Method to automatically control a drivetrain provided with a servo-assisted transmission

Abstract

A method to automatically control a drivetrain provided with a servo-assisted transmission; the method presents the steps of: measuring a rotation speed of the internal combustion engine; carrying out a downshift to a lower gear in an autonomous manner, when the rotation speed of the internal combustion engine reaches a lower threshold; carrying out an upshift to a higher gear in an autonomous manner, when the rotation speed of the internal combustion engine reaches an upper threshold; detecting a release of an accelerator pedal in a first instant; waiting, starting from the first instant, a time interval until a second instant, which is subsequent to the first instant; and increasing a value of the lower threshold starting from the second instant until a following pressing of the accelerator pedal, if in the second instant the rotation speed of the internal combustion engine still exceeds the lower threshold.

Claims

1. A method to automatically control a drivetrain (6) provided with a servo-assisted transmission (7); the control method comprises the steps of: measuring a rotation speed (ω.sub.E) of an internal combustion engine (4); carrying out a downshift to a lower gear in an autonomous manner and independently of an intervention of a driver, when the rotation speed (ω.sub.E) of the internal combustion engine (4) reaches a lower threshold (TH.sub.DOWN); carrying out an upshift to a higher gear in an autonomous manner and independently of an intervention of the driver, when the rotation speed (ω.sub.E) of the internal combustion engine (4) reaches an upper threshold (TH.sub.UP); detecting a release of an accelerator pedal (22) in a first instant (t.sub.1); waiting, starting from the first instant (t1), a time interval (TO) until a second instant (t.sub.2), which is subsequent to the first instant (t.sub.1); and increasing a value of the lower threshold (TH.sub.DOWN) starting from the second instant (t.sub.2) until a following pressing of the accelerator pedal (22), if in the second instant (t.sub.2) the rotation speed (ω.sub.E) of the internal combustion engine (4) still exceeds the lower threshold (TH.sub.DOWN).

2. The control method according to claim 1, wherein the lower threshold (TH.sub.DOWN) has a standard value (VS) before the second instant (t.sub.2) and has an increased value (VA), which is greater than the standard value (VS), after the second instant (t.sub.2).

3. The control method according to claim 2, wherein the lower threshold (TH.sub.DOWN) is decreased and assumes the standard value (VS) again after a following pressing of the accelerator pedal (22).

4. The control method according to claim 1, wherein: if in the second instant (t2) the rotation speed (ω.sub.E) of the internal combustion engine (4) is greater than or equal to an increased value (VA), in the second instant (t2) the lower threshold (TH.sub.DOWN) is brought to the increased value (VA); and if in the second instant (t2) the rotation speed (ω.sub.E) of the internal combustion engine (4) is smaller than the increased value (VA), in the second instant (t2) the lower threshold (TH.sub.DOWN) is brought to an extemporaneous value (V*) equal to the rotation speed (ω.sub.E) of the internal combustion engine (4) in the second instant (t2).

5. The control method according to claim 4 and comprising the further step of carrying out, in the second instant (t.sub.2), a downshift to a lower gear in an autonomous manner and independently of an intervention of the driver, if in the second instant (t2) the rotation speed (ω.sub.E) of the internal combustion engine (4) is smaller than the increased value (VA).

6. The control method according to claim 1 and comprising the further step of carrying out, in the second instant (t.sub.2), a downshift to a lower gear in an autonomous manner and independently of an intervention of the driver, if in the second instant (t.sub.2) the rotation speed (ω.sub.E) of the internal combustion engine (4) is smaller than the increased value (VA).

7. The control method according to claim 1, wherein a duration of the time interval (TO) is variable.

8. The control method according to claim 1 and comprising the further steps of: measuring a pressing of a brake pedal (23); and changing a duration of the time interval (TO) based on the pressing of a brake pedal (23).

9. The control method according to claim 8, wherein the duration of the time interval (TO) is reduced as the pressing of the brake pedal (23) increases.

10. The control method according to claim 1, wherein the duration of the time interval (TO) is chosen in such a way that in the second instant (t.sub.2) the rotation speed (ω.sub.E) of the internal combustion engine (4) still exceeds the lower threshold (TH.sub.DOWN) and, hence, no downshift to a lower gear has been carried out yet since the first instant (t.sub.1).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described with reference to the accompanying drawings, showing a non-limiting embodiment thereof, wherein:

(2) FIG. 1 is a schematic plan view of a rear-wheel drive road vehicle provided with a drivetrain with a dual-clutch, servo-assisted transmission, which is controlled according to the control method of the invention;

(3) FIG. 2 is a schematic view of the drivetrain of FIG. 1; and

(4) FIGS. 3 and 4 show, according to two different embodiments, the time development of the rotation speed of an internal combustion engine during the execution of some downshifts following the release of the accelerator pedal.

PREFERRED EMBODIMENTS OF THE INVENTION

(5) In FIG. 1, number 1 indicates, as a whole, a road vehicle (in particular, a car) provided with two front driven (namely, non-drive) wheels 2 and with two rear drive wheels 3. In a front position there is an internal combustion engine 4, which is provided with a drive shaft 5, which produces a torque, which is transmitted to the drive wheels 3 by means of a drivetrain 6. The drivetrain 6 comprises a dual-clutch, servo-assisted transmission 7 arranged in the rear-wheel-drive assembly and a transmission shaft 8, which connects the drive shaft 5 to an input of the dual-clutch, servo-assisted transmission 7. The dual-clutch, servo-assisted transmission 7 is connected, in a train-like manner, to a self-locking differential 9, from which a pair of axle shafts 10 start, each integral to a drive wheel 3.

(6) The road vehicle 1 comprises a control unit 11 of the engine 4, which controls the engine 4, a control unit 12 of the drivetrain 6, which controls the drivetrain 6, and a BUS line 13, which is manufactured, for example, according to the CAN (Car Area Network) protocol, extends to the entire road vehicle 1 and allows the two control units 11 and 12 to communicate with one another. In other words, the control unit 11 of the engine 4 and the control unit 12 of the drivetrain 6 are connected to the BUS line 13 and, therefore, can communicate with one another by means of messages sent through the BUS line 13. Furthermore, the control unit 11 of the engine 4 and the control unit 12 of the drivetrain 6 can be directly connected to one another by means of a dedicated synchronization cable 14, which is capable of directly transmitting a signal from the control unit 12 of the drivetrain 6 to the control unit 11 of the engine 4 without the delays caused by the BUS line 13. Alternatively, the synchronization cable 14 could be absent and all communications between the two control units 11 and 12 could be exchanged using the BUS line 13.

(7) According to FIG. 2, the dual-clutch, servo-assisted transmission 7 comprises a pair of primary shafts 15, which are coaxial to one another, independent of one another and inserted inside one another. Furthermore, the dual-clutch, servo-assisted transmission 7 comprises two coaxial clutches 16, each designed to connect a respective primary shaft 15 to the drive shaft 5 of the internal combustion engine 4 through the interposition of the transmission shaft 8; each clutch 16 is an oil bath clutch and, hence, is pressure-controlled (i.e. the degree of opening/closing of the clutch 16 is determined by the pressure of the oil inside the clutch 16); according to an alternative embodiment, each clutch 16 is a dry clutch and, hence, is position-controlled (i.e. the degree of opening/closing of the clutch 16 is determined by the position of a movable element of the clutch 16). The dual-clutch, servo-assisted transmission 7 comprises one single secondary shaft 17 connected to the differential 9 that transmits the motion to the drive wheels 3; according to an alternative and equivalent embodiment, the dual-clutch, servo-assisted transmission 7 comprises two secondary shafts 17, both connected to the differential 9.

(8) The dual-clutch, servo-assisted transmission 7 has seven forward gears indicated with Roman numerals (first gear I, second gear II, third gear III, fourth gear IV, fifth gear V, sixth gear VI and seventh gear VII) and a reverse gear (indicated with R). The primary shaft 15 and the secondary shaft 17 are mechanically coupled to one another by a plurality of gear trains, each defining a respective gear and comprising a primary gear wheel 18 fitted on the primary shaft 15 and a secondary gear wheel 19 fitted on the secondary shaft 17. In order to allow for a correct operation of the dual-clutch, servo-assisted transmission 7, all odd gears (first gear I, third gear III, fifth gear V, seventh gear VII) are coupled to a same primary shaft 15, whereas all even gears (second gear II, fourth gear IV and sixth gear VI) are coupled to the other primary shaft 15.

(9) Each primary gear wheel 18 is splined to a respective primary shaft 15, so as to always rotate with the primary shaft 15 in an integral manner, and permanently meshes with the respective secondary gear wheel 19; on the other hand, each secondary gear wheel 19 is mounted on the secondary shaft 17 in an idle manner. Furthermore, the dual-clutch, servo-assisted transmission 7 comprises four synchronizers 20, each mounted coaxial to the secondary shaft 17, arranged between two secondary gear wheels 19 and designed to be operated so as to alternatively fit the two respective secondary gear wheels 19 to the secondary shaft 17 (i.e. so as to alternatively cause the two respective secondary gear wheels 19 to become angularly integral to the secondary shaft 17). In other words, each synchronizer 20 can be moved in one direction to fit a secondary gear wheel 19 to the secondary shaft 17 or can be moved in the other direction to fit the other secondary gear wheel 19 to the secondary shaft 17.

(10) The dual-clutch transmission 7 comprises one single secondary shaft 17 connected to the differential 9 that transmits the motion to the drive wheels 3; according to an alternative and equivalent embodiment, the dual-clutch transmission 7 comprises two secondary shafts 17, both connected to the differential 9.

(11) According to FIG. 1, the road vehicle 1 comprises a passenger compartment housing a driving position for the driver; the driving position comprises a seat (which is not shown), a steering wheel 21, an accelerator pedal 22, a brake pedal 23 and two paddle shifters 24 and 25, which control the dual-clutch, servo-assisted transmission 7 and are connected to the opposite sides of the steering wheel 21. The upshift paddle shifter 24 is operated by the driver (by means of a short pressure) in order to request an upshift (namely, the engagement of a new gear, which is higher than the current gear and contiguous with the current gear), whereas the downshift paddle shifter 25 is operated by the driver (by means of short pressure) in order to request a downshift (namely, the engagement of a new gear, which is lower than the current gear and is contiguous with the current gear).

(12) In use, the drivetrain 6 can operate in an automatic mode, namely the gear shifts are not requested by the driver through the paddle shifters 24 and 25, but are autonomously decided by the control unit 12 of the drivetrain 6 (simulating the behaviour of an automatic transmission). According to FIG. 3, when the drivetrain 6 operates in an automatic mode, the control unit 12 of the drivetrain 6 uses a lower threshold TH.sub.DOWN and an upper threshold TH.sub.UP: when the rotation speed ω.sub.E of the internal combustion engine 4 falls below the lower threshold TH.sub.DOWN, a downshift is carried out (namely, a new, lower gear is engaged) (in an autonomous manner and independently of an intervention of the driver), whereas, when the rotation speed ω.sub.E of the internal combustion engine 4 exceeds the upper threshold TH.sub.UP, an upshift is carried out (namely, a new, higher gear is engaged) (in an autonomous manner and independently of an intervention of a driver).

(13) Furthermore, the control unit 12 of the drivetrain 6 detects a release of the accelerator pedal 22 in an instant t.sub.1 (namely, the driver releases the accelerator pedal 22 in the instant t.sub.1) and waits, starting from the instant t.sub.1, a time interval TO (generally having a duration of some seconds, for example the time interval TO could last 2-6 seconds) until a second instant t.sub.2, which is subsequent to the instant t.sub.1 (and separated from the instant t.sub.1 by the time interval TO); finally, in the instant t.sub.2, the control unit 12 of the drivetrain 6 increases the value of the lower threshold TH.sub.DOWN until a following pressing of the accelerator pedal 22 (if in the second instant t.sub.2 the rotation speed ω.sub.E of the internal combustion engine 4 still exceeds the lower threshold TH.sub.DOWN, but, generally speaking, this condition is always met as the duration of the time interval TO is chosen so that it is not long enough to provoke a great slowing-down of the road vehicle 1). Namely, the lower threshold TH.sub.DOWN has a standard value (shown lower in FIG. 3) before the instant t.sub.2 and has an increased value VA (shown higher in FIG. 3), which is higher than the standard value VS, after the instant t.sub.2; by way of example, the standard value VS could be equal to 850-950 revolutions/minute, whereas the increased value could be equal to 1,400-1,600 revolutions/minute.

(14) The lower threshold TH.sub.DOWN is decreased and takes on again the standard value VS after a following pressing of the accelerator pedal 22; namely, a following pressing of the accelerator pedal 22 “resets” the lower threshold TH.sub.DOWN, which goes back to having the standard value.

(15) According to a preferred embodiment, if in the instant t.sub.2 the rotation speed ω.sub.E of the internal combustion engine 4 is greater than or equal to the increased value VA, in the instant t.sub.2 the lower threshold TH.sub.DOWN is brought to the increased value VA (according to FIG. 3); on the other hand, if in the instant t.sub.2 the rotation speed ω.sub.E of the internal combustion engine 4 is smaller than the increased value VA, in the instant t.sub.2 the lower threshold TH.sub.DOWN is brought to an extemporaneous value V* equal to the rotation speed ω.sub.E of the internal combustion engine 4 in the second instant t.sub.2 (according to FIG. 4) and, therefore, the extemporaneous value V* of the lower threshold TH.sub.DOWN is smaller than the increased value VA (in this case, again, a following pressing of the accelerator pedal 22 “resets” the lower threshold TH.sub.DOWN, which goes back to having the standard value.

(16) According to a preferred embodiment, the duration of the time interval TO is variable and is preferably variable depending on a pressing of the brake pedal 23; namely, the control unit 12 of the drivetrain 6 measures the pressing of the brake pedal 23 (for example, by detecting the pressure of the brake fluid in the hydraulic circuit of the braking system) and changes the duration of the time interval TO based on the pressing of the brake pedal 23. In particular, the duration of the time interval TO is reduced as the pressing of the brake pedal 23 increases, namely the greater the pressing of the brake pedal 23, the shorter the duration of the time interval TO.

(17) As already mentioned above, the duration of the time interval TO is chose so that it is not long enough to provoke a great slowing-down of the road vehicle 1 and, therefore, so that in the instant t.sub.2 (i.e. after the time interval TO has elapsed since the instant t.sub.1 in which the accelerator pedal 22 was released) the rotation speed ω.sub.E of the internal combustion engine 4 still exceeds the lower threshold TH.sub.DOWN and, hence, no downshift has been carried out yet since the instant t.sub.1 in which the accelerator pedal 22 was released.

(18) According to FIG. 3, until the instant t.sub.1 the accelerator pedal 22 is pressed (see the lower diagram, which shows the position GAS of the accelerator pedal 22) and, therefore, the rotation speed ω.sub.E of the internal combustion engine 4 increases until the instant t.sub.1. In the instant t.sub.1 the accelerator pedal 22 is released, then the control unit 12 of the drivetrain 6 waits, starting from the instant t.sub.1, the time interval TO until the instant t.sub.2 in which the value of the lower threshold TH.sub.DOWN is increased. In the following instants t.sub.3, t.sub.4 and is downshifts are automatically carried out each time the rotation speed ω.sub.E of the internal combustion engine 4 reaches the lower threshold TH.sub.DOWN (equal to the increased value VA). It should be pointed out that the lower threshold TH.sub.DOWN is equal to the standard value VS until the instant t.sub.2 and is equal to the increased value VA after the instant t.sub.2. If after the instant is the accelerator pedal 22 were pressed again, the lower threshold TH.sub.DOWN would go back to the standard value VS.

(19) According to FIG. 4, until the instant t.sub.1 the accelerator pedal 22 is pressed (see the lower diagram, which shows the position GAS of the accelerator pedal 22) and, therefore, the rotation speed ω.sub.E of the internal combustion engine 4 increases until the instant t.sub.1. In the instant t.sub.1 the accelerator pedal 22 is released, then the control unit 12 of the drivetrain 6 waits, starting from the instant t.sub.1, the time interval TO until the instant t.sub.2 in which the value of the lower threshold TH.sub.DOWN is increased; in the instant t.sub.2 the rotation speed ω.sub.E of the internal combustion engine 4 is smaller than the increased value and, therefore, in the instant t.sub.2 the lower threshold TH.sub.DOWN is brought to the extemporaneous value V*, which is equal to the rotation speed ω.sub.E of the internal combustion engine 4 in the instant t.sub.2 (and, hence, the extemporaneous value V* of the lower threshold TH.sub.DOWN is smaller than the increased value VA). As a result, a downshift is automatically carried out in the instant t.sub.2 (since the lower threshold TH.sub.DOWN is equal to the rotation speed ω.sub.E of the internal combustion engine 4 in the instant t.sub.2). In the following instants t.sub.3 and t.sub.4 downshifts are automatically carried out each time the rotation speed ω.sub.E of the internal combustion engine 4 reaches the lower threshold TH.sub.DOWN (equal to the extemporaneous value V* and smaller than the increased value VA). It should be pointed out that the lower threshold TH.sub.DOWN is equal to the standard value VS until the instant t.sub.2 and is greater (even though smaller than the increased value VA) after the instant t.sub.2. If after the instant t.sub.4 the accelerator pedal 22 were pressed again, the lower threshold TH.sub.DOWN would go back to the standard value VS.

(20) What disclosed above can be applied, with no significant changes, even when the drivetrain 6 of the road vehicle 1 is provided with a single-clutch, servo-assisted transmission.

(21) The control method described above has different advantages.

(22) First of all, the control method described above allows the threshold THD.sub.DOWN to be maintained very low (close to the minimum rpm), though avoiding the aforesaid “bouncing” between gears when the driver releases the accelerator pedal 22; indeed, the control method described above increases the lower threshold THD.sub.DOWN only when needed in order to avoid, by so doing, the aforesaid “bouncing” between gears when the driver releases the accelerator pedal 22.

(23) Furthermore, the control method described above controls the dual-clutch, servo-assisted transmission 7 in ways that are generally appreciated by drivers, who deem them to be “natural” (namely, corresponding to the drivers' expectations).

(24) Finally, the control method described above is easy and economic to be implemented as its execution requires a limited memory space and a reduced calculation ability.

LIST OF THE REFERENCE NUMBERS OF THE FIGURES

(25) 1 road vehicle 2 front wheels 3 rear wheels 4 engine 5 drive shaft 6 drivetrain 7 transmission 8 transmission shaft 9 differential 10 axle shafts 11 engine control unit 12 drivetrain control unit 13 BUS line 14 synchronization cable 15 primary shafts 16 clutches 17 secondary shaft 18 primary gear wheel 19 secondary gear wheel 20 synchronizers 21 steering wheel 22 accelerator pedal 23 brake pedal 24 upshift paddle shifter 25 downshift paddle shifter ω.sub.E rotation speed ω.sub.A rotation speed ω.sub.B rotation speed t.sub.1 time instant t.sub.2 time instant t.sub.3 time instant t.sub.4 time instant t.sub.5 time instant TO time interval GAS position of the accelerator pedal TH.sub.DOWN lower threshold TH.sub.UP upper threshold VS standard value VA increased value V* extemporaneous value