Electricity: motive power systems – Plural – diverse or diversely controlled electric motors – Plural – diverse motor controls
Reexamination Certificate
2001-07-17
2003-03-04
Leykin, Rita (Department: 2837)
Electricity: motive power systems
Plural, diverse or diversely controlled electric motors
Plural, diverse motor controls
C318S052000, C318S058000, C318S064000, C180S065100
Reexamination Certificate
active
06528959
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a driving force control system for a type of front-and-rear wheel drive vehicle that drives one pair of a pair of front wheels and a pair of rear wheels by an engine, and another pair of the pairs by an electric motor.
2. Description of the Prior Art
Conventionally, a driving force control system of the above-mentioned kind was proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 2000-79831. The front-and-rear wheel drive vehicle has front wheels thereof driven by an engine and rear wheels thereof driven by an electric motor. This driving force control system reduces the driving force for driving the front wheels for slip control, e.g. when the front wheels undergo a slip when the vehicle starts on a low-friction road surface, such as that of a snowy road. Further, under such a slip control of the front wheels, when it is determined e.g. from the vehicle speed that the vehicle is in a traveling condition in which the vehicle can move forward, the operation of the electric motor is inhibited, thereby saving electric energy of the vehicle.
The conventional driving force control system, however, simply inhibits the operation of the electric motor to thereby completely stop the assistance thereof, when it is determined during the slip control of the front wheels that the vehicle can move forward. Therefore, the total driving force for driving the vehicle tends to become short, and the slip of the front wheels is liable to be increased. Further, the determination as to whether the vehicle can move forward or not is carried out only by estimation based on the vehicle speed detected then, and therefore, depending on a subsequent operation of the accelerator pedal and the like, the slip of the front wheels can become excessively large, so that the front wheels cannot be maintained in an optimum slip condition, which makes it impossible to perverse traveling stability of the vehicle on a low-friction road surface.
Another driving force control system of the above-mentioned type was also proposed which causes the engine braking force to act on the front wheels and the braking torque caused by the electric motor to act on the rear wheels during a decelerating travel condition in which the accelerator pedal is released, to thereby brake the vehicle (e.g. Japanese Laid-Open Patent Publication No. 9-298802). In this case, the braking by the electric motor is carried out by producing a resisting force or drag force against the rotation of the rear wheels, during the decelerating travel of the vehicle.
According to the proposed driving force control system, however, when the decelerating travel is being carried out on a low-friction road surface, such as that of a snowy road, when a lateral force acts on the rear wheels by the user's operation of the steering wheel, the rear wheels can loose their grip on the road surface to skid sideways, and in worst cases, the vehicle undergoes a spin. This is because the larger the braking force (braking torque) of the electric motor applied to the rear wheels, the smaller the lateral grip of the rear wheels on the road surface, and the deceleration causes the center of the gravity of the vehicle to be shifted forward to cause the axle load distribution to be unevenly shifted forward, whereby the lateral grip of the rear wheels is further reduced to make the above problem more conspicuous.
Further, the present assignee has already proposed another driving force control system of the above-mentioned type e.g. by Japanese Patent Application No. 11-366934. A front-and-rear wheel drive vehicles incorporating this control system has its front wheels driven by an engine via a torque converter, and its rear wheels driven by an electric motor. In this driving force control system, the driving conditions for driving the rear wheels by the electric motor, i.e. the conditions for executing the four-wheel drive include one determined based on a speed ratio of the torque converter. More specifically, the control system is configured such that if the detected speed ratio of the torque converter is equal to or larger than a predetermined value, the electric motor is stopped to execute the two-wheel drive, whereas if the former is lower than the latter, the electric motor is operated to execute the four-wheel drive. For instance, when the vehicle is started, the speed ratio tends to be small and hence the torque amplification factor of the torque converter is high, which makes the vehicle more prone to slippage. Therefore, the control system causes the electric motor to be operated to drive the rear wheels to assist the driving of the vehicle by the rear wheels, thereby improving the startability of the vehicle. Further, when the vehicle is being accelerated by the driver stepping on the accelerator pedal, the sliding of the torque converter increases, so that the speed ratio of the same temporarily drops. In such a case as well, the assistance of driving of the vehicle by the rear wheels is carried out to thereby ensure excellent acceleration of the vehicle. It should be noted that in the four-wheel drive state, first, the total driving force required for driving the vehicle is determined, and the driving force of the electric motor is determined such that the maximum output therefrom is an upper limit value thereof, and the driving force of the engine is determined as a difference obtained by subtracting the driving force of the electric motor from the total driving force.
In this driving force control system, however, there is room for improvement, because a torque step can be produced during acceleration for the following reason: In this driving force control system, as described above, when the vehicle is accelerated, a decrease in the speed ratio of the torque converter is expected which is caused by sliding of the torque converter, and the assistance of the electric motor is started on condition that the speed ratio becomes lower than the predetermined value. However, the sliding of the torque converter does not occur instantly, but progressively increases, so that there is a time lag before the speed ratio actually decreases below the predetermined value, i.e. before the assistance of the electric motor is started. On the other hand, the total driving force continues to be increased in response to the demand for acceleration, and since the driving force of the electric motor remains set to 0, the driving force of the engine continues to be increased at the same ratio. As a result, when the speed ratio becomes lower than the predetermined value, a large driving force of the electric motor is suddenly generated, and the driving force of the engine suddenly drops by an extent equal to the large driving force of the electric motor. In this case, it is difficult to synchronize the generation of the driving force of the electric motor and the drop of the driving force of the engine, so that occurrence of a certain amount of torque step is inevitable. Particularly, when the vehicle is accelerated from a low-speed traveling condition, or from a decelerating condition, the total driving force is largely increased even when the accelerator pedal is stepped on by a small amount, and therefore, the above-mentioned problem tend to be conspicuous.
SUMMARY OF THE INVENTION
It is a first object of the invention to provide a driving force control system for a front-and-rear wheel drive vehicle, which is capable of properly controlling the driving force of an engine without inhibiting the assistance of an electric motor when drive wheels driven by the engine undergo a slip, such that an optimum slip condition of the drive wheels can be ensured even on a low-friction road surface, thereby making it possible to preserve traveling stability of the vehicle.
It is a second object of the invention to provide a driving force control system for a front-and-rear wheel drive vehicle, which enables rear wheels of the vehicle to have an appropriate lateral grip even when the driver operates the
Fukuda Toshihiko
Honda Kenji
Kitano Kazuhiko
Nakasako Tooru
Uchiyama Naoki
Honda Giken Kogyo Kabushiki Kaisha
Leykin Rita
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