Abstract:
A propulsion system for use in a hybrid vehicle, wherein a first propulsion system provides a driving force to a first pair of wheels and a second propulsion system provides a driving force to a second pair of wheels. A system controller actuates the propulsion systems determines the necessary commands to be provided to the first and second propulsion systems so as to provide the vehicle with the most efficient driving and/or stopping force.

Description:
The present invention is related to a method and apparatus for maintaining a state of charge (SOC) for electric batteries in a parallel hybrid electric vehicle (PHEV). 
     BACKGROUND OF THE INVENTION 
     Passenger comfort and fuel efficiency have set forth increasing demands on automotive vehicle designs. It is a primary goal of most vehicle designs to provide a more efficient vehicle without having to sacrifice passenger comfort and satisfaction. 
     Moreover, and as alternative vehicle propulsion systems are implemented, passenger comfort and fuel efficiency are sometimes in opposition to each other. This is particularly true in hybrid vehicle designs. 
     A Hybrid Vehicle is a vehicle that has two sources of propulsion. A hybrid electric vehicle (HEV) is a vehicle wherein one of the sources of propulsion is electric and the other source of propulsion may be derived from fuel cells or an internal combustion engine (ICE) that burns diesel, gasoline or any other source of fuel. 
     Generally, a hybrid vehicle utilizes either one or two drive trains wherein the internal combustion engine (ICE) provides torque to one of the drive trains and an electrical driving force is applied to either of both of the drive trains. 
     In addition and in order to provide a secondary source of power, hybrid vehicles also utilize a concept known as regenerative braking. Generally, regenerative braking is the conversion of the vehicle&#39;s kinetic energy into a source of electrical power. The vehicle&#39;s kinetic energy is converted from the spinning wheels, in response to a user request to slow or stop the vehicle. A generator is manipulated, and accordingly, produces electrical energy as it applies a stopping force to the vehicle&#39;s axle and/or drive train in response to a stopping request. Therefore, and in accordance with regenerative braking, the kinetic energy is converted to electric energy, as the vehicle begins to slow down. 
     In order to operate the internal combustion engine (ICE) of a hybrid vehicle a fuel source must be consumed. This causes the engine to generate emissions that are harmful to the environment and the reduction of such emissions is a primary goal of any hybrid vehicle design. On the other hand, an electric drive system produces little or no emissions, however, the operation of such a system draws energy from a battery or plurality of batteries which ultimately must be recharged. 
     Accordingly, and in order to operate in a most efficient manner, either one or both of the energy sources of a hybrid vehicle should be operated in accordance with the most efficient usage of energy. 
     Additionally, a hybrid electric vehicle (HEV) encounters many operational states which affect the performance and or efficiency of the vehicle&#39;s operation. 
     Moreover, and as driving conditions vary, these operational states also vary. 
     For example, and during typical driving and/or operating conditions, the vehicle&#39;s battery system will lose a state-of-charge (SOC) in any one of the following instances: providing tractive energy to the vehicles drive train for either vehicle launch or maintaining speed in a pure electric vehicle (EV) mode; supplying energy to the vehicles climate control system, this is even more apparent when the ICE is not running; providing a synchronizing or active damping energy to a motor/generator system for synchronizing the drive train coupled to the vehicle&#39;s ICE; and providing operational energy for the vehicle&#39;s electrical accessories. 
     Of the aforementioned states only the first two (providing tractive energy and supplying energy to the climate control system) are controllable for state of charge (SOC) management. 
     On the other hand, the vehicle&#39;s batteries or battery system gains a state of charge (SOC) from the following sources: regenerative mechanical energy generated while the vehicle is braking or coasting down, this is known as “regenerative braking”; and electrical energy generated by a generator coupled to the vehicle&#39;s engine, that applies mechanical energy. 
     Of the aforementioned regenerative states only the charge generated by the engine is fully controllable, whereas the charge generated through regenerative braking can only be controlled by reducing the energy flowing into the batteries. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a hybrid vehicle having a parallel propulsion system wherein the state of charge (SOC) is maintained at or near its nominal value by using the controllable quantities, during discharging and/or charging. 
     In an exemplary embodiment of the present invention a controller system configures the propulsion system of a hybrid vehicle to provide the most energy efficient means for meeting requested demands. 
     In another exemplary embodiment of the present invention a system configures the propulsion system of a hybrid vehicle to provide the lowest amount of emissions while at the same time meeting the requested driver demands. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a partial schematic of a hybrid vehicle; 
     FIG. 2 is a partial schematic illustrating the rear propulsion configuration of the hybrid vehicle illustrated in FIG. 1; 
     FIG. 3 is a flowchart illustrating the state of charge management system of an exemplary embodiment of the present invention; 
     FIG. 4 illustrates the state of charge management of the FIG. 3 embodiment; 
     FIG. 5 illustrates the state of charge management of the FIG. 3 embodiment; 
     FIG. 6 is a partial schematic of a hybrid vehicle; 
     FIG. 7 is a flowchart illustrating the command sequence of an alternative embodiment of the present invention; 
     FIGS. 8 and 9 illustrate the power management considerations of the FIG. 7 embodiment; and 
     FIG. 10 illustrates a flowchart depicting the command sequence of an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a hybrid vehicle system configuration contemplated for use with the present invention is illustrated. 
     A hybrid vehicle  10  is configured to have a rear propulsion system  12  and a front propulsion system  14 . Rear propulsion system  12  has an internal combustion engine  16  which provides a driving force to an automated manual transmission  18  which converts the driving force of internal combustion engine  16  into the required torque for driving the rear wheels of hybrid vehicle  10 . 
     In the preferred embodiment, front propulsion system  14  is an electric traction drive with a continuous torque output. Alternatively, vehicle  10  is equipped with only a rear propulsion system. 
     In the preferred embodiment, internal combustion engine  16  is a high-efficiency diesel engine such as a compression-ignition direct-injection CIDI engine. However, and in accordance with the present invention, engine  16  can be a high efficiency gasoline engine, employing various forms of combustion controls. 
     Referring now to FIGS. 1 and 2, component parts of rear propulsion system  12  are illustrated. In response to a user or drivers manipulation of an accelerator mechanism (not shown) internal combustion engine  16  provides a rotational force to a driveshaft  20  that is coupled to a flywheel  22 . In order to transfer the rotational force from flywheel  22  to automated manual transmission  18  a clutch  24  is positioned to engage and disengage flywheel  22 . 
     Generally, clutch  24  is a planetary gear having a surface area that frictionally engages the surface area of flywheel  22 . 
     Clutch  24  is coupled to an input shaft  26  of automated manual transmission  18 . Input shaft  26  is coupled to a plurality of input gears  28 . Each one of input gears  28  has a differing diameter and/or gear tooth ratio which provides a differing torque value which in response to a gearshift command makes contact with a corresponding one of a plurality of output gears  30  which are secured to an output shaft of  32  of automated manual transmission  18 . Similarly, output gears  30  each have a differing diameter and/or gear tooth ratio. 
     Output shaft  32  ultimately provides a resulting driving force to a rear differential  34  and a respective axle  36  that provides a rotational force to rear wheels  38  of hybrid vehicle  10 . 
     An electric motor/generator system  40  is also coupled to input shaft  26 . Motor/generator system  40  is coupled to input shaft  26  at a position remote from clutch  24 . Accordingly, and as a rotational force is applied to input shaft  26  motor/generator system  40  can be rotated to provide a source of electrical power for use in hybrid vehicle  10 , as well as a rotational driving force to shaft  26 , drawing electrical energy from the batteries. 
     In addition, and since motor generator system  40  is coupled to input shaft  26  of automated manual transmission  18 , motor generator system  40  can apply a torque to input shaft  26  in order to synchronize the appropriate gear  28  with gear  30 . 
     A hybrid system controller  42  provides command inputs to rear propulsion system  12  and front propulsion system  14 . Hybrid system controller  42  also controls the motor/generator system  40  when transmission  18  is shifted. Since motor/generator system  40  is coupled to input shaft  26  controller  42  can send a signal to motor/generator system  40  in order to provide a force to input shaft  26  in order to rapidly synchronize input shaft  26  to the proper speed for engaging the next one of plurality of gears  30 . 
     Motor/generator system  40  provides either a rotational positive force or a negative force to input shaft  26 . For example, and in situations where the rotation of input shaft  26  must be increased in order to mesh the gears of input shaft  26  to the gears of output shaft  32 , the motor portion of motor/generator system  40  provides a rotational positive force to input shaft  26 . 
     Conversely, and in situations where the rotation of input shaft  26  must be slowed in order to mesh the gears of input shaft  26  with the gears of output shaft  32 , the generator portion of motor/generator system  40  provides a rotational negative force to input shaft  26 . 
     Accordingly, this feature allows the gears of transmission  18  to be shifted without having to open the clutch. This will result in faster shifts and higher overall efficiency with less interruption of the more efficient primary drive train, namely, the propulsion force of internal combustion engine  16 . 
     Moreover, and in contravention to systems where the clutch is disengaged to shift the gears, there is no loss of the output energy of internal combustion engine  16  as the clutch remains engaged to the flywheel. 
     For example, as transmission  18  shifts from a lower gear to a higher gear input shaft  26  must be slowed to allow for the meshing of the appropriate one of the gears  28  to the appropriate one of gears  30 . Accordingly, and in particular the generator system of motor/generator system  40 , applies a torque force to input shaft  26  in order to slow its rotation thereby allowing the gears of transmission  18  to be meshed to allow for upshifting. 
     In addition, and as the rotation of input shaft  26  is slowed the generator portion of motor/generator system  40  is also rotated and accordingly, provides an electrical output which is either stored or used by hybrid vehicle  10 . 
     Charge-sustaining operation is defined as the operation of the vehicle without having external means of (off-board) electric charging for the batteries in the vehicle. In the case of charge-sustaining hybrids, the on-board engine and generator do of the battery charging, which is similar to what is currently the norm for conventional vehicles. The control algorithms proposed in the instant application take the concept a little further, to improve the operating efficiency of the engine as well as a resulting fuel economy of the vehicle. The control algorithms proposed herein use the ability of the battery to store energy to “load-level” the engine, i.e., storing the energy by being charged from the engine at a higher efficiency, and then turning the engine off later, to send his energy to the wheels through the electric motors. 
     In accordance with the instant application, the controllable discharging events are used to intentionally reduce the state of charge (SOC) of the vehicle&#39;s battery system, if the SOC&#39;s value is close to 100%, thereby allowing better regenerative braking headroom. Therefore, and in accordance with the instant application, the climate control compressor-powering algorithm of a hybrid vehicle is based on the battery&#39;s state of charge. If the SOC is close to 100%, the engine&#39;s duty cycle, i.e., fraction of time the engine is on versus off, is reduced, and the engine is kept off for longer durations during light accelerations and cruising situations, when the vehicle is being driven in a pure electric mode, i.e., electric motors providing the propulsion force, drawing energy from the battery. 
     Also, the climate compressor is driven electrically to load-level the batteries when the SOC is higher than nominal and the ICE is turned off. Turning the engine off during light accelerations and cruising has a huge beneficial impact on the vehicle&#39;s fuel economy and emissions. Therefore, controllable discharging (providing energy to the vehicle&#39;s drive train and climate control compressor) is used to load-level the vehicle&#39;s batteries. 
     On the other hand, the controllable charging events are used to force the battery&#39;s SOC back to its nominal value, if the SOC has fallen below the nominal value due to higher rates of discharging, which may be caused by excessive loading upon the vehicles battery system, such as providing energy to drive the vehicle, its climate control compressor and accessories. 
     The vehicle&#39;s batteries are charged using mechanical energy generated by the ICE. In addition, the priority with which the battery charging torque is added to engine torque demand depends on how low the SOC has actually depleted. 
     Referring now to FIG. 3, a flow chart  50  illustrates portions of a possible command sequence of the state of charge management strategy used in accordance with an exemplary embodiment of the present invention. Here a first step or decision node  52  determines whether the state of charge of hybrid vehicle  10  is greater than 80% of its nominal value. The nominal value of SOC is a function of the battery technology. If so, a second step  54  instructs the controllable discharging events, namely, supplying energy for climate control and vehicle propulsion, to be operated solely on electrical power and draw their energy from the vehicle&#39;s battery system. In this operational configuration there is no need for hybrid vehicle  10  to operate ICE  16 . Accordingly, the fuel efficiency and emission output of hybrid vehicle  10  in this configuration is at an optimal level. 
     If however, the SOC is less than or equal to 80% of the nominal valuea third step or decision node  56  determines whether the SOC is greater than 70%. If so, a fourth step  58  instructs ICE  16  to be started. In this operational configuration the torque load of ICE  16  is altered to provide a charge to hybrid vehicle  10  through motor/generator system  40  in addition to supplying a driving force in response to the vehicle&#39;s light acceleration and/or cruising power demands. Accordingly, and in this configuration, a percentage of the ICE&#39;s torque load is reserved for battery recharging. 
     If however, the SOC is less than or equal to 70% a fifth step or decision node  60  determines whether the SOC is greater than 55%. If so, the ICE of hybrid vehicle  10  is instructed by a six step  62  to provide the vehicle&#39;s hard acceleration power demands while at the same time also providing a charge to the vehicle&#39;s batteries. In this configuration, high-energy demands such as vehicle acceleration are now powered by the ICE. This produces the energy load upon the vehicles battery system in addition to a portion of the engine torque being devoted to battery recharging. 
     If the SOC is less than or equal to 55% a seventh step or decision node  64  determines whether the SOC is greater than 40%. If so, an eighth step  66  instructs the internal combustion engine to charge the vehicle&#39;s batteries at all times. 
     If however, the SOC is less than or equal to 40% battery recharging becomes a high priority, critical function and a ninth step  68  disconnects non-essential electrical loads of significant value, such as, the vehicle&#39;s climate control compressor, and, in some situations requiring aggressive acceleration, will even reduce the torque being supplied to propel the vehicle. 
     Accordingly, the command sequence illustrated in flow chart  50  utilizes controllable charging states, namely, mechanical energy from the ICE and proportionate load-level of the ICE, in order to offset the deviations of the SOC from its nominal value. The controllable charging states are manipulated in accordance with the deviations of the SOC. 
     In addition, and in order to replenish the depleted battery system in the quickest and the most energy efficient manner energy-efficiency maps are utilized to identify the most optimal way to replenish the vehicle&#39;s battery energy without having negative effects on the vehicle&#39;s fuel economy. Efficiency maps are 2-dimensional look-up tables, representing the efficiency of each component at different torque and speeds. These are measured by testing. 
     Flow chart  50 , illustrates one possible command sequence for maintaining the state of charge in a hybrid vehicle. Of course, and in accordance with the instant application, it is contemplated that the percentage values of the vehicle&#39;s state of charge in each of the aforementioned decision nodes may vary in accordance with the vehicle&#39;s component parts and/or systems. 
     Referring now in particular to FIGS. 6 and 7, an alternative embodiment of the present invention is illustrated. In this embodiment component parts performing similar or analogous functions are numbered in multiples of 100. 
     Here a hybrid vehicle  110  has a hybrid system controller  142  that employs a torque management strategy in order to most efficiently supply a driving force in response to a user or command request. 
     Hybrid vehicle  110  has a rear propulsion system  112  and a front propulsion system  114 . Rear propulsion system  112  has, among other elements an internal combustion engine  116 , an automated manual transmission  118  and an electric motor/generator system  140 . 
     Referring now in particular to FIG. 7, a possible command sequence employed by system controller  142  is illustrated by a flow chart  176 . 
     The torque management strategy uses an algorithm that splits the driver&#39;s torque request into a torque demand for the ICE engine and a torque demand for the electric drive (front and a lower rear) in a parallel hybrid-electric vehicle, in order to deliver the driver&#39;s requested torque. This strategy is incorporated in the software, and enables the vehicle to maintain maximum fuel economy while generating a minimal amount of emissions. This is achieved irrespective of the driving conditions encountered and/or the vehicle operator&#39;s driving habits. 
     The algorithm uses the following information which is inputted into a first step  178 : engine (ICE and electric) fuel economy and emission maps as functions of torque, speed and temperature; driver requested accelerator or interpreted coast-down torque from the accelerator pedal; interpreted regenerative braking torque from the brake controller; available maximum torque values from all traction components (the engine with transmission gear ratio, the front electric traction motor and the starter motor/generator); maximum allowable charge/discharge power for the traction battery pack; and speed of operation as well as temperature of each of the traction components. 
     A second step  180  determines a net axle torque request from the acceleration or coast-down torque request from the accelerator pedal and regenerative braking torque request from the brake controller. These inputs are combined to determine a net axle torque request (which is either positive or negative). 
     A third step or decision node  182  determines whether the net axle torque request is either positive (acceleration) or negative (coasting or decelerating or braking). If the net axle torque request is positive a fourth step or decision node  184  determines whether the net axle torque request is greater than the ICE maximum emission limited torque output which, of course, is a known value depending on the design characteristics of the hybrid vehicle&#39;s ICE and accompanying drivetrain. 
     If the net axle torque request is less than the ICE maximum machine limited torque output, a fifth step  186  determines the more efficient drivetrain (i.e. electric or ICE). This is calculated using the efficiency maps of electric drives and the ICE and its accompanying transmission whereby the energy losses are compared to for each drive in order to provide the necessary net axle torque force. For example, and when determining whether the electric drive is to be used, the calculation is made knowing that the ICE will be needed to the recharge the vehicle&#39;s battery. 
     Therefore, and based on these calculations the mechanical path with the least amount of energy losses is chosen to provide the necessary torque for the vehicle. A decision node or sixth step  188  determines whether the electrical drive is more efficient. 
     If not, the ICE is used to drive the vehicle. This command is executed by step  190 . 
     If on the other hand the electrical drive is deemed more efficient, the battery available energy or state of charge (SOC) is determined by a decision node  192  in order to determine whether the electric drive of the vehicle can supply the necessary torque based upon the current SOC. If the SOC is acceptable a command step  194  instructs the electric drive to provide the necessary torque. 
     Conversely, if the SOC is too low, the command step  196  instructs the ICE to provide the necessary torque while at the same time providing an electrical charge to the vehicles batteries. 
     Alternatively, and as illustrated by the dashed lines, a state of charge management control system  150  is used to replace decision node  192 , and steps  194  and  196 . State of charge management control system  150  is similar to the system illustrated in FIGS. 1-3. 
     Referring back now to decision node  184  and if the requested torque is greater than the maximum emission limited torque of the ICE a step  198  instructs the ICE to provide its maximum emission limited torque to the axle. 
     A decision node  200  determines whether the remaining torque (requested torque-maximum emission limited torque of ICE) is greater than the maximum allowable torque of the electric drives (front and rear). 
     If not a step  202  instructs the electric drives to provide the remaining torque. If yes, a step  204  instructs the electric drives to provide their maximum torque. 
     Referring now to decision node  182 , and if the requested torque is negative (braking or coast-down situation) a command step  206  determines the battery charge power limit and instructs either a brake controller-imposed or a pre-determined front to rear regenerative braking torque split of 70%/30% to be acted upon the vehicles axles, and/or drivetrain (motor/generator system  140 ), in order to provide the necessary negative axle torque request. 
     The algorithm for this embodiment achieves high fuel economy and low emissions for a parallel hybrid vehicle. 
     With the correct sizing of the powertrain components, a high-performance parallel hybrid electric vehicle (car or truck) with an improved fuel economy can be realized. 
     The algorithm calibrations can be adjusted to provide customer satisfaction such as all-time traction control system as well as all-wheel drive system for this parallel hybrid vehicle. 
     The aforementioned methodology is general-purpose and it is not customized to a particular vehicle or powertrain configuration, and accordingly, it can be used to determine the torque split for any type of parallel hybrid vehicle where the electric drive torque and engine torque can be controlled independently of each other. 
     In addition, the command system illustrated in FIGS. 4 and 5 utilizes an algorithm which facilitates four-wheel regenerative prioritized braking (in response to a negative axle torque request) for the vehicle. This is also determined by step  206 . 
     Referring now to in FIG. 10, another alternative embodiment of the present invention is illustrated. Here, a possible command sequence employed by system controller  142  is illustrated by a flow chart  276 . 
     The battery state of charge management strategy uses an algorithm that utilizes a target state of charge and the vehicles actual state of charge. This strategy is incorporated in the software, and enables the vehicle to maintain a state of charge while also attaining maximum fuel economy. This is achieved irrespective of the driving conditions encountered and/or the vehicle operator&#39;s driving habits. 
     The algorithm uses the following information, which is inputted into a first step  278 : target state of charge, actual state of charge, compression-ignition direct-injection CIDI engine and motor/generator charge and torque headroom. 
     A first step or decision node  282  determines whether the actual state of charge is less than the target state of charge. If the actual state of charge is not less than the target state of charge a second step or decision node  284  determines whether the actual state of charge is less than the maximum allowable state of charge. If the actual state of charge is greater than the maximum allowable state of charge of a command step  286  sets the charge torque 2  to equal zero. A command step  288  performs the following function charge torque=charge torque 1 +charge torque 2 . A next step  290  determines the charge torque request from the following equation charge torque request=min (charge torque headroom, charge torque). 
     If on the other hand the actual state of charge is less than the maximum allowable state of charge a command step  292  sets the charge torque 2  to equal f 2  (Delta SOC). 
     If decision node  282  determines that the actual state of charge is less than the target state of charge a command step  294  sets charge torque 1 =f 1  (delta SOC) 
     Accordingly, and as contemplated in accordance with the instant application, the vehicles batteries are charged at a predetermined rate through either the engine or motor/generator. The batteries can also be charged at an acceptable rate through regenerative braking. For example, acceptable can be determined as follows acceptable equals allowable by the batteries at the specific state of charge and temperature. 
     In addition, and as contemplated with the instant application, the vehicles batteries are discharged during heavy accelerations when the SOC is “sufficient”, during light to moderate accelerations when SOC is “acceptably high” and it is more efficient to use the electric drive as opposed to the CIDI and during all accelerations when SOC is “very high”. 
     The following calibrations are used to determine what is “sufficient”, “acceptably high” and “very high”. 
     KE 13  HybridVehicleBattery 13  Operation 13  SOC=30%—minimum acceptable electric drive SOC for NiMH batteries. 
     KE 13  Electric 13  Traction 13  SOC 13  Min=46%—in them SOC level at which motor/generator will begin to recharge the batteries. 
     KE 13  Electric 13  Traction 13  SOC 13  Max=52%—maximum SOC level at which motor/generator will stop recharge in the batteries. 
     KE 13  Pure 13  Elec 13  Trac 13  SOC=53%—minimum SOC for pure electric traction. 
     Referring now to NiMH batteries, sufficient SOC can be defined as follows, between 30-46%. Acceptably high SOC is defined as follows, between 46-52% and very high SOC is defined as greater than or equal to 52%. Of course, and as contemplated in accordance with the instant application, and as different types of batteries with varying discharge and charge rates are used, the aforementioned values may have a lesser or greater value and has indicated herein. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.