1. Field of the Invention
The present invention relates to a vehicle control apparatus equipped with an automatic transmission and, particularly, to a control apparatus for immediately responding to a driver's intention to effect acceleration in a vehicle having a mechanism for keeping a following distance between an own vehicle and a preceding vehicle coincident with a target distance.
2. Description of the Background Art
An automatic transmission mounted to a vehicle is connected to an engine through a torque converter and the like, includes a speed change mechanism having a plurality of power transmission paths, and is formed to automatically switch between the power transmission paths, i.e., switch between speed change ratios (traveling speed gears) on the basis of an acceleration pedal opening degree (throttle opening degree) and a vehicle speed. In general, a vehicle having an automatic transmission is provided with a shift lever to be operated by a driver, speed change positions (e.g., a reverse traveling position, a neutral position, and a forward traveling position) are set on the basis of operation of the shift lever. Within the speed change positions set in this way (normally, within the forward traveling position), an automatic speed change control is carried out.
Normally, in the forward traveling position selected during traveling of the vehicle, a speed change control is executed on the basis of speed change lines (speed change maps) determined by the vehicle speed and the throttle opening degree. Such speed change lines are set separately in a case of upshift and in a case of downshift. If the same speed change line is used in the case of the upshift and the case of the downshift, when the vehicle speed increases to effect the upshift, for example, the speed is reduced and the speed change line is crossed to execute the downshift. When this downshift is executed, the vehicle speed increases again to effect the upshift to cause a hunting phenomenon in which the upshift and the downshift are repeated in the vicinity of the speed change line. In order to avoid this hunting phenomenon, while an upshift speed change line is made agree with a fuel economy optimum line or an emission cleanup optimum line, a downshift speed change line is set so that hysteresis is secured on a lower-speed side than the upshift speed change line, for example.
There is also a system called an automatic follow-up system or the like with which a following distance between the own vehicle and the preceding vehicle is kept coincident with a target following distance and the vehicle travels at a set cruising speed. With such a system, even if a speed of the preceding vehicle changes, a driving force and a braking force of the vehicle are controlled so as to maintain the set target following distance.
Japanese Patent Laying-Open No. 10-306872 discloses an automatic shift down device related to such a technique. This automatic shift down device effects downshift of an automatic transmission when an actual following distance between an own vehicle and a preceding vehicle traveling ahead of the own vehicle is shorter than a target following distance based on a vehicle speed and includes a following distance detector for detecting the following distance between the own vehicle and the preceding vehicle, a vehicle speed detector for detecting a speed of the own vehicle, an accelerator operation detector for detecting operation of an accelerator by a driver, and a controller for effecting the downshift of the automatic transmission when a detection value of the following distance detector is smaller than the target following distance based on the vehicle speed and the accelerator operation detector detects operation of the accelerator in a returning direction.
With this automatic shift down device for the vehicle, the controller effects the downshift of the automatic transmission to effect deceleration when the detection value of the following distance detector is smaller than the target following distance based on the vehicle speed and the accelerator operation detector detects operation of the accelerator in a returning direction, i.e., in synchronization with sensing of the driver's intension to effect deceleration. In other words, this controller determines whether the driver effects deceleration to obtain a safe following distance or wants to effect acceleration for overrunning and overtaking on the basis of whether or not the accelerator has been operated in the returning direction when the detection value of the following distance detector is smaller than the target following distance based on the vehicle speed. In a case of deceleration, i.e., when the accelerator operation detector has detected operation of the accelerator in the returning direction, the downshift of the automatic transmission is effected. As a result, deceleration by the automatic transmission is effected until a foot brake is depressed and deceleration by the foot brake and the automatic transmission is effected after the depression to thereby perform safe and reliable braking without delay in response. On the other hand, when the accelerator operation detector has not detected operation of the accelerator in the returning direction, it is determined that the driver is not effecting deceleration but effecting acceleration for overrunning or overtaking. As a result, acceleration becomes possible and overtaking or the like can be carried out without a hitch.
However, in the automatic shift down device disclosed in Japanese Patent Laying-Open No. 10-306872, if the driver depresses the acceleration pedal to effect acceleration so as to overtake the preceding vehicle during an automatic follow-up control, the downshift is not effected. Therefore, it is impossible to immediately accelerate the vehicle and it takes much time to overtake. Moreover, when the downshift is effected while passing the down shift speed change line by depression of the acceleration pedal, a delay time is caused until the downshift of the automatic transmission is effected after the driver demands acceleration and the driver's intention to effect acceleration cannot sufficiently be realized.