Control apparatus method for four-wheel drive vehicle

A control apparatus for a four-wheel drive vehicle, said four-wheel drive vehicle having mainly driven tire wheels which are either one of front tire wheels or rear tire wheels and auxiliarily driven tire wheels which are the others of either the front tire wheels or the rear tire wheels, said control apparatus comprising:

• driving force distribution adjustment mechanism having a frictional clutch arranged for receiving a control signal according to which an engaging force thereof is controlled so that a distribution of a driving force from a vehicular engine to said mainly driven tire wheels and to said auxiliarily driven wheels is carried out;
• a first sensor arranged for detecting a rotation speed of one of the mainly driven tire wheels;
• a second sensor arranged for detecting a rotation speed of one of the auxiliarily driven tire wheels;
• a third sensor arranged for detecting a running speed of the vehicle;
• and
• a control unit arranged for determining a tire wheel rotation difference in the rotation speed of the mainly driven driven tire wheel to that of the auxiliarily driven tire wheel detected by said first and second sensors, setting a rate of the distribution of the driving force between the mainly driven tire wheels and the auxiliarily driven tire wheels on the basis of the determined difference in the rotation speed of the tire wheel so that as the determined speed difference becomes increased, the rate of the distribution of the driving force to the auxiliarily driven tire wheels becomes increased, outputting the control signal toward the frictional clutch so as to control the engaging force of the frictional clutch on the basis of the set rate of distribution of the driving force to the auxiliarily driven tire wheels, setting a first reference running speed (V.sub.WF1) exceeding a vehicular low running speed, and modifying the set rate of the distribution so that the driving force distributed to the auxiliarily driven tire wheels is maintained constantly at its minimum value when the vehicle is running at a speed equal to or above the first reference running speed.

A control method for a four-wheel drive vehicle, said four-wheel drive vehicle having mainly driven tire wheels which are either one of front tire wheels or rear tire wheels and auxiliarily driven tire wheels which are the others of either the front tire wheels or the rear tire wheels and driving force distribution adjustment mechanism having a frictional clutch arranged for receiving a control signal according to which an engaging force thereof is controlled so that a distribution of a driving force from a vehicular engine to said mainly driven tire wheels and to said auxiliarily driven wheels is carried out, said control method comprising the steps of;

• a) detecting a rotation speed of one of the mainly driven tire wheels;
• b) detecting a rotation speed of one of the auxiliarily driven tire wheels;
• c) detecting a running speed of the vehicle;
• d) determining a tire wheel rotation speed difference from the rotation speed of the auxiliarily driven tire wheel to that of the mainly driven tire wheel;
• e) setting a rate of the distribution of the driving force between the mainly driven tire wheels and the auxiliarily driven tire wheels on the basis of the determined difference in the rotation speed of the tire wheel so that as the determined difference becomes increased, the rate of the distribution of the driving force to the auxiliarily driven tire wheels becomes increased;
• f) outputting the control signal toward the frictional clutch so as to control the engaging force of the frictional clutch on the basis of the set rate of distribution of the driving force to the auxiliarily driven tire wheels;
• g) setting a first reference running speed (V.sub.WF1) exceeding a vehicular low running speed;
• and
• h) modifying the set rate of the distribution so that the driving force distributed to the auxiliarily driven tire wheels is maintained constantly at its minimum limit when the vehicle is running at a speed equal to or above the first reference running speed.