1. Field of the Invention
This invention relates to a slip control system for a vehicle.
2. Description of the Prior Art
There has been put into practice a traction control system which detects the amount of slip of the driving wheels and controls the engine output and/or application of braking force to the wheels so that the amount of slip of the driving wheels converges on a target value, thereby preventing deterioration of acceleration of the vehicle due to an excessive driving torque transmitted to the driving wheels. Many vehicles are provided with a traction control system as well as an anti-skid brake system. See, for instance, Japanese Unexamined Patent Publication No. 1(1989)-197160.
When the amount of slip of the driving wheels increases, for instance, during running on an ascending slope, the traction control is initiated to suppress the engine output. Suppression of the engine output tends to reduce the engine speed, which can result in engine stall, and to deteriorate accelerating performance of the vehicle. Such a tendency is enhanced when the friction coefficient .mu. of the road surface is high.
If the traction control system is arranged so that the traction control is interrupted when the engine speed falls below a predetermined value, the amount of slip of the driving wheels can increase again upon resumption of the traction control due to poor engine output suppression, which result in deterioration of the acceleration performance.
Further if the traction control is interrupted when the friction coefficient .mu. of the road surface is low, the amount of slip increases and not only the accelerating performance cannot be improved but also the driving stability is lost due to an excessive slip.
In Japanese Patent Publication No. 4(1992)-32930, there is disclosed a traction control system for a vehicle in which the wheel speed of the driving wheels is prevented from excessively falling at a low engine speed by less suppressing the engine torque when the engine speed is low than when the engine speed is high.
However since the amount of slip of the driving wheels for a given engine speed differs depending on the friction coefficient .mu. of the road surface, the traction control system in which the degree of reduction of the engine torque is determined depending solely on the engine speed cannot effect a fine control for preventing engine stall while ensuring an excellent accelerating performance.
Thus there has been a demand for a slip control system in which the slip control can be finely effected according to the friction coefficient .mu. of the road surface so that engine stall can be effectively prevented without sacrificing the accelerating performance.
Especially in a manual transmission vehicle, when the clutch is engaged upon start on an ascending slope while the vehicle slightly reverses, the amount of slip of the driving wheels abruptly increases and the traction control is initiated, which can result in reduction of the engine speed and engine stall.