Abstract:
A method for controlling a vehicle driveline includes determining, for current vehicle operating conditions, an initial engine output torque, at which the driveline meets predetermined structural criteria, establishing a factor that varies with torque converter slip, determining an engine torque limit by multiplying the factor corresponding to a current torque converter slip by the initial engine output torque, and limiting torque produced by an engine connected to the driveline to the engine torque limit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to controlling vehicle driveline torque, particularly through engine output torque delivery during high demand operating conditions, which should occur infrequently in the life of a vehicle, while limiting torque to protect driveline components from fatigue type failures during more normal operation. 
         [0003]    2. Description of the Prior Art 
         [0004]    Vehicle driveline components, such as transmissions, driveshaft, axles, etc., have a finite fatigue life when subjected to input torque loads during the life of the vehicle. Empirical data is often used to establish a relation between the magnitude of the torque load and the number of applied cyclic applications of that load before fatigue failure of the tested component occurs. 
         [0005]    To ensure an acceptable length of service life of the component under normal use, torque limitations may be applied to prevent the application of high magnitudes of torsional loading on a regular, recurring basis. Because such high magnitudes of torque are lower than the ultimate strength and yield strength limits of the component, a limited number of cycles at high torque magnitudes would not have a substantial adverse affect on the component&#39;s fatigue strength. 
         [0006]    A need exists in the field of vehicle driveline control for a strategy that permits application of higher magnitudes of torque in certain non-typical conditions, yet excludes such torque magnitudes during regular use when larger numbers of cycle occur. 
       SUMMARY OF THE INVENTION 
       [0007]    A method for controlling a vehicle driveline includes determining, for current vehicle operating conditions, an initial engine output torque, at which the driveline meets predetermined structural criteria, establishing a factor that varies with torque converter slip, determining an engine torque limit by multiplying the factor corresponding to a current torque converter slip by the initial engine output torque, and limiting torque produced by an engine connected to the driveline to the engine torque limit 
         [0008]    A vehicle driveline includes an engine, a torque converter connected to a crankshaft of the engine, a transmission connected to a turbine of the torque converter, a differential connected to an output of the transmission, axle shafts connecting the differential to wheels, a controller including a processor configured to determine for current vehicle operating conditions an initial engine output torque at which the torque converter, transmission and differential meet predetermined structural criteria, and to determine an engine torque limit by multiplying a factor corresponding to a current torque converter slip by the initial engine output torque, and an engine controller configured to limit torque produced by an engine connected to the driveline to the engine torque limit 
         [0009]    The method creates an operational area where extra engine torque is delivered through the driveline, while limiting the effect of fatigue cycles on failure of driveline components. The method allows for high levels of torque to launch or accelerate the vehicle in challenging situations, such as when the vehicle is fully loaded and starting from a stop on a steep grade. This method utilizes converter slip because slip is a readily-available signal to the powertrain controller and it differentiates between extreme and normal operating conditions of vehicle mass, road grade, resistance to motion, etc. 
         [0010]    The method enhances the ability to launch the vehicle on a grade, without requiring axle ratio increases that could increase engineering development costs and decrease fuel economy. 
         [0011]    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 
         [0012]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0013]      FIG. 1  is a top view of a motor vehicle driveline having a transmission, transfer case, and drive shafts extending to the front wheels and rear wheels; 
           [0014]      FIG. 2  is a schematic diagram showing a vehicle driveline including a power source, torque converter, and the transmission input; 
           [0015]      FIG. 3  is algorithm for determining the final engine torque limit and controlling the engine output torque; 
           [0016]      FIG. 4  is a graph showing the variation a function that corrects an initial engine torque limit 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    Referring to  FIG. 1 , a motor vehicle driveline includes front and rear wheels  10 ,  12 , a power transmission assembly  14  that produces multiple forward speed ratios and at least one reverse speed ratio, and a transfer case assembly  16 . 
         [0018]    When two-wheel drive (2WD) operation of the driveline is selected, the transfer case assembly  16  driveably connects the transmission output to a rear drive shaft  18 . When four-wheel-drive (4WD) operation is selected, the transfer case  16  connects the transmission output concurrently to both the front drive shaft  20  and rear drive shaft  18 . Shaft  18  transmits power to a rear wheel differential mechanism  22 , from which power is transmitted differentially to the rear wheels  12  through axle shafts  24 ,  26 , which are contained within a differential housing. The front wheels are driveably connected to right-hand and left-hand axle shafts  32 ,  34 , to which power is transmitted from the front drive shaft  20  through a front differential  36 . 
         [0019]      FIG. 2  shows a portion of the driveline between an internal combustion engine  39 , or another power source, and the transmission  14 , wherein the hydrokinetic torque converter  38  is driveably connected to the engine crankshaft  40 , which is connected to a bladed impeller  42  of the torque converter. A bladed turbine  44 , a bladed stator  48 , and the impeller  42  define a toroidal fluid flow circuit within the casing of the torque converter. The stator  48  is supported on a stationary sleeve shaft  50 , and an overrunning brake  52  anchors the stator to shaft  50  to prevent rotation of stator  48  in a direction opposite to the direction of rotation of the impeller, although free-wheeling motion in the opposite direction is permitted. Turbine  44  is connected to transmission input shaft  54 . Torsion damper  56  and lock-up clutch  58  are arranged in series between crankshaft  40  and the transmission input shaft  54 . When clutch  58  is engaged, the engine drives input shaft  54  through a direct mechanical connection, but when clutch  58  is disengaged, shaft  54  is connected hydrodynamically to the engine through the torque converter. 
         [0020]    The transmission  14  includes gears, shafts, bearings, clutches, brakes, hydraulically controlled servos, and other components, which operate to produce the forward and reverse drive ratios. 
         [0021]    Each of the major components of the various assemblies of the driveline must have capacity to meet and exceed predetermined structural criteria including ultimate torsional strength, yield torsional strength, fatigue torsional strength, torsional endurance limit, torsional service life, torsional displacement, etc. associated with the magnitude of torque transmitted from the engine  39  through the driveline. Factors that affect these structural criteria include the transmission gear ratio, engine torque amplification produced by the torque converter  38 , the selected 2WD and 4WD operation, vehicle acceleration, vehicle mass, road grade, resistance to motion, etc. 
         [0022]    Torque converter speed ratio, the turbine speed divided by engine speed, is used in the algorithm  60  of  FIG. 3  as a calibration function that is implemented to provide an increase in engine output torque, which is transmitted to the input of the vehicle driveline. 
         [0023]    The control method steps of algorithm  60  are stored in electronic memory in a computer-readable medium accessible by an electronic processor  64 , which is able to control operation of the transmission  14 , transfer case  16  and differential  22 . The method of the algorithm is implemented using the electronic processor  64 . 
         [0024]    When the vehicle is being launched, i.e., accelerated from a stop or nearly stopped condition, the processor  64  determines an initial engine output torque limit for the current driving conditions, represented by vehicle load, vehicle acceleration, gear ratio, torque converter speed ratio, etc. The initial engine output torque limit accounts for the torque transmitted to each of the critical driveline components and the effects of amplification of engine output torque produced by the torque converter  38  and the gear ratio produced by the transmission  14 . The acceptable engine output torque limit for the combination of current driving conditions accounts for (i) the torsional load applied to the most torsionally critical of the driveline components due to engine output torque, and (ii) the structural criteria that each such component must meet. 
         [0025]    Preferably the processor  64  determines an initial engine output torque limit at step  66  for each of the driveline components from look-up tables for each driveline component, the tables being indexed by the combination of the current driving conditions. 
         [0026]    At step  68 , processor  64  uses the current measured slip across the torque converter  38 , i.e., the difference between turbine speed and engine speed to determine from a function  70 , stored in computer-readable medium accessible by an electronic processor  64 , the torque uplift factor  72 . 
         [0027]      FIG. 4  shows a preferred variation of factor  72  as a function  70  of torque converter slip. 
         [0028]    Processor  64  produces at step  74  the final engine torque limit, which is the product of factor  70  multiplied by the initial engine torque limit 
         [0029]    An engine controller  76  responds to final engine torque limit signal  78  by producing engine output torque that is no greater than the final engine torque limit 
         [0030]    The function  70  produces an increase in allowed engine output torque at lower speed ratios, close to torque converter stall, and then blends back to normal allowed levels at the higher speed ratios. In practice, this allows increased engine torque in rare full-stall conditions, but ramps out the engine torque increase shortly after the wheels start rolling. 
         [0031]    When the power source  50  is a diesel engine, the higher speed ratios coincide with the time when boost from a turbocharger is achieved. Therefore, higher levels of combustion torque during full torque converter stall is allowed when the engine is not able to meet the torque limitations, but engine torque is restored to normal limits when the engine is able to meet the torque limits 
         [0032]    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.