Abstract:
A gear shift control system for vehicles is provided which mathematically calculates a rate of deceleration of the vehicle equipped an automatic transmission upon start of deceleration of the vehicle and inhibits the automatic transmission from upshifting or changes a permissible gear shift range of the automatic transmission as a function of the rate of deceleration of the vehicle, thereby ensuring a desired degree of braking force including an engine braking force while the vehicle is decelerating.

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
       [0001]    The present application claims the benefit of priority of Japanese Patent Application No. 2012-196314 filed on Sep. 6, 2012, the disclosure of which is totally incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field of the Invention 
         [0003]    The present invention relates generally to a gear shift control system for an automatic transmission for vehicles, and more particularly to such a control system designed to control an operation of the automatic transmission when it is required to decelerate the vehicle. 
         [0004]    2. Background Art 
         [0005]    Usually, an internal combustion engine in automotive vehicles is rotated in a selected one of multiple gear ranges in order to drive road wheels of the vehicle at a desired torque. Specifically, the vehicle has an automatic transmission disposed between a drive shaft and the wheels to provide a given torque to the wheels either in a low-speed range or in a high-speed range 
         [0006]    Typical automatic transmissions work to automatically change gear ratios as a function of relation of the position of an accelerator operated by the driver of the vehicle and the speed of the vehicle to transmit torque, as outputted by the internal combustion engine, to the wheels. For instance, the automatic transmission is, as demonstrated in FIG.  14 , designed to change the gear ratios at the time when the relation (i.e., a locus) between the position of the accelerator and the speed of the vehicle meets one of upshift gear shift lines US or downshift gear shift lines DS prepared one for each of a set of gear ranges (see Japanese Patent First Publication listed below). The automatic transmission, as taught in this publication, is engineered to inhibit an upshift thereof (i.e., fix a selected one of the gear ranges) or establish a downshift by one gear to enhance the efficiency of engine braking when the vehicle is running on an slope.
   Japanese Patent First Publication No. 10-238621.   
 
         [0008]    The driver of the vehicle, as demonstrated by a speed reduction locus DT in  FIG. 14 , may release the accelerator pedal suddenly for some reason while the vehicle is accelerating, and then depress the brake pedal strongly to decelerate the vehicle rapidly. This causes the speed reduction locus DT to intersect with the upshift gear shift lines US several times. The automatic transmission, thus, upshifts several times in a gear shift control mode even though the speed of the vehicle is not very high, which results in an increased possibility that the speed of the internal combustion engine falls in an engine stall range. Additionally, the speed reduction locus DT also intersects with the downshift gear shift lines DS several times to establish the downshift of the automatic transmission. This results in sequential application of sudden engine braking to the vehicle, which will give occupants of the vehicle an uncomfortable ride and also lead to a bad maneuverability of the vehicle. 
         [0009]    When the vehicle is decelerating on a slope, the gear shift control system, as taught in the above publication, works to control the operation of the automatic transmission to provide the engine braking to ensure the continuity of speed reduction control. Specifically, the gear shift control system calculates an inclination of the road and, when the inclination exceeds a given threshold, controls an upshift of the automatic transmission. The gear shift control system also calculates a rate of deceleration of the vehicle and, when such a rate exceeds a given threshold, instructs the automatic transmission to downshift to produce engine braking. Usually, the rate of deceleration is derived by calculating a change in speed of the vehicle for a set period of time. The comparison of the rate of deceleration with the threshold is, therefore, made after a lapse of the set period of time following the start of deceleration of the vehicle. This may cause the automatic transmission to upshift or downshift undesirably depending upon the relation between the speed of the vehicle and the position of the accelerator pedal, which may result in a lack of braking force when the vehicle has started decelerating suddenly during acceleration. 
         [0010]    It is, therefore, an object to provide a gear shift control system for an automatic transmission which is mounted in a vehicle along with a power source such as an internal combustion engine or an electric motor and designed to inhibit the automatic transmission from shifting gear ratios undesirably frequently, avoid the stall of the power source without sacrificing a comfortable ride and maneuverability of the vehicle. 
         [0011]    According to one aspect of the invention, there is provided a gear shift control system for an automatic transmission mounted in a vehicle to transmit torque, as produced by a power source, to a wheel of the vehicle. The gear shift control system comprises: (a) a speed determiner which works to determine a speed of the vehicle; (b) a deceleration detector which detects deceleration of the vehicle when a braking force is being applied to the wheel; (c) a braking force determiner which determines the braking force applied to the wheel when the deceleration detector is detecting the deceleration of the vehicle; (d) a deceleration determiner which determines a deceleration of the vehicle as a function of the speed of the vehicle, as determined by the speed determiner, and the braking force, as determined by the braking force determiner; and (e) a gear shift controller which determines whether the deceleration of the vehicle, as determined by the deceleration determiner, is greater than a given deceleration threshold. When the deceleration of the vehicle is determined to be greater than the given deceleration threshold, the gear shift controller inhibits the automatic transmission from upshifting gears thereof. 
         [0012]    In the first preferred mode of the invention, the vehicle is equipped with a brake pedal which is responsive to depression thereof to produce the braking force. The gear shift control system further includes an amount-of-braking effort determiner which works to determine an amount by which the brake pedal is depressed. The deceleration detector detects the deceleration of the vehicle when the braking force is being applied to the wheel in response to the depression of the brake pedal. The braking force determiner calculates the braking force applied to the wheel when the deceleration detector is detecting the deceleration of the vehicle as a function of the speed of the vehicle, as determined by the speed determiner, and the amount by which the brake pedal is depressed, as determined by the amount-of-braking effort determiner. 
         [0013]    In the second preferred mode of the invention, the vehicle is equipped with an accelerator pedal which is responsive to depression thereof to accelerate the vehicle. The gear shift control system may further include an amount-of-acceleration determiner which works to determine an amount by which the accelerator pedal is depressed. The deceleration detector may detect the deceleration of the vehicle when the braking force is being applied to the wheel in response to release of the accelerator pedal. The braking force determiner may calculate the braking force applied to the wheel which arises from the release of the accelerator pedal. 
         [0014]    In the third preferred mode of the invention, the vehicle may be equipped with either or both an internal combustion engine and an electric generator which serve to produce the braking force applied to the wheel. The braking force determiner may determine one or the sum of an engine braking force, as produced by the internal combustion engine, and a regenerative braking force, as produced by the electric generator, as the braking force applied to the wheel. 
         [0015]    In the fourth preferred mode of the invention, the braking force determiner may determine a regenerative electric energy which is produced by the electric generator and being charged in a storage battery mounted in the vehicle and calculate the regenerative braking force based on the determined regenerative electric energy to determine the braking force. 
         [0016]    In the fifth preferred mode of the invention, the given deceleration threshold may be set to change to have one of a plurality of values as a function of the speed of the vehicle. The plurality of values are different from each other. When the deceleration of the vehicle is determined to be greater than a greatest one of the values, the gear shift controller may inhibit the automatic transmission from upshifting gears thereof. When the deceleration of the vehicle is determined to be greater than one of the values other than the greatest one, the gear shift controller permits the automatic transmission from upshifting in a range of a given number of gear ratios. The smaller the values, the greater the given number of gear ratios. 
       EFFECT OF THE INVENTION 
       [0017]    In the one aspect of the invention, at the time when finding the deceleration of the vehicle subjected to the braking force, the gear shift control system immediately starts to derive the speed of the vehicle and the braking force to calculate the deceleration (i.e. the rate of deceleration) of the vehicle. When the deceleration is greater than the deceleration threshold, the gear shift control system inhibits the automatic transmission from upshifting. In other words, even when the vehicle has started to decelerate suddenly during acceleration, the automatic transmission is immediately inhibited from upshifting before a lapse of a set period of time needed to monitor a change in speed of the vehicle to determine the deceleration of the vehicle. This avoids lots of unwanted gear shifts during sudden deceleration of the vehicle and ensures a comfortable ride and maneuverability of the vehicle. 
         [0018]    In the first preferred mode of the invention, the gear shift control system is responsive to the depression of the brake pedal to detect the event of deceleration of the vehicle. The gear shift control system, therefore, works to use the braking force resulting at least from the depression of the brake pedal to determine the deceleration of the vehicle, thus resulting in an enhanced quality of the gear shift inhibition control. 
         [0019]    In the second preferred mode of the invention, the gear shift control system is responsive to the release of the accelerator pedal to detect the event of deceleration of the vehicle. The gear shift control system, therefore, works to use the braking force resulting at least from the release of the accelerator pedal to determine the deceleration of the vehicle, thus resulting in an enhanced quality of the gear shift inhibition control. 
         [0020]    In the third preferred mode of the invention, the gear shift control system is capable of using the engine braking, as produced by the internal combustion engine, and/or the regenerative braking force, as produced by the electric motor, to calculate the braking force, thus resulting in an enhanced accuracy in calculating the braking force to improve the quality of the gear shift inhibition control. 
         [0021]    In the fourth preferred mode of the invention, the gear shift control system is capable of determining the regenerative braking force, as produced by the electric generator upon the release of the accelerator pedal, by directly measuring the amount of current created by the regenerative electric energy which is produced by the electric generator and being charged into the storage battery. This facilitates the ease of calculating the braking force acting on the wheel of the vehicle equipped with the electric generator, thus realizing the high-quality gear shift inhibition control at a decreased cost. 
         [0022]    In the fifth preferred mode of the invention, the gear shift control system works to change an permissible gear shift range of the automatic transmission as a function of the degree of deceleration of the vehicle. This is achieved by changing the value of the deceleration threshold to select the permissible gear shift range of gear ratios to which the automatic transmission is permitted to upshift based on comparison of the value of the deceleration threshold and the deceleration of the vehicle, thereby enhancing the quality of the gear shift inhibition control and efficiency in application of the braking force to the wheel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a block diagram which illustrates a gear shift control system of the first embodiment for an automatic transmission which is mounted in an automotive vehicle; 
           [0024]      FIG. 2  is an illustration which shows a braking force map for use in gear shift control of the gear shift control system of  FIG. 1 ; 
           [0025]      FIG. 3  is an illustration which shows a first deceleration decision map for use in gear shift control of the gear shift control system of  FIG. 1 ; 
           [0026]      FIG. 4  is a flowchart of a gear shift control program to be executed by the gear shift control system of  FIG. 1 ; 
           [0027]      FIG. 5A  is a graph which demonstrates a stroke of a brake pedal; 
           [0028]      FIG. 5B  is a graph which demonstrates a change in braking force applied to a wheel of a vehicle in gear shift inhibition control and standard gear shift control; 
           [0029]      FIG. 5C  is a graph which demonstrates a relation between the speed of a vehicle and the rate of deceleration of the vehicle in gear shift inhibition control and standard gear shift control; 
           [0030]      FIG. 5D  is a graph which illustrates a deceleration flag used in initiating gear shift inhibition control of the gear shift control system of  FIG. 1 ; 
           [0031]      FIG. 5E  is a graph which demonstrates a change in speed of a vehicle in gear shift inhibition control and standard gear shift control; 
           [0032]      FIG. 6  is a graph which demonstrates an example of a change in speed of an internal combustion engine arising from gear shifting of an automatic transmission; 
           [0033]      FIG. 7  is a graph which demonstrates an example of changes in speed of an internal combustion engine arising from gear shifting of an automatic transmission in gear shift inhibition control of the gear shift control system of  FIG. 1  and standard gear shift control; 
           [0034]      FIG. 8  is a flowchart of a gear shift control program to be executed by a gear shift control system of the second embodiment; 
           [0035]      FIG. 9  is a block diagram which illustrates a gear shift control system of the third embodiment for an automatic transmission which is mounted in an automotive vehicle; 
           [0036]      FIG. 10  is a flowchart of a gear shift control program to be executed by the gear shift control system of  FIG. 9 ; 
           [0037]      FIG. 11  is a block diagram which illustrates a gear shift control system of the fourth embodiment for an automatic transmission which is mounted in an automotive vehicle; 
           [0038]      FIG. 12  is a flowchart of a gear shift control program to be executed by the gear shift control system of  FIG. 11 ; 
           [0039]      FIG. 13  is an illustration which shows a fourth deceleration decision map for use in gear shift control of the gear shift control system of  FIG. 11 ; and 
           [0040]      FIG. 14  is a map representing a gear shifting schedule for an automatic transmission in standard gear shift control. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]    Embodiments will be described below with reference to the drawings.  FIGS. 1 to 7  illustrate a gear shift control system  10  for an automatic transmission according to the first embodiment. 
         [0042]    The gear shift control system  10  is designed to control an operation of the automatic transmission which is mounted on an automotive vehicle along with an internal combustion engine. The automatic transmission works to automatically change the speed of a drive shaft of the internal combustion engine and transmit the torque of the drive shaft to a drive axle of road wheels  60  to run the vehicle. 
         [0043]    The internal combustion engine, as referred to herein, is engineered to burn a mixture of air and fuel sprayed into combustion chambers to reciprocate pistons, thereby rotating the drive shaft through a crankshaft to output the torque. An engine system  100  which includes the internal combustion engine and components or actuators such as fuel injectors is generally controlled in operation by an engine ECU (Electronic Control Unit)  101 . The engine ECU  101  works to execute an engine control program, as stored in a memory  102 , to control the operation of the engine system  100  based on outputs from a sensor group  104  and/or given parameters. 
         [0044]    The automatic transmission is disposed between the drive shaft of the internal combustion engine (i.e., the engine system  100 ) and the drive axle of the wheels  60  of the vehicle and engineered to automatically change the gear ratios thereof as a function of the rotational speed of the engine and the speed of the vehicle when the torque is transmitted from the drive shaft of the engine to the drive axle of the wheels  60 . The gear shift control system  10  works to automatically control engagement of clutches of the automatic transmission to select one of the gear ratios, that is, upshift or downshift the automatic transmission. Specifically, the gear shift control system  10  includes an automatic transmission ECU (Electronic Control Unit)  11  which works to execute a gear shift control program, as stored in the memory  12 , to control an operation of an automatic transmission system  50  based on the outputs from the sensor group  104  and/or given parameters. The automatic transmission system  50  includes the automatic transmission and other components such as the clutches. The automatic transmission ECU  11  works to control operations of the components of the automatic transmission system  50  using the outputs from the sensor group  104  and data stored in the memory  12  as a function of a vehicle running condition established by a driver&#39;s effort on the accelerator pedal  70  of the vehicle. 
         [0045]    The engine ECU  101  monitors the outputs from the sensor group  104  to derive drive information needed for drive control of the vehicle. The sensor group  104  includes an engine torque determiner  105 , a vehicle speed determiner  106 , a brake pedal  80  stroke determiner  107 , and an accelerator position determiner  108  which are connected to the engine ECU  101 . The engine ECU  101  controls the operation of the internal combustion engine (i.e., the engine system  100 ) using the outputs from the sensor group  104  and data stored in the memory  102  as a function of a vehicle running condition established by a driver&#39;s effort on the brake pedal  80  (also called a foot brake) or the accelerator pedal  70 . 
         [0046]    The engine torque determiner  105  is implemented by, for example, a sensor designed to measure the speed Nt of the drive shaft of the internal combustion engine. The engine torque determiner  105  works to determine an engine torque Er as a function of the speed Nt and a gear ratio S, as inputted from the automatic transmission ECU  11 . The vehicle speed determiner  106  is equipped with a vehicle speed sensor which works to measure the speed (the number of revolutions per unit time) of the drive axle of the wheels  60 . The vehicle speed determiner  106  calculates the speed of the vehicle as a function of the measured speed of the drive axle of the wheels  60  and an effective radius D of the wheels  60 . The effective radius D, as referred to herein, is the distance between the axis of rotation of tires inflated at a standard air pressure and the circumference of the tires which makes contact with the road surface. 
         [0047]    The brake pedal stroke determiner  107  is equipped with a brake pedal stroke sensor (also called a brake pedal position sensor) which works as an amount-of-braking effort determiner to measure an amount by which the brake pedal  80  is depressed by the driver (i.e., a stroke BS of the brake pedal  80 ) to decelerate or stop the vehicle. The brake pedal stroke determiner  107  calculates the stroke BS of the brake pedal  80  in terms of percentage (%) where the stroke BS immediately before brakes start to be applied to the wheels  60  is 0%, and the stroke BS when the brakes are fully applied to the wheels  60  is 100%. 
         [0048]    The accelerator position determiner  108  is equipped with an accelerator stroke sensor (also called an accelerator position sensor) which works as an amount-of-acceleration determiner to measure an amount by which the accelerator pedal  70  is depressed by the driver (i.e., a stroke AS of the accelerator pedal  70 ) to accelerate the vehicle. The accelerator position determiner  108  calculates the stroke AS of the accelerator pedal  70  in terms of percentage (%) where the stroke AS when the fuel injectors are deactivated so that no fuel is sprayed into the combustion chambers of the engine is 0%, and the stroke AS when the fuel injectors are activated to spray a maximum amount of fuel into the combustion chambers of the engine to accelerate the vehicle fully is 100%. 
         [0049]    The automatic transmission ECU  11  shares the vehicle speed determiner  106 , the brake pedal stroke determiner  107 , and the accelerator position determiner  108  with the engine ECU  101  to perform a gear shift control operation to select one of the gears of the automatic transmission which is suitable for the running condition of the vehicle. Specifically, the gear shift control system  10  consists essentially of the automatic transmission ECU  11 , the memory  12 , the vehicle speed determiner  106 , the brake pedal stroke determiner  107 , and the accelerator position determiner  108 . 
         [0050]    Specifically, the automatic transmission ECU  11  performs a gear shift control task (which will also be referred to below as a normal gear shift control operation) to shift, as illustrated in  FIG. 14 , the gear ratios at the time when the locus representing the relation between the position of the accelerator pedal  70  and the speed of the vehicle during acceleration or deceleration of the vehicle intersects with one of the upshift gear shift lines US or the downshift gear shift lines DS. The automatic transmission ECU  11  also uses a braking force map (i.e., a table), as illustrated in  FIG. 2 , and a first deceleration decision map (i.e., a table), as illustrated in  FIG. 3 , to control the upshift or downshift of the vehicle as a function of a rate of deceleration of the vehicle while the vehicle is decelerating. 
         [0051]    Specifically, the automatic transmission ECU  11  executes a gear shift control program, as shown in a flowchart of  FIG. 4 , and determines in step S 11  whether the brake pedal stroke determiner  107  has detected the stroke BS of the brake pedal  80  which is greater than 0% or not, that is, whether the brake pedal  80  has been depressed or not. If a NO answer is obtained, the routine repeats step S 11 . 
         [0052]    If the brake pedal  80  is determined in step S 11  as having been depressed, in other words, the vehicle has begun to decelerate, the routine proceeds to step S 21  wherein the stroke BS of the brake pedal  80 , as determined by the brake pedal stroke determiner  107 , and the vehicle speed Vs, as determined by the vehicle speed determiner  106 , are derived. 
         [0053]    The routine proceeds to step S 31  wherein a feed-forward control mode is entered to calculate a braking force F 1  (N) applied to the wheels  60  by look-up using the braking force map of  FIG. 2  in relation to the stroke BS of the brake pedal  80  and the vehicle speed Vs, as derived in step S 21 . The routine then proceeds to step S 41  wherein the braking force F 1 , as derived in step S 31 , a travel resistance F 2  of the vehicle, and a weight M of the vehicle which are pre-stored in the memory  12 , are substituted into an equation (1) below to determine a deceleration a 1  representing the rate of deceleration of the vehicle. The value of the travel resistance F 2  may be derived by look-up using a map stored in the memory  12  which lists relations of values of the travel resistance F 2  to values of air resistance defined as a function of the vehicle speed Vs, values of rolling resistance (also called rolling drag) of the wheels  60 , and values of grade resistance acting on the vehicle. 
         [0000]        a 1=( F 1+ F 2)/ M   (1)
 
         [0054]    The routine then proceeds to step S 51  wherein the deceleration a 1 , as derived in step S 41 , is compared with a first deceleration threshold value Xn to determine whether the deceleration a 1  is greater than the first deceleration threshold value Xn or not. The first deceleration threshold value Xn has a correlation with the speed of the vehicle and is selected from the first deceleration decision map, as illustrated in  FIG. 3 , which lists the first deceleration threshold value Xn in relation to the vehicle speed Vs and is stored in the memory  12 . The first deceleration threshold value Xn is set to be increased with an increase in speed of the vehicle. For instance, when the speed (km/h) of the vehicle is 10, 20, 30 . . . , the first deceleration threshold value Xn is selected to have X 1 , X 2 , X 3 , . . . (X 1 &lt;X 2 &lt;X 3  . . . ). If a YES answer is obtained in step S 51  meaning that the deceleration a 1  is greater than the first deceleration threshold value Xn, then the routine proceeds to step S 61  wherein the normal gear shift control operation, as described above with reference to  FIG. 14 , is disabled to inhibit the upshift of the automatic transmission. 
         [0055]    Next, the routine proceeds to step S 71  wherein it is determined whether the locus defined by the relation between the position of the accelerator pedal  70  and the speed of the vehicle during acceleration or deceleration thereof has now intersected with one of the downshift gear shift lines DS, as illustrated in  FIG. 14 , or not which instructs the automatic transmission to shift down to a lower speed gear. If a YES answer is obtained meaning that the fact that the vehicle has decelerated, so that the speed of the vehicle has reached a given value at which the vehicle should be downshifted is detected, then the routine proceeds to step S 81  wherein the gear shift inhibition operation which inhibits the upshift of the automatic transmission is terminated. The routine proceeds to step S 91  wherein the normal gear shift control operation, as shown in  FIG. 14 , is resumed. The routine then terminates. Alternatively, if a NO answer is obtained in step S 51  meaning that the deceleration a 1  is smaller than or equal to the first deceleration threshold value Xn, then the routine proceeds to step S 91  wherein the normal gear shift control operation, as shown in  FIG. 14 , is resumed. 
         [0056]    For instance, when the brake pedal  80  has been depressed suddenly to decelerate the vehicle greatly immediately before the vehicle is required to be upshifted from the first to the second speed, as demonstrated in  FIG. 14 , and accelerated, the gear shift control system  10  immediately inhibits, as demonstrated by solid lines in  FIGS. 5A to 5E , the automatic transmission from upshifting according to the gear shift control program of  FIG. 4 , thereby producing a degree of engine braking which is greater than that, as demonstrated by broken lines in  FIGS. 5A to 5E , when the automatic transmission is not inhibited from upshifting. 
         [0057]      FIGS. 5A to 5E  show the example where the brake pedal  80  is depressed fully, so that the stroke BS reaches 100%. When the deceleration a 1  which is derived in step S 41  as a function of the braking force F 1  in relation to the stroke BS of the brake pedal  80  and the vehicle speed Vs exceeds the first deceleration threshold value Xn, the automatic transmission ECU  11  sets, as illustrated in  FIG. 5D , a deceleration flag to “on” and inhibits the automatic transmission from upshifting automatically. This causes, as can be indicated by solid lines in  FIGS. 5B and 5C , the degree of braking force (i.e., the rate of deceleration of the vehicle) which is greater than that when the automatic transmission is permitted to upshift to be produced, thereby resulting in, as illustrated in  FIG. 5C , a quick drop in vehicle speed Vs. The automatic transmission ECU  11  of this embodiment is, as described above, designed to initiate the gear shift inhibition operation of  FIG. 4  when the stroke BS of the brake pedal  80  is less than 100%, but the deceleration a 1  exceeds the first threshold value Xn, however, may alternatively be engineered to execute the gear shift inhibition operation, provided that the stroke BS reaches 100%. 
         [0058]    In the case where the automatic transmission ECU  11  continues to execute the normal gear shift control operation, as demonstrated by dashed lines in  FIGS. 5A to 5E , without executing the gear shift inhibition operation of  FIG. 4 , a great degree of engine braking is initially applied to the wheels  60  as well as the application of braking force, as produced by the brake actuators of the vehicle, upon depression of the brake pedal  80 , so that the vehicle decelerates greatly. The automatic transmission, however, may be upshifted depending upon the speed of the vehicle and the position of the accelerator pedal  70 . Such upshifting will result in a decrease in degree of the engine braking, that is, a decrease in total amount of braking force applied to the wheels  60 , so that the deceleration of the vehicle drops. The prior art gear shift control system, like in the publication referred to in the introductory part of this application, usually takes a given period of time to derive the rate of deceleration of the vehicle (i.e., a drop in speed VL, as illustrated in  FIG. 5C  after the vehicle begins to decelerate. This may cause the automatic transmission to have already upshifted at the time when the deceleration flag is set to “on”. 
         [0059]    When the deceleration a 1  exceeds the first deceleration threshold value Xn upon depression of the brake pedal  80 , so that the deceleration flag is set to “on”, the gear shift control system  10  of this embodiment, as demonstrated by solid lines in  FIGS. 5A to 5E , decelerates the vehicle without upshifting the automatic transmission from, for example, the second speed, to decrease the speed of the vehicle and subsequently downshifts the automatic transmission to the first speed. Upon the downshift to the first speed, the speed of the vehicle temporarily rises and then drops. The gear shift control system  10 , therefore, permits the internal combustion engine to run at a set idle speed without decreasing the engine speed Nt below a limit speed SL which will result in stall of the engine. Specifically, when it is required to decelerate the vehicle quickly while the automatic transmission is accelerating, for example, at the second speed, the gear shift control system  10  works to avoid a sudden drop in engine speed Nt, as indicated by a two-dot chain line in  FIG. 7 , which exceeds the limit speed SL at which the internal combustion engine will stall. 
         [0060]    When the automatic transmission ECU  11  continues to execute the normal gear shift control operation, it may cause the automatic transmission, as indicated by dashed lines in  FIGS. 6 and 7 , to upshift from the second to third speed and to the fourth speed as a function of the relation between the position of the accelerator pedal  70  and the speed of the vehicle. This may result in an undesirable great drop in speed of the internal combustion engine below the limit speed SL at which the internal combustion engine will stall. Such a speed drop will cause the automatic transmission to subsequently downshift from the fourth to third speed and to the second speed, thus resulting in sudden application of the engine braking, which leads to the deterioration in ride quality and drivability of the vehicle. 
         [0061]    Upon detection of depression of the brake pedal  80  of the vehicle to apply the brakes, the gear shift control system  10  of this embodiment begins to calculate the deceleration a 1  as a function of the vehicle speed Vs and the braking force F 1  and, when the deceleration a 1  is greater than the first deceleration threshold value Xn, inhibits the automatic transmission from upshifting. This eliminates the possibility of engine stall when the internal combustion engine starts decelerating suddenly during acceleration thereof and ensures a great amount of braking force applied to the wheels  60  without undue gear shifts of the automatic transmission which will result in deterioration of ride quality of drivability of the vehicle, thus enabling the vehicle to decelerate or stop quickly and safely. 
         [0062]      FIG. 8  illustrates the second embodiment of the gear shift control system  10  for the automatic transmission (i.e., the automatic transmission system  50 ). The gear shift control system  10  of this embodiment is identical in structure with that in the first embodiment. The same reference numbers, as employed in the first embodiment, will refer to the same parts. The same applies to other embodiments, as will be discussed later. 
         [0063]    The gear shift control system  10  has three kinds of deceleration decision maps, such as the one in  FIG. 3 , stored in the memory  12 . The memory  12  may, however, store two or more than three kinds of deceleration decision maps. Specifically, the memory  12  holds a first deceleration decision map which is identical with the one in  FIG. 3  and used to calculate the first deceleration threshold value Xn at which the automatic transmission is inhibited from upshifting, a second deceleration decision map for use in calculate a second deceleration threshold value Xn which inhibits the automatic transmission from shifting up two or more gear ratios at once, in other words, permits the automatic transmission to perform a single upshift, and a third deceleration decision map for use in calculate a third deceleration threshold value Xn which inhibits the automatic transmission from shifting up three or more gear ratios at once, in other words, permits the automatic transmission to perform only an upshift of two or less gear ratios. 
         [0064]    The automatic transmission ECU  11  of the gear shift control system  10  executes a gear shift control program, as shown in a flowchart of  FIG. 8 . When detecting the fact that the vehicle has started decelerating (step S 11 ), the automatic transmission ECU  11  calculates the braking force F 1  applied to the wheels  60  as a function of the stroke BS of the brake pedal  80  and the vehicle speed Vs (steps S 21  and S 31 ) and then substitutes the braking force F 1 , the travel resistance F 2 , as stored in the memory  12 , and the weight M of the vehicle into the above Eq. (1) to derive the deceleration a 1  (step S 41 ). 
         [0065]    Subsequently, it is determined whether the deceleration a 1  is greater than the third deceleration threshold value Xn or not which is derived from the third deceleration decision map in the memory  12  as a function of the vehicle speed Vs (step S 53 ). 
         [0066]    If it is determined that the deceleration a 1  is less than or equal to the third deceleration threshold value Xn, as derived as a function of the vehicle speed Vs, the normal gear shift control operation, as described above in  FIG. 14 , is executed (step S 91 ). The gear shift control program is then terminated. 
         [0067]    If the deceleration a 1  is determined in step S 53  to be greater than the third deceleration threshold value Xn, as derived as a function of the vehicle speed Vs, then it is determined whether the deceleration a 1  is greater than the second deceleration threshold value Xn or not which is derived from the second deceleration decision map in the memory  12  as a function of the vehicle speed Vs (step S 54 ). 
         [0068]    If it is determined in step S 54  that the deceleration a 1  is less than or equal to the second deceleration threshold value Xn, as derived as a function of the vehicle speed Vs, a first gear shift inhibition operation is executed to inhibit the automatic transmission from shifting up three or more gear ratios, in other words, to permit the automatic transmission from upshifting within a range of two higher speed gear ratios (step S 63 ). The routine then proceeds to step S 71 , as will be described later in detail. 
         [0069]    Alternatively, if the deceleration a 1  is determined in step S 54  to be greater than the second deceleration threshold value Xn, as derived as a function of the vehicle speed Vs, it is determined whether the deceleration a 1  is greater than the first deceleration threshold value Xn or not which is derived from the first deceleration decision map in the memory  12  as a function of the vehicle speed Vs (step S 55 ). 
         [0070]    If it is determined in step S 55  that the deceleration a 1  is less than or equal to the first deceleration threshold value Xn, as derived as a function of the vehicle speed Vs, a second gear shift inhibition operation is executed to inhibit the automatic transmission from shifting up two or more gear ratios, in other words, to permit the automatic transmission from upshifting only to one higher speed gear ratio (step S 64 ). The routine then proceeds to step S 71 , as will be described later in detail. 
         [0071]    Alternatively, if it is determined in step S 55  that the deceleration a 1  is greater than the first deceleration threshold value Xn, as derived as a function of the vehicle speed Vs, a third gear shift inhibition operation is executed to inhibit the automatic transmission from shifting up completely (step S 65 ). The routine then proceeds to step S 71 , as will be described later in detail. Note that the first deceleration threshold value Xn is greater than the second and third deceleration threshold value Xn, and the second deceleration threshold value Xn is greater than the third deceleration threshold value Xn. 
         [0072]    In step S 71 , it is determined whether the locus representing the relation between the position of the accelerator pedal  70  and the speed of the vehicle during deceleration thereof has now intersected with one of the downshift gear shift lines DS, as illustrated in  FIG. 14 , or not which instructs the automatic transmission to shift down to a lower gear ratio, in other words, whether the speed of the vehicle has dropped to the value which requires the automatic transmission to downshift. If a YES answer is obtained, the gear shift inhibition operation which inhibits the upshift is terminated (step S 81 ). The routine then proceeds to step S 91  wherein the normal gear shift control operation, as shown in  FIG. 14 , is resumed and then terminates. 
         [0073]    As apparent from the above discussion, the gear shift control system  10  works to decrease the number of gear ratios to which the automatic transmission is permitted to upshift or inhibit the automatic transmission from upshifting as the speed of the vehicle increases to provide a degree of engine braking depending upon the deceleration of the vehicle. 
         [0074]    The automatic transmission ECU  11  of this embodiment is, as described above, engineered not merely to inhibit the automatic transmission from upshifting, but to partially permit the automatic transmission to upshift depending upon the rate of deceleration of the vehicle, thereby applying an amount of engine braking force to the wheels  60  of the vehicle which is suitable for the rate of deceleration of the vehicle without the engine stall or undesirable frequent shifting operations of the automatic transmission. This avoids over-application of the engine braking to the wheels  60  even when the deceleration of the vehicle is low and also ensures the ride quality of the vehicle. 
         [0075]      FIGS. 9 and 10  illustrate an example of a vehicle in which the gear shift control system  10  of the third embodiment is mounted. 
         [0076]    In  FIG. 9 , the automatic transmission ECU  11  of the gear shift control system  10  includes the vehicle speed determiner  106 , the brake pedal stroke determiner  107 , and the accelerator position determiner  108  which are shared with the engine ECU  101  and also includes the engine torque determiner  105  to select one of the gear ratios of the automatic transmission based on running conditions of the vehicle. Specifically, the gear shift control system  10  is equipped with the automatic transmission ECU  11 , the memory  12 , the engine torque determiner  105 , the vehicle speed determiner  106 , the brake pedal stroke determiner  107 , and the accelerator position determiner  108 . 
         [0077]    The automatic transmission ECU  11  of the gear shift control system  10  executes a gear shift control program, as shown in a flowchart of  FIG. 10 . The same step numbers as employed in  FIGS. 4 and 8  refer to the same operations, and explanation thereof in detail will be omitted here. When detecting the fact that the vehicle has started decelerating (step S 11 ), the automatic transmission ECU  11  derives the stroke BS of the brake pedal  80 , the vehicle speed Vs, the engine torque Er outputted from the internal combustion engine, as determined by the engine torque determiner  107 , the gear ratio S, as provided by the automatic transmission ECU  11  by itself, and the effective radius D of the wheels  60 , as pre-stored in the memory  12  (step S 22 ). 
         [0078]    Subsequently, the automatic transmission ECU  11  calculates the braking force F 1  applied to the wheels  60  and then substitutes the engine torque Er, the gear ratio S, and the effective radius D of the wheels  60  into an equation (2) below to derive a power train drive force F 3  (i.e., the engine braking force) which acts on to drive a power train of the vehicle (step S 32 ). The automatic transmission ECU  11  also substitutes the braking force F 1 , as derived in step S 32 , the travel resistance F 2 , as stored in the memory  12 , the weight M of the vehicle, and the power train drive force F 3  into the above an equation (3), as shown below, to derive a deceleration a 2  (step S 42 ). Note that the gear ratio S is given by the product of a gear ratio in a gear position selected in the automatic transmission and a final gear ratio. The final gear ratio is a gear ratio of a final gear. The final gear is usually mounted in a differential which allows the right and left wheels  60  to rotate at different speeds while the vehicle is turning. The differential works to reduce through the final gear the speed of the internal combustion engine which has been already reduced by the automatic transmission and transmit it to the wheels  60 . 
         [0000]        F 3= Er×S/D   (2)
 
         [0000]        a 2=( F 1+ F 2+ F 3)/ M   (3)
 
         [0079]    Subsequently, it is determined in the same manner, as in the first embodiment, whether the deceleration a 2  is greater than the first deceleration threshold value Xn or not which is derived as a function of the vehicle speed Vs (step S 51 ). 
         [0080]    If it is determined that the deceleration a 2  is greater than the first deceleration threshold value Xn, as derived as a function of the vehicle speed Vs, the gear shift inhibition operation is executed to inhibit the automatic transmission from shifting up (step S 61 ). In step S 71 , it is determined whether the locus representing the relation between the position of the accelerator pedal  70  and the speed of the vehicle during deceleration thereof has now intersected with one of the downshift gear shift lines DS, as illustrated in  FIG. 14 , or not which instructs the automatic transmission to shift down to a lower gear ratio, in other words, whether the speed of the vehicle has dropped to the value at which the automatic transmission should be downshifted. If a YES answer is obtained, the gear shift inhibition operation which inhibits the upshift is terminated (step S 81 ). The routine then proceeds to step S 91  wherein the normal gear shift control operation, as shown in  FIG. 14 , is resumed. The routine then terminates. 
         [0081]    Alternatively, if it is determined in step S 51  that the deceleration a 2  is less than or equal to the first deceleration threshold value Xn, the routine proceeds directly to step S 91  wherein the normal gear shift control operation, as shown in  FIG. 14 , is resumed and then terminates. 
         [0082]    As apparent from the above discussion, the gear shift control system  10  is designed to calculate the deceleration a 2  of the vehicle as a function of an additional parameter, i.e., the power train drive force F 3 , thus enhancing the accuracy in inhibiting the automatic transmission from upshifting in the gear shift inhibition mode. 
         [0083]      FIGS. 11 to 13  illustrates the gear shift control system  10  according to the fourth embodiment. 
         [0084]    In  FIG. 11 , the gear shift control system  10  is mounted in a hybrid electric vehicle (HEV) equipped with a combination of a gasoline-powered internal combustion engine (i.e., the engine system  100 ) and an electric motor  200  driven by electric power stored in a storage battery  90  which are used as a drive source to produce torque to rotate wheels  60  of the vehicle. The storage battery  90  of the vehicle can be recharged by an external power supply. The electric motor  200  may be engineered as an electric generator to produce electric energy. Alternatively, a separate electric generator may be installed in the vehicle. The electric generator (i.e., the electric motor  200  or the separate electric generator), which will also be referred as a generator motor  200  below, is driven in a regenerative braking mode during running or decelerating of the vehicle to produce the regenerative electric energy for use in recharging the storage battery  90  in the vehicle. 
         [0085]    The engine ECU  101  executes coordination control to drive the engine as a function of an amount of electric energy remaining in the storage battery  90 , as measured by an SOC determiner  109 , for charging the battery  90 . The SOC determiner  109  is designed to determine a state of charge (SOC) of the battery  90  as indicating the amount of electric energy remaining in the battery  90 . The storage battery  90  is, as described above, mounted in the vehicle to supply the electric power to the electric motor  200 , spark plugs installed in the engine, and electric components of an air conditioner. 
         [0086]    The automatic transmission ECU  11  of the gear shift control system  10  includes the engine torque determiner  105 , the vehicle speed determiner  106 , the brake pedal stroke determiner  107 , and the accelerator position determiner  108  and the SOC determiner  109  which are shared with the engine ECU  101  and works to execute gear shift control to select one of the gear ratios of the automatic transmission based on running conditions of the vehicle. Specifically, the gear shift control system  10  is equipped with the automatic transmission ECU  11 , the memory  12 , the engine torque determiner  105 , the vehicle speed determiner  106 , the brake pedal stroke determiner  107 , the accelerator position determiner  108 , and the SOC determiner  109 . 
         [0087]    The automatic transmission ECU  11  executes a gear shift control program, as shown in a flowchart of  FIG. 12 . The same step numbers as employed in  FIGS. 4 ,  8 , and  10  refer to the same operations, and explanation thereof in detail will be omitted here. When detecting the fact that the vehicle has started decelerating (step S 11 ), the automatic transmission ECU  11  derives the stroke BS of the brake pedal  80 , the vehicle speed Vs, the engine torque Er, the gear ratio S, the effective radius D of the wheels  60 , and the SOC of the battery  90 , as measured by the SOC determiner  109  (step S 24 ). 
         [0088]    Subsequently, the automatic transmission ECU  11  determines the braking force F 1  applied to the wheels  60  and the power train drive force F 3  in the same manner, as described above. Additionally, the automatic transmission ECU  11  also calculates a regenerative electric energy being charged in the storage battery  90  as a function of a change in the SOC, as measured by the SOC determiner  109 , per unit time, and derives a regenerative drive force F 4  (i.e., regenerative braking force acting on the wheels  60 ) that is force used to drive the generator motor  200  to produce the calculated regenerative electric energy (step S 34 ). The automatic transmission ECU  11  then substitutes the braking force F 1 , the travel resistance F 2 , the power train drive force F 3 , the weight M of the vehicle, and the regenerative drive force F 4  into an equation (4), as shown below, to derive a deceleration a 3  (step S 42 ). 
         [0000]        a 3=( F 1+ F 2+ F 3+ F 4)/ M   (4)
 
         [0089]    Subsequently, the automatic transmission ECU  11  adds a threshold value Yn (also referred to as a fourth deceleration threshold value) to the first deceleration threshold value Xn, as described in the first embodiment, to derive a fifth deceleration threshold value Zn. The fourth deceleration threshold value Yn is determined by look-up using a fourth deceleration decision map, as illustrated in  FIG. 13 , stored in the memory  12 . The fourth deceleration threshold value Yn is listed in relation to the regenerative drive force produced as a function of the speed of the vehicle. The automatic transmission ECU  11  determines whether the deceleration a 3  is greater than the fifth deceleration threshold value Zn or not (step S 52 ). 
         [0090]    If it is determined that the deceleration a 3  is greater than the fifth deceleration threshold value Yn, the gear shift inhibition operation is executed to inhibit the automatic transmission from shifting up (step S 61 ). In step S 71 , it is determined whether the locus representing the relation between the position of the accelerator pedal  70  and the speed of the vehicle during deceleration thereof has now intersected with one of the downshift gear shift lines DS, as illustrated in  FIG. 14 , or not which instructs the automatic transmission to shift down to a lower gear ratio, in other words, whether the speed of the vehicle has dropped to the value at which the automatic transmission should be downshifted. If a YES answer is obtained, the gear shift inhibition operation which inhibits the upshift is terminated (step S 81 ). The routine then proceeds to step S 91  wherein the normal gear shift control operation, as shown in  FIG. 14 , is resumed and then terminates. 
         [0091]    Alternatively, if it is determined in step S 52  that the deceleration a 3  is less than or equal to the fifth deceleration threshold value Zn, the routine proceeds directly to step S 91  wherein the normal gear shift control operation, as shown in  FIG. 14 , is resumed and then terminates. 
         [0092]    As apparent from the above discussion, the gear shift control system  10  is designed to calculate the deceleration a 3  of the vehicle as a function of an additional parameter, i.e., the regenerative drive force F 4 , thus enhancing the accuracy in inhibiting the automatic transmission from upshifting when the hybrid electric vehicle is decelerating in the regenerative braking mode. 
         [0093]    The gear shift control system  10  of this embodiment is, as described above, mounted in the hybrid vehicle, but may be used with an electric vehicle in which an internal combustion engine is not mounted and which is equipped with a gear shift mechanism. 
         [0094]    The regenerative drive force is calculated based on the regenerative electric energy that is derived as a function of a change in the SOC of the battery  90 , as measured by the SOC determiner  109 , per unit time, but may be derived by directly measuring the amount of current charged in the battery  90  to determine the regenerative electric energy. 
         [0095]    The fact that the vehicle has started decelerating is found by monitoring the amount by which the brake pedal  80  is depressed, but the calculation of the braking forces, as described above, may be initiated in response to release of the accelerator pedal  70 , as detected by the accelerator position determiner  108 . For instance, the automatic transmission ECU  11  may calculate the braking force (i.e., the braking force F 1 ) which arises from the release of the accelerator pedal  70 , as sensed y the accelerator position determiner  108 . 
         [0096]    The automatic transmission ECU  11  may work to determine one or the sum of an engine braking force, as produced by the internal combustion engine (i.e., the engine system  100 ), and a regenerative braking force, as produced by the electric generator  200 , as the braking force (e.g., the braking force F 1 ) applied to the wheel  60  and calculate the degree of deceleration (e.g., the deceleration a 1 , a 2 , or a 3 ) of the vehicle as a function of such a braking force. 
         [0097]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.