Abstract:
An improved engine torque control method uses existing powertrain sensors and controls to reliably detect and suppress power-hop with minimum degradation of vehicle acceleration. Power-hop is detected by identifying a characteristic wheel jerk magnitude and oscillation frequency based on driven wheel speeds. Once a power-hop condition is detected, the control method computes a desired engine torque output for suppressing the detected power-hop without unnecessarily degrading vehicle performance, based on the wheel jerk magnitude, the engine speed and vehicle acceleration. A combination of engine cylinder fuel cut-off and spark retard is then scheduled for reducing the engine output torque to the desired level for the duration of the power-hop condition. The control method has minimal impact on vehicle cost since it is performed (preferably) by engine or other control software, and has been shown to quickly and effectively suppresses power-hop and its disadvantages without significantly degrading vehicle performance.

Description:
TECHNICAL FIELD 
     This invention relates to a torque control method for a vehicle engine, and more particularly to a control method for suppressing a detected power-hop condition while minimizing degradation of vehicle acceleration. 
     BACKGROUND OF THE INVENTION 
     Vehicles equipped with high torque engines, aggressive tires and lightweight driveline components can experience an undesired condition referred to as power-hop during a high torque vehicle launch. In general, power-hop is a condition of driveline instability initiated when the tractive effort decreases due to tire slip beyond an optimal slip value. If the engine torque is sufficiently high, the power-hop condition can be sustained, resulting in both torsional oscillation of the vehicle driveline and vertical oscillation of suspension members. Various techniques for suppressing power-hop include increasing driveshaft and half-shaft stiffness, decreasing tire aggressiveness, and equipping the driveline with dampers or auxiliary flywheels. However, each of these techniques increases cost and/or decreases performance. Accordingly, what is needed is a technique that effectively suppresses power-hop with minimal impact on vehicle cost and performance. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved engine torque control method that uses existing vehicle sensors and controls to reliably detect and suppress power-hop with minimum degradation of vehicle acceleration. Power-hop is detected by identifying a characteristic oscillation (magnitude and frequency) of the driven wheels. Once a power-hop condition is detected, the control method computes a desired engine torque output for suppressing the detected power-hop without unnecessarily degrading vehicle performance, based on the wheel oscillation magnitude, the engine speed and vehicle acceleration. A combination of engine cylinder fuel cut-off and spark retard is then scheduled for reducing the engine output torque to the desired level for the duration of the power-hop condition. The control method has minimal impact on vehicle cost since it is performed (preferably) by engine or other control software, and has been shown to quickly and effectively suppresses power-hop and its disadvantages without significantly degrading vehicle performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a vehicle powertrain, including an electronic control unit programmed to carry out the control method of this invention. 
     FIG. 2 is a block diagram of the control method of this invention. 
     FIGS. 3,  4  and  5  are flow diagrams representative of computer program instructions executed by the electronic control unit of FIG. 1 in carrying out the control method of FIG.  2 . FIG. 3 depicts a portion of the control method relating to detection of a power-hop condition; FIG. 4 depicts a portion of the control method relating to calculation of a desired torque output for suppressing a detected power-hop condition; and FIG. 5 depicts a portion of the control method relating to determination of engine fuel and spark controls for reducing the engine torque to the desired value. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, and particularly to FIG. 1, the reference numeral  10  generally designates a vehicle drive train including an engine  12  coupled to a multiple-speed ratio transmission  14 , which in turn is coupled via drive shaft  16  and differential gearset (DG)  18  to a pair of driven wheels  20   a - 20   b.  Engine  12  includes conventional spark and fuel control mechanisms  22 ,  24  operated under the control of an electronic control module (ECM)  26  via lines  28 ,  30  as indicated. The ECM  26  is microprocessor based, and operates in response to a number of inputs, including an engine speed signal ES on line  34  and a vehicle speed signal VS on line  36 . Preferably, the vehicle speed signal VS is a true representation of vehicle speed, and may be obtained based on the speed of un-driven wheels, or from a vehicle anti-lock braking system (not shown). Additional inputs relevant to the control of this invention include the driven wheel speed signals WS 1 , WS 2  on lines  38 ,  40 , which signals may also be obtained from an anti-lock braking system. The ECM  26  carries out a number of conventional engine control and diagnostic algorithms, and according to this invention carries out an additional algorithm for monitoring the illustrated inputs to identify an oscillation of the driven wheels that is characteristic of a power-hop condition, and to adjust the engine fuel and spark controls in a manner to alleviate the power-hop condition without significantly degrading the vehicle performance. In the illustrated embodiment, the oscillation component of the respective driven wheel speeds WS 1 , WS 2  is determined by twice differentiating the wheel speeds to form respective jerk signals. However, it will be recognized that the oscillation component may alternatively be obtained by suitable filtering of the wheel speeds WS 1 , WS 2  or the corresponding wheel acceleration values ACCEL 1 , ACCEL 2 . 
     The block diagram of FIG. 2 generally illustrates the control method of this invention as comprising three main portions, signified by the blocks  50 ,  52  and  54 . The control portion signified by block  50  pertains to detection of a power-hop condition. The inputs to block  50  include the accelerations ACCEL 1 , ACCEL 2  of the driven wheels  20   a,    20   b  and the vehicle speed VS; in this regard, the blocks  56 ,  58  signify a differentiation function for obtaining the acceleration values ACCEL 1 , ACCEL 2  from the respective wheel speed signals WS 1 , WS 2 . The outputs of block  50  include a POWER-HOP_DET flag on line  60  for indicating whether a power-hop condition is detected and a peak-to-peak jerk signal JERK_P 2 P on line  62  for indicating the severity of a detected power-hop condition. The control portion signified by block  52  is responsive to the outputs of block  50 , and pertains to calculation of a desired engine torque for TQ_DES for suppressing a detected power-hop condition of the indicated magnitude. Additional inputs for block  52  include a vehicle acceleration signal ACCELv on line  64  and the engine speed signal ES on line  34 , the block  66  signifying a differentiation function for developing the acceleration signal ACCELv based on the vehicle speed VS signal on line  36 . The control portion signified by block  54  pertains to an engine torque control for quickly reducing the engine torque to a value corresponding to the TQ_DES signal on line  68 , and is additionally responsive to the POWER-HOP_DET flag on line  60  and a model based torque (MBT) signal on line  70 . The outputs of block  54  are applied to conventional engine control software residing within ECM  26  (signified by block  72 ), such outputs including the number of engine cylinders enabled for fuel control (#CYL_EN) and a spark retard variable (SPK_RET) on lines  74  and  76 , respectively. As indicated in FIG. 2, the functionality of block  50 ,  52  and  54  are depicted in detail by the flow diagrams of FIGS. 3,  4  and  5 , respectively. 
     Referring to the flow diagram of FIG. 3, it will be seen that power-hop is detected according to this invention when a characteristic wheel jerk oscillation is identified. As such, the detection routine of FIG. 3 is periodically executed at a given rate to sample and process the wheel speed signals WS 1 , WS 2 . The signal processing utilizes a number of flags and variables, including the POWER-HOP_DET flag, a flag JERK_FLAG to indicate if the wheel jerk is in a positive or negative cycle, a timer JERK_TIMER to measure the elapsed time between wheel jerk oscillation cycles, a counter CYCLE_CTR to count the number of positive and negative wheel jerk cycles, and variables JERK_MAX and JERK_MIN to track the peak positive and negative wheel jerk values. 
     The POWER-HOP_DET flag is initially set to NO at each execution of the detection routine, as indicated at block  80 , and the blocks  84 - 132  are then executed for each of the driven wheels  20   a,    20   b,  as indicated by the FOR, NEXT blocks  82 ,  134  to compute peak-to-peak jerk values JERK_P 2 P( 20   a ), JERK_P 2 P( 20   b ) for each drive wheel  20   a,    20   b.  Thereafter, the block  136  computes the jerk magnitude JERK_P 2 P according to the maximum of the individual peak-to-peak jerk values JERK_P 2 P( 20   a ) and JERK_P 2 P( 20   b ). 
     Referring to block  84 , a wheel jerk signal WJ is computed by differentiating the respective wheel acceleration signal (ACCEL 1  or ACCEL 2 ). The blocks  86  and  88  are then executed to detect a change in polarity (in excess of minimum magnitudes POS_ENTRY and NEG_ENTRY) of the computed wheel jerk signal WJ, based on the status of JERK_FLAG and the current polarity of WJ. At each negative-to-positive transition, block  86  is answered in the affirmative, and blocks  90 ,  92 ,  94  are executed to increment CYCLE_CTR so long as the time denoted by JERK_TIMER is less than a predetermined reference time REF_TIME, to reverse the state of JERK_FLAG, and to set JERK_TIMER to zero. In a similar manner, block  88  is answered in the affirmative at each positive-to-negative transition of WJ, in which case blocks  96 ,  98 ,  100  are executed to increment CYCLE_CTR so long as the time denoted by JERK_TIMER is less than REF_TIME, to reverse the state of JERK_FLAG, and to set JERK_TIMER to zero. If a polarity transition of WJ does not occur, blocks  86  and  88  are answered in the negative, and block  102  is executed to increment JERK_TIMER. Thus, it will be seen that the entry magnitudes POS_ENTRY, NEG_ENTRY define the minimum characteristic jerk magnitude of power-hop, while the timer JERK_TIMER defines the minimum characteristic jerk oscillation frequency of power-hop. 
     The blocks  104 - 106 ,  110 ,  114  and  118  compare the current wheel jerk value WJ to the wheel jerk value (WJ_OLD) for the same wheel computed in the previous execution of the power-hop detection routine. In the event of a negative-to-positive transition, blocks  104  and  106  will be answered in the affirmative, and block  108  is executed to reset JERK_MAX to zero. Similarly, blocks  104  and  110  detect a positive-to-negative transition, in which case, block  112  resets JERK_MIN to zero. If the polarities of WJ and WJ_OLD are both positive, and WJ is more positive than WJ_OLD, as detected by blocks  104 ,  106  and  114 , block  116  is executed to set JERK_MAX equal to WJ. Similarly, if the polarities of WJ and WJ_OLD are both negative, and WJ is more negative than WJ_OLD, as detected by blocks  104 ,  110  and  118 , block  120  is executed to set JERK_MIN equal to WJ. In this way, JERK_MAX tracks WJ when WJ is positive and increasing, and JERK_MIN tracks WJ when WJ is negative and decreasing. The blocks  104 ,  106  and  114  also detect when WJ is positive but decreasing; in such case, the peak positive value of WJ is stored in JERK_MAX, and block  122  is executed to compute the peak-to-peak jerk JERK_P 2 P for the respective wheel  20   a,    20   b  according to the difference (JERK_MAX−JERK_MIN). Similarly, the blocks  104 ,  110  and  118  also detect when WJ is negative but increasing; in such case, the peak negative value of WJ is stored in JERK_MIN, and block  124  is executed to compute the peak-to-peak jerk JERK_P 2 P according to the difference (JERK_MAX−JERK_MIN). 
     The block  126  defines exit criteria for power-hop detection by comparing JERK_TIMER and vehicle speed VS to respective thresholds RESET_TIME and EXIT_SPD. The event criteria are satisfied if JERK_TIMER exceeds RESET_TIME or VS exceeds EXIT_SPD, in which case block  128  sets the POWER-HOP_DET flag to NO and sets CYCLE_CTR to zero. If the exit criteria are not satisfied, block  130  compares CYCLE_CTR to a reference count RE_FCT. If CYCLE_CTR exceeds REF_CT, a power-hop condition is detected, and block  132  is executed to set the POWER-HOP_DET flag to YES. Thus, the POWER-HOP_DET flag is set to indicate a power-hop condition when a characteristic oscillation is observed in either of the drive wheels  20   a  or  20   b.    
     As indicated above, the functionality of calculating and implementing the power-hop torque request (blocks  52  and  54  of FIG. 2) is detailed in the flow diagrams of FIGS. 4 and 5, respectively. Thus, whenever a power-hop condition is detected—that is, when the status of the POWER-HOP_DET flag is YES—software routines corresponding to the flow diagram blocks of FIGS. 4 and 5 are periodically executed to determine the appropriate inputs for the engine control algorithm software, designated by block  72  in FIG.  2 . 
     Referring to FIG. 4, the blocks  140 - 142  are first executed to compute three torque terms: TQ_P 2 P, TQ_RPM and TQ_ACCEL. The term TQ_P 2 P is determined according to the product of the wheel jerk magnitude (JERK_P 2 P) from block  136  of FIG. 3 and a vehicle acceleration dependent constant, CAL_TQ_P 2 P. The term TQ_RPM is determined according to the product of the engine speed ES and a vehicle acceleration dependent constant, CAL_TQ_RPM, and the term TQ_ACCEL is determined according to the product of the vehicle acceleration ACCELv and a vehicle acceleration dependent constant, CAL_TQ_ACCEL. The constants CAL_TQ_P 2 P, CAL_TQ_RPM, CAL_TQ_ACCEL are determined by table look-up as a function of vehicle acceleration (ACCELv), and serve to convert the respective jerk, speed and acceleration terms into corresponding torque loss/gain quantities associated with the detected power-hop condition. The torque terms TQ_P 2 P, TQ_RPM and TQ_ACCEL are then combined at block  144  to determine the desired engine output torque PHOP_TQ_% as a percentage of the maximum output torque (i.e., 100%). Referring to block  144 , it will be seen that the terms TQ_P 2 P and TQ_RPM reduce PHOP_TQ_%, while the term TQ_ACCEL increases PHOP_TQ_%. That is, PHOP_TQ_% decreases with increasing power-hop severity and with increasing engine speed, but increases with increasing vehicle acceleration. This serves, along with the calibration values determined at block  140 , to tailor PHOP_TQ_% so that the power-hop condition is curtailed while sustaining (as much as possible) the vehicle acceleration level. If the calculated PHOP_TQ_% is greater than a reference MIN_REF such as 95%, as determined at block  146 , the power-hop condition is not sufficiently severe to warrant engine torque reduction, and the block  148  is executed to set PHOP_TQ_% equal to 100%. Blocks  146 - 148  similarly prevent the engine torque reduction if it is determined that the power-hop condition is under control, meaning that the measured severity of the power-hop condition is decreasing (dampening). Finally, block  150  is executed to determine an engine output torque value TQ_DES corresponding to PHOP_TQ_% according to the product (PHOP_TQ_% * CAL_MAX_TQ), where CAL_MAX_TQ is a calibrated value representing the maximum engine output torque, in N-m for example. 
     Referring to FIG. 5, the blocks  160 - 174  are executed in sequence as shown to convert the desired torque TQ_DES into corresponding fuel cut-off and spark retard control signals #CLY_EN, SPK_RET. In general, the routine determines the minimum number of engine cylinders #CLY_EN required to produce TQ_DES, and then determines a spark timing retardation value SPK_RET for reducing the produced torque to TQ_DES. First, block  160  converts TQ_DES to a percentage TQ_% of a model-based representation MBT of the current engine torque with normal fueling being delivered to each of the engine cylinders. Block  162  then determines the minimum number of engine cylinders (ENABLED_#CYL) required to achieve TQ_%; this is achieved by rounding up the product (TQ_% * #CYL), where #CYL is the total number of engine cylinders. Block  164  limits ENABLED_#CYL to be at least two, but no more than #CYL, and block  166  computes the corresponding number of disabled cylinders, #DISABLED_CYL. The block  168  then determines the output #CYL_EN by table look-up as a function of #DISABLED_CYL to satisfy driveability concerns. Blocks  170 - 174  then determine how much spark retard is required to achieve the torque reduction not obtained by disabling cylinder fueling. At block  170 , the additional torque reduction to be achieved by spark retard (SPARK_FRACTION) is computed according to the difference between #CYL_EN and the product (TQ% * #CYL). Block  172  expresses SPARK_FRACTION as a percentage of torque reduction per enabled (fueled) cylinder (SPARK_TQ_RED%), and block  174  determines a corresponding amount of spark retard (SPK_RET) by table look-up. As indicated above, the computed values of #CYL_EN and SPK_RET are subsequently used by the engine control algorithm software (block  72  of FIG. 2) to correspondingly control the engine spark and fuel control mechanisms  22 ,  24 . 
     In summary, the control of this invention provides a practical and cost-effective solution for suppressing power-hop. The control is enabled when a characteristic driven wheel oscillation is identified, and torque reduction is scheduled based on the severity of the power-hop to ensure that the detected power-hop is suppressed without significantly affecting vehicle performance. Additionally, the torque is reduced with a combination of engine cylinder fuel cut-off and spark retard so that the suppression occurs quickly and accurately. While described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. In this regard, it will be understood that the scope of this invention is not limited to the illustrated embodiment, and that control methods incorporating such modifications may fall within the scope of this invention, which is defined by the appended claims.