Abstract:
In a powertrain for a motor vehicle that includes an engine, a transmission driveably connected to the engine and wheels of the vehicle, an electric machine for transmitting power to at least some of the vehicle wheels, a method for controlling the powertrain includes operating the engine and transmission to produce a wheel torque at a desired vehicle speed with the transmission operating in a desired gear in response to a demanded wheel torque, increasing the demanded wheel torque due to the vehicle ascending a grade, determining a magnitude of wheel torque able to be produced in the desired gear, using the electric motor to provide a second wheel torque in combination with wheel torque whose magnitude is equal to or greater than the wheel torque being produced in the desired gear, and continuing to operate the transmission in the desired gear.

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 a method for avoiding shift hunting among gears of an automatic transmission. 
     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 level ground since the vehicle generally operates in these conditions. 
     When the vehicle is on a significant positive slope, the level ground calibration can cause a condition described generally referred to a shift hunting, wherein the transmission frequently upshifts and downshifts between gears when the driver is trying to maintain vehicle speed on a grade having positive slope. The transmission hunts between upshifts and downshifts because the driver is unable to find an accelerator pedal range that will maintain a desired level of vehicle speed. This condition can occur on a grade having positive slope where the driver is trying to maintain a constant speed. 
     Shift hunting occurs when the vehicle begins to decelerate on the grade due to the increased road load, causing the driver to tip-in to the accelerator pedal 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 shift hunting continues until the slope levels or the vehicle leaves the grade. Shift hunting is undesirable due to the excessive upshifting and downshifting, which cause inability to maintain a desired constant speed on the grade. 
     If a downshift is executed and a subsequent upshift is prevented, shift hunting can be prevented but the driver is dissatisfied because the transmission is perceived to be unresponsive. Inhibiting the upshift causes a reduction in fuel economy due to remaining in the lower gear. 
     SUMMARY OF THE INVENTION 
     In a powertrain for a motor vehicle that includes an engine, a transmission driveably connected to the engine and wheels of the vehicle, an electric machine for transmitting power to at least some of the vehicle wheels, a method for controlling the powertrain includes operating the engine and transmission to produce a wheel torque at a desired vehicle speed with the transmission operating in a desired gear in response to a demanded wheel torque, increasing the wheel torque due to the vehicle ascending a grade, determining a magnitude of wheel torque able to be produced in the desired gear, using the electric motor to provide a second wheel torque in combination with wheel torque provided by the engine in the desired gear, and continuing to operate the transmission in the desired gear. 
     When conditions for shift hunting are detected, torque produced by an electric machine torque increases the current wheel torque so that the resulting wheel torque remains above the road load. The wheel torque provided by the electric machine maintains the desired vehicle speed and allows the transmission to stay in the higher gear until road load decreases. 
     Cyclic shift hunting between a lower gear and higher gear is prevented. The vehicle operator perceives that the powertrain is automatically responsive to the increased road load caused by the vehicle&#39;s ascending a grade that requires more wheel torque. 
     Continued operation in the higher gear improves fuel economy compared to operation in a lower gear, yet the desired vehicle speed is maintained on the grade. 
     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 graph relating road load torque at the wheels and vehicle speed for a range of positive grades; 
         FIG. 3  is graph showing a gear shift schedule between third and fourth gears with available engine torque in those gears superimposed on the schedule; 
         FIG. 4  is a graph showing the variation of accelerator pedal position, demanded torque, road load, vehicle speed and gears during a shift hunting condition; 
         FIG. 5  is a logic flow diagram of a algorithm for controlling shift hunting; and 
         FIG. 6  is a logic flow diagram of an algorithm for determining whether a vehicle is operating on a grade having positive slope. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIGS. 1 and 2 , the powertrain  10  configuration 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)  26  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 graphs of  FIGS. 2 and 3  show that a road load caused by the vehicle operating on a grade having positive slope can affect a transmission shift schedule, which is defined to establish, for the current vehicle operating state, the desired gear produced by transmission  14 . The current operating state is a function of current vehicle speed and current driver demanded torque, which is usually represented by the degree to which an accelerator pedal is depressed. For purposes of this description, road load is the minimum torque that must be transmitted to the driven wheels  32 - 35  to maintain vehicle speed in order that the vehicle can ascend a grade having positive slope. 
     In a vehicle powertrain having an automatic transmission that produces discrete gear ratios, the shift schedule is optimized for unloaded, level ground vehicle operation. When the vehicle is climbing a steep grade, road load can increase significantly compared to road load on level ground.  FIG. 2  illustrates the variation of required wheel torque with vehicle speed for three grades of positive slope  52 ,  54  and  56 . Line  58  represents the required wheel torque on a three percent grade at 35 mph; line  60  represents the required wheel torque on a six percent grade at 35 mph; line  62  represents the required wheel torque on a nine percent grade at 35 mph. 
       FIG. 3  illustrates a boundary line  64 , at which a downshift to third gear from fourth gear occurs when the current operating state moves from above line  64  to below that line. Similarly, line  66  represents the boundary at which an upshift to fourth gear from third gear occurs when the current operating state moves from below line  66  to above line  66 . 
     Line  70  represents the maximum available engine torque transmitted to the wheels over a range of vehicle speed when transmission  14  operates in fourth gear. Line  72  represents the maximum available engine torque transmitted to the wheels over a range of vehicle speed when the transmission operates in third gear. 
     As  FIG. 3  shows, with the vehicle operating on a three percent positive grade at 35 mph (line  58 ), the driver demanded torque for the current operating state  74  is less than the torque available  70  in fourth gear. Therefore, the current driver demanded torque can be delivered to the wheels and the desired constant vehicle speed can be maintained in fourth gear. 
     With the vehicle operating on a six percent positive grade at 35 mph (line  60 ), the driver demanded torque for the current operating state  76  is greater than the torque available  70  in fourth gear, and operating state  76  is above line  64  and in a range where the desired gear is third gear. Consequently, the vehicle will decelerate on the six percent grade. 
     To maintain the desired speed of 35 mph, the driver will likely demand greater torque by executing a tip-in, i.e., depressing the accelerator pedal, which causes a downshift to third gear. The downshift will then accelerate the vehicle above the desired 35 mph speed and the driver will tip-out of the accelerator pedal to slow the vehicle. The tip-out and the resulting vehicle deceleration will cause the transmission  14  to upshift to fourth gear. 
     As illustrated in  FIG. 4 , this undesired cycle of upshifts and downshifts  80 , called shift hunting, occurs in zone B, which begins at  82  when the vehicle enters a grade with positive slope and continues until either shift hunting control is activated at  84  or the vehicle exits or crests the grade. Zone A represents vehicle operation on level ground. Zones B and C represent vehicle operation on a six percent grade having positive slope. Cyclic variation of the accelerator pedal displacement  86 , wheel torque  88 , vehicle speed  90 , and gear  80  are shown in  FIG. 4  with a phase lag between each successive variable. These cyclic variations occur after road load  92  increases due to the vehicle entering the grade. When shift hunting control is activated at  84 , wheel torque  88  increases to  94  due to an increase in wheel torque  96  being produced by at least one of the electric machines  16 ,  20 . 
     In zone C, torque produced by at least one of the electric machines  16 , is used to increase wheel torque currently being transmitted to the wheels. In this way, the increased magnitude of wheel torque meets or exceeds the road load, vehicle speed is constant in the current gear, and no down shift is required. Engine torque is transmitted through transmission  14 , final drive unit and differential mechanism  26  to the front axles  28 ,  30  and front wheels  32 ,  33 . Powertrain  10  also includes a second power path driveably connected to the load that includes ERAD  20 , gearing  48 , differential mechanism  36 , axles  22 ,  23  and wheels  34 ,  35 . 
       FIG. 5  shows the steps of an algorithm for controlling shift hunting. At step  100  a test is made to determine whether the vehicle is on a positive slope grade. If the result of test  100  is logically false, at step  102  execution of the algorithm is terminated and control returns to step  84 . The algorithm is executed repetitively at about 8 ms intervals. 
     If the result of test  100  is logically true, at step  104  a test is made to determine whether a downshift is scheduled to occur upon reference to the current operating state and the shift schedule. If the result of test  104  is false, control advances to step  106  to determine whether an upshift is scheduled to occur. If the result of test  106  is false, at step  108  execution of the algorithm is terminated and control returns to step  84 . 
     The preferred result of the shift hunting control algorithm is to remain in the current gear as long as possible while transcending the grade. The least preferable shift to occur on the grade is a downshift since it can accelerate the vehicle beyond the current driver demand and initiate shift hunting. 
     If at step  104  a downshift is scheduled on the positive slope grade, at step  110  a check is made to determine whether an electric machine  16 ,  20  can provide sufficient additional wheel torque to avoid a downshift. At step  110 , the available electric machine torque is compared to the difference between the desired wheel torque and the engine torque available in the current gear. 
     If the result of test  110  is false, control advances to step  112 , where transmission  14  performs a downshift, whereupon, at step  114 , execution of the algorithm is terminated and control returns to step  84 . 
     If the result of test  110  is true, at step  116  the scheduled downshift is cancelled, and at step  118  torque produced by at least one of the electric machines  16 ,  20  is used in addition to the current wheel torque to maintain a constant vehicle speed without requiring a downshift to occur. Then, at step  120  execution of the algorithm is terminated and control returns to step  84 . 
     If the result of test  106  is true, a check is performed at step  122  to determine if the available wheel torque in the scheduled gear is greater than the desired wheel torque. 
     If the result of test  122  is true, at step  124  transmission  14  performs the upshift since the desired wheel torque can be produced in the next higher gear, which prevents the vehicle from decelerating in the higher gear due to the grade. Then, at step  126  execution of the algorithm is terminated and control returns to step  84 . 
     If the result of test  122  is false, indicating that the desired wheel torque is not available in the next higher gear, a test is made at step  128  to determine whether the available wheel torque from an electric machine exceeds the difference between the desired wheel torque and the available torque in the next higher gear. 
     If the result of test  128  is false, indicating that electric machine torque does not meet the difference between the desired wheel torque and the available torque in the next higher gear, at step  130  the upshift is cancelled. Then, at step  132  execution of the algorithm is terminated and control returns to step  84 . 
     If the result of test  128  is true, at step  134  the electric machine torque is used in addition to the current wheel torque to meet the desired wheel torque. At step  124 , transmission  14  performs the upshift and stays in the higher gear while meeting the driver demand. 
       FIG. 6  illustrates the steps of a grade detection algorithm for detecting whether the vehicle is operating on a grade of positive slope. The algorithm is initialized at step  140 . At step  142 , a test is made to determine whether the vehicle&#39;s accelerator pedal position, i.e., the distance the pedal is displaced from its neutral position, is greater than a reference displacement. When the accelerator pedal is displaced more than the reference displacement, the driver is demanding a sufficient wheel torque magnitude. 
     If the result of test  142  is true, at step  144  a test is made to determine whether the rate of change of accelerator pedal position is greater than a reference rate, which is a calibrated reference. A true result of test  144  indicates that the driver desires to maintain vehicle speed or to accelerate the vehicle. 
     Modern vehicle control systems with an electronic throttle can send a command to limit or reduce wheel torque by retarding spark timing relative to a maximum brake torque spark or by reducing torque through electronic throttle control by a reduction in air flow. If the result of test  144  is true, at step  146  a test is made to determine whether a command, such as that produced by an electronic vehicle control system, has been made to reduce wheel torque. 
     If the result of test  146  is false, which indicates that the driver demanded wheel torque is not overridden by a torque reduction command issued by a vehicle control system or by another request or command, a test is made at step  147  to determine whether a traction control event is occurring. 
     If the result of test  147  is false, a test is made at step  148  to determine whether the vehicle is decelerating at less than a reference deceleration, which is a calibrated reference. 
     Vehicle deceleration can be determined from the time rate of change of vehicle speed as represented by: (1) the rotational speed of the ERAD electric motor  20  upon accounting for the speed ratio of a power path between the rear wheels  34 ,  35  and motor  20 , or (2) the rotational speed of the crankshaft-integrated starter/generator CISG  16  upon accounting for the speed ratio of a power path between the front wheels  32 ,  34  and CISG  16 , or (3) a speed sensor whose output speed signal represents the speed of transmission output shaft  24 , or (4) the speed sensor of a wheel brake system. Preferably vehicle speed is determined with reference to a signal representing the speed of one of the electric machines, CISG  16  or ERAD  20  because that signal has a higher resolution than the alternative speed sensors. 
     If the result of each of tests  142 ,  144  and  148  is true, and the result of each of tests  146  and  147  is false, the control strategy concludes that the driver is attempting to maintain vehicle speed or to accelerate the vehicle, no internal functions are overriding the driver wheel torque demand, yet the vehicle is decelerating. Then, at step  150 , road load is assumed to have increased due to a grade of positive slope hence a positive slope grade has been detected. 
     If the result of any of tests  142 ,  144  and  148  is false or the result of any of tests  146  and  147  is true, at step  152  execution of the grade detection algorithm is terminated. 
     The transmission  14  to which the control strategy can be applied is an automatic transmission that produces multiple speed ratios, i.e., the speed of its input to the speed of its output. The transmission  14  may shift among discrete gears, such as a conventional multiple-speed transmission having a torque converter, or a powershift transmission having multiple input clutches for driveably connecting the engine to the transmission input. In either case, the transmission produces a speed ratio that varies inversely with the number of the gear, i.e., a lower speed ratio in a higher gear, a higher speed ratio in a lower gear. Alternatively, transmission  14  may produce an infinite range of speed ratios, such as a belt-drive continually variable transmission or a traction-drive continually variable transmission. 
     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.