be operational when there are no conditions present that inhibit the transfer of power through the power converter158.

The power output of the second power converter204may be controlled to an amount of power that is the difference between the power demand and the power limit of the power converter158. In some configuration, the power output may be controlled to a predetermined amount of power. In other configurations, the power output of the second power converter204may be dynamically changed such that power flowing into or from the batteries212,214is less than a predetermined threshold.

Under some conditions, the power converter158may be unable to transfer power from the high-voltage bus154. For example, the power converter158may be inoperable. For example, high temperature operation of the power converter158may cause internal switching devices to reach a critical temperature. To protect the switching devices, the switching devices may be switched off to prevent further degradation. The switching devices may be switched off until the temperatures falls below a predetermined temperature. The power converter158may be permanently or temporarily unable to transfer power. In response to the power converter158being unable to transfer power between the high-voltage bus154and the low-voltage power network166, the controller220may be programmed to operate the electric machine226to generate power. The amount of power generated may be the lesser of the power demand of the low-voltage power network166and the power limit of the electric machine226.

The controller220may be further programmed to operate the electric machine226to generate power in response to activation of predetermined electrical loads. The predetermined electrical loads may include infrequently used electrical loads that are configured to request high levels of power demand. For example, the predetermined electrical loads may include the engine cooling fan. It may be anticipated that the engine cooling fan may be activated during heavy engine loads that occur infrequently.

FIG. 3depicts a flow chart of a possible sequence of operations to implement the power distribution system. The operations may be implemented in a controller (e.g.,220). At operation302, inputs are collected and monitored. For example, status information may be received from the power converter158, voltages and currents from the sensors may be measured and stored. Information for determining the power demand and power limit may be received and/or computed based on the received and measured inputs. The power converter158may be configured to communicate status information. For example, the power converter158may communicate any diagnostic codes that may render the power converter158unable to transfer power. In addition, the power converter158may communicate any power limit information.

At operation304, a check may be made to determine if the power converter158is unable to transfer power. For example, in the event of a malfunction, the power converter158may be unable to operate and the power converter158may send a diagnostic code or indicator indicative of the condition. If the power converter158is unable to transfer power, operation314may be performed.

If the power converter158is determined to be capable of power transfer, then operation306may be performed. At operation306, power demand of the low-voltage power network166may be determined. The power demand may be determined by voltage and current measurements as described previously herein. In addition, the inputs may include status and operational information regarding the electrical loads. At operation308, the power limit of the power converter158may be determined. The power limit may be a stored value. The power limit may be a table of stored values indexed by an operational parameter (e.g., temperature). The power limit may be a value received from the power converter158. At operation310, the power demand is compared to the power limit. If the power demand is less than or equal to the power limit, then operation312may be performed. At operation312, the electric machine226may be operated normally. For example, the electric machine226may be normally idle. That is, the electric machine226may not be generating power nor operating as a motor. If the power demand is greater than the power limit, then operation314may be performed.

At operation314, the electric machine226may be operated as a generator to generate power for the low-voltage power network166. The second power converter204may be controlled to convert the power generated by the electric machine226to a form compatible with the low-voltage power network166. At operation316, the controller may manage the power output of the electric machine226by controlling the second power converter204. For example, the electric machine226may be controlled to output an amount of power that is the difference between the power demand and the power limit. The power may be controlled by controlling the voltage and/or current supplied to the low-voltage power network166. At operation318, the controller may manage the engine114. Managing the engine114may include controlling the engine speed so that the electric machine226may generate a sufficient amount of power. Managing the engine114may also include starting the engine114if the engine114is in a non-running state.

Operation of the electric machine226to generate power provides several benefits. The power converter158may be sized for nominal power demands on the low-voltage power network166. The electric machine226and second power converter204may supplement the power converter158during peak power demands. By sizing the power converter158for nominal power demands instead of peak power demands, cost of the power converter158may be reduced. In addition, the electric machine226and second power converter204provide a secondary means of powering the low-voltage power network166when the power converter158is unable to transfer power. For example, some power may be provided to the low-voltage power network166in the event of an inoperable power converter158. In such cases, the engine114may remain running to power the electric machine226.