1. Field of the Invention
This invention relates to a driving force control system for a four-wheel drive vehicle including a pair of front wheels, and a pair of rear wheels, one of the pairs being main drive wheels, and another of the pairs being auxiliary drive wheels, the driving force control system controlling engagement forces of clutches to thereby control driving forces distributed to the auxiliary drive wheels.
2. Description of the Related Art
Conventionally, a driving force controls system of this kind has been proposed, e.g., by Japanese Laid-Open Patent Publication (Kokai) No. 9-109716, for a four-wheel drive vehicle (hereinafter referred to as he vehicle including rear wheels as main drive wheels, and front wheels as auxiliary drive wheels. This driving force control system controls the engagement forces of variable torque clutches to thereby control torque distributed or allocated to the front wheels, i.e., the auxiliary drive wheels. More specifically, a first front-wheel distribution torque is calculated based on the difference (hereinafter referred to as ear-front wheel speed difference obtained by subtracting the wheel speed of the front wheels from the wheel speed of the rear wheels. A second front-wheel distribution torque is also calculated based on the oil temperature of the variable torque clutches. Further, a third front-wheel distribution torque is calculated based on a throttle opening when the wheel speed of the front wheels is equal to or lower than a predetermined value (i.e., during the start of the vehicle), and set to a value of 0 when the wheel speed of the front wheels is larger than the predetermined value (i.e., after the start of the vehicle), or when the rear-front wheel speed difference assumes a negative value (i.e., during deceleration of the vehicle). Then, the maximum value of the above first to third front-wheel distribution torques is determined to be a final control value, based on which the variable torque clutches are driven to control the engagement forces thereof to thereby control the torque distribution to the front wheels.
According to the proposed prior art driving force control system, the torque distributed to the front wheels is calculated based on the above-mentioned three parameters: the rear-front wheel speed difference, the oil temperature, and the throttle opening. Therefore, it is impossible to perform delicate or fine control of torque distribution to the front wheels based on a state of slippage of the individual rear wheels. As a result, when the vehicle is being started on a road surface having a small frictional resistance xcexc (hereinafter referred to as xe2x80x9clow-xcexc road surfacexe2x80x9d, such as an icy road surface), for instance, only one of the rear wheels might undergo slippage, but the system cannot perform the driving force control such that suitable torque distribution to the front wheels is effected in response to this slipping condition of one rear wheel, which makes it impossible for the vehicle to start smoothly. Further, the throttle opening indicates a driver demand for acceleration, but does not faithfully reflect an actual accelerating condition of the vehicle, so that the torque distribution to the front wheels cannot be properly carried out accordingly to the actual accelerating condition of the vehicle. This sometimes results in an insufficient torque distribution to the front wheels, or inversely, an excessive torque distribution to the same, which leads to decreased response of the vehicle and degraded fuel economy.
It is an object of the invention to provide a driving force control system for a four-wheel drive vehicle, which enables the vehicle to smoothly start on a low-xcexc road surface, and is capable of distributing only the required amounts of driving forces to auxiliary drive wheels depending on an actual accelerating condition of the vehicle, thereby improving the response and fuel economy of the vehicle.
To attain the above object, according to a first aspect of the invention, there is provided a driving force control system for a four-wheel drive vehicle including a pair of front wheels, and a pair of rear wheels, one of the pairs being main drive wheels, and another of the pairs being auxiliary drive wheels, the driving force control system controlling engagement forces of clutches to thereby control driving forces distributed to the auxiliary drive wheels.
The driving force control system according to the first aspect of the invention comprises:
driving force control means for controlling the engagement forces of the clutches such that the driving forces are distributed to the auxiliary drive wheels, on condition that there is satisfied at least one of the following conditions that a wheel speed of one of the main drive wheels is equal to or higher than a first predetermined speed, and at the same time, a wheel speed of another of the main drive wheels is lower than the first predetermined speed, and that an average wheel speed of the main drive wheels is equal to or higher than a second predetermined speed, and at the same time, an average wheel speed of the auxiliary drive wheels is lower than the second predetermined speed, until the average wheel speed of the auxiliary drive wheels reaches a third predetermined speed equal to or higher than the second predetermined speed.
According to this driving force control system for a four-wheel drive vehicle, when the wheel speed of one of the main drive wheels is equal to or higher than the first predetermined speed, and at the same time, the wheel speed of another of the main drive wheels is lower than the first predetermined speed, i.e., when one of the main drive wheels is slipping, driving forces are distributed to the auxiliary drive wheels until the average wheel speed of the auxiliary drive wheels reaches the third predetermined speed. Thus, the control of distribution of the driving forces to the auxiliary drive wheels can be delicately or finely carried out depending on a slipping condition of each of the individual main drive wheels. Further, even when the wheel speeds of the respective main drive wheels are equal to each other, if the average wheel speed of the main drive wheels is equal to or higher than the second predetermined speed, and at the same time, the average wheel speed of the auxiliary drive wheels is lower than the second predetermined speed, i.e., when both the main drive wheels are slipping, driving forces are distributed to the auxiliary drive wheels until the average wheel speed of the auxiliary drive wheels reaches the third predetermined speed. This make it possible to carry out control of distribution of the driving forces to the auxiliary drive wheels delicately or finely depending on a slipping condition of both of the main drive wheels, and hence enables the vehicle to smoothly start even on a low-xcexc road surface.
To attain the above object, according to a second aspect of the invention, there is provided a driving force control system for a four-wheel drive vehicle including a pair of front wheels, and a pair of rear wheels, one of the pairs being main drive wheels, and another of the pairs being auxiliary drive wheels, the driving force control system controlling engagement forces of clutches to thereby control driving forces distributed to the auxiliary drive wheels.
The driving force control system according to the second aspect of the invention comprises:
vehicle acceleration-calculating means for calculating a vehicle acceleration of the vehicle based on a demanded driving force demanded by the main drive wheels; and
driving force-calculating means for calculating the driving forces to be distributed to the auxiliary drive wheels based on the calculate vehicle acceleration.
According to this driving force control system for a four-wheel drive vehicle, the vehicle acceleration of the vehicle is calculated based on a demanded driving force demanded by the main drive wheels. Based on the calculated vehicle acceleration, the driving forces to be distributed to the auxiliary drive wheels are calculated, and the engagement forces of the clutches are controlled such that the calculated driving forces are distributed to the auxiliary drive wheels. Thus, based on the demanded driving force actually demanded by the main drive wheels, the vehicle acceleration is calculated, which can faithfully reflect an actual accelerating condition of the vehicle on the calculated driving forces. Therefore, because the driving forces to be distributed to the auxiliary drive wheels are calculated based the vehicle acceleration calculated as described above, differently from conventional cases where the driving forces are calculated based on the throttle opening, only the required amounts of the driving forces can be distributed to the auxiliary drive wheels while taking the actual accelerating condition of the vehicle into account. This makes it possible to improve the response and fuel economy of the vehicle.
Preferably, the driving force control system further includes driving force-increasing means for increasing the driving forces distributed to the auxiliary drive wheels, on condition that there is satisfied at least one of the following conditions that a wheel speed of one of the main drive wheels is equal to or higher than a first predetermined speed, and at the same time, a wheel speed of another of the main drive wheels is lower than the first predetermined speed, and that an average wheel speed of the main drive wheels is equal to or higher than a second predetermined speed, and at the same time, an average wheel speed of the auxiliary drive wheels is lower than the second predetermined speed, until the average wheel speed of the auxiliary drive wheels reaches a third predetermined speed equal to or higher than the second predetermined speed.
According to this preferred embodiment, when wheel speeds of the respective main drive wheels are different from each other, or the average wheel speed of the main drive wheels is higher than that of the auxiliary drive wheels, i.e. when one or both of the main drive wheels is/are slipping, the driving forces distributed to the auxiliary drive wheels are increased until the average wheel speed of the auxiliary drive wheels reaches the third predetermined speed. Therefore, when the vehicle is starting on an icy road surface, and the vehicle acceleration is small due to a small throttle opening, for instance, if one or both of the main drive wheels is/are slipping, the driving forces distributed to the auxiliary drive wheels are increased, thereby enabling the vehicle to start smoothly. As a result, the startability of the four-wheel drive vehicle can be further improved.
More preferably, the driving force-increasing means includes progressively-increasing means for progressively increasing the driving forces distributed to the auxiliary drive wheels with the lapse of time.
More preferably, the driving force control system further includes driving force-progressively decreasing means for progressively decreasing the driving forces distributed to the auxiliary drive wheels with the lapse of time, after the average wheel speed of the auxiliary drive wheels reaches the third predetermined speed.
Preferably, the driving force control system further includes vehicle speed detection means for detecting a vehicle speed of the vehicle, and correction means for correcting the driving forces for prevention of tight turn, such that the driving forces are made smaller as the vehicle speed detected by the vehicle speed detection means is larger, and at the same time that the driving forces are made larger as the demanded driving force is larger.
Preferably, the demanded driving force is calculated based on a basic driving force calculated based on an engine rotational speed and an intake pipe absolute pressure, by using a coefficient corresponding to a gear ratio of the vehicle, and a correction term corresponding to an inertial force of the vehicle.
Preferably, the vehicle acceleration-calculating means calculates the vehicle acceleration by subtracting a value corresponding to a running resistance of the vehicle from a vehicle acceleration value calculated based on the demanded driving force.