This application claims priority from Japanese Patent Application No. 2020-080809 filed on Apr. 30, 2020, the disclosure of which is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a four-wheel drive vehicle in which a ratio of distribution of a drive power between main and auxiliary drive wheels is adjustable.

BACKGROUND OF THE INVENTION

There is well-known a four-wheel drive vehicle including (a) main drive wheels and auxiliary drive wheels, (b) a drive power source including at least an engine, (c) a drive-power distribution device configured to transmit a drive power of the drive power source toward the main drive wheels and the auxiliary drive wheels, at a drive-power distribution ratio that is a ratio of distribution of the drive power between the main drive wheels and the auxiliary drive wheels, and (d) a control apparatus configured to execute a drive-power distribution control for adjusting the drive-power distribution ratio, and configured to execute an engine automatic-start control for causing the engine to be automatically started upon satisfaction of an engine-start condition. A four-wheel drive vehicle is disclosed in WO/2011/042951 is an example of such a vehicle.

SUMMARY OF THE INVENTION

By the way, a drive system, to which the engine is connected in a power transmittable manner, has a resonance rotational speed that is dependent on its mass and torsional rigidity. Therefore, when an engine automatic-start control is executed, resonance of the drive system could occur depending on a frequency of torque fluctuation of the engine, and vibration of the drive system is increased upon occurrence of the resonance. Thus, there is a risk of increase of NV as a result of the increased vibration of the drive system, wherein the NV is a generic term including noise generated in the vehicle and vibration sensible by passengers in the vehicle. In the four-wheel drive vehicle as described above, the mass and the torsional rigidity of the drive system are changed depending on the drive-power distribution ratio, so that the resonance rotational speed of the drive system is changed depending on the drive-power distribution ratio. Therefore, in the four-wheel drive vehicle as described above, there is a problem that, when the engine automatic-start control is executed, the resonance of the drive system could easily occur due to the torque fluctuation of the engine whereby the NV could be easily increased.

The present invention was made in view of the background art described above. It is therefore an object of the present invention to provide a four-wheel drive vehicle capable of suppressing or preventing increase of the NV due to increase of vibration of a drive system upon execution of an engine automatic-start control.

The object indicated above is achieved according to the following aspects of the present invention.

According to a first aspect of the invention, there is provided a four-wheel drive vehicle comprising: (a) main drive wheels and auxiliary drive wheels; (b) at least one drive power source including an engine; (c) a drive-power distribution device configured to transmit a drive power of the drive power source toward the main drive wheels and the auxiliary drive wheels, at a drive-power distribution ratio that is a ratio of distribution of the drive power between the auxiliary drive wheels and the main drive wheels; and (d) a control apparatus configured to execute a drive-power distribution control for adjusting the drive-power distribution ratio, and configured to execute an engine automatic-start control for causing the engine to be automatically started upon satisfaction of an engine-start condition, wherein the control apparatus is configured, upon execution of the engine automatic-start control, to change a target engine rotational speed that is a target speed value of a rotational speed of the engine after a complete combustion of the engine, from a predetermined engine-start rotational speed, to a changed engine rotational speed, depending on the drive-power distribution ratio, such that a difference of the changed engine rotational speed from a resonance rotational speed that causes resonance of a drive system to which the engine is connected in a power transmittable manner, is larger than a difference of the predetermined engine-start rotational speed from the resonance rotational speed.

According to a second aspect of the invention, in the four-wheel drive vehicle according to the first aspect of the invention, the control apparatus is configured to set the target engine rotational speed to the changed engine rotational speed that is different, by a predetermined speed value, from the resonance rotational speed that is calculated based on the drive-power distribution ratio, wherein the predetermined speed value is a predetermined value for setting the changed engine rotational speed as the target engine rotational speed, which makes it possible to avoid or suppress occurrence of resonance of the drive system and to suppress an amount of change of the target engine rotational speed from the predetermined engine-start rotational speed to the changed engine rotational speed.

According to a third aspect of the invention, in the four-wheel drive vehicle according to the first or second aspect of the invention, the predetermined engine-start rotational speed is an optimum speed value of the rotational speed of the engine, which maximizes an energy efficiency in the four-wheel drive vehicle.

According to a fourth aspect of the invention, in the four-wheel drive vehicle according to any one of the first through third aspects of the invention, the control apparatus includes, in addition to a function of changing the target engine rotational speed, a function of changing the drive-power distribution ratio upon the execution of the engine automatic-start control, from an unchanged distribution ratio that is a ratio value of the drive-power distribution ratio when the engine automatic-start control is not executed, to a changed distribution ratio, such that the difference of the predetermined engine-start rotational speed from the resonance rotational speed is increased by change of the drive-power distribution ratio from the unchanged distribution ratio to the changed distribution ratio, wherein the control apparatus is configured, upon the execution of the engine automatic-start control, to execute one of change of the target engine rotational speed and change of the drive-power distribution ratio, which is selected depending on a state of the four-wheel drive vehicle.

According to a fifth aspect of the invention, in the four-wheel drive vehicle according to the fourth aspect of the invention, the control apparatus is configured, when an accelerating operation amount or a requested drive amount is not smaller than a threshold value, to change the target engine rotational speed such that the difference of the changed engine rotational speed from the resonance rotational speed is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed, and is configured, when the accelerating operation amount or the requested drive amount is smaller than the threshold value, to change the drive-power distribution ratio such that the difference of the predetermined engine-start rotational speed from the resonance rotational speed is increased by the changed ratio value of the drive-power distribution ratio.

According to a sixth aspect of the invention, in the four-wheel drive vehicle according to the fourth or fifth aspect of the invention, the control apparatus is configured, when a yaw rate is not smaller than a threshold value, to change the target engine rotational speed such that the difference of the changed engine rotational speed from the resonance rotational speed is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed, and is configured, when the yaw rate is smaller than the threshold value, to change the drive-power distribution ratio such that the difference of the predetermined engine-start rotational speed from the resonance rotational speed is increased by the changed ratio value of the drive-power distribution ratio.

According to a seventh aspect of the invention, in the four-wheel drive vehicle according to any one of the fourth through sixth aspects of the invention, the control apparatus is configured, when a steering angle is not smaller than a threshold value, to change the target engine rotational speed such that the difference of the changed engine rotational speed from the resonance rotational speed is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed, and is configured, when the steering angle is smaller than the threshold value, to change the drive-power distribution ratio such that the difference of the predetermined engine-start rotational speed from the resonance rotational speed is increased by the changed ratio value of the drive-power distribution ratio.

According to an eighth aspect of the invention, in the four-wheel drive vehicle according to any one of the fourth through seventh aspects of the invention, the control apparatus is configured, when the four-wheel drive vehicle is turning, to change the target engine rotational speed such that the difference of the changed engine rotational speed from the resonance rotational speed is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed, and is configured, when the four-wheel drive vehicle is running straight, to change the drive-power distribution ratio such that the difference of the predetermined engine-start rotational speed from the resonance rotational speed is increased by the changed ratio value of the drive-power distribution ratio.

In the four-wheel drive vehicle according to the first aspect of the invention, the target engine rotational speed in the engine automatic-start control is changed, based on the drive-power distribution ratio, from the predetermined engine-start rotational speed to the changed engine rotational speed, such that the difference of the changed engine rotational speed from the resonance rotational speed that causes the resonance of the drive system is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed, whereby occurrence of the resonance of the drive system due to torque fluctuation of the engine is suppressed or avoided upon execution of the engine automatic-start control. Therefore, when the engine automatic-start control is executed, it is possible to suppress or prevent increase of the NV due to increase of vibration of the drive system.

In the four-wheel drive vehicle according to the second aspect of the invention, the target engine rotational speed is set to the changed engine rotational speed that is different, by the predetermined speed value, from the resonance rotational speed that is calculated based on the drive-power distribution ratio, so that the occurrence of the resonance of the drive system due to the torque fluctuation of the engine is appropriately suppressed or avoided. Further, the predetermined speed value is a predetermined value for setting the changed engine rotational speed as the target engine rotational speed, which makes it possible to avoid or suppress the occurrence of resonance of the drive system and to suppress an amount of change of the target engine rotational speed from the predetermined engine-start rotational speed to the changed engine rotational speed, so that, upon execution of the engine automatic-start control, the occurrence of the resonance of the drive system is appropriately suppressed or avoided with the amount of change of the target engine rotational speed from the predetermined engine-start rotational speed being suppressed.

In the four-wheel drive vehicle according to the third aspect of the invention, the predetermined engine-start rotational speed is an optimum speed value of the engine rotational speed, which maximizes an energy efficiency in the four-wheel drive vehicle. With the target engine rotational speed being changed from the an optimum speed value of the engine rotational speed to the changed engine rotational speed, the difference of the target engine rotational speed from the resonance rotational speed of the drive system is increased, so that increase of the NV can be suppressed or prevented while reduction of the energy efficiency can be suppressed. Further, where the target engine rotational speed is set to the changed engine rotational speed that is different, by the predetermined speed value, from the resonance rotational speed, the amount of change of the target engine rotational speed from the optimum speed value of the engine rotational speed is suppressed whereby the reduction of the energy efficiency is appropriately suppressed.

In the four-wheel drive vehicle according to the fourth aspect of the invention, when the engine automatic-start control is executed, a selected one of the change of the target engine rotational speed and the change of the drive-power distribution ratio is executed whereby the difference of the target engine rotational speed from the resonance rotational speed of the drive system is increased. Thus, the increase of the NV is suppressed or prevented with affection to a vehicle motion controllability being suppressed and with the amount of change of the target engine rotational speed from the predetermined engine-start rotational speed being suppressed. Where the predetermined engine-start rotational speed is the optimum speed value of the engine rotational speed, the reduction of the energy efficiency is suppressed.

In the four-wheel drive vehicle according to the fifth aspect of the invention, when the accelerating operation amount or the requested drive amount is not smaller than the threshold value, the target engine rotational speed is changed such that the difference of the changed engine rotational speed from the resonance rotational speed is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed. Thus, in a situation with sudden start operation or sudden acceleration operation, a higher priority is given to the vehicle motion controllability by the drive-power distribution control rather than to improvement of the energy efficiency. On the other hand, when the accelerating operation amount or the requested drive amount is smaller than the threshold value, the drive-power distribution ratio is changed whereby the difference of the target engine rotational speed from the resonance rotational speed of the drive system is increased. Thus, in a situation with slow start operation or slow acceleration operation, a higher priority is given to the improvement of the energy efficiency rather than to the vehicle motion controllability by the drive-power distribution control. Therefore, the increase of the NV is suppressed or prevented with affection to the vehicle motion controllability being suppressed and with the amount of change of the target engine rotational speed from the predetermined engine-start rotational speed being suppressed. Where the predetermined engine-start rotational speed is the optimum speed value of the engine rotational speed, the reduction of the energy efficiency is suppressed.

In the four-wheel drive vehicle according to the sixth aspect of the invention, when the yaw rate is not smaller than the threshold value, the target engine rotational speed is changed such that the difference of the changed engine rotational speed from the resonance rotational speed is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed. Thus, in a situation with large change of an attitude of the vehicle, a higher priority is given to the vehicle motion controllability by the drive-power distribution control rather than to improvement of the energy efficiency. On the other hand, when the yaw rate is smaller than the threshold value, the drive-power distribution ratio is changed whereby the difference of the target engine rotational speed from the resonance rotational speed of the drive system is increased. Thus, in a situation with small change of the attitude of the vehicle, a higher priority is given to the improvement of the energy efficiency rather than to the vehicle motion controllability by the drive-power distribution control. Therefore, the increase of the NV is suppressed or prevented with affection to the vehicle motion controllability being suppressed and with the amount of change of the target engine rotational speed from the predetermined engine-start rotational speed being suppressed. Where the predetermined engine-start rotational speed is the optimum speed value of the engine rotational speed, the reduction of the energy efficiency is suppressed.

In the four-wheel drive vehicle according to the seventh aspect of the invention, when the steering angle is not smaller than the threshold value, the target engine rotational speed is changed such that the difference of the changed engine rotational speed from the resonance rotational speed is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed. Thus, in a situation with large change of the attitude of the vehicle, a higher priority is given to the vehicle motion controllability by the drive-power distribution control rather than to improvement of the energy efficiency. On the other hand, when the steering angle is smaller than the threshold value, the drive-power distribution ratio is changed whereby the difference of the target engine rotational speed from the resonance rotational speed of the drive system is increased. Thus, in a situation with small change of the attitude of the vehicle, a higher priority is given to the improvement of the energy efficiency rather than to the vehicle motion controllability by the drive-power distribution control. Therefore, the increase of the NV is suppressed or prevented with affection to the vehicle motion controllability being suppressed and with the amount of change of the target engine rotational speed from the predetermined engine-start rotational speed being suppressed. Where the predetermined engine-start rotational speed is the optimum speed value of the engine rotational speed, the reduction of the energy efficiency is suppressed.

In the four-wheel drive vehicle according to the eighth aspect of the invention, when the four-wheel drive vehicle is turning, the target engine rotational speed is changed such that the difference of the changed engine rotational speed from the resonance rotational speed is larger than the difference of the predetermined engine-start rotational speed from the resonance rotational speed. Thus, in a situation with large change of the attitude of the vehicle, a higher priority is given to the vehicle motion controllability by the drive-power distribution control rather than to improvement of the energy efficiency. On the other hand, when the when the four-wheel drive vehicle is running straight, the drive-power distribution ratio is changed whereby the difference of the target engine rotational speed from the resonance rotational speed of the drive system is increased. Thus, in a situation with small change of the attitude of the vehicle, a higher priority is given to the improvement of the energy efficiency rather than to the vehicle motion controllability by the drive-power distribution control. Therefore, the increase of the NV is suppressed or prevented with affection to the vehicle motion controllability being suppressed and with the amount of change of the target engine rotational speed from the predetermined engine-start rotational speed being suppressed. Where the predetermined engine-start rotational speed is the optimum speed value of the engine rotational speed, the reduction of the energy efficiency is suppressed.

