Abstract:
In a vehicle driving-force control device, when a brake switch is turned on, a transmission control unit determines a currently-selected engine mode and sets a deceleration determination value by referring to a determination-value table corresponding to the currently-selected engine mode, using a vehicle speed as a parameter. Then, the transmission control unit compares an absolute value of a deceleration with an absolute value of the deceleration determination value, and shifts down the gear stage if the absolute value of the deceleration exceeds the absolute value of the deceleration determination value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2010-280668 filed on Dec. 16, 2010, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vehicle driving-force control device that applies adaptive control of an automatic transmission to control of an engine having at least two engine modes. 
     2. Description of the Related Art 
     In the related art, examples of an automatic transmission include a multistage transmission that switches gear ratios in a stepwise manner and a continuously variable transmission that switches gear ratios in a stepless manner. A known representative type of a multistage transmission is a planetary-pinion type. On the other hand, known types of a continuously variable transmission include a belt or chain based type that uses a metallic belt or a metallic chain as a power shift component and a toroidal type that performs a shift operation by changing a contact radius of a power roller relative to a disk. 
     Shift control of such an automatic transmission is performed by a transmission control unit (TCU). Specifically, the shift control is performed according to a driving state of a vehicle by referring to a shift map that stores a shift characteristic for each gear ratio, on the basis of parameters such as a vehicle speed and a throttle opening for detecting the driving state. 
     In this case, a technology of performing so-called driving-condition adaptive control (simply referred to as “adaptive control” hereinafter) is also known, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2010-169111 (referred to as “Patent Document 1” hereinafter). Specifically, in this adaptive control, the TCU performs upshifting or downshifting or performs a shift hold (that is, fixes a gear ratio) according to the driver&#39;s intention when the vehicle accelerates or decelerates, or is running along a curved road. 
     Since the maximum driving force differs between a fuel efficient vehicle and a high performance vehicle such as a turbo vehicle, the maximum acceleration naturally differs therebetween. Therefore, when driving along a slope with continuous curves, for example, an amount of operation (that is, degree of depression) of a brake pedal and an accelerator pedal also differs between a fuel efficient vehicle and a high performance vehicle. 
     In another similar known technology, a single vehicle has at least two different driving-force characteristics. In this case, when the driver selects one mode from multiple engine modes, engine output characteristics are switched, and a shift characteristic of the automatic transmission is correspondingly changed. For example, Japanese Patent No. 3930529 (referred to as “Patent Document 2” hereinafter) discloses a technology in which a vehicle has three kinds of mode maps with different engine driving-force characteristics. When the driver selects one of the engine modes, the engine is controlled according to the corresponding mode map, and shift control is performed on the basis of a shift pattern that corresponds to the selected engine mode. 
     In the technology disclosed in Patent Document 2, the engine modes include three modes, which are a normal mode (referred to as “S mode” hereinafter) suitable for normal driving, a save mode (referred to as “I mode” hereinafter) in which a rise in a target torque is reduced relative to the S mode, and a power mode (referred to as “S# mode” hereinafter) in which a rate of change in the target torque relative to a change in the accelerator opening is set to a large value over substantially the entire driving range. 
     Specifically, as shown in  FIG. 6 , in the S mode, the degree of opening of a throttle valve (throttle opening SV) changes substantially linearly with respect to the degree of depression of the accelerator pedal (accelerator opening AP). In other words, the characteristic of the S mode is set similar to that of a naturally aspirated DOHC engine. In the I mode, the characteristic thereof is set so as to be suitable for driving at a low-to-medium speed range since the throttle opening SV does not reach full throttle even when the accelerator pedal is fully pressed. In other words, the characteristic of the I mode is set similar to that of an SOHC engine. In contrast, in the S# mode, the characteristic thereof is set such that the rate of change in the throttle opening SV relative to a change in the accelerator opening AP is set to a large value over the entire driving range. Therefore, the characteristic of the S# mode is set similar to that of a turbo engine, which is suitable for sporty driving in which the engine can exhibit its maximum potential. 
     Accordingly, in the technology disclosed in Patent Document 2, a single vehicle can exhibit the driving performance of three types of vehicles, which are a normal vehicle (NA vehicle), a fuel efficient vehicle, and a high performance vehicle, depending on the selected engine mode. 
     In the aforementioned adaptive control, if sudden deceleration due to braking is detected during traveling on a road with continuous curves, for example, the automatic transmission is shifted down to prepare for subsequent acceleration. When releasing of the accelerator pedal is subsequently detected, upshifting is performed based on the assumption that the vehicle will make a turn after braking. If the vehicle is running up a slope, downshifting is performed for achieving acceleration performance. 
     When this is applied to a vehicle having the aforementioned three engine modes, the braking performance of the vehicle is naturally set so as to assure the driving performance of the S# mode that can exhibit the maximum driving-force performance. Therefore, the adaptive control is also set so as to assure the performance of the S# mode. 
     In this case, since the driving force in the I mode is limited, as mentioned above, the acceleration performance like that of the S# mode cannot be obtained. Therefore, the I mode does not need to have the braking performance like that of the S# mode. In other words, if a braking force similar to that in the S# mode is exhibited in the I mode, the vehicle speed would rapidly decrease. 
     Therefore, when the vehicle is running in the I mode, the driver drives the vehicle with a reduced braking force so that the speed of the vehicle entering a road with continuous curves is naturally lower than that in the S# mode. As a result, the speed of the vehicle upon making a turn on a curved road in the I mode is entirely lower than that in the S# mode. Consequently, when braking is detected in such a condition and downshifting is performed by the adaptive control, the driver would feel a sense of sudden deceleration beyond the driver&#39;s prediction. 
     SUMMARY OF THE INVENTION 
     In view of the circumstances described above, it is an object of the present invention to provide a vehicle driving-force control device that, when the device performs driving-condition adaptive control of a gear ratio of an automatic transmission in a vehicle equipped with an engine having at least two engine modes, can achieve good driving performance in each engine mode so as to reduce a sense of discomfort felt a the driver. 
     An aspect of the present invention provides a vehicle driving-force control device including a mode selecting unit, an engine control unit, a shift control unit, and an adaptive control unit. The mode selecting unit selects one engine mode from at least two engine modes having different engine driving-force characteristics. The engine control unit controls an engine according to the one engine mode selected by the mode selecting unit. The shift control means performs shift control of an automatic transmission according to a driving state of a vehicle. The adaptive control unit compares a deceleration of the vehicle with a preset deceleration determination value and outputs a downshift command to the automatic transmission if the deceleration is greater than the deceleration determination value. The adaptive control unit includes the deceleration determination value for each of the engine modes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a system configuration of an engine and an automatic transmission; 
         FIG. 2  is a flowchart illustrating an engine control routine; 
         FIG. 3  is a flowchart illustrating a braking downshift control subroutine; 
         FIG. 4  illustrates a shift map; 
         FIG. 5  illustrates deceleration-determination-value tables for respective modes; and 
         FIG. 6  is a characteristic diagram illustrating the relationship between an accelerator opening and a throttle opening for each engine mode. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. Reference numeral  1  in  FIG. 1  denotes an engine. An automatic transmission  3  is coupled to an output side of the engine  1  via a torque converter  2 . An output from the engine  1  is transmitted to the automatic transmission  3  via a fluid in the torque converter  2  or via a lock-up clutch (not shown) when in a locked-up state. In the case of a multistage transmission, the automatic transmission  3  shifts a gear to a predetermined gear stage. In the case of a continuously variable transmission, the automatic transmission  3  shifts a gear to a predetermined gear ratio. Subsequently, the output is transmitted from an output shaft  4  toward driving wheels, such as rear wheels or front wheels, so that a driving force is generated. The following description is directed to an example in which the automatic transmission  3  is a multistage transmission. In the case of a continuously variable transmission, the term “gear stage” may be changed to the term “gear ratio”. 
     An electronic throttle controller (ETC)  6  is disposed at an intermediate section of an intake passage  5  that communicates with an intake port (not shown) of the engine  1 . The ETC  6  has a throttle actuator  7  that opens and closes a throttle valve  6   a . The throttle actuator  7  is driven in response to a drive signal from a later-described engine control unit (ECU)  11  serving as an engine control unit of the present invention. 
     The automatic transmission  3  includes a transmission mechanism constituted of planetary pinions, and a friction fastener formed of a clutch, a brake, and the like that appropriately drives this transmission mechanism. 
     Furthermore, the automatic transmission  3  is provided with a control valve unit  3   a  that performs a fastening or releasing operation of the friction fastener as well as a releasing or connecting operation of the lock-up clutch provided in the torque converter  2 . The control valve unit  3   a  is activated in response to a transmission command (downshift command or upshift command) signal from a later-described transmission control unit (TCU)  12  serving as a shift control unit of the present invention. 
     Each of the ECU  11  and the TCU  12  is a microcomputer equipped with a CPU, a ROM, a RAM, and a nonvolatile memory and such as an EEPROM. The ECU  11  and the TCU  12  are connected with each other via a bus line so as to allow communication therewith. 
     An input side of the ECU  11  is connected to various sensors that detect parameters required for engine control. Examples of such sensors include an engine-rotation-speed sensor  14  that detects an engine rotation speed Ne (rpm) from a rotation speed of an output shaft of the engine  1 , an accelerator-opening sensor  15  that detects an accelerator opening AP (%) indicating how much an accelerator pedal (not shown) is depressed, and a mode selection switch  16  serving as a mode selecting unit of the present invention. An output side of the ECU  11  is connected to the throttle actuator  7  of the ETC  6 . 
     The ECU  11  sets a target torque as a driving-force instruction value by performing map searching using the engine rotation speed Ne and the accelerator opening AP as parameters, and sets a throttle opening SV corresponding to the thus set target torque. Then, the ECU  11  performs feedback control on the throttle actuator  7  such that the throttle opening SV of the throttle valve  6   a  is equal to a target throttle opening. 
     In this embodiment, a target torque map used for setting the target torque is set for each preset output characteristic (engine mode). A driver can select any one of the engine modes by operating the mode selection switch  16 . As is the case with Patent Document 2 described above, the engine modes in this embodiment include three modes, which are a normal mode (S mode), a save mode (I mode), and a power mode (S# mode). Since the S, I, and S# engine modes have different engine-driving-force characteristics, a target torque map is provided for each engine mode. 
     The characteristics of the target torque maps for the respective engine modes will now be briefly explained. An S-mode target torque map (referred to as “S-mode map” hereinafter) has a characteristic in which the target torque changes substantially linearly with respect to the accelerator opening AP, and is set so as to achieve comfortable driving. An I-mode target torque map (referred to as “I-mode map” hereinafter) is set such that a rise in the target torque is suppressed as compared with the aforementioned S-mode map, so as to achieve both comfortable driving and high fuel economy in a well-balanced manner in a low-to-medium speed range. An S#-mode target torque map (referred to as “S#-mode map” hereinafter) is set such that the rate of change in the target torque relative to a change in the accelerator opening AP is increased over substantially the entire driving range, whereby the engine  1  can exhibit its maximum potential. The relationship between the accelerator opening AP and the throttle opening SV in each of the S, I, and S# modes is schematically shown in  FIG. 6 . 
     An input side of the TCU  12  is connected to various sensors and switches that detect parameters required for shift control. Examples of such sensors and switches include a vehicle-speed sensor  17  that detects a vehicle speed Vsp on the basis of a rotational speed of the output shaft  4  of the automatic transmission  3 , a brake switch  18  that outputs an ON signal in response to depression of a brake pedal (not shown), a select position sensor  19  that detects a set position of a select lever, a throttle-opening sensor  20  that detects the opening (throttle opening) SV of the throttle valve  6   a , a longitudinal-acceleration (longitudinal G) sensor  21  that detects longitudinal acceleration Gx of the vehicle, and a lateral-acceleration (lateral G) sensor  22  that detects lateral acceleration Gy of the vehicle. Furthermore, an output side of the TCU  12  is connected to actuators provided in the control valve unit  3   a.    
     The TCU  12  performs shift control for setting an optimal gear stage by referring to a shift map using the vehicle speed Vsp and the throttle opening SV as parameters.  FIG. 4  illustrates an example of the shift map. In  FIG. 4 , upshift lines are denoted by solid lines, whereas downshift lines are denoted by dotted lines. In the shift map shown in  FIG. 4 , the upshift lines are disposed at the higher speed side relative to the downshift lines so as to provide hysteresis, thereby preventing control hunting during shifting. This shift map is set by previously obtaining a shift schedule suitable for the output characteristics of the engine  1  for each of the aforementioned S, I, and S# modes from an experiment or the like. Since the shift control is similar to that in the related art, a detailed description thereof is omitted. 
     When the vehicle rapidly decelerates in response to depression of the brake pedal, the TCU  12  performs braking downshift control as adaptive control for shifting down the gear stage of the automatic transmission  3 . In this braking downshift control, downshifting is performed at a timing corresponding to any one of the S, I, or S# engine mode selected by the driver (a detailed description will be given hereinbelow). 
     The aforementioned engine control in the ECU  11  is performed according to an engine control routine shown in  FIG. 2 . The braking downshift control in the TCU  12  is performed according to a braking downshift control routine shown in  FIG. 3 . 
     First, the engine control routine shown in  FIG. 2  will be described. This routine is performed at every predetermined calculation cycle. In step S 1 , it is determined whether or not the mode selection switch  16  has been operated by the driver. If the mode selection switch  16  has not been operated, the process proceeds to step S 6 . If it is detected that the mode selection switch  16  has been operated, the process proceeds to step S 2 . 
     In step S 2 , it is determined which one of the S, I, and S# engine modes has been selected by the driver via the mode selection switch  16 . If it is determined that the S mode has been selected, the process proceeds to step S 3 . If it is determined that the I mode has been selected, the process proceeds to step S 4 . If it is determined that the S# mode has been selected, the process proceeds to step S 5 . 
     When the process proceeds to any one of steps S 3  to S 5 , the corresponding mode map (S, I, or S#) is read from the nonvolatile memory in the ECU  11  so as to be selected as a current mode map. The process then proceeds to step  56 . 
     When the process proceeds to step S 6  from step S 1  or from any one of steps S 3  to S 5 , a target torque τe is set on the basis of the selected mode map using the engine rotation speed Ne and the accelerator opening AP as parameters. Then, the process proceeds to step S 7  where a target throttle opening SVo corresponding to the target torque τe is set. 
     Subsequently, in step S 8 , the throttle opening SV detected by the throttle-opening sensor  20  is read, and the degree of opening of the throttle valve  6   a  is feedback-controlled by controlling the throttle actuator  7  provided in the ETC  6  such that the throttle opening SV matches the target throttle opening SVo. The process then exits the routine. 
     As mentioned above, the engine  1  according to this embodiment has three engine modes, which are S mode, I mode, and S# mode, and the driver can drive the vehicle while selecting the preferred mode. When the TCU  12  detects deceleration in response to braking operation, the TCU  12  performs the braking downshift control as adaptive control for automatically shifting down the current gear stage according to the deceleration. 
     In this case, if the downshift characteristic of the braking downshift control is set on the basis of the S# mode independently of the S, I, or S# engine mode selected by the driver, the downshift timing in he I mode tends to be too early. This not only leads to higher fuel consumption due to more frequent downshifting, but also to a large decrease in the engine rotation speed due to excessive downshifting, thereby giving the driver a sense of discomfort. If the downshift characteristic is set on the basis of the I mode, the downshift timing in the power-oriented S# mode tends to be delayed, thereby causing the driver to feel a lack of torque. 
     On the other hand, in the S mode, whether the downshift characteristic is set on the basis of the S# mode or the I mode, the downshift timing tends to be too early in the downshift characteristic set on the basis of the S# mode, or the downshift timing tends to be delayed in the downshift characteristic set on the basis of the I mode. In either case, it is difficult to achieve good driving performance. 
     Therefore, in the TCU  12  according to this embodiment, the downshift characteristic during braking is set such that the downshift timing is set early for the S# mode and set late for the I mode, and the braking downshift control is prohibited in the S mode, whereby good driving performance can be achieved. 
     The term “deceleration” refers to a negative value of acceleration. Specifically, a vehicle speed per time decreases as deceleration increases. 
     The braking downshift control performed by the TCU  12  will be described below according to the braking downshift control routine shown in  FIG. 3 . The process of this routine corresponds to an adaptive control unit of the present invention. 
     This routine is activated in response to an ON signal from the brake switch  18  as a trigger signal. Specifically, the TCU  12  monitors depressing of the brake pedal by the driver, and when the TCU  12  detects that the brake pedal has been depressed, the TCU  12  activates the braking downshift control routine. First, in step S 11 , the engine mode is read. In step  512 , it is determined whether the current engine mode is set in the S mode, the I mode, or the S# mode. If it is determined that the current engine mode is set in the S mode, the process proceeds to step S 18 . If it is determined that the current engine mode is set in the I mode, the process proceeds to step S 13 . If it is determined that the current engine mode is set in the S# mode, the process proceeds to step S 14 . 
     In the case where the process proceeds to step S 13 , a deceleration determination value Dn is set by referring to an I-mode deceleration-determination-value table using the current gear stage and the vehicle speed Vsp as parameters, and the process then proceeds to step S 15 . On the other hand, in the case where the process proceeds to step S 14 , a deceleration determination value Dn is set by referring to an S#-mode deceleration-determination-value table using the current gear stage and the vehicle speed Vsp as parameters, and the process then proceeds to step S 15 . 
     In  FIG. 5 , downshift determination lines S#D 1  to S#D 4  for respective gear stages stored in the S#-mode deceleration-determination-value table are denoted by solid lines, whereas downshift determination lines ID 1  to ID 4  for respective gear stages stored in the I-mode deceleration-determination-value table are denoted by dotted lines. 
     As shown in  FIG. 5 , the downshift determination lines set in both deceleration-determination-value tables are set such that downshifting is performed at a lower deceleration as the vehicle speed Vsp increases. Specifically, downshifting is performed at a lower deceleration as the gear stage is located more toward the lower stage side. Furthermore, the downshift determination lines (solid lines) for respective gear stages stored in the S#-mode deceleration-determination-value table are set at the lower deceleration side compared to the downshift determination lines (dotted lines) for respective gear stages stored in the I-mode deceleration-determination-value table. 
     Therefore, in this embodiment, the I mode and the S# mode have different braking downshift timings, such that the downshift control in the S# mode is performed at a relatively earlier timing than in the I mode. For this reason, the vehicle runs at a relatively high engine rotation speed in the S# mode, thereby achieving good acceleration performance when the vehicle exits a curved road. On the other hand, in the I mode with lower acceleration performance than that of the S# mode, the downshift timing is late compared to the S# mode and thus engine braking does not become excessive, thereby preventing a sharp decrease in the vehicle speed. 
     Subsequently, when the process proceeds to step S 15  from step S 13  or S 14 , the deceleration determination value Dn set in step S 13  or S 14  is compared with a deceleration D (m/s 2 ) obtained on the basis of the time derivative or the like of the vehicle speed Vsp. If it is determined that the deceleration D is greater than the deceleration determination value Dn, the process proceeds to step S 16 . If it is determined that the deceleration D is smaller than the deceleration determination value Dn, the process branches to step S 18 . 
     When the process proceeds to step S 18  from step S 12  or S 15 , the braking downshift control is prohibited, and the process exits the routine. Accordingly, when the engine mode is set in the S mode, downshifting is not performed even in the case of sudden braking by the driver. Instead, the normal shift control simply performed, and thus excessive engine braking is prevented, thereby achieving comfortable driving performance. 
     On the other hand, when the process proceeds to step S 16  from step S 15 , a gear stage lower by one stage than the current gear stage is set as a target gear stage. Subsequently, in step S 17 , a downshift command corresponding to the target gear stage is output to the control valve unit  3   a , and the process then exits the routine. The control valve unit  3   a  shifts down the gear stage of the automatic transmission  3  to the target gear stage according to the downshift command from the TCU  12 . 
     Accordingly, the braking downshift control according to this embodiment adapts to the S, I, and S# engine modes controlled by the ECU  11 . Specifically, when the engine mode is in the S mode, the braking downshift control is prohibited, or when the engine mode is in the I mode or the S# mode, a deceleration determination value Dn according to the corresponding mode is set. Therefore, when the vehicle runs in the S# mode, downshifting is performed at an early timing, and thus the vehicle runs with a higher engine rotation speed, thereby preventing a lack of torque and achieving good driving performance. On the other hand, since the downshift timing is set to a later timing for the I mode, downshifting is performed less frequently, thereby preventing not only high fuel consumption, but also a sharp decrease in the vehicle speed caused by downshifting. 
     As a result, good adaptive control (braking downshift control) reflecting the driver&#39;s intention can be achieved for all of the engine modes, that is, the S mode, the I mode, and the S# mode, thereby reducing a sense of discomfort felt by the driver. 
     The present invention is not to be limited to the above-described embodiment. For example, a deceleration determination value Dn based on the vehicle speed Vsp may also be set also for the S mode. In that case, the deceleration determination value Dn for the S mode is set to a value between the deceleration determination value Dn set for the I mode and the deceleration determination value Dn set for the S# mode. Furthermore, the engine modes may be modes other than the aforementioned three kinds, that is, the S mode, the I mode, and the S# mode. In that case, a deceleration determination value Dn corresponding to each of the modes is set. 
     As a further alternative, in the aforementioned step S 12  in the braking downshift control routine shown in  FIG. 3 , a determination process may be performed only for the I mode and the S# mode. With regard to the S mode in that case, the same control can be achieved by simply ending the routine.