Abstract:
A method of controlling a driveline retarder of a powertrain including a transmission having a rotatable input shaft connected to an engine and an output shaft connected to the driveline retarder is provided. The method includes determining a set speed value and comparing the set speed value plus a customer modifiable constant value to a current rotational speed value of the output shaft. A driveline retarder cruise modulation torque request value is then calculated when the current rotational speed value of the output shaft is greater than or equal to the set speed value plus the customer modifiable constant value. The driveline retarder is controlled as a function of the driveline retarder cruise modulation torque request value. A powertrain configured to implement the aforementioned method is also disclosed.

Description:
TECHNICAL FIELD 
     The present invention relates to a powertrain and method of controlling the operation of a driveline retarder for a vehicle. 
     BACKGROUND OF THE INVENTION 
     The use of driveline retarders as a means for applying braking torque to a vehicle transmission is well known. Typically, such driveline retarders employ a rotary member connected to the hub of a transmission output shaft that is operative to circulate fluid between vane members on a stationary housing. The fluid circulation results in power absorption and retardation of the rotational speed of the transmission output shaft. A reduction in vehicle speed typically results from such action, assuming that the roadway or other supportive surface of the vehicle demonstrates a frictional coefficient of sufficient magnitude to support such braking torque. The driveline retarder is especially suited to slow or control the speed of a vehicle so equipped when traversing steep grades. 
     It is generally known to employ a solenoid valve in hydrodynamic-type driveline retarders to function as a pressure control valve under electro-hydraulic solenoid modulation as controlled by an appropriate electronic control unit such as a dedicated microprocessor or the like. The duty cycle at which the solenoid valve is actuated or modulated regulates the driveline retarder capacity or the load applied to the output shaft of the transmission and, accordingly, controls the braking activity of the driveline retarder. The mechanical structure and control architecture of the previously known driveline retarders are generally adaptable for implementation in accordance with the present invention. 
     SUMMARY OF THE INVENTION 
     A method of controlling a driveline retarder of a vehicular powertrain is provided. The powertrain includes a transmission having a rotatable input shaft operatively connected to an engine and an output shaft operatively connected to the driveline retarder. The method includes comparing a set speed value plus a customer modifiable constant value to a current rotational speed value of the output shaft. Additionally, the method includes calculating a driveline retarder cruise modulation torque request value as a function of at least one of set speed value, current rotational speed value of the output shaft, acceleration rate of the output shaft, and time, when the set speed value plus the customer modifiable constant value is greater than or equal to the current rotational speed value of the output shaft. The driveline retarder is then controlled as a function of the driveline retarder cruise modulation torque request value. 
     The method also includes determining the set speed value if cruise control is active and driveline retarder cruise is enabled. The set speed value may be set equal to the current rotational speed value of the output shaft if engine load is greater than or equal to a predetermined calibration constant. The driveline retarder cruise modulation torque request value may be set equal to zero if engine load is less than a predetermined calibration value. Additionally, the driveline retarder cruise modulation torque request value may be set equal to zero if the current rotational speed value of the output shaft is greater than or equal to the set speed value plus the customer modifiable constant value. 
     A vehicular powertrain is also provided. The powertrain includes a transmission having a rotatable input shaft and a rotatable output shaft. An engine is operatively connected to the input shaft, while a driveline retarder is operatively connected to the output shaft. A first electronic control unit is provided in communication with the engine and operable in a cruise control mode. A second electronic control unit is provided in communication with at least one of the transmission and the driveline retarder. A data link communicates with the first and second electronic control units and operates to provide communication between the first and second electronic control units. The second electronic control unit is preferably configured to compare a set speed value plus a customer modifiable constant value to a current rotational speed value of an output shaft. Subsequently, the second electronic control unit calculates a driveline retarder cruise modulation torque request value as a function of at least one of the set speed value, the current rotational speed value of the output shaft, acceleration rate of the output shaft, and time when the current rotational speed of the output shaft is greater than or equal to the set speed value plus the customer modifiable constant value. Finally, the second electronic control unit operates to control the driveline retarder as a function of the driveline retarder cruise modulation torque request value. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagrammatic representation of a vehicular powertrain incorporating a driveline retarder controlled by an electronic control unit in accordance with a method of the present invention; and 
         FIG. 2  is a method, presented in flowchart form, of controlling the driveline retarder of  FIG. 1  in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures and more particularly  FIG. 1 , there is shown a vehicular powertrain, a portion of which is generally indicated at  10 . The vehicular powertrain  10  includes a transmission  12  interconnected to an engine  14  through a transmission input shaft  16 . The powertrain  10  further includes a final drive  18 , such as a differential, interconnected to the transmission  12  though a transmission output shaft  20 . The transmission  12  includes a plurality of gear sets, not shown, operable to accommodate or provide various power transmission ratios between the transmission input shaft  16  and the transmission output shaft  20 . 
     A driveline retarder  22  is interconnected with the transmission output shaft  20  and is operable to selectively and variably provide a braking or resistive force to the rotation of the transmission output shaft  20 . The driveline retarder  22  may be of any type generally known in the art, such as a hydrodynamic device. An electronic control unit, or ECU  24 , is interconnected with the transmission  12  and driveline retarder  22  by respective buses  26  and  28 . An ECU  25  in interconnected with the engine  14  and a data link  29 . Additionally, the ECU  24  may receive signals from the engine  14  and vehicle sensors over the data link  29 . The data link  29  may be of the type SAE J1939, SAE J1587, IES-CAN, or other similar data link. The ECUs  24  and  25  are preferably a programmable microprocessor device, which is known to those skilled in the art. It should be appreciated that the ECU  24  receives, for example, data indicating the active state of the cruise control system, active state of the driveline retarder  22 , the speed of the vehicle, the rotational speed of the transmission output shaft  20 , the cruise control set speed (if available and if cruise control is enabled), throttle position, etc. The ECU  24  is operable to control the shifting of the transmission  12  through the bus  26  and controls the duty cycle or modulation torque of the driveline retarder  22  over the bus  28 . It should also be appreciated that the ECU  25  receives, for example, data indicating the active state of the cruise control system, throttle position, etc. and is operable to control the operation of the engine  14  over a bus  27 . Additionally, the ECU  25  includes the required software to operate the powertrain  10  in a cruise control mode. That is by providing a vehicle set speed, usually determined, by the vehicle operator, the speed of the vehicle can be maintained at or near that vehicle set speed. This cruise control mode may also include a selectively activatable coast mode, i.e. when the cruise control mode is active but disengaged. Since the transmission output shaft  20  is operatively connected to the final drive  18 , the vehicle set speed is proportional to the rotational speed of the transmission output shaft  20 . 
     Referring now to  FIG. 2  and with continued reference to  FIG. 1 , a method  30  for controlling the driveline retarder  22  in conjunction with the cruise control system is shown in accordance with the present invention. More precisely,  FIG. 2  illustrates a block diagram or flowchart representing steps performed by a control device such as the ECU  24 . The method  30  begins at step  32  and proceeds to step  34  where a volatile random access memory, or VRAM, set speed value is initialized to clear any stored values. The method  30  will then proceed to step  36  where a determination is made as to whether the cruise control system is active and whether the cruise control option for the driveline retarder  22  is enabled. If either the cruise control is inactive or the cruise control option for the driveline retarder  22  is disabled, the method proceeds to step  38  where a determination is made as to whether the driveline retarder  22  is active. If the driveline retarder  22  is not active, the method  30  will loop to step  36 . Alternately, if the driveline retarder  22  is active, the method  30  will loop to step  34 . Returning to step  36 , if both the cruise control is active and the cruise control option for the driveline retarder  22  is enabled, the method  30  proceeds to step  40 . 
     At step  40 , a determination is made as to whether a cruise control set speed value is received via the data link  29 . If the cruise control set speed is available over the data link  30 , then this value is stored as the VRAM set speed as illustrated at step  42 . Alternately, if the cruise control set speed is not received or available over the data link  29 , the method proceeds to step  44 . At step  44 , a determination is made as to whether the value for filtered or conditioned engine load, such as the percentage of engine fueling or throttle opening, is greater than or equal to a predetermined calibrated constant stored within the ECU  24 . The value of the predetermined calibrated constant is preferably non-modifiable and constitutes part of the preprogrammed calibration contained within the ECU  24 . Additionally, a determination is made as to whether the cruise control coast mode is active. If the filtered engine load is greater than or equal to the calibrated constant and/or the cruise control coast mode is active, the method  30  proceeds to step  46  where the current rotational speed value of the transmission output shaft  20  of the transmission  12  is stored as the VRAM set speed. Alternately, if the filtered engine load is less than the calibrated constant and the cruise control coast mode is not active, the method  30  proceeds to step  48  where the driveline retarder cruise modulation torque value for the control of the driveline retarder  22  is set equal to zero. 
     From step  42  or step  46 , the method  30  proceeds to step  50  where a determination is made whether the current rotational speed of the transmission output shaft  20  is greater than or equal to the VRAM set speed, stored at step  42  or  46 , added to a customer modifiable constant, or CMC, value. The CMC value may be modified or adjusted via a service tool known to those skilled in the art and may also include a hysteresis or offset. If the current rotational speed of the transmission output shaft  20  is less than the VRAM set speed value plus the CMC value, the method  30  will proceed to step  48  where the driveline retarder cruise modulation torque value for the control of the driveline retarder  22  is set equal to zero. If the current rotational speed of the transmission output shaft  20  is greater than or equal to the VRAM set speed value plus the CMC value, the method  30  will proceed to step  52 . 
     At step  52 , a value for the driveline retarder cruise modulation torque request is calculated as a function of the VRAM set speed, current rotational speed of the output shaft  20 , rate of acceleration of the output shaft  20 , time, and other related gains. Once the calculation is made at step  52 , the method  30  will proceed to step  54 . Additionally, step  48  proceeds to step  54 . At step  54 , the calculated driveline retarder cruise control modulation torque request is arbitrated or compared with other driveline retarder requests, such as manual activation requests from the operator of the vehicle. The driveline retarder  22  is then operated or controlled based on the overall driveline retarder requests. Subsequently, the method  30  of the present invention will loop from step  54  to step  36 . 
     Since the vehicle weight and road grade are often difficult to determine, a precise braking toque for the driveline retarder  22  may be difficult to calculate. The method  30  of the present invention provides a closed loop algorithm to determine the requested torque value for driveline retarder  22  based on current rotational speed of the output shaft  20  and the desired speed of the transmission output shaft  20  multiplied by a variable gain. Additionally, the method  30  of the present invention determines the requested torque value for driveline retarder  22  based on the acceleration of the transmission output shaft  20  versus the desired acceleration that will achieve the desired speed of the transmission output shaft  20 . 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.