vehicle powertrain torque control 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 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.

1. A torque control for a vehicle having a powertrain driving at least one driven wheel, the vehicle also having at least one undriven wheel, the control comprising: wheel speed sensors responsive to the driven and undriven wheels; means responsive to the wheel speed sensors to determine an engine drag control mode; means responsive to the wheel speed sensor of the undriven wheel in the engine drag control mode to derive a vehicle speed; means responsive to the wheel speed sensors in the engine drag control mode to derive a target velocity lower than the vehicle speed by a predetermined target velocity difference; means in the engine drag control mode to derive a velocity error as the difference between the driven wheel speed and the target velocity; and means responsive to the wheel speed sensors in the engine drag control mode to derive and deliver to the powertrain a torque command for reducing the velocity error, whereby the driven wheel speed is controlled in closed loop to the target velocity.

2. The apparatus of claim 1 in which the target velocity difference is derived as a function of the vehicle speed.

3. The apparatus of claim 2 in which the target velocity difference is reduced as a function of vehicle turn curvature.

4. The apparatus of claim 1 in which the target velocity difference is derived as a function of vehicle turn curvature.

5. The apparatus of claim 1 in which the means responsive to the wheel speed sensors to determine the engine drag control mode are also responsive to a torque delivered by the powertrain.

6. The apparatus of claim 1 having a pair of driven wheels in which the driven wheel speed used is the driven wheel speed that is closest in speed to the vehicle speed.

7. The apparatus of claim 6 further having a pair of undriven wheels in which the vehicle speed is a function of the wheel speeds of both of the undriven wheels.

8. A method of controlling torque of a powertrain in a vehicle, the powertrain driving a pair of driven wheels and the vehicle also having a pair of undriven wheels, the control comprising the steps: determining wheel speeds of the driven wheels and the undriven wheels; responsive at least to the wheel speeds of the driven wheels and the undriven wheels, determining an engine drag control mode; in the engine drag control mode, deriving a vehicle speed from the wheel speed of at least one of the undriven wheels; in the engine drag control mode, deriving a target velocity lower than the vehicle speed by a predetermined target velocity difference determined at least from the vehicle speed; in the engine drag control mode, calculating a velocity error as the difference between the wheel speed of one of the driven wheels and the target velocity; and in the engine drag control mode, deriving from the wheel speeds and delivering to the powertrain a torque command for reducing the velocity error, whereby the driven wheel speed is controlled in closed loop to the target velocity.

9. The method of claim 8 wherein, in the step of calculating a velocity error, the one of the driven wheels is the driven wheel having a wheel speed closest to the vehicle speed.

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.

If car has problems with engine drag via the OBD scanner viewing PCM and ETBCM you can see all the PIDs related to engine drag to determine what the problem is

JR, obviously the descriptions in the first 3 posts were written by some pompous, verbose, anal-retentive engineer...worse doublespeak than the latest version of Nobama's stimulus package! I had to read parts of it 4 or 5 times to make sure I understand it, and I know I'm NOT stupid! This is likely the reason for the complaints about racing the C6 Z06 because you get zero engine braking when you lift off the throttle, especially when turning the car in tight radius turns. Can you just turn this shit off? Tweak my tune and I'll do some autocross testing.

The only way people can understand it is a real function and exactly as the engineers defined it is show their content exactly as they wrote it.

Written any other way and people will just say it is just my view and ignore it.

Hence in research and review of flash files I find all major tuning products are ignoring this function and in fact if tire or wheel changes are made can induce engine drag on when it should not and the cause of crashes.

At this point I am looking at either a fix or a workaround.

In existing testing C6 Z06 owner simply went from a 325/30/19 tire to 345/30/19 which is a slight change ( changes made also in the PCM flash calibration) and it caused inducing engine drag to come on even at slow speeds with stock and modified tune.