Shift control method for an automatic transmission of a vehicle

Abstract: A shift controlling method for an automatic transmission of a vehicle includes a feedback controlling method by a transmission control unit (8) so as to return fully a kick down piston of a front brake by the automatic transmission in a driving state of the vehicle before the vehicle stops, whereby the shift operation can be smoothly accomplished and can effectively prevent a shift shock.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a shift down control method for an automatic transmission of a vehicle and more particularly, to an improved shift control method for a vehicle automatic transmission, which can prevent a shift shock of the automatic transmission. Description of Related Art Generally, a transmission increases or decreases rotating power and rotating speed of an engine according to a driving condition of a vehicle, and functions in an off-position between the engine and driving wheels for a certain period of time so as to warm-up the engine. The transmission industry classifies a manual transmission by manual operation, and an automatic transmission by automatic operation. The automatic transmission related to the present invention is convenient to operate, enables the vehicle to start smoothly, to increase and decrease the car speed, and has few chances to stop the engine by a beginning driver. Therefore, this automatic transmission is a popular choice among drivers. The automatic transmission comprises a "D" range which shifts in multiple stages according to a pushing degree of an accelerator pedal in driving of a vehicle, a "2" range which matches with a long downward slope of a driving road, an an "L" range which matches with an extreme downward slope and a long extreme curve. The automatic transmission further comprises an "N" range which ignites the engine free of all elements of the engine, a "P" range which fixes an outputting shaft while the vehicle is parking, and "R" range which operates a reverse motion of the vehicle. The shift motion in the "D" range starts the vehicle in a 1-speed or 2-speed state without any relation of an opening degree of a throttle valve, and causes an up-shift to the 2-speed or 3-speed according to the car speed. Also, if the throttle valve is gradualy opened, a driving gear of the 1-speed, 2-speed, and 3-speed has an increasingly higher shift point thereof. If the car speed is down, the downshift automatically occurs in the 3-speed, 2-speed, and 1-speed. In this 3-speed or 2-speed state of the "D" range, if the accelerator pedal is pushed completely in order to suddenly accelerate the car speed, the automatic transmission conducts a kick down state by down-shifting in the 3-speed to 2-speed or 2-speed to 1-speed over the shifting point thereof. The automatic transmission is provided with a hydraulic control system for shifting by automatically operating brake bands of star gear unit and clutches according to the car speed, the engine loading degree, and the shifting lever. Each valve of the hydraulic control system is controlled in an open loop system by a transmission control unit TCU and a fluid load of a line pressure can be controlled from the manual valves. Therefore, the gear shifting device located at a rear portion of a torque converter conducts a shift of the upshift or the downshift. In the "D" range of the driving function, the 1-speed to 4-speed, or recently, the 1-speed to 5-speed is accomplished in multiple stages. In detail, a rear clutch and a one-way clutch operate in the 1-speed shift, the rear clutch and a kick down brake band operate in the 2-speed shift operation, a front clutch and the rear clutch operate in the 3-speed shift operation, and an end clutch and the kick down brake band operate in the 4-speed shift operation. Each element of the star gear device in order to accomplish the shift operation conducts one of the shift control solenoid valves operated by a control signal which is permitted from the TCU according to the driving state of the engine and the car speed, operates one of shift control valves of valve bodies, and a pressure control solenoid valve controls the pressure which is permitted from each element of the vehicle. However, such conventional shift control systems suffer from a number of problems, such as for example, a shift shock occurs since such conventional shift control systems do not utilize a feedback control method by the TCU due to an incomplete return operation of a kick down piston in the 3 to 2-speed downshift operation while a downshift proceeds to shift from 4-speed to 1-speed when the engine and the vehicle drive in a lower speed. US 5, 431, 604 relates to control system for an automatic transmission and describes a transmission control method for downshift operation wherein in a first step the existence or nonexistence of a downshift signal of a 4- to 3-speed shift down is determined and if a 4- to 3- speed shift order exist 4- to 3- speed shift down is immediately initiated. After checking wether or not the 4- to 3-shift down operation has been completed the existence or nonexistence of a 3- to 2-speed shift down signal is judged, when the 4- to 3-shift down operation has not been accomplished. If the 3- to 2-speed shift down signal exists and the 4- to 3-speed shift operates the 3- to 2-speed is not performed until the completion of the 4- to 3-speed shift down has been determined. US 5, 131, 298 discloses a method for controlling shift of an automatic transmission having a sub-transmission and a main-transmission. When for a speed change both the sub-transmission and the main-transmission have to be downshifted or upshifted the downshift or upshift operation of one of the transmissions is delayed for a certain time and a pilot pressure controlling the shift valve of the hydraulic shift control is governed so as to ensure that both downshift or upshift operations of each of the sub-transmission and the main-transmission are completed within a certain time interval. SUMMARY OF THE INVENTION It is an object of the invention to provide a shift controlling method for an automatic transmission of the vehicle which eliminates shift shocks effectively when a downshift from the 4-speed to 1-speed have to be performed before the vehicle stops. This object is achieved by the shift down control method according the claim. Thus, the present invention provides a shift controlling method for an automatic transmission of the vehicle, which comprises feed back control conducted by transmission control unit so as to return fully a kick-down piston of kick-down break and to supply the necessary hydraulic pressure to a rear clutch by the automatic transmission in a driving stage of the vehicle before the vehicle stops, whereby the shift operation as smoothly accomplished and can effectively prevent a shift shock. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein: Fig. 1 is a schematic view of a shift control system for an automatic transmission of a vehicle of the present invention; and Fig. 2 is a program of the shift control method of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Refering now in detail to the drawings for the purpose of illustrating preferred embodiments of the present invention, the shift controlling method for an automatic transmission of a vehicle as shown in Figs. 1 and 2, comprises a hydraulic system including fluid pump 6 operated by a torque converter 4, and a plurality of valve bodies which have a plurality of valves for controlling a flowing channel of a line pressure generated by the fluid pump 6. The hydraulic system for accomplishing the shift controlling method of the present invention enables shifting by automatically operating a star gear device according to a car speed, a load of an engine, and a position of a shift lever. Each valve of the plurality of valve bodies is controlled by a transmission control unit (TCU) 8 and a shift control system of the star gear device conducts a shifting operation of an upshift or a downshift according to a state of the engine and the car speed. Thus, the shift system of the star gear device conducts a selectively rotating operation according to a pressure state of the line pressure by each valve of the plurality of valves bodies. Specifically, in the case of a "D" range which performs the driving function, multistage shifts of 1-speed to 4-speed is accomplished. That is, in a 1-speed shift operation state a rear clutch 14 from a plurality of clutches forming the star gear and a one-way clutch (not shown) operate, in a 2-speed shift operation state, the rear clutch 14 and a kick down brake band 12 operate, in a 3-speed shift operation state, a front clutch 10 and the rear clutch 14 operate, and in a 4-speed shift operation state, an end clutch 15 and the kick down brake band 12 operate. In the "D" range, each element of the star gear device for operating the 1-speed to 4-speed enables one ofthe shift control solenoids 16 operated by a control signal which is permitted from the TCU 8 depending on the driving state of the engine and the car speed to operate a shift control valve 18, and a pressure control solenoid valve 17 controls the pressure permitted from each element and operates properly so as to match with each shift state. As shown in Fig. 2, when the downshift operation of the 4-speed to 1-speed occurs in the driving stage before the vehicle stops, the TCU 8 provides the 4-3 speed downshift signal conformation stage 20 for judging existence or nonexistence of the 4-3 speed downshift signal before the vehicle stops, and 3-2 speed downshift signal conformation stage 22 for judging existence or nonexistence of the 3-2 downshift signal. If the signal is not existing in the 4-3 speed downshift signal confirmation stage 20 and 3-2 speed downshift signal conformation stage 22, it initially returns to the relative confirmation stages 20 and 22. If the signal is existing in 4-3 speed and 3-2 speed downshift signal confirmation stages 20 and 22, the 4-3 speed downshift operation is conducted by a hydraulic control system. Also, the TCU 8 further provides a 3-2 speed downshift operation holding stage 24 for controlling the 3-2 speed downshift operation for a setting time "T" so as to fully accomplish a returning operation of a kick down piston of the kick down brake after the 4-3 speed downshift operates, and a downshift operation stage 26 of the 3-2 speed for actuating after passing the "T" time. Accordingly, the shift control system for an automatic transmission of a vehicle according to the present invention provides a hydraulic pressure to the 3-2 speed downshift, and enables the TCU 8 to actuate a feedback control, so that the shift shock can be effectively prevented by being performed smoothly and softly. The invention being thus decribed, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: Shift down control method for an automatic transmission of a vehicle, performed before the vehicle stops, wherein a rear clutch (14) and a kick down brake (12) operate in the 2nd speed operation, a front clutch (10) and the rear clutch (14) operate in the 3rd speed operation and an end clutch (15) and the kick down brake (12) operate in the 4th speed operation, comprising the steps of: judging existence or nonexistence of the 4-3 downshift signal in a first confirmation stage (20) of a 4-3 downshift when a downshift operates from the 4th to 1st speed in a driving state before the vehicle stops; judging existence or nonexistence of the 3-2 downshift signal in a second confirmation stage (22) of a 3-2 downshift; holding a downshift operation of the 3rd to 2nd speed before controlling the downshift operation of the 3rd to 2nd speed for a setting time (T) during which a returning operation of a kick down piston of the kick down brake (12) is fully completed after the 4-3 downshift operation; and operating a 3-2 downshift after said setting time (T), whereby the downshift operation can be smoothly accomplished and effectively prevent a shift shock since a necessary hydraulic pressure for the 3-2 downshift is fully supplied to the rear clutch (14) and a transmission control unit (8) can conduct a feedback control.

Inventors: LEE HEE-YOUNG, LEE, HEE-YOUNG
Applicants: HYUNDAI MOTOR CO LTD, HYUNDAI MOTOR CO., LTD.
Application year: 1995
Main classification: F16H61
IPC codes: F16H59, F16H61