The technical field of this invention is powertrain torque control for a motor vehicle.
When a driven wheel of a vehicle slips with respect to the road surface, either by slowing down or speeding up, lateral adhesion of the tire to the road can decrease quickly and significantly. Loss of lateral adhesion can allow the tire to slip sideways and thus cause understeer (if a front wheel) or oversteer (if a rear wheel). Wheel slip is controlled in many vehicles by traction control systems during powertrain produced acceleration and by anti-lock braking controls during application of the vehicle brakes. But such slip may sometimes be produced by engine braking during vehicle deceleration when the throttle is closed with no vehicle brakes applied, when the engine slows down faster than the vehicle body and causes a braking torque to be applied to the driven wheels. Anti-lock braking controls are of no use when the vehicle brakes are not applied; and most acceleration traction controls are generally not designed to deal with wheel slip due to engine braking.
One system for sensing a difference in wheel speeds between driven and undriven wheels due to engine braking and increasing fuel to provide increased engine torque and spin up the slipping wheel has been suggested in U.S. Pat. No. 3,802,528; but modern computerized powertrain controls permit a far more finely tuned and accurate closed loop control to more advantageously balance the opposing goals of controlling wheel slip and providing engine braking.
The powertrain torque control of this invention provides an engine drag control mode of operation during periods of undesired engine drag induced wheel slip by modifying the torque of the vehicle engine in closed loop control to maintain a driven wheel speed at a predetermined target velocity lower than the vehicle speed by a target velocity difference providing as much engine braking as is consistent with a desired degree of lateral traction. The control derives a velocity error as the difference between the driven wheel speed and the target velocity and derives and delivers to the powertrain a torque command for reducing the velocity error.
The torque control determines the engine drag control mode in response to the wheel speed sensors, preferably causing entry of the engine drag control mode when the driven wheel speed that is closest to the vehicle speed falls below the target velocity while powertrain delivered torque and throttle position are below predetermined values indicative of deceleration. The driven wheel speed closest to the vehicle speed is preferably chosen because only one driven wheel is necessary for lateral traction and this wheel requires the smallest reduction in engine braking. The target velocity difference is preferably determined as a weighted difference between vehicle speed and vehicle turn curvature, the difference being reduced by the latter for quicker response in vehicle turns.