Method and system for use with a vehicle electric storage system

A method and system for controlling a vehicle having an electric powertrain and an electric energy storage system. The electric energy storage system includes a capacitor, DC/DC converter, and a battery. The electric energy storage system is controlled to maximize use of the capacitor relative to use of the battery.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to powertrain hybrid electric and electric vehicles having an electric storage system (ESS).

2. Background Art

In a hybrid electric vehicle powertrain with an electric energy storage sources (ESS), electric power can flow between the powertrain and the ESS. In some cases, the power flows to the ESS for storage. In other cases, the power flows to the powertrain for consumption.

U.S. Pat. No. 5,318,142 discloses one configuration for an ESS. It describes a system having a battery and a supercapacitor connected to a bus by separate energy conversion and control devices. The separate conversion and control devices add cost and control complexity to the system. It would be desirable to eliminate one or both of these devices.

Another shortcoming of the configuration of the ESS of the '142 patent is that it fails to maximize usage of the capacitor. Maximum capacitor usage is desirable because of the performance advantages of a capacitor relative to a battery. In particular, a capacitor has better charge and discharge rates and efficiencies relative to a battery. Further, less frequent charging and discharging of a battery increases its life expectancy.

SUMMARY OF THE INVENTION

The present invention relates to a vehicle having a hybrid electric or electric powertrain and an electric energy storage system (ESS). The powertrain includes structures and features that allow the vehicle to use electric power for driving. Typically, the powertrain consists of an electric power generation unit and an electric drive unit.

The powertrain can receive power from the ESS, which converts it to mechanical power to drive the vehicle. In addition, the powertrain can generate electric power, using a fuel cell or an internal combustion engine, for powering a generator. Power can be generated also by regenerative braking. The power is provided to the ESS for storage.

The present invention includes a battery, a capacitor, and a DC/DC converter. The DC/DC converter is controllable by a vehicle system controller to control power flow between the powertrain and the ESS. This controls the powertrain and the DC/DC converter to maximize capacitor usage relative to battery usage.

In accordance with one aspect of the present invention, capacitor usage is maximized by controlling power flow to and from the battery. Capacitor usage can be maximized by controlling the DC/DC converter to prevent discharging of the battery until after the capacitor has been discharged to a low discharge threshold. In addition, capacitor usage can be maximized by controlling the DC/DC converter to prevent charging of the battery until after the capacitor has been charged to a high charge threshold.

Another aspect of the present invention relates to calculating an ESS power demand for maintaining the state of charge (SOC) of the ESS that may change due to ESS charge and discharge during vehicle operation. The ESS power demand can be used by the vehicle system controller to control the electric powertrain and the DC/DC converter. The electric powertrain can be controlled to provide power to the ESS if the ESS power demand is positive, and to accept power from the ESS if the ESS power demand is negative. Simultaneously, the controller can control the DC/DC converter to maximize capacitor usage during charging/discharging of the ESS. Power demand can be based both on capacitor state of charge (SOC) and battery SOC.

An aspect of the present invention relates to utilizing the capacitor in the ESS to compensate for a transient nature of vehicle operation in which a constantly changing motor power demand makes it difficult to quickly balance power from an electric generator unit with a power demanded by a motor. In particular, the capacitor is charged and discharged prior to charging and discharging the battery so as to maximize capacitor usage. This increases battery life and makes it possible to use a smaller battery.

One advantage of the present invention is that it includes an electric energy storage system (ESS) that includes fewer controllers.

Another advantage of the present invention is that it maximizes capacitor usage and takes advantage of the greater durability and charging/discharging power capabilities of the capacitor relative to the battery.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1illustrates vehicle10having electric energy storage system (ESS)14in electric communication with powertrain16over bus18. The ESS14can be configured for operation with hybrid or a purely electric vehicle powertrain16, including a series hybrid vehicle (SHEV), a parallel hybrid vehicle (PHEV), a parallel-series hybrid vehicle (PSHEV), or a fuel cell hybrid vehicle (FCHEV). The scope of the present invention, however, is not limited to these configurations.

FIG. 2illustrate one embodiment of the invention wherein powertrain16includes an electric power generator unit20and a traction motor24, and wherein ESS14includes capacitor28, battery30, and DC/DC converter32. Traction motor24receives electric power from generator unit20and/or ESS14for driving wheels34.

Electric generator unit20generates electric power, such as with a fuel cell or an engine/generator. Some or all of the electric power flows to motor24and/or to ESS14over electric bus18. Likewise, some or all of the electric energy stored by the ESS14flows to powertrain16over bus18. In this manner, electric power can flow between powertrain16and ESS14.

Vehicle system controller (VSC)40controls operation of powertrain16and ESS14. Communication buses42and44extend from VSC40to ESS14and powertrain16and control signals are transferred therebetween. This allows VSC24to determine whether the powertrain16is to accept or provide power to and from bus18and whether battery30is to accept or provide power to and from bus18, as described below.

In one aspect of the present invention, power generator unit20is controlled such that P*gen=P*ess+P*mot, wherein P*gen is the power demand of electric generator unit20, P*essis the power demand of ESS14, and P*motis the power demand of traction motor24. The power demand of ESS14is further defined as P*ess=P*cap+P*bat, wherein P*capis the power demand of capacitor28and P*batis the power demand of battery30.

In general, with load-following strategy for power and torque control, VSC40estimates motor power demand (P*mot) based on a driver's torque demand—other vehicle operating parameters can also be included. VSC40controls the power output of electric generator unit20(Pgen) to meet the motor power demand (P*mot).

The transient nature of vehicle operation, and in particular, the constantly changing motor power demand (P*mot) make it difficult to quickly balance power from electric generator unit20with the power to the motor (Pmot). An imbalance occurs where the power output of generator unit20is either more or less than the motor power consumption. ESS14acts as a buffer to make up for the imbalance of power by providing power when Pgenis less than Pmotand by taking power when Pgenis greater than Pmot.

Other power imbalances can arise during starting of generator unit20and regenerative braking of motor24. At start-up, the driver may demand power for driving vehicle, which generator unit20is unable to immediately provide. ESS14can make up for a lack of immediate power by discharging to motor24. Regenerative braking is another condition where motor24is producing power rather than consuming power. The power produced can be consumed by vehicle auxiliary loads (not shown) and, in accordance with the present invention, received by ESS14if ESS14is not fully charged.

Capacitor28, connected directly to electric power generator unit20and motor24over bus18, serves primarily as a power buffer by compensating for transient charge and discharge spikes between generator unit20and motor24. Battery30is connected to bus18by DC/DC converter32. It serves as an energy buffer for either dumping surplus energy from generator unit20, motor24, and capacitor28when the charge of capacitor28is high or the power rating of capacitor28is not high enough to meet the ESS power demand (P*ess), or for delivering energy back to generator unit20, motor24, and capacitor28when the charge of capacitor28is low or the power rating of capacitor28is not high enough to meet the ESS power demand (P*ess).

The ability of ESS14to receive or discharge power is determined based on its power demand (P*ess). The ESS power demand values (P*ess) can be positive or negative. Positive power demand values indicate a need for ESS14to receive energy. Negative power demand values indicate a need for ESS14to discharge electric energy. The power demand needs of ESS14are included in the vehicle control equation: P*gen=P*ess+P*mot.

VSC40transfers command signals over signal flow path42to control electric generator unit20and motor24. VSC40transfers command signals over signal flow path44to control DC/DC converter32. Additional signals are transferred over signal flow paths42and44to monitor the operation of electric generator unit20, motor24, capacitor28, battery30, and DC/DC converter32.

Capacitor28is a typical high voltage capacitor commonly used in electric vehicles. It is an electric energy storage device of low energy density, high power density, and high durability and provides fast charging/discharging. Battery30is a typical high voltage battery commonly used in electric vehicles. It is an electric storage device of high energy density, low power density, and low durability and provides slow charging/discharging.

The invention takes advantage of the properties by maximizing charging and discharging of capacitor28so that response time of ESS14is short. At the same time, increase reliance on capacitor28allows the charging and discharging of battery30to be limited, so that battery size, and therefore cost, can be lowered and usage prolonged.

The graph54inFIG. 3illustrates how to monitor the capability of ESS14to receive or discharge power. Graph54is one means for establishing the ESS power demand value (p*ess) based on the respective SOC values of capacitor28(SOCcap) and battery30SOCbat) calculated by VSC40. Graph58shows capacitor power demand curve60and graph64shows battery power demand curve66.

Capacitor28is preferably maintained within a neutral charge band defined by SOCcap—lb(lower band) and SOCcap—up(upper band). If the capacitor SOC deviates beyond this range, a need arises for charging or discharging capacitor28. If the capacitor SOC is within the range, then capacitor28acts as a power buffer as described above, wherein capacitor28receives or discharges power to make up an imbalance in power output of generator unit20and power consumption and power production of motor24.

VSC40determines a need for discharging capacitor28and calculates a corresponding negative value for P*capif its SOC is greater than SOCcap—ub. VSC40determines a need for charging capacitor28and calculates a corresponding positive value for P*capif its SOC is less than SOCcap—lb. P*capis zero if SOCcapis within the neutral charge band, which indicates no need for charging or discharging of capacitor. Capacitor28then can be used to buffer power.

The power demand values corresponding with the rate of charging and discharging capacitor28are variable. Power demand curve60gradually increases negatively from zero at SOCcap—ubto a maximum negative Pcap—minat maximum capacitor SOC (SOCcap—max). Power demand curve60gradually increases positively from zero at SOCcap—lbto a maximum positive Pcap—maxat minimum capacitor SOC (SOCcap—min).

P*batis determined in a manner similar to the determination of P*cap. Battery30is preferably maintained within a neutral charge band defined by SOCbat—lb(lower band) and SOCbat—up(upper band). If the battery SOC deviates beyond this range, a need arises for charging or discharging battery30. If the battery SOC is within the range, then battery30is sufficiently charged and the battery can additionally act as the power buffer described above. Preferably, the use of battery30as a power buffer is limited to conditions where operation of capacitor28is insufficient to buffer power so that charging and discharge of battery30is limited.

VSC40determines a need for discharging battery30and calculates a corresponding negative value for P*batif its SOC is greater than SOCbat—ub. VSC40determines a need for charging battery and calculates a corresponding positive value for P*batif its SOC is less than SOCbat—lb. P*batis zero if SOCcapis within the neutral charge band to indicate no need for charging or discharging of battery and to indicate that battery30can be used to buffer power.

The power demand values corresponding with the rate of charging and discharging of battery30are variable. Power demand curve66gradually increases negatively from zero at SOCbat—ubto a maximum negative Pbat—minat maximum capacitor SOC (SOCbat—max). The power demand curve gradually increases positively from zero at SOCbat—lbto a maximum positive Pbat—maxat minimum battery SOC (SOCbat—min).

VSC40preferably modifies the P*batvalue based on the efficiency of DC/DC converter32, as shown in box68. This is done to compensate for energy losses due to DC/DC converter32passing energy to battery30or receiving energy from battery30. The modified P*batvalue, for purposes of clarity, is still referred to as P*bat.

Graphs58and64are merely an exemplary means for determining P*capand P*batfrom the respective SOC values and are not intended to limit the scope of the present invention. Algorithms, fuzzy logic, neural networks, and the like could also be used to determine P*capand P*bat.

The value for P*capand the modified value for P*batare outputted to summer74. The output of summer74corresponds with the total ESS power demand (P*ess=P*cap+P*bat).

FIG. 3relates to one means for determining the ESS power demand value (P*ess) from the SOC of battery30and capacitor28. P*esscan also be determined based on the operating conditions of electric generator unit20and motor24.

Electric generator unit20may not supply sufficient power to meet the motor power demand (P*mot), in which case VSC40may assign a negative P*essvalue and thereby control ESS14to discharge power to motor to make up to the lack of power provided by electric generator unit20. Preferably, the assigned negative value is limited such that battery30and capacitor28are not discharged beyond their respective SOC low limit values (SOCcap—minand SOCbat—min).

Likewise, traction motor24may be generate electric energy during a regenerative braking event, in which case VSC40may assign a positive ESS value and thereby control ESS14to receive at least some of the power generated by the regenerative braking of traction motor24. Preferably, the assigned positive value is limited such that battery30and capacitor28are not charged beyond their respective SOC upper limit values (SOCcap—maxand SOCbat—max).

FIG. 4illustrates a flowchart80for controlling DC/DC converter32according to the ESS power flow (Pess). The control of ESS consists of at least one of controlling electric generator unit20and motor24to produce power, controlling motor24to consume power, and controlling DC/DC converter32with DC/DC converter control signal (P*dcdc) to permit charging and discharging of battery30. The control of ESS14maximizes use of capacitor28relative to use of battery30to take advantage of its improved performance characteristics relative to battery30.

Pessis determined at decision block82. If block82indicates Pessis negative, one or both of capacitor28and battery30will be discharged. At decision block84it is determined whether capacitor28will be discharged based on whether capacitor SOC (SOCcap) is greater than or less than its minimum SOC (SOCcap—min). If SOCcapis greater than or equal to SOCcap—min, DC/DC converter is controlled at block88so that both capacitor28and battery30can be discharged. VSC40sets the DC/DC converter command signal (P*dcdc) at block88so that DC/DC converter32limits battery30discharge to a difference between the ESS power flow (Pess) and power demand of capacitor (P*cap). The power demand of capacitor (P*cap) corresponds with the difference between the minimum capacitor SOC (SOCcap—min) and the actual capacitor SOC (SOCcap). Limiting battery30discharge in this manner maximizes use of capacitor28to take advantage of its improved characteristics relative to battery30.

If SOCcapis less than SOCcap—min, VSC40controls DC/DC converter32so that only battery30can be discharged. VSC40sets the DC/DC converter command signal (P*dcdc) at action block90so that DC/DC converter32sets the battery30discharge below its low limit (SOCcap—min) Limiting capacitor28discharge in this manner limits capacitor28degradation, which may otherwise occur if capacitor28is discharged too much.

If block82indicates Pessis positive, one or both of capacitor28and battery30is to be charged. It is determined at decision block94whether battery30and capacitor28are to be charged based on whether capacitor SOC (SOCcap) is greater than or less than its maximum SOC (SOCcap—max).

It SOCcapis less than SOCcap—max, VSC40DC/DC converter32is controlled at96so that both capacitor28and battery30can be charged. VSC40sets the DC/DC converter command signal (P*dcdc) so that DC/DC converter32limits battery30charge to a difference between the ESS power flow (Pess) and power demand of capacitor (P*cap). The power demand of capacitor (P*cap) corresponds to the difference between the maximum capacitor SOC (SOCcap—max) and the actual capacitor SOC (SOCcap). Limiting battery30charge so that capacitor28is charged first maximizes capacitor28usage to take advantage of its improved characteristics relative to battery30.

If SOCcap—maxis less than SOCcap, VSC40controls DC/DC converter32at action block98so that only battery30can be charged. VSC40sets the DC/DC converter command signal (P*dcdc) so that DC/DC converter32sets the battery charge to cover the entire the ESS power flow (Pess), thereby limiting any charging of capacitor above its max limit (SOCcap—max). Limiting capacitor28charge in this manner limits capacitor28degradation, which may otherwise occur if capacitor28is charged too much.