Patent Publication Number: US-7900726-B2

Title: Method and system for hybrid energy management control

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to the field of gas/electric hybrid vehicles and, more specifically, to a method and system for hybrid energy management control. 
     BACKGROUND OF THE INVENTION 
     The desire to both increase the gas mileage of vehicles and decrease the amount of pollutants emitted by vehicles has lead led to the development of hybrid electric vehicles (HEVs). HEVs use a combination of an internal combustion engine and an electric motor to power the vehicle. Different types of HEVs exist. Parallel hybrid vehicles use an internal combustion engine in tandem with a battery powered electric motor to propel a vehicle. Serial hybrid vehicles use a battery powered electric motor to propel a vehicle and a secondary power source, such as a fuel cell or internal combustion engine, to recharge the battery. A third type of hybrid vehicle, known as a start/stop hybrid, shuts down the internal combustion engine when the vehicle comes to a stop and utilizes a battery to power the vehicle system. In this configuration, when the vehicle starts to move again the internal combustion engines restarts. 
     Common to all of these types of hybrids is that the on-board batteries become discharged and, therefore, need to be charged. Typically, there are two different ways to charge the battery. The first way is to use regenerative braking to charge the battery. In regenerative braking, when the electric motor provides braking torque, energy is produced to recharge the battery. 
     The second way to charge the battery is through the use of active charging. In active charging, the internal combustion engine uses the electric motor as a generator to charge the battery. In order to maximize the efficiency of the HEV, active recharging of the battery should occur at times when the engine is running efficiently. Therefore, there is a need for a method and system for hybrid energy management control. 
     SUMMARY OF THE INVENTION 
     In one exemplary embodiment, a charging system for a hybrid vehicle is disclosed. The charging system comprises an internal combustion engine coupled to an electric motor generator, the electric motor generator to be driven as a generator to produce a charging voltage and a battery coupled to the electric motor generator and configured to receive a charge voltage. The charging system further comprises an engine control computer coupled to the internal combustion engine, the electric motor generator and the battery. The engine control computer is configured to determine a threshold CSFC, calculate an instantaneous CSFC, and initiate active charging of the battery if the instantaneous CSFC is less than or equal to the threshold CSFC. 
     In another exemplary embodiment, a method for optimizing the active charging of a battery in a hybrid vehicle is provided. First, a change in a state of charge of the battery is calculated. Next, a threshold CSFC is determined using, at least in part, the change in the state of charge of the battery and an instantaneous CSFC is determined. Then, the battery is charged if the instantaneous CSFC is less than the threshold CSFC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and: 
         FIG. 1  is a block diagram of an exemplary hybrid vehicle illustrating an exemplary embodiment of a charging system; 
         FIG. 2  is a flowchart of an exemplary embodiment of a method to provide energy management for a hybrid vehicle; and 
         FIG. 3  is a graph of the change of state of charge versus specific fuel consumption threshold for various vehicle speeds. 
     
    
    
     DETAILED DESCRIPTIONS OF THE DRAWINGS 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
       FIG. 1  is a block diagram of charging system  101  for an exemplary hybrid vehicle  100  in accordance with the teachings of the present invention. In the exemplary embodiment of  FIG. 1 , vehicle  100  is illustrated as a parallel hybrid vehicle, although other types of hybrid vehicles such as serial hybrid vehicles and start and stop hybrid vehicles are within the scope of the present invention. Charging system  101  includes an internal combustion engine  102  and an electric motor generator  104  both of which couple to a drive train  106 . Electric motor generator  104  couples to the battery  110  via an inverter  107  and a DC-to-DC converter  108 . An engine control computer  112  is coupled to the internal combustion engine  102 , the electric motor generator  104  and the battery  110 . 
     Electric motor generator  104  is of conventional design. Electric motor generator  104  can function as a motor to propel vehicle  100 . Additionally, electric motor generator  104  can operate as a generator to charge battery  110 . When operating as a generator to charge the battery, the electric motor generator  104  operates at a certain efficiency which can be represented as motor eff . In a typical embodiment, the motor eff  is the ratio of the power generated by the electric motor generator to the power provided to generate the power. In  FIG. 1 , a single electric motor generator  104  is shown coupled to the drive train  106  to power rear wheels  114 . Alternatively, the electric motor generator  104  can be coupled to a set of front wheels  116  or both the front and rear wheels. In one exemplary embodiment, a separate electric motor generator is associated with the front wheels and the back wheels. In  FIG. 1 , the electric motor generator  104  acts as both a motor and a generator. However, in one exemplary embodiment, a separate electric motor and a separate generator can also be provided. 
     Internal combustion engine  102  is also of conventional design. Internal combustion engine  102  can be used to propel the vehicle  100 . Also, when the battery  110  needs to be charged actively, the internal combustion engine  102  operates the electric motor generator  104  as a generator to generate an AC voltage. That AC voltage is converted to a DC voltage by the inverter  107 . Inverter  107  can be integrated with the electric motor generator  104  or provided separately. 
     The generated DC voltage can also be converted, in a typical embodiment, to a higher DC voltage, by use of the DC-to-DC converter  108 . In one exemplary embodiment, the DC-to-DC converter  108  converts the DC voltage generated by the electric motor generator  104  into a higher DC voltage needed to charge the battery  110 . The DC-to-DC converter  108  can also step down the voltage from the battery when the battery is used to run the electric motor generator  104 . When converting the DC voltage, the DC-to-DC converter  108  operates at a certain efficiency, which can be expressed as DC eff . In a typical embodiment, DC eff  is the ratio of the power input to the DC-to-DC converter  108  and the power produced by the DC-to-DC converter  108 . 
     Engine control computer  112  receives data from various components of the automobile, processes the data and outputs processed data or commands for other vehicular systems. In one embodiment, engine control computer  112  can include non-volatile memory, input/output ports, a central processor, units and communication interfaces for networking with an automotive communication network. In an exemplary embodiment of the present invention, the engine control computer  112  receives data regarding engine efficiency and the state of charge of the battery. Using this information, the engine control computer  112  can then determine the most efficient time to actively charge the battery  110 . Engine control computer  112  is of conventional design. In one exemplary embodiment, the engine control computer  112  can be the engine control module (ECM) manufactured by General Motors Corporation of Detroit, Mich. 
     Battery  110 , in one embodiment comprises a large number of low voltage batteries connected in series to form a high voltage battery pack. In one embodiment, the battery  110  is a lead acid (PbA) battery, although other battery chemistries are within the scope of the present invention. 
     A method for determining the best time to actively charge the battery  110  is illustrated in  FIG. 2 . In general, the initiation of active charging is preferred when the internal combustion engine and other components are operating at a high efficiency level. Other factors, such as the state of charge of the battery  110 , influence the active charging decision. 
     In a first step, step  202 , the current vehicle speed is received by engine control computer  112 . Next, in step  204 , a delta state of charge (ΔSOC) is calculated. The ΔSOC is the difference between the present state of charge of the battery and a minimum state of charge. Typically, the state of charge is expressed as a percentage of a fully charged battery. The minimum state of charge is typically defined as the lowest state of charge that the battery can be at while still providing all vehicle electrical demands. The present state of charge can be received by the engine control computer  112  from a sensor associated with the battery  110 . 
     Next, in step  206 , the ΔSOC and the vehicle speed are used to determine a charging specific fuel consumption threshold (CSFC Threshold ). The CSFC is a measure of the efficiency of the internal combustion engine and the charging system used in active charging. The lower the CSFC the more efficiently the vehicle is operating. The CSFC threshold is a cutoff that the vehicle must fall below before the initiation of active charging. Note that when a vehicle falls below the CSFC Threshold , it is operating at a more efficient level than the level represented by the CSFC Threshold . The CSFC Threshold , in one exemplary embodiment, can be determined by using a look-up table or graph representing the CSFC Threshold  for a given speed and ΔSOC. An exemplary graph  300  for CSFC Threshold  versus ΔSOC for various vehicle speeds is shown in  FIG. 3 . For example, in  FIG. 3 , a 0 kph curve  302 , a 30 kph curve  304 , a 50 kph curve  306 , a 60 kph curve  308  an 80 kph curve  310  and a 150 kph curve are illustrated. Each curve represents the CSFC Threshold  as a function of the ΔSOC for a given vehicular speed. For example, the 50 kph curve  306  illustrates the CSFC Threshold  value for a given ΔSOC. As an example, if the vehicular speed is 50 kph and the ΔSOC is 5, the CSFC Threshold  is approximately 95. 
     Note that as the vehicular speed increases the ΔSOC versus CSFC Threshold  curve shifts, for most data points on the curves, downward. As vehicle speed increases, the amount of regenerative charging available when braking initiates increases. Therefore, the need to actively charge the battery is reduced. Thus, as vehicle speed for a given ΔSOC increases, the CSFC Threshold  drops, indicating that the vehicle&#39;s operating efficiency must increase before active charging can be initiated. 
     Additionally, as can be seen in  FIG. 3 , as the ΔSOC increases for a given vehicular speed, the CSFC Threshold  decreases. This reflects the fact that as the present state of charge gets further away from the minimum state of charge, the need to charge the battery decreases. Conversely, as the ΔSOC decreases the present SOC becomes close to the minimum SOC necessitating the active charging of the battery. 
     Also, as seen in  FIG. 3 , each curve stops at a ΔSOC of ten percent. This is because when the present state of charge is much greater than the minimum state of charge, active charging is not a priority because active charging can be started later when the ΔSOC has decreased. Of course, in  FIG. 3 , a ΔSOC of ten percent is used for exemplary purposes only. The margin between the present state of charge and the minimum state of charge at which initiation of active charging is prevented can be set at differing amounts without departing from the scope of the present invention. 
     Next, in step  208 , it is determined if an instantaneous CSFC is less than or equal to the CSFC threshold. The instantaneous CSFC (CSFC inst ), which represents the present CSFC, is calculated by first determining a current brake specific fuel consumption (BSFC). The BSFC is a measure of engine efficiency and is expressed in terms of micrograms of fuel burned per joule of energy produced. The larger the BSFC, the less efficient the engine operation. The BSFC can be computed by the engine control computer  112  from data collected by engine sensors associated with internal combustion engine  102 . Once the BSFC is determined, the CSFC inst  can be determined by dividing the BSFC by a measurement of the charging system efficiency, which, in one embodiment, can be expressed as the product of the motor generator efficiency (Motor eff ) and the DC to DC conversion efficiency (DC eff ). 
     
       
         
           
             
               CSCF 
               inst 
             
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               BSCF 
               
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                     motor 
                     eff 
                   
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     If the CSFC inst  exceeds the CSFC Threshold  then, in step  210 , no active charging is done. If the CSFC inst  is less than the CSFC Threshold , active charging is started in step  212 . The active charging will continue until the instantaneous CSCF exceeds the CSFC inst  by a predetermined amount. This is to prevent the charging system  101  from cycling between charging and discharging as the instantaneous CSFC varies between just greater than the CSFC Threshold  and just less than the CSFC Threshold . 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.