Automatic Transmission for Automobile and Method of Controlling Same

1. Field of the Invention

This invention relates to an automatic transmission for automobiles utilizing a continuously variable transmission (hereinafter called a CVT) and a method of controlling the same.

2. Description of the Prior Art

The speed ratio in conventional automatic transmissions for automobiles is changed stepwise from the first to fourth steps, for example, and the change of step is determined by the rotational speed of the output shaft of the transmission, i.e. by the vehicle speed and the throttle (valve) position of an intake system to provide a predetermined hysterisis between shift-up and shift-down. Also, when the first speed is changed to the second one at a predetermined rotational speed of an output shaft within the low range, the first speed is thereafter maintained irrespective of the rotational speed of the output shaft. By such control of the change of step, sufficient driveability is assured over a wide running range of the engine. However the fact that the speed ratio in the respective speed change step region is fixed, the provision of the hysteresis, etc. are all disadvantageous to fuel consumption, and the low efficiency of power transmission of a hydraulic torque converter used together with a stepped automatic transmission results in a reduction in fuel consumption. Further, the occurrence of impulses in the speed change also provides a drawback.

While in general methods of controlling a CVT, the speed ratio e of the CVT is controlled so that (1) the actual engine speed Ne provides the desired engine speed Ne', (2) the amount of change .DELTA.Ne per a predetermined time of the engine speed Ne in the speed change, i.e. the change of the speed ratio e is controlled to be a desired value or (3) the speed ratio e provides a desired speed ratio e', the rate of speed change, i.e. the differential value de/dt of the speed change with respect to time is not limited in any of these controlling methods. Since the transmission loss of the CVT in the speed change of the CVT is increased, the specific fuel consumption in the speed change is degraded by such controlling methods.

An object of the present invention is to provide an automatic transmission for automobiles which can provide the optimum variable speed ratio for the vehicle running conditions and avoid the occurrence of shock caused by speed change, as distinguished from conventional stepped automatic transmissions for automobiles.

A further object of the present invention is to provide a method and apparatus for controlling a CVT which can ensure driveability while improving the specific fuel consumption.

In the automatic transmission for automobiles according to the present invention, to achieve these objects a CVT for transmitting the rotation of an engine to the drive wheels is provided with means to calculate the desired speed ratio as an approximately continuous function of the intake system throttle position and vehicle system throttle position and the vehicle speed and to control the speed ratio of CVT on the basis of these calculated values.

Thus, the speed ratio is controlled precisely and continuously by the CVT to be suited to the running condition of vehicle so that the fuel consumption and running performance can be remarkably improved and the occurrence of shock in speed change can be avoided, in comparison with power transmitting units of stepped transmissions. Also, when the desired speed change is corrected by a change in engine temperature or the like, speed change patterns fixed in conventional stepped automatic transmissions irrespective of the engine temperature can be alternated by the control of the speed change corresponding to the engine temperature so that a large advantage is provided in the performance of running engine and the purification of exhaust gas.

Also, according to the present invention, the rate of speed change de/dt of the CVT is made a function of the amount of pedalling Ac of an accelerator pedal and the amount of change .DELTA.Ac in this pedalling amount Ac or as a function of the differential value dAc/dt of the pedalling amount Ac with respect to time and the vehicle speed V of the rotational speed Nout of output shaft of CVT.

Thus, the rate of speed change de/dt is set according to the running condition of an engine, i.e. to the minimum value for ensuring necessarily sufficient running performance. As a result, the increase of the rate of speed change de/dt causing the increase of transmission loss can be restrained to be as little as possible to ensure the running performance while improving the specific fuel consumption. In addition, if a driver expects superior driveability for a CVT car, the de/dt can be set in proportion to the difference between the desired speed ratio e' and the actual speed ratio e.

Intake pipe pressure P, throttle position .theta., or intake air flow Q in corresponding relation to the pedalling amount Ac can be obtained as data substituted for the pedalling amount Ac, and P, .theta. and Q and their change amounts .DELTA.P, .DELTA..theta., .DELTA.Q, or their differential values dP/dt, d.theta./dt, dQ/dt with respect to time can be used instead of the pedalling amount Ac, its change amount .DELTA.Ac or the differential value dAc/dt with respect to time.

Also, assuming that the change amount in the speed ratio e of the CVT per unit time is .DELTA.e, this .DELTA.e can be reflected as the rate of speed change in the speed change operating amount of the CVT. The operating amount for changing the speed ratio (e) of the continuously variable transmission up to a desired value may be a function of .DELTA.e.

Other objects and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows an embodiment according to the present invention;

FIG. 2 shows, in a block diagram format, an electronic control unit used with the CVT;

FIG. 3 is a block diagram illustrating certain features of the preferred embodiment of the present invention;

FIG. 4 is a flow chart showing an example of a control routine for the speed ratio of the CVT;

FIG. 5 is a chart illustrating the relationship between the output speed of the CVT and the throttle position;

FIG. 6 is a graph showing the relationship between a shift range and the shift range correction;

FIG. 7 is a graph showing the relationship between coolant temperature and the coolant temperature correction;

FIG. 8 is a chart illustrating the relationship between the output speed of the CVT and the differential of throttle position with respect to time;

FIG. 9 is another block diagram illustrating features of the preferred embodiment of an apparatus of the present invention; and

FIG. 10 is a prospective view of a vehicle of a type in which a transmission according to the invention may be used.

In FIG. 1, CVT 10 of a vehicle 1 (FIG. 10) is provided with an input shaft 12 and an output shaft 14 parallel to each other. The output shaft 14 delivers power to selected ones of the wheels 2 of the vehicle 1. The input shaft 12 is provided coaxially with a crankshaft 18 of an engine 16 and connected to the crankshaft 18 through a clutch 20. Input side pulleys 22a, 22b are provided opposed to each other, one input side pulley 22a being provided axially movably as a movable pulley and circumferentially fixedly on the input shaft 12, and the other input side pulley 22b being secured fixedly to the input shaft 12 as a fixed pulley. Similarly, output side pulleys 24a, 24b are provided opposedly to each other, one output side pulley 24a being secured fixedly to the output shaft 14 as a fixed pulley and the other one 24b being axially movable as a movable pulley and circumferentially fixed on the output shaft 14. Opposed surfaces of the input side pulleys 22a, 22b and the output side pulleys 24a, 24b are formed to be tapered and a belt 26 having an isosceles trapezoidal section is trained over the input side pulleys 22a, 22b and the output side pulleys 24a, 24b. An oil pump 28 sends oil from an oil reservoir 30 to a pressure regulating valve 32. The pressure regulating valve 32 varies the amount of oil escaping to a drain 34 to control line pressure in an oil path 36 which is sent to a hydraulic cylinder and a flow controlling valve 38 of the output side pulley 24b. The flow controlling valve 38 controls the oil flow supplied from the oil path 36 to an oil path 40 connected to the hydraulic cylinder of the input side pulley 22a and the oil flow drained from the oil path 40 to the drain 34. The pressing force of the input side pulleys 22a, 22b and the output side pulleys 24a, 24b to the belt 26 is controlled by oil pressure in the input and output side hydraulic cylinders so that the belt 26 engaging the radii on the tapered surfaces of the input side pulleys 22a, 22b and the output side pulleys 24a, 24b are varied in relation to this pressing force and thereby the speed ratio e of CVT 10(=Nout/Nin, provided Nout is the rotational speed of the output shaft, Nin is the rotational speed of the input shaft 12, and in this embodiment, Nin is equal to engine speed Ne) is varied. The line pressure in the output side hydraulic cylinder is controlled to the necessary minimum value for avoiding slipping of the belt 26 and ensuring the power transmission to restrain the drive loss of the oil pump 28, and the speed ratio e is controlled by the oil pressure in the input side hydraulic cylinder. Further while the oil pressure in the input side hydraulic cylinder is lower than or equal to that in the output side hydraulic cylinder, since the pressure receiving surface of the input side hydraulic cylinder is larger than that of the output side hydraulic cylinder, the pressing force of the input side pulleys 22a, 22b can be larger than that of the output side pulleys 24a, 24b. An input side rotational angle sensor 42 and an output side rotational angle sensor 44 detect respectively the rotational speeds Nin, Nout of the input shaft 12 and the output shaft 14, and a water temperature sensor 46 detects cooling water temperature in the engine 16. An accelerator pedal 50 is provided in a cab 48 and a throttle valve in an intake path is interlocked with the accelerator pedal 50 so that a throttle position sensor 52 detects throttle position .theta.. A shift position sensor 54 detects the shift range of a shift lever provided near the cab.

FIG. 2 is a block diagram of an electronic control unit. An address data bus 56 interconnects a CPU 58, RAM 60, ROM 62, I/F (interface) 64, A/D (Analog/Digital converter) 66 and D/A (Digital/Analog converter) 68. The I/F 64 receives pulse signals from the input side rotational angle sensor 42, the output side rotational angle sensor 44 and the shift position sensor 54, the A/D 66 receives analog signals from the water temperature sensor 46 and the throttle position sensor 52, and the D/A 68 sends output pulses to the pressure regulating valve 32 and the flow controlling valve 38.

FIG. 3 is a block diagram of an embodiment according to the present invention. In block 70 is calculated a basic speed ratio eb from the rotational speed Nout (=vehicle speed) of the output side disk 7a, and throttle position .theta.th. FIG. 5 shows data on a map storing the basic speed ratio eb. In FIG. 5, .fwdarw., .rarw. mean that they equal respectively right and left data. Further the speed ratio ##EQU1## The basic speed ratio eb is set as a desired speed ratio e' in the drive range after completion of warming-up. The basic speed ratio eb other than data points in FIG. 5 is calculated by interpolation. In block 72 is calculated the correction amount es of shift range by shift range Ps. FIG. 6 shows the relationship between the shift range and the correction amount es of thereof. In L(low), 2(second) and D(drive) ranges, the es values are respectively 1.0, 0.5 and 0.0. In block 74 is calculated a water temperature correction amount ew on the basis of cooling water temperature Tw. FIG. 7 shows the relationship between the cooling water temperature Tw and the water temperature correction amount ew. As the cooling water temperature Tw is lowered, the water temperature correction amount ew is increased. In block 76 is calculated the actual speed ratio e of CVT 10. At point 78 is calculated a deviation (.dbd.eb-es-ew-e). A feedback gain is designated by block 80 and an amplifier for the flow controlling valve 38 by 82. As a result, the speed ratio of CVT 10 is controlled to be a desired speed ratio e'=eb-es-ew. The speed ratios in L range, second speed range and warming-up are corrected and reduced by es and ew corrections to prevent the driveability from degrading.

FIG. 4 is a flow chart of the speed ratio controlling routine of the CVT. A change amount .DELTA.e per unit time to change the speed ratio e to the desired speed ratio e' is calculated, and when the actual speed ratio e is less than the desired speed ratio e' the oil flow transmitted via the flow controlling valve 38 to the input side hydraulic cylinder is increased by an amount corresponding to .DELTA.e. When the actual speed ratio e is larger than the desired speed ratio e', the oil flow drained from the input side hydraulic cylinder through the flow controlling valve 38 is increased by an amount corresponding to .DELTA.e. .DELTA.e is used for the rate of speed change, and the greater is .DELTA.e, the greater is the absolute value of the rate of speed change. Since .DELTA.e is set as a function of the differential value d.theta./dt of throttle position .theta. with respect to time and the rotational speed Nout of the output shaft, .DELTA.e specified to the minimum value for preventing acceleration problems in the running performance. As a result, an abrupt change in the speed ratio e causing the degradation of the specific fuel consumption can be restrained to improve the specific fuel consumption. Detailing the respective steps, in step 84 are read the rotational speed Nin of the input shaft 12, the rotational speed Nout of the output shaft 14, the throttle position .theta., the engine cooling water temperature Tw and the shift range Ps. In step 86 is calculated the basic speed ratio eb from the rotational speed Nout of the output shaft and the throttle position .theta..

The table in FIG. 5 shows the relationship between the rotational speed Nout of the output shaft, throttle position .theta. and the basic speed ratio eb. In FIG. 5, .fwdarw., .rarw. means that data values are respectively left or right data. The basic speed raio eb is set such that for the same throttle position, the more the rotational speed Nout of the output shaft is increased, the more the basic speed ratio eb is increased, and, for the same rotational speed of the output shaft, the more the throttle position .theta. is increased, the more the basic speed ratio eb is decreased in principle. Moreover, the basic speed ratio eb corresponding to any throttle position .theta. and the rotational speed Nout of the output shaft is obtained by utilizing interpolation. In step 88 is calculated the shift range correction amount es from the shift range Ps.

FIG. 6 shows the relationship between the shift range Ps and the shift range correction amount es. The shift range correction amount es is decreased in the order of L range, second range and D range. Thus, as is apparent from a formula in step 92 which will be described later, the desired speed ratio e' is increased in the order of L range, second speed range and D range. In step 90 is calculated the water temperature correction amount ew from the engine cooling water temperature Tw.

FIG. 7 shows the relationship between the cooling water temperature Tw and the water temperature correction amount ew, and the water temperature correction amount ew is increased as the cooling water temperature Tw is lowered. Thus, as is understood from the formula in step 92, the desired speed ratio e' is decreased as the engine cooling water temperature Tw is lowered. In step 92 is calculated the desired speed ratio e' from the formula e'=eb-es-ew. In steps 94-98 are set the maximum and minimum desired speed ratio e' to the upper limit emax and the lower limit emin respectively. In step 100 is calculated the differential value d.theta./dt or throttle position .theta. with respect to time from the throttle position .theta. detected this time and the throttle position .theta.p detected at the previous time. The sampling time of .theta.p should be taken at a suitable value. The shorter sampling time causes a differential value fluctuation and the longer time causes a time delay of vehicle acceleration. The suitable value may be about 0.2-2.0 sec. In order to secure the sampling time, in step 99 the throttle position memory registers are shifted one after the other. In step 101 is calculated the change amount .DELTA.e in the speed ratio e from the rotational speed Nout of the output shaft and d.theta./dt.

FIG. 8 shows the relationship between the rotational speed Nout of the output shaft, d.theta./dt and the change amount .DELTA.e. The change amount .DELTA.e in FIG. 8 is represented as the change in the speed ratio e per 1 second, and .rarw. means that the data equal the left values. The change amount .DELTA.e is set to increase as the rotational speed Nout of the output shaft and the differential value d.theta./dt of the throttle position .theta. with respect to time are increased. Further, in FIG. 8, d.theta./dt is used for an alternative data of differential value dAc/dt of pedalling amount Ac of the accelerator pedal with respect to time, and instead of d.theta./dt may be used Ac, dAc/dt or change amount .DELTA.Ac in Ac and further the intake pipe pressure P, throttle position .theta., intake air flow Q, their change amounts .DELTA.P, .DELTA..theta., .DELTA.Q or their differential value dP/dt, d.theta./dt with respect to time in corresponding relation to Ac. Also, instead of the rotational speed Nout of the output shaft, the corresponding vehicle speed V may be used. In step 102 is calculated control voltage Vin of the flow controlling valve 38 from the following formula;

provided K is a constant. As a result, if e'-e>0, Vin>0, and the flow of hydraulic medium to the input side hydraulic cylinder is increased to increase the belt 26 engaging radii on the input side pulleys 22a, 22b. Also if e'-e>0, the oil flow drained from the input side hydraulic cylinder through the flow controlling valve 38 is increased to reduce the belt 26 engaging radii on the input side pulleys 22a, 22b.

FIG. 9 is a block diagram of an apparatus according to the present invention. A first detecting means 110 detects the pedalling amount Ac of accelerator pedal 52, the change amount .DELTA.Ac in this Ac or the differential value .DELTA.Ac/dt of the .DELTA.Ac with respect to time. A second detecting means 112 detects the vehicle speed V or the rotational speed Nout of the output shaft of CVT 10. A means 114 for calculating the rate of speed change calculates the change amount .DELTA.e in the speed ratio e (since CVT 10 has the speed ratio e changed by .DELTA.e in a predetermined time cycle, .DELTA.e corresponds to the rate of speed change.) on the basis of the detected values of the detecting means 110, 112. A means 116 for controlling the speed ratio changes the speed ratio e by .DELTA.e in a predetermined time cycle when the actual speed ratio e differs from the desired speed ratio e'.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.