Abstract:
An energy management system includes a first energy source having a first well-to-wheels greenhouse gas emissions content, a second energy source having a second well-to-wheels greenhouse gas emissions content and a drive mechanism powered by the first energy source and the second energy source. The drive mechanism is powered by the first energy source when the first well-to-wheels greenhouse gas emissions content is less than the second well-to-wheels greenhouse gas emissions content and the drive mechanism is powered by the second energy source when the second well-to-wheels greenhouse gas emissions content is less than the first well-to-wheels greenhouse gas emissions content.

Description:
FIELD 
     The present disclosure relates to hybrid electric vehicles. More particularly, the present disclosure relates to an energy management system and method which minimize well-to-wheels greenhouse gas emissions in a hybrid electric vehicle. 
     BACKGROUND 
     Carbon dioxide emissions from sources such as internal combustion engines may contribute to global climate change. Therefore, a large-scale reduction in the quantity of carbon dioxide in automobile engine exhaust may provide environmental benefits. The development of electric and hybrid electric vehicles is currently being pursued as a partial solution to the problem of increasing carbon dioxide emissions. 
     Plug-in hybrid electric vehicles are designed to consume less fuel than conventional vehicles and non plug-in hybrid electric vehicles. In some cases, however, the well-to-wheels greenhouse gas emissions associated with plug-in electric energy may be equal to or greater than the well-to-wheels greenhouse gas emissions of the fuel itself. This is particularly true in cases in which the fuel is a biofuel or hydrogen. 
     Therefore, an energy management system and method are needed to reduce greenhouse gas emissions of a hybrid electric vehicle by balancing the energy sources which are utilized during operation of the vehicle. 
     SUMMARY 
     The present disclosure is generally directed to an energy management system. An illustrative embodiment of the energy management system includes a first energy source having a first well-to-wheels greenhouse gas emissions content, a second energy source having a second well-to-wheels greenhouse gas emissions content and a drive mechanism powered by the first energy source and the second energy source. The drive mechanism is powered by the first energy source when the first well-to-wheels greenhouse gas emissions content is less than the second well-to-wheels greenhouse gas emissions content and the drive mechanism is powered by the second energy source when the second well-to-wheels greenhouse gas emissions content is less than the first well-to-wheels greenhouse gas emissions content. 
     The present disclosure is further generally directed to an energy management method for minimizing greenhouse gas emissions of a vehicle. An illustrative embodiment of the method includes providing a first energy source; providing a second energy source; determining a first well-to-wheels greenhouse gas emissions content of the first energy source; determining a second well-to-wheels greenhouse gas emissions content of the second energy source; and operating the vehicle using the first energy source when the first well-to-wheels greenhouse gas emissions content is less than the second well-to-wheels greenhouse gas emissions content and operating the vehicle using the second energy source when the second well-to-wheels greenhouse gas emissions content is less than the first well-to-wheels greenhouse gas emissions content. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will now be made, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of an illustrative embodiment of the energy management system for hybrid electric vehicles. 
         FIG. 2  is a flow diagram which illustrates recalculation of the average greenhouse gas emissions per unit of fuel upon refueling of a hybrid electric vehicle. 
         FIG. 3  is a flow diagram which illustrates recalculation of the average greenhouse gas emissions per unit of electrical energy stored in a hybrid electric vehicle battery upon plug-in recharging of the battery. 
         FIG. 4  is a flow diagram which illustrates implementation of the energy management system during operation of a hybrid electric vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is generally directed to an energy management system which reduces greenhouse gas emissions of a hybrid electric vehicle by balancing the electric and fuel energy sources which are utilized during operation of the vehicle. The energy management system maintains a running estimate of the average well-to-wheels greenhouse gas emissions per unit mass of fuel and the average well-to-wheels greenhouse gas emissions per unit of electrical energy stored in the vehicle battery. During fuel fill and plug-in battery charging of the vehicle, the vehicle controller receives information which indicates the greenhouse gas emissions per unit mass of fuel and the greenhouse gas emissions per unit of plug-in electrical energy stored in the vehicle battery. The vehicle controller uses this information to update the running estimate of the greenhouse gas emissions of the fuel and of the plug-in electrical energy. Alternatively, the vehicle controller may use GPS and time information to estimate the greenhouse gas emissions content of the plug-in electrical energy based on regional grid mix averages. 
     During vehicle operation, the vehicle controller monitors the state of charge of the vehicle battery and the power demand of the vehicle. Using look-up efficiency tables, the vehicle controller determines which of the current running total of greenhouse gas emissions of the fuel energy or of the plug-in electrical energy from the battery is higher. Using this information, the vehicle controller adjusts the power distribution between the fuel energy and the plug-in electrical energy to power the vehicle in such a manner as to minimize the greenhouse gas emissions consistent with other constraints on vehicle performance. 
     Also during vehicle operation, the average well-to-battery greenhouse gas emissions per unit of electrical energy stored in the vehicle battery may be continuously updated to account for charging and discharging of the battery. When the battery is charging during vehicle operation, the vehicle controller sets the power of the engine, fuel cell or other prime mover to charge the battery in such a manner as to minimize the average greenhouse gas emissions content per unit of electrical energy stored in the battery. Applied to a plug-in hybrid electric vehicle, the system preferentially uses battery energy in the event that the source of plug-in electrical energy has a lower well-to-wheels greenhouse gas emissions content than the well-to-wheels greenhouse gas emissions content of the fuel. If the battery energy has a higher greenhouse gas emissions content than the greenhouse gas emissions content of the fuel per unit mass of the fuel, then the fuel is consumed under normal vehicle operational conditions and battery energy is used only when necessary to meet power demands. 
     Referring initially to  FIG. 1 , an illustrative embodiment of the energy management system for hybrid vehicles, hereinafter energy management system, is generally indicated by reference numeral  100 . The energy management system  100  includes a filling station (block  102 ) at which a hybrid electric vehicle (not shown) is refueled. The fuel from the filling station  102  is stored in an onboard fuel storage facility  104  on the hybrid electric vehicle. Fuel from the onboard fuel storage facility  104  is distributed to an internal combustion engine, fuel cell or other prime mover  106  of the hybrid electric vehicle. The prime mover  106  converts chemical energy into electrical energy which is subjected to power distribution and conversion by a power distribution and conversion mechanism  108 . The electrical energy from the power distribution and conversion mechanism  108  is operable to drive an electric drive mechanism  110  which propels the vehicle. 
     The energy management system  100  further includes an electrical grid  114  at which an onboard vehicle battery  112  of the hybrid electric vehicle (not shown) is charged. The onboard vehicle battery  112  may be recharged from the electrical grid  114  using plug-in technology which is known to those skilled in the art. Electric power from the onboard vehicle battery  112  is subjected to power distribution and conversion by the power distribution and conversion mechanism  108 . Electric power from the power distribution and conversion mechanism  108  is used to operate the electric drive mechanism  110  of the vehicle. During operation of the hybrid electric vehicle, a portion of the electrical energy from the power distribution and conversion mechanism  108  is used to recharge the onboard vehicle battery  112 . A vehicle controller  116  of the hybrid electric vehicle interfaces with the onboard fuel storage facility  104 , the power distribution and conversion mechanism  108  and the battery  112 . 
     As fuel from the filling station  102  is stored in the onboard fuel storage facility  104  of the hybrid electric vehicle, information which indicates the greenhouse gas (GHG) emissions that was expended in manufacture and processing of the fuel is received and stored in the onboard vehicle controller  116  of the hybrid electric vehicle. Also, as the onboard vehicle battery  112  is charged from the electrical grid  114 , information which indicates the greenhouse gas (GHG) emissions that was expended to generate and store the electrical power in the electrical grid  114  is received and stored in the onboard vehicle controller  116  of the hybrid electric vehicle. During operation of the hybrid electric vehicle, the vehicle controller  116  uses the fuel GHG emissions information and the electrical power GHG emissions information to determine a running total of the GHG emissions which results from consumption of the fuel and the electrical power. The vehicle controller  116  then uses this information to balance the power consumption of the fuel and the electrical power which is used to operate the electric drive mechanism  110  in such a manner as to minimize the greenhouse gas emissions of the hybrid electric vehicle. 
     Referring next to  FIG. 2 , a flow diagram  200  which illustrates recalculation of the average greenhouse gas emissions per unit of fuel upon refueling of the hybrid electric vehicle at the filling station  102  ( FIG. 1 ) is shown. In block  202 , during refueling of the hybrid electric vehicle, the vehicle controller receives fuel greenhouse gas (GHG) emissions information from a fuel fill infrastructure at the filling station  102  ( FIG. 1 ). The fuel GHG emissions information may include, for example, the greenhouse gas emissions per kilogram of fuel delivered (G F     —     del ) and the total mass of fuel delivered (M F     —     del ). In block  204 , the average greenhouse gas emissions per unit mass of fuel (G F ) after filling of the vehicle is recalculated according to the formula:
 
 G   F     —     post     —     fill =( G   F     —     pre     —     fill   M   F     —     pre     —     fill   +G   F     —     del   M   F     —     del )/ M   F     —     pre     —     fill   +M   F     —     del  
 
     Referring next to  FIG. 3 , a flow diagram  300  which illustrates recalculation of the average greenhouse gas emissions per unit of electrical energy stored in a hybrid electric vehicle battery is shown. In block  302 , during recharging of a vehicle battery onboard the hybrid electric vehicle, the vehicle controller receives electrical energy GHG emissions information from a charging infrastructure at the electrical grid  114  ( FIG. 1 ). The electrical energy GHG emissions information may include, for example, the greenhouse gas emissions per kilowatt-hour of energy stored in the onboard vehicle battery (G Batt     —     del ) and the total quantity of energy stored in the battery (E Batt     —     del ). In block  304 , the average greenhouse gas emissions per unit electrical energy stored in the battery (G Batt ) is recalculated according to the formula:
 
 G   Batt     —     post     —     charge =( G   Batt     —     pre     —     charge   E   Batt     —     pre     —     charge   +G   Batt     —     del   E   Batt     —     del )/ E   Batt     —     pre     —     charge   +E   Batt     —     del  
 
     Referring next to  FIG. 4 , a flow diagram  400  which illustrates implementation of the energy management system during normal operation of a hybrid electric vehicle is shown. In block  402 , a determination is made as to whether the state of charge (SOC) of the onboard vehicle battery is below the charge threshold. In the event that the SOC is not below the charge threshold, in block  404  the power demand of the vehicle is determined. This determination may be made according to the equation:
 
 P   demand   =P   prop     —     demand   +P   acc     —     demand  
 
     In block  406 , a determination is made as to which power setting of the prime mover of the hybrid electric vehicle minimizes the rate of greenhouse gas emissions according to the formula:
 
 P   demand   =P   \PM   +P   batt     —     dis  
 
     The rate of greenhouse gas emissions is expressed by the equation:
 
 G =( P   PM ) G   F /ψ pm   [P   pm ]+( P   batt     —     dis ) G   batt /ψ batt   [P   batt     —     dis ,SOC]
 
     Where ψ pm [P pm ]=P pm /M fuel [P pm ] is the prime mover power divided by prime mover fuel consumption rate, and ψ batt     —     dis [P batt     —     dis ,SOC]=P batt     —     dis /E batt [P batt     —   dis,SOC] is the battery discharge power divided by energy depletion rate of the battery. Both of these can be stored as look-up tables for the specific hardware being used. 
     The constraint that prime mover and battery power must meet vehicle demand may be imposed by the equation:
 
 G =( P   PM ) G   F /ψ pm   [P   pm ]+( P   demand   −P   PM ) G   batt /ψ batt   [P   demand   −P   PM ,SOC]
 
     The prime mover power is then set to the value that minimizes the previous function subject to maximum and minimum power constraints for the prime mover and the battery. 
     If ψ pm  and ψ batt     —     dis  are relatively constant with respect to power, the following simple strategy will be very close to optimum. Calculate the following sensitivity:
 
 dG/dP   pm   =G   F /ψ pm   −G   batt /ψ batt  
 
     If dG/dP pm &gt;0, the battery discharge power is set as high as possible and the FCS power is set as low as possible. If dG/dP pm &lt;0, the FCS power is set at the vehicle demand if possible and the battery power is used only to fill in when needed. 
     In block  408  of the flow diagram  400 , the electrical energy content of the vehicle battery is updated. In block  410 , steps  1 - 4  are repeated. In block  412 , the vehicle power demand is determined. In block  414 , the minimum charge rate required by the vehicle battery is determined. In block  416 , the power setting of the prime mover which both meets vehicle demand and minimizes greenhouse gas emissions content of the electrical energy from the battery is determined: 
     
       
         
           
             
               
                 
                   
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     This is minimized by choosing the value of P pm  that minimizes ψ Batt     —     charge (P pm −P dem )ψ pm (P pm ). 
     In block  418 , the electrical energy content of the battery and the average greenhouse gas emission content of the electrical energy stored in the battery are updated. In block  420 , steps  1 - 9  are repeated. 
     In the event that the determination made in block  402  reveals that the battery SOC is below the charge threshold, then in block  404   a  the power level of the prime mover is set to meet vehicle demand and to a battery power charge rate that minimizes the average greenhouse gas emissions content per unit electrical energy stored in the battery. 
     During regeneration events, the same process applies as in normal operation of the hybrid electric vehicle which was heretofore described with respect to  FIG. 4 , but with, the following adjustments: when the vehicle battery is in a discharge mode and dG/dP pm &gt;0, regeneration energy is used to offset the battery discharge power and the FCS power is kept as low as possible. When dG/dP pm &lt;0, regeneration energy is used to offset the FCS power and battery power is used only to fill in when needed. When the battery is in charge mode, the value of P pm  that maximizes ψ Batt     —     charge  (P pm +P reg −P dem )ψ pm (P pm ) is selected. 
     While the preferred embodiments of the disclosure have been described above, it will be recognized and understood that various modifications can be made in the disclosure and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the disclosure.