Abstract:
A starter/alternator system ( 24 ) for hybrid electric vehicle ( 10 ) having an internal combustion engine ( 12 ) and an energy storage device (34) has a controller ( 30 ) coupled to the starter/alternator ( 26 ). The controller ( 30 ) has a state of charge manager ( 40 ) that monitors the state of charge of the energy storage device. The controller has eight battery state-of-charge threshold values that determine the hybrid operating mode of the hybrid electric vehicle. The value of the battery state-of-charge relative to the threshold values is a factor in the determination of the hybrid mode, for example; regenerative braking, charging, battery bleed, boost. The starter/alternator may be operated as a generator or a motor, depending upon the mode.

Description:
GOVERNMENTAL RIGHTS 
     This invention was made with government support under Prime Contract No. DE-AC36-83CH10093, Subcontract No. ZCB-4-13032-02, awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to control systems and methods for controlling hybrid electric vehicles, more specifically, the present invention relates generally to battery charge management for hybrid electric vehicles. 
     BACKGROUND OF THE INVENTION 
     Automotive vehicles with internal combustion engines are typically provided with both a starter motor and alternator. In recent years, a combined alternator and starter motor has been proposed particularly for use in hybrid electric vehicles. During initial startup of the vehicle, the starter/alternator functions as a starter. While functioning as a starter, the starter/alternator rotates the crankshaft of the engine while the cylinders are fired. 
     After the engine is started, the starter/alternator is used as a generator to charge the electrical system of the vehicle. 
     In automotive applications, the engine may be shut down during stops (e.g., red lights). When the accelerator is depressed, the starter/alternator starts the motor and the engine will resume firing. Thus, many startups may occur over the course of a trip. 
     The starter/alternator can also be operated as a motor if the state-of-charge management strategy deems it desirable to reduce the state-of-charge of the battery (i.e., “bleed model”) to allow for the collection of regenerative energy via braking. 
     Another mode of the starter/alternator is providing additional torque (i.e., “boost”) to the wheels when the torque of the engine is not enough to meet the driver demand. 
     During braking hybrid electric vehicles seek to recharge the batteries by recapturing the kinetic energy of the vehicle. Batteries for the hybrid electric vehicles by their nature have a limited operating life. If, during operation, an adequate charge is not maintained the battery life may be shortened. 
     It would therefore be desirable to control the state of charge of the battery to prolong battery life while achieving the required hybrid functionality. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the invention to create calibrateable state-of-charge boundary parameters for this state-of-charge management algorithm for the primary purpose of operating the hybrid electric vehicle in the desired hybrid operating mode, which will have the secondary effect of increasing the battery life of a hybrid electric vehicle. 
     In one aspect of the invention, a method of charging a battery in a hybrid electric vehicle having a starter/alternator and a battery having a state of charge comprises the step of: 
     When the state of charge is below a first predetermined value, allowing the charging of the battery through operation of the starter/alternator as a generator; and similarly when the state of charge is below a another predetermined value, allowing the battery to be charged via regenerative braking; and when the state-of-charge is below another predetermined value, keeping the engine on because the battery charge may be too low to restart the engine if it is stopped. 
     When the state of charge is above another predetermined value, allowing the battery to bleed by, operating the starter/alternator as a motor and when the state-of-charge is above another predetermined value allowing the vehicle to “boost” the torque at the wheels to meet driver demand that could not be met by the engine alone, by operating the starter/alternator as a motor, consequently reducing the state-of-charge. 
     In a further aspect of the invention, a system for an automotive vehicle having an internal combustion engine, an energy storage device and a starter/alternator coupled to the engine includes a controller coupled to the starter/alternator and to the energy storage device. The controller monitors a state of charge of the energy storage device. When the state of charge is below predetermined values for the various hybrid modes of charging, regenerative braking or for keeping the engine on if the SOC is very low, the controller allows charging of the battery through operation of the starter/alternator as a generator. When the state of charge is above predetermined values, the controller allows the battery to reduce the state-of-charge for both the “bleed” and “boost” hybrid modes by operating the starter/alternator as a motor. 
     One advantage of the invention is the state of charge manager may be set with some hysteresis to avoid rapid changing between the different modes. 
     Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an automotive vehicle having a starter/alternator system according to the present invention. 
     FIG. 2 is a state diagram of a state of charge management according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is described with respect to a particular state of charge diagram and a particular hybrid vehicle configuration. However, the teachings of the present invention may be applied to various alternative state-of-charge diagrams and hybrid vehicle configurations would be evident to those in the art. 
     Referring now to FIG. 1, an automotive vehicle  10  is illustrated having an internal combustion engine  12  having cylinders with pistons (not shown) located therein. Each cylinder is coupled to a spark plug and fuel pump through a fuel injector (not shown) or other fuel delivery system as is common in the art. Each cylinder also has a spark plug or other ignition source coupled to a powertrain control unit  14 . A powertrain control unit  14  controls the ignition timing and fuel pump operation in a conventional manner subject to the improvements of the present invention. 
     Engine  12  is coupled to a transmission  16 . Transmission  16  may be automatic, manual or continuously variable. Transmission  16  is coupled to a differential  18  to drive an axle  20  which in turn provides power to wheels  22 . Of course, the present invention is also applicable to four wheel drive systems in which all of the wheels  22  are driven. 
     A starter/alternator system  24  that includes a starter/alternator  26  and its associated control electronics is coupled to engine  12 . In one embodiment of the present invention, starter/alternator  26  is positioned between a housing of transmission  16  and the engine  12 . A clutch  28  is used to engage and disengage engine  12  from transmission  16 . As will be further described below, starter/alternator  26  is used as a starter during engine startup and as an alternator to supply power to recharge the batteries of the vehicle and to supply electrical loads. Clutch  28  also allows starter/alternator  26  to start the engine prior to engagement of the transmission. 
     Starter/alternator system  24  has a vehicle system controller  30  that is coupled to powertrain control unit  14  and to a power inverter  32 . In practice, the power inverter  32  and system controller  30  may be contained in a single package. The inverter  32  is used to convert DC power to AC power in the startup mode and AC power to DC power in power generation mode as will be further described below. 
     Power inverter  32  is coupled to an energy storage device  34  such as a high power battery or an ultra capacitor. Of course, those skilled in the art would recognize that DC to DC converter (not shown) may be interposed between energy storage device  34  and inverter  32 . Also, the energy storage device  34  may be, for example, comprised of many batteries including a 12 volt battery to provide power to the vehicle electronics. Of course, the actual battery voltage is dependent on the particular system to which it is attached. 
     Vehicle system controller  30  has a state of charge (SOC) manager  40  that monitors the state of charge of energy storage device  34 . The goal of the SOC manager is to prevent the state of charge of energy storage device  34  from overcharging or operating in an undesirably low state that may prevent the vehicle from restarting. Various thresholds may be set for the different modes of operation as will be described below. These thresholds may overlap to provide some hysterisis to prevent rapid movement back and forth between two different states. Also, because some of the thresholds overlap, priority may be set between them to establish desirable operation. 
     Referring now to FIG. 2, a state diagram of state of manager  40  is illustrated to illustrate its desired operation. Various state of charge thresholds are established. The first threshold is an engine running threshold (SOC_ENG_RUN) below which the state of charge manager keeps the engine running because the power left in the energy management device is less than that required to restart the engine. Another threshold greater than the engine running threshold is the boost off threshold (SOC_BOOST_OFF). When the energy storage device has a state of charge that is below the boost off threshold, the state of charge manager does not allow the starter/alternator  26  to act as a motor and boost the power output of the hybrid electric powertrain. The boost off threshold is set because if boost were provided from the energy storage device, the energy storage device would likely not have enough power after boosting to allow the engine to restart. 
     Another threshold greater than the boost off threshold is the engine off threshold (SOC_ENG_OFF). Above the engine off threshold the state of charge is such that it is safe to shut the engine off. Between SOC_ENG_OFF and SOC_ENG_RUN, the engine should be restarted if it is off to prevent SOC from crossing below SOC_ENG_RUN. 
     Another threshold that may be simultaneous with the engine off threshold or above the engine off threshold is the boost ok threshold (SOC_BOOST_OK). Above the boost ok threshold the starter/alternator is allowed to operate as a motor to provide additional torque to maximize the engine torque to meet driver demand in a high power situation. 
     Another threshold established is the charge on threshold (SOC_CHARGE_ON). The charge on threshold allows the battery to be charged by the starter/alternator acting as a generator. The charge on threshold is preferably above the boost ok threshold. 
     Another threshold is the charge off threshold (SOC_CHARGE_OFF). The charge off threshold is set so that above the charge of f threshold the starter/alternator is not allowed to be in charge mode. Above this threshold the hybrid electric vehicle is able to recapture kinetic energy through regenerative braking. The charge off threshold and the charge on threshold may be simultaneous. However, in a preferred embodiment, the charge on threshold is located between the charge off threshold and the boost ok threshold. 
     A bleed off threshold (SOC_BLEED_OFF) is established above the charge off threshold. When battery state-of-charge is above the bleed off threshold, the bleeding of the battery by operating the starter/alternator as a motor is permitted. This reduces battery state-of-charge to allow room in the battery to be able to accept recaptured kinetic energy through regenerative braking. 
     The final threshold is. the regenerative braking off threshold (SOC_REGEN_OFF). The regenerative braking off threshold prevents the battery from becoming overcharged. Thus, if the state of charge reaches the regenerative braking off threshold, no regenerative braking is allowed. Also recall that the charge off threshold is also set below the regenerative braking threshold so that the battery will not be charged or try to be charged over the regenerative braking threshold. Thus, below the regenerative braking threshold, regenerative braking may take place. 
     As those skilled in the art will recognize, the operation of the vehicle changes under various driving conditions. Thus, it is desirable to take advantage of regenerative braking whenever possible. Thus, anytime below the regenerative braking threshold the energy storage device may be regeneratively braked. Also, anywhere below the charge on threshold, the starter/alternator is acting as a generator to allow the battery to be charged by the starter/alternator. 
     Thus, to summarize the operating limit constraints: 
     SOC_ENG_RUN&lt;SOC_BOOST_OFF&lt;SOC_ENG_OFF 
     SOC_ENG_OFF&lt;=SOC_BOOST_OK&lt;SOC_CHARGE_ON 
     SOC_CHARGE_ON&lt;SOC_CHARGE_OFF&lt;SOC_BLEED_OFF 
     SOC_BLEED_OFF&lt;SOC_REGEN_OFF 
     While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.