Abstract:
A system and method for accurate control of engine torque for a parallel/series hybrid electric vehicle (PSHEV) is disclosed. An accurate estimate of engine torque is determined from the generator motor torque of a PSHEV. The estimated engine torque can then be used to control engine torque in a closed loop torque control strategy. The invention comprises at least one controller to receive, process and output torque signals; a first control strategy to determine a modified engine torque signal from at least a desired engine torque signal; and a second control strategy to determine variables for air, fuel and spark from said modified engine torque signal. The first control strategy can include a proportional integral (PI) controller. The estimated engine torque signal can be a function of an estimated generator motor torque signal, a generator motor speed signal and an engine torque loss signal.

Description:
FIELD OF INVENTION 
   The present invention relates generally to an hybrid electric vehicle (HEV), and specifically a strategy for controlling engine torque in an HEV. 
   BACKGROUND OF THE INVENTION 
   The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 (Severinsky). 
   The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set. 
   Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE. 
   A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a parallel/series “split” configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier gear. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed. 
   The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or driveability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shut down. Nevertheless, new ways must be developed to optimize the HEV&#39;s potential benefits. 
   One such area of HEV development is torque control of the engine, which requires an accurate estimate of engine torque. 
   HEV systems to control or determine engine torque or motor torque are generally known in the art. For example, Tabata et al., U.S. Pat. No. 5,951,614, teaches an apparatus for controlling an HEV drive system having a transmission disposed between a vehicle drive wheel and an assembly of an engine and a motor/generator, the apparatus including a torque reduction control device for reducing the input torque of the transmission during a shifting action of the transmission. 
   Bader, U.S. Pat. No. 6,307,276, teaches a method for operating a parallel hybrid electric vehicle, with an internal combustion engine which is connected to a drive shaft via a clutch and a manual transmission, and with a three-phase machine (a traction motor) which is directly coupled with its rotor to a countershaft of the manual transmission and is connected to an electrical energy store (a battery) via a three-phase converter. A time average of the driving torque required during a respective predeterminable travel time interval is determined by a hybrid drive control unit. The power outputs of the internal combustion engine and of the three-phase machine are controlled so that the internal combustion engine outputs driving torque corresponding to the time average determined, and the three-phase machine outputs the difference between the driving torque currently required and the driving torque delivered by the internal combustion engine. 
   Deguchi et al., U.S. Pat. No. 6,233,508, teaches a system where a target drive torque is calculated based on a detected value for vehicle speed and a detected value for an accelerator pedal depression amount. A generator torque is calculated for a motor based on a battery state of charge (SOC). An engine is controlled to a torque value that achieves a target drive torque and a generator torque as a target engine torque. The motor is controlled to a value that is the difference of a target drive torque and an engine torque estimation value as a target motor torque. 
   Tabata et al., U.S. Pat. No. 6,081,042, teaches a hybrid drive system for a motor vehicle, wherein a controllable device such as an automatic transmission or a center differential device is disposed between drive wheels of the vehicle and a drive power source consisting of an engine operated by combustion of a fuel, and an electric motor operated with an electric energy, and the engine and/or the electric motor is/are operated for driving the motor vehicle in different running modes. The controllable device is controlled by a control device on the basis of an input torque received by the controllable device. The control device is adapted to estimate the input torque of the controllable device depending upon a currently selected one of the running modes, or effect learning control of the controllable device in different manners corresponding to the different running modes. 
   The prior art has met the general needs of controlling an HEV&#39;s engine. Nevertheless, to fully achieve the goals of an HEV&#39;s performance, drivability, and efficiency, a more accurate system for controlling engine torque is needed. 
   SUMMARY OF INVENTION 
   Accordingly, the present invention provides a system and method for accurate control of engine torque in a parallel/series hybrid electric vehicle (PSHEV). An accurate estimate of engine torque is determined from the generator motor for the PSHEV. The estimated engine torque can then be used to control engine torque in a closed loop torque control strategy. 
   According to the invention, a system and method for controlling engine torque in a parallel/series hybrid electric vehicle utilizes at least one controller to receive, process and output torque signals. A first control strategy embodied within this controller can determine a modified engine torque signal from a signal representing desired engine torque. A second control strategy embodied within the controller can determine variables for air, fuel and spark from the modified engine torque signal. The first control strategy can include use of a proportional integral (PI) controller. The first control strategy can also determine the modified engine torque signal from the desired engine torque signal and an estimated engine torque signal. The estimated engine torque signal can be a function of an estimated generator motor torque signal, a generator motor speed signal and an engine torque loss signal. 
   The present invention can improve vehicle drivability by providing accurate engine torque control. The present invention can also reduce violations of battery power limits by accurately controlling engine torque. 
   The present invention can also improve the performance of an active neutral function by accurately controlling engine torque about a point where zero torque is applied to the vehicle drive wheels such as when operation of an air conditioning compressor is desired, but no torque is applied to the vehicle drive wheels. 
   Other features of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing advantages, and features, as well as other advantages, will become apparent with reference to the description and figures below, in which like numerals represent like elements and in which: 
       FIG. 1  illustrates a general hybrid electric vehicle (HEV) configuration. 
       FIG. 2  illustrates an engine torque control strategy using open loop control and closed loop control. 
       FIG. 3  illustrates a strategy to map generator motor torque estimation accuracy. 
       FIG. 4  illustrates a strategy to schedule the gain of a proportional integral controller. 
   

   DETAILED DESCRIPTION 
   The present invention relates to electric vehicles and, more particularly, hybrid electric vehicles (HEVs).  FIG. 1  demonstrates just one possible configuration, specifically a parallel/series hybrid electric vehicle (split) configuration. 
   In a basic HEV, a planetary gear set  20  mechanically couples a carrier gear  22  to an engine  24  via a one way clutch  26 . The planetary gear set  20  also mechanically couples a sun gear  28  to a generator motor  30  and a ring (output) gear  32 . The generator motor  30  also mechanically links to a generator brake  34  and is electrically linked to a battery  36 . A traction motor  38  is mechanically coupled to the ring gear  32  of the planetary gear set  20  via a second gear set  40  and is electrically linked to the battery  36 . The ring gear  32  of the planetary gear set  20  and the traction motor  38  are mechanically coupled to drive wheels  42  via an output shaft  44 . 
   The planetary gear set  20 , splits the engine  24  output energy into a series path from the engine  24  to the generator motor  30  and a parallel path from the engine  24  to the drive wheels  42 . Engine  24  speed can be controlled by varying the split to the series path while maintaining the mechanical connection through the parallel path. The traction motor  38  augments the engine  24  power to the drive wheels  42  on the parallel path through the second gear set  40 . The traction motor  38  also provides the opportunity to use energy directly from the series path, essentially running off power created by the generator motor  30 . This reduces losses associated with converting energy into and out of chemical energy in the battery  36  and allows all engine  24  energy, minus conversion losses, to reach the drive wheels  42 . 
   A vehicle system controller (VSC)  46  controls many components in this HEV configuration by connecting to each component&#39;s controller. An engine control unit (ECU)  48  connects to the Engine  24  via a hardwire interface. The ECU  48  and VSC  46  can be based in the same unit, but are actually separate controllers. The VSC  46  communicates with the ECU  48 , as well as a battery control unit (BCU)  50  and a transaxle management unit (TMU)  52  through a communication network such as a controller area network (CAN)  54 . The BCU  50  connects to the battery  36  via a hardware interface. The TMU  52  controls the generator motor  30  and the traction motor  38  via a hardwire interface. The control units  46 ,  48 ,  50  and  52 , and controller area network  54  can include one or more microprocessors, computers, or central processing units; one or more computer readable storage devices; one or more memory management units; and one or more input/output devices for communicating with various sensors, actuators and control circuits. 
   To efficiently control engine  24  torque, generator motor  30  torque, and traction motor  38  torque, an accurate determination of engine  24  torque is needed. The present invention utilizes a strategy to accurately determine engine  24  torque from generator motor  30  torque. The strategies of the present invention can be in a computer readable format embodied in one or more of the computing devices described above. 
   To determine an estimated engine  24  torque (T eng     —     est ) from generator motor  30  torque, the following relationship can be used:
 
 T   eng   est   =−G   eng2gen *( T   gen   est )−J gen+sun   *dw   gen   /dt )+ T   loss  
 
   Where, the following definitions apply:
         T eng     —     est  Estimated Engine  24  Torque;   G eng2gen  Gear Ratio from Engine  24  to Generator Motor  30 , G eng2gen =(R+1)/R.   R Planetary Gear Set  20  Ratio (Ratio of Sun Gear  28  to Ring Gear  32 ), R=N s /N r ;   N s  Number of teeth on Sun Gear  28 ;   N R  Number of teeth on Ring Gear  32 ;   T gen     —     est  Estimated Generator Motor  30  Torque;   J gen+sun  Lumped Moment Inertia of Generator Motor  30  and Sun Gear  28 ;   w gen  Generator Motor  30  Speed; and   T loss  Engine  24  Torque Loss.       

   As shown in the above relationship, estimated engine  24  torque is a function of estimated generator motor  30  torque, generator motor  30  speed and engine  24  torque loss. 
     FIG. 2  illustrates an engine  24  torque control strategy, shown generally at  100  using open loop control and closed loop control. In closed loop control, the difference between a desired engine  24  torque (T eng     —     des ) signal  102  and an estimated engine  24  torque (T eng     —     est ) signal  104 , calculated using the relationship shown above, are used to create an engine  24  torque error (T eng     —     err ) signal  106 . T eng     —     des 102  can come from the VSC  46  and is a function of driver demand and other demands placed on the vehicle. The T eng     —     err  signal  106  can be used by a proportional integral (PI) controller  108 , known in the art, to produce a modified engine  24  torque (T eng     —     mod ) signal  110 . The T eng     —     mod  signal  110  is input into the ECU  48 , where a torque based engine  24  control strategy  112 , known in the art, can use the T eng     —     mod  signal  110  to calculate variables for fuel, air and spark, shown collectively at  114 . 
   When the vehicle is operating in parallel mode, i.e., both the engine  24  and the traction motor  38  provide torque to the output shaft  44 , the estimated generator motor  30  torque (T gen     —     est ) is not available. Therefore, the estimated engine  24  torque (T eng     —     est )  104  cannot be calculated using the relationship shown above. In this mode, the engine  24  torque control strategy  100  operates in an open loop control mode. In the open loop control mode, the T eng     —     mod  signal  110  is set equal to the desired engine  24  torque T end     —     des  signal  102 , which is input into the ECU  48  as described above. 
   To achieve accurate closed loop control, the PI controller  108  is tuned as a function of the accuracy of the estimated engine  24  torque (T eng     —     est ) signal  104 , which in turn is a function of the accuracy of the estimated generator motor  30  torque (T gen     —     est ). The accuracy of T gen     —     est  is a function of the generator motor&#39;s  30  operating point, torque, and speed. 
     FIG. 3  illustrates a strategy to map estimated generator motor  30  torque accuracy using a dynamometer  210 . This strategy, shown generally at  200 , is accomplished by comparing a transfer function map generated estimate of generator motor  30  torque (T gen     —     1 )  206  to a measured generator motor  30  torque (T gen   —     2 )  212 . In the strategy  200 , a signal for generator motor  30  current (I gen )  202  is inputted into a transfer function map (K map )  204 . The transfer function map  204  outputs a first estimate of generator motor  30  torque* (T gen     —     1 )  206 . The same generator motor  30  current (I gen )  202  is used to drive the generator motor  30  on a dynamometer  210 . The dynamometer  210  can measure actual generator motor  30  torque (T gen     —     2 )  212  and is known in the art. Comparing T gen     —1    and T gen     —     2  at  208  results in a generator motor  30  torque estimation accuracy  214 . Trends of the generator motor  30  torque estimation accuracy  214  can be used to schedule the gain (the degree to which the controller adjusts the signal, i.e., how much correction is applied) in the PI controller  108 . 
     FIG. 4  illustrates a strategy, shown generally at  300 , to schedule of the gain of the PI controller  108 . The PI controller  108  can be scheduled using the trends of the generator motor  30  torque estimation accuracy  214  as a function of generator motor  30  torque  302  and speed  304 . Gain scheduling can be accomplished by choosing different PI controller  108  constants in the regions where the generator motor  30  torque estimation accuracy  214  is different. For example, if the generator motor  30  torque estimation accuracy  214  is roughly constant in each of the four quadrants of generator motor  30  speed  304  versus torque  302 , the gains of the PI controller  108  can be chosen as follows: 
   Positive Speed  304 , Positive Torque  302 =Kp 1 , Ki 1    306 ; 
   Positive Speed  304 , Negative Torque  302 =Kp 2 , Ki 2    308 ; 
   Negative Speed  304 , Negative Torque  302 =Kp 3 , Ki 3    310 ; and 
   Negative Speed  304 , Positive Torque  302 =Kp 4 , Ki 4    312 . 
   Where Kp n  and Ki n  are the proportional and integral constants of the PI controller  108 . 
   By following the aforementioned strategies, the task of controlling torque to the drive wheels  42  becomes easier because engine  24  torque is more accurately controlled, which results in improved vehicle drivability. Accurate engine  24  torque control also results in fewer violations of battery  36  power limits, since energy from the battery  36  can be used when torque demand exceeds available engine  24  torque. Lastly, accurate control of engine  24  torque allows the vehicle to perform an active neutral function more easily. 
   Active neutral is an operating condition where desired drive wheel  42  torque is zero and generator motor  30  torque is commanded to effectively cancel out engine  24  torque. An example of an active neutral condition could be an instance when the engine  24  may need to run an air conditioning compressor, but no engine  24  torque is needed for drive purposes. Accurate engine  24  torque control allows for reduced variation about a point where no torque is applied to the drive wheels  42 . 
   The above-described embodiments of the invention are provided purely for purposes of example. Many other variations, modifications, and applications of the invention may be made.