Abstract:
In a powertrain for a motor vehicle that includes a power source, a transmission driveably connected to the power source and wheels of the vehicle, an electric machine able to operate as an electric motor for transmitting power to at least some of the vehicle wheels, a method for controlling the powertrain includes operating the power source and the transmission in a desired gear to produce a first wheel torque in response to a demanded wheel torque, increasing the demanded wheel torque while turning the vehicle along a curved path, and using the electric motor to provide a second wheel torque, such that a combined magnitude of the first wheel torque produced in the desired gear and the second wheel torque is equal to or greater than the increased demanded wheel torque.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a powertrain for a hybrid electric vehicle (HEV), and in particular to avoiding cyclic shifting among gears of a transmission while the vehicle corners or turns on a winding road. 
     2. Description of the Prior Art 
     A motor vehicle powertrain having an internal combustion engine and an automatic transmission, in which discrete gear ratios are produced, typically has a shift schedule calibrated to schedule downshifts when the engine runs out of reserve power and to schedule upshifts as close as possible to tractive effort crossovers. The shift schedule is calibrated for use in a vehicle that is unloaded and on straight road since the vehicle generally operates in these conditions. 
     When the vehicle is negotiating a curve or turning a corner, the straight road calibration can cause a condition, wherein the transmission repetitively upshifts and downshifts between gears. Excessive gear shifting occurs when the vehicle begins to decelerate in the turn the driver tips-in, i.e., depresses the accelerator pedal either at a high rate or to a substantial portion of its travel, to maintain vehicle speed. As a result of the tip-in, the transmission downshifts. Because the vehicle begins to accelerate beyond the desired speed after the downshift is completed, the driver tips-out of the throttle causing an upshift to occur. This cyclic gear shifting continues until the vehicle exits the curve. 
     To avoid this frequent gear shifting, it is conventional to perform the downshift and to prevent a subsequent upshift. Although this procedure mitigates the shift frequency problem, it can lead to driver dissatisfaction due to the downshift that occurs upon entering the turn. Furthermore, the early downshift causes a fuel economy penalty attributable to remaining in the lower gear for longer period, whereas fuel economy is maximized in the higher gear. 
     SUMMARY OF THE INVENTION 
     In a powertrain for a motor vehicle that includes a power source, a transmission driveably connected to the power source and wheels of the vehicle, an electric machine able to operate as an electric motor for transmitting power to at least some of the vehicle wheels, a method for controlling the powertrain includes operating the power source and the transmission in a desired gear to produce a first wheel torque in response to a demanded wheel torque, increasing the demanded wheel torque while turning the vehicle along a curved path, and using the electric motor to provide a second wheel torque, such that a combined magnitude of the first wheel torque produced in the desired gear and the second wheel torque is equal to or greater than the increased demanded wheel torque. 
     The method uses an electric motor to provide torque to the wheels during cornering, in addition to the torque that transmitted from the transmission output, and to allow the vehicle to stay in the higher gear and therefore to reduce gear shift frequency. Preferably, the electric motor may be an electric rear axle drive (ERAD) or a crankshaft-integrated starter/generator (CISG), or a combination of these. 
     The electric machine torque can increase the available torque to supplement the current gear torque so that the desired wheel torque can be attained while remaining in the higher gear. Shift cycle frequency is reduced and a greater range of authority is attained with the accelerator pedal while remaining in the higher gear. 
     Vehicle acceleration is proportional to the degree of accelerator pedal displacement as the driver tips in to the accelerator pedal when beyond the midpoint of the corner without unnecessary gear changes. This reduces the effort of the driver while cornering by enabling the driver to adjust vehicle acceleration smoothly and precisely. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram showing an automotive vehicle powertrain for a hybrid electric vehicle; 
         FIG. 2  is a schematic diagram showing the variation of speed, acceleration and gear of a vehicle while negotiating a curve; 
         FIG. 3  is graph showing a shift schedule of gear changes produced by an automatic transmission; 
         FIG. 4  is a diagram that illustrates multiple upshifts and downshifts between third and fourth gears as a vehicle turns through a series of curves; 
         FIG. 5  is a shift schedule that illustrates gear changes of  FIG. 4  occurring as the operating state crosses the gear change lines; 
         FIG. 6  is a logic flow diagram representing an algorithm for preventing gear shift cycling in a HEV; and 
         FIG. 7  is a schematic diagram showing the variation of speed, acceleration and gear of a vehicle while negotiating a curve under control of the algorithm of  FIG. 6 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIG. 1 , the powertrain  10  for a hybrid electric vehicle includes a first power source such as an internal combustion engine  12 , a diesel engine or a gasoline engine; an automatic transmission  14  producing multiple forward and reverse gear ratios; an electric machine  16  driveably connected to the engine crankshaft and transmission input  18 , such as a crankshaft-integrated starter/generator (CISG) for providing starter/generator capability; and an additional electric machine  20  driveably connected to a rear axle differential mechanism  36 , such as an electric rear axle drive (ERAD), for providing additional propulsion capability in either an electric drive or hybrid drive mode. The transmission output  24  is connected through a final drive unit and differential mechanism  26  to the front axles  28 ,  30 , which drive the front wheels  32 ,  33 , respectively. ERAD  20  drives the rear wheels  34 ,  35  through ERAD gearing  36 , a differential mechanism  36 , rear axles  22 ,  23  and wheels  34 ,  35 . 
     The powertrain  10  comprises a first power path driveably connected to the load that includes CISG  16 , transmission  14 , final drive unit  26 , axles  28 ,  30  and the wheels  32 ,  33 . A gear of the transmission must be engaged between input  18  and output  24  and the input clutch  38  or  39  that is associated with the engaged gear must be engaged to complete a drive path between CISG  16  and the vehicle wheels  32 ,  33 . Powertrain  10  also comprises a second power path driveably connected to the load that includes ERAD  20 , ERAD gearing  48 , a differential mechanism  36 , rear axles  22 ,  23  and wheels  34 ,  35 . 
     An electronic engine control module (ECM)  24  controls operation of engine  12 . An electronic transmission control module (TCM)  27  controls operation of transmission  14  and the input clutches  38 ,  39 . An integrated starter controller (ISC)  40  controls operation of CISG  16 , ERAD  20  and the system for charging an electric storage battery  42 , which is electrically coupled to the electric machines  16 ,  20 . 
     The undesired gear shifting is illustrated in  FIG. 2  with the HEV  42  entering a curve  44  while in fourth gear and having its wheel brakes applied at  45 . The accelerator pedal  46  is depressed at  48  as the vehicle starts to exit the curve, causing a downshift to third gear. The driver eases off accelerator pedal  46  at  50  as the vehicle  10  accelerates to the desired speed  52 . In response to this movement of the accelerator pedal  46 , transmission  14  upshifts at  54  to fourth gear. 
     If the vehicle  42  were entering the curve  44  while accelerating in third gear, an upshift to fourth gear occurs at  56  when the curve is entered. Thereafter, the driver would tip-out of the accelerator pedal  46 , potentially causing a 3-4-3-4 gear sequence through the curve. The shift hunting condition can be even more extreme if the vehicle were operating in several switch back curves (s-curves), which are frequently encountered in mountainous driving. 
     Gear changes in a discrete ratio transmission  14  are produced according to a gear shift schedule  60 , such as that illustrated in  FIG. 3 . The gear shift schedule is calibrated to schedule downshifts when the engine runs out of reserve power and upshifts as close as possible to the tractive effort crossovers. Driver demanded wheel torque is represented either by engine throttle position, for a vehicle without electronic throttle control (ETC); or by accelerator pedal position, for a vehicle with ETC. One of the criteria for shift scheduling calibration is to have good shift spacing so that the transmission does not cycle between upshifts  62 - 64  and downshifts  65 - 67  too frequently. 
       FIGS. 4 and 5  illustrate the occurrence of multiple upshifts and downshifts as the vehicle  42  turns through a series of curves  44 . The driving pattern is a deceleration of the vehicle and a downshift  67  to third gear from fourth gear as it enters the curve, the wheel brakes are applied and the operating state, defined by demanded wheel torque and vehicle speed, crosses the 3-4 gear shift curve. This is followed by acceleration of the vehicle and an upshift  64  to fourth gear as the vehicle exits the curve when the driver tips into the accelerator pedal  46 . 
       FIG. 6  shows the steps of an algorithm for preventing gear shift cycling of a HEV  42  while the vehicle is turning through a curving roadway  44  or cornering. The algorithm is executed repetitively at intervals of about 8 ms. 
     After entering the cornering shift control at  80 , a test is made at step  82  to determine whether the vehicle is cornering or entering a curve. Various techniques for detecting whether the vehicle is entering a curve include referencing a global position system, a steering angle sensor that produces a signal representing angular displacement of the vehicle&#39;s steering wheel from a reference position, or speed sensors that produce a signal representing the difference in speed of wheels on opposite sides of the vehicle, preferably wheels  34 ,  35  that are not being driven by a power source. 
     If the result of test  82  is logically true, at step  84  a test is made to determine whether an electric machine  16  that is driveably connected to the transmission input shaft  18  can provide sufficient wheel torque in the current gear in additional to the wheel torque provided by the engine  12  in the current gear to meet or exceed the wheel torque being demanded by the driver. Step  84  determines whether the CISG  16  is able currently to produce wheel torque in the current gear that is equal to or greater than the difference between the demanded wheel torque and the wheel torque produced by the engine in the current gear. 
     If the result of test  82  is logically false, at step  86  a flag called “Upshift Inhibit” is cleared to allow upshifts to occur, and at step  88  the algorithm is terminated and control returns to step  80 . 
     If the result of test  84  is true, control advances to step  90  to determine whether a downshift is scheduled to occur. If the result of test  90  is false, at step  92  execution of the algorithm is terminated and control returns to step  80 . 
     The preferred result of the algorithm is to remain in the current gear, the higher gear as long as possible while passing through the curve. The least preferable result is to downshift since it can accelerate the vehicle beyond the current driver demand and initiate shift cycling and reduce fuel economy. 
     If the result of test  90  is true, at step  94  the scheduled downshift is prevented. 
     At step  96 , a check is made to determine whether the demanded wheel torque is greater than the wheel torque that can produced in the current gear by the engine  12 ; any other power source or sources, such as CISG  16 , driveably connected to the input shaft; or a combination of the engine and the other power source or sources. If the result of test  96  is false, at step  98  execution of the algorithm is terminated and control returns to step  80 . 
     If the result of test  96  is true, at step  100  an electric machine that is not driveably connected to transmission input shaft  18 , such as ERAD  20 , is used to provide sufficient wheel torque in addition to the torque transmitted to input shaft  18 , thereby avoiding a downshift. At step  102 , execution of the algorithm is terminated and control returns to step  80 . 
     If the result of test  84  is false, control advances to step  104 , to determine whether a downshift is scheduled. If the result of test  104  is false, execution of the algorithm is terminated at step  106  and control returns to step  80 . 
     If the result of test  104  is true, the transmission  14  performs the scheduled downshift at step  108 . 
     At step  110 , the torque transmitted to input shaft  18  is used to produce the demanded wheel torque in the lower gear. 
     At step  112 , the flag is set to prevent an upshift from the lower gear. At step  114 , the algorithm is terminated and control returns to step  80 . 
       FIG. 7  illustrates use of torque  120  produced by electric machine  20  in response to depressing the accelerator pedal  46  at  48 . Torque  120  increases the wheel torque produced in the current gear, fourth gear, by torque transmitted to transmission input shaft  18  in order to meet the demanded wheel torque without need for a downshift. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.