CONTROL SYSTEM FOR A CONTINUOUSLY VEHICLE TRANSMISSION IN A VEHICLE DRIVE SYSTEM

Abstract

Vehicle propulsion system (100), comprising: a drive machine (110) with a drive machine output shaft; a continuously variable transmission (140, 322) with a variator input shaft (51) coupled to the drive machine output shaft, and with a variator output shaft (61); a driver torque request module (326) in conjunction with a driver input and for outputting a driver torque request (416); a motor main train (304) in conjunction with the drive motor (110); and a transmission main train (302) in conjunction with the continuously variable transmission (140) and the engine main train (304) in the vehicle drive system (100), the transmission main train (302) comprising: a clutch filling and clutch pressure control module (316) that generates a positive torque request (412); and a monitor (338) for the positive torque request (412) which limits a torque request from the transmission main train (302) to the motor main train (304) to a maximum of a predetermined threshold value; wherein the positive torque request (412) is an amount of engine torque above an amount of engine torque requested by the drive motor (110) as a result of a driver torque request (416); where the torque request is a sum of the driver torque request (416) and the positive torque request (412); wherein the torque request results in a net engine torque (414) which provides a flat and jerk-free axle torque (408) in accordance with a driver axle torque request to enable smooth downshifting; and where the predetermined threshold prevents the application of additional protection algorithms that would negatively affect drivability.

Description

Technical area

[0001] The present disclosure relates to a control system for a continuously variable transmission in a vehicle drive system. Introduction

[0002] A vehicle propulsion system comprising a prime mover, such as an internal combustion engine, coupled to a continuously variable transmission (CVT) can be used to generate traction power in vehicles. A CVT is capable of varying an input / output speed ratio over a range between a minimum (downdrive) and a maximum (overdrive) ratio, allowing an infinitely variable selection of engine operation that achieves a desired balance of fuel consumption and engine power in response to a torque demand.

[0003] Common chain-type continuously variable transmissions may include two pulleys, each with two sheaves. A chain or belt runs between the two pulleys, with the two sheaves of each pulley sandwiching the chain. Frictional engagement between the sheaves of each pulley and the chain couples the chain to each pulley to transmit torque from one pulley to the other. One of the pulleys can function as the drive, or input, pulley, and the other pulley can function as the driven, or output, pulley. The gear ratio (also known as the torque ratio) is the ratio of the torque of the driven (output) pulley to the torque of the drive (input) pulley.The gear ratio can be changed by moving the two pulleys of one pulley closer together and the two pulleys of the other pulley farther apart, causing the chain to ride higher or lower on the corresponding pulley. The gear ratio can also be obtained by dividing a transmission input speed by a transmission output speed. The desired gear ratio can be determined based on a number of factors, including, by way of example and without limitation, driver pedal effort, vehicle speed, and the like.

[0004] JP 2012-026363 A describes a drive unit control device for shift control of a continuously variable transmission mechanism in a multi-stage shift mode. The drive unit control device calculates a maximum torque increase or decrease amount Tmax based on the operating state of an engine, which can be increased or decreased by engine torque increase control and engine torque decrease control. Furthermore, the control device calculates the moment of inertia Ti, which cancels out an increase or decrease in engine torque, from the maximum torque increase or decrease amount Tmax, and calculates the shift speed v1 of the continuously variable transmission at which the moment of inertia Ti is generated. Thus, the continuously variable transmission can be shifted at the maximum commandable shift speed in the current operating state.

[0005] It is an object of the invention to improve the drivability and smoothness of a downshift in a vehicle drive system with a continuously variable transmission while maintaining full protection, rationalization and security of torque values. Description of the invention

[0006] According to the invention, a vehicle propulsion system includes a prime mover having an engine output shaft, a continuously variable transmission having a variator input shaft coupled to the prime mover output shaft and a variator output shaft, a driver torque request module in communication with a driver input and for outputting a driver torque request, an engine main train in communication with the prime mover, and a transmission main train in communication with the continuously variable transmission and the engine main train in the vehicle propulsion system. The transmission main train includes a clutch fill and clutch pressure control module that generates a positive torque request, and a positive torque request monitor that limits a torque request from the transmission main train to the engine main train to a maximum of a predetermined threshold.The positive torque request is an amount of engine torque above an amount of engine torque requested by the prime mover as a result of a driver torque request. The torque request is a sum of the driver torque request and the positive torque request. The torque request results in a net engine torque that provides flat and smooth axle torque in accordance with a driver axle torque request to enable smooth downshifts. The predetermined threshold prevents the application of additional protection algorithms that would negatively impact drivability.

[0007] In this way, the drivability and smoothness of a downshift in a vehicle drive system with a continuously variable transmission can be improved while maintaining full protection, rationalization, and security of torque values. Monitoring a torque request in a transmission main train before passing the request to an engine main train improves responsiveness, accuracy, smoothness, and drivability. Furthermore, this eliminates the need for monitoring or screening in an engine main train and avoids the need to make torque requests that potentially exceed a driver-requested torque for a timer.

[0008] According to one embodiment, the clutch fill and clutch pressure control module determines the positive torque request based on engine acceleration and engine inertia.

[0009] In another embodiment, the clutch fill and clutch pressure control module determines the positive torque request during a decrease in the gear ratio of the continuously variable transmission such that a resultant torque at an axle of the vehicle drive system substantially matches a driver requested axle torque.

[0010] According to a further embodiment, the axle torque requested by the driver corresponds to the driver torque request.

[0011] According to another embodiment, the positive torque request monitor limits a torque request from the transmission mainline to the engine mainline to a maximum of a predetermined threshold from a table of predetermined thresholds.

[0012] According to another embodiment, the positive torque request monitor determines the predetermined threshold from the table based on an engine acceleration and a continuously variable transmission gear ratio.

[0013] Further areas of applicability of the present disclosure will become apparent from the detailed description provided below. It should be understood that the detailed description and specific examples are provided for purposes of illustration only and are not intended to limit the scope of the disclosure.

[0014] The above features and advantages, as well as other features and advantages of the invention, are readily apparent from the following detailed description, including the claims and embodiments, when taken in conjunction with the accompanying drawings. Brief description of the drawings

[0015] The present disclosure will become more fully understood with the aid of the detailed description and the accompanying drawings, in which: Fig. 1 schematically shows elements of a vehicle drive system 100; Fig. Figure 2 schematically shows elements of a variator 30 of a chain-type continuously variable transmission (CVT); Fig. 3 schematically illustrates an exemplary control system 300 of a vehicle propulsion system according to the present disclosure; and Fig. 4 shows a diagram of signals from a continuously variable transmission being downshifted according to an exemplary embodiment of the present disclosure.

[0016] In the drawings, the same reference numerals are used for similar and / or identical elements. Detailed description

[0017] Referring to the drawings, in which the representations are for illustrating certain exemplary embodiments and not for the purpose of limiting the same, Fig. 1 schematically illustrates elements of a vehicle propulsion system 100 including a prime mover 110 rotatably connected to a continuously variable transmission (CVT) 140 via a torque converter 120 and a transmission housing 130. The vehicle propulsion system 100 is connected to a vehicle wheel 160 via a transmission 150 for generating traction when used in a vehicle. The operation of the vehicle propulsion system 100 is monitored and controlled by a control system 10 in response to driver commands and other factors.

[0018] The prime mover 110 may, for example, be an internal combustion engine, a motor, or other system, without limitation, capable of producing torque in response to commands from the control system 10. The torque converter 120 may provide fluid coupling between its input and output elements for transmitting torque, and may preferably include a pump 122 coupled to the prime mover 110, a turbine 124 coupled to the transmission 130 via the input element, and a torque converter clutch 126 that locks rotation of the pump 122 and the turbine 124 and is controllable by the control system 10.The output member of the torque converter 120 is rotatably coupled to the transmission case 130, which may include meshing gears or other suitable gearing mechanisms providing a reduction ratio between the torque converter 120 and the CVT 140. Alternatively, the transmission 130 may be any other suitable gear configuration for establishing a gear ratio between the engine 110, the torque converter 120, and the CVT 140, including, by way of non-limiting example, a chain or planetary gear configuration. In alternative embodiments, the torque converter 120 and the transmission case 130 may be omitted.

[0019] The transmission case 130 may include an output member rotatably connected to the CVT 140 via an input member 51. An exemplary embodiment of the CTV 140 is described with reference to Fig. 2. An output member 61 of the CVT 140 is rotatably connected to the driveline 150, which is rotatably connected to the vehicle wheels 160 via an axle, halfshaft, or other suitable torque-transmitting member. The driveline 150 may include a differential, a chain drive, or other suitable gear arrangement for transmitting torque to one or more vehicle wheels 160.

[0020] The vehicle propulsion system 100 preferably includes one or more sensor devices for monitoring speeds of various devices, including, for example, an engine speed sensor 112, a torque converter turbine speed sensor 125, a CVT variator input speed sensor 32, a CVT variator output speed sensor 34, and a wheel speed sensor 162 for monitoring vehicle speed (Vss). Each of the aforementioned speed sensors may be any suitable rotational position / speed measuring device, such as a Hall-effect sensor. Each of the aforementioned speed sensors is in communication with the control system 10.

[0021] The control system 10 preferably includes one or a plurality of controllers 12 and a user interface 14. A single controller 12 is shown for simplicity of illustration. The control unit 12 may include a plurality of control units, each of which may be associated with a controller 12 for monitoring and controlling a single system. This may include an engine control module (ECM) for controlling the engine 110 and a transmission control module (TCM) for controlling the CVT 140 and for monitoring and controlling a single subsystem, such as a torque converter clutch. The controller 12 preferably includes a memory device 11 containing executable instruction sets. The user interface 14 is in communication with user input devices, including, for example, an accelerator pedal 15, a brake pedal 16, and a gearshift 17.

[0022] Fig. Figure 2 schematically shows elements of a variator 30 of a chain-type continuously variable transmission (CVT) 140, which is advantageously controlled by a controller 12. The variator 30 transmits torque between the first rotating element 51 and the second rotating element 61. The first rotating element 51 may be referred to as the input element 51, and the second rotating element 61 may be referred to as the output element 61.

[0023] The variator 30 includes a first or primary pulley 36, a second or secondary pulley 38, and a flexible continuously rotatable device 40 rotatably connected to the first and second pulleys 36, 38 for transmitting torque therebetween. The first pulley 36 is rotatably connected to the input member 51, and the second pulley 38 is rotatably connected to the output member 61. The rotatable device 40 is adapted to transmit torque between the first and second pulleys 36, 38, and thus between the input and output members 51, 61. The first pulley 36 and input member 51 rotate about a first axis 48, and the second pulley 38 and output member 61 rotate about a second axis 46. The continuously rotatable device 40 may be a belt, a chain, or other suitable flexible continuous device.

[0024] The first pulley 36 is sectioned perpendicular to the first axis 48 to define a first annular groove 50 formed between a movable pulley 52 and a stationary pulley 54. The movable pulley 52 moves or is transmitted along the first axis 48, which is located opposite the stationary pulley 54. For example, the first movable pulley 52 may be connected to a drive member 51 via a spline connection, thereby enabling axial movement of the first movable pulley 52 along the first axis 48. The first stationary pulley 54 is disposed opposite the first movable pulley 52. ​​The first stationary pulley 54 is axially connected to the drive member 51 along the first axis 48. As such, the stationary first pulley 54 does not move in the axial direction of the first axis 48.The first movable pulley 52 and the first stationary pulley 54 each include a first groove surface 56. The first groove surfaces 56 of the movable first pulley 52 and the fixed first pulley 54 are disposed opposite each other to define the first annular groove 50 therebetween. The opposed first groove surfaces 56 preferably form an inverted frustoconical shape such that movement of the movable first pulley 52 toward the fixed first pulley 54 increases an outer pulley diameter of the annular first groove 50. An actuator 55 is arranged such that the first pulleys 36 control an axial position of the first movable pulley 52 in response to a control signal 53, including compressing the first movable pulley 52 toward the first stationary pulley 54.In one embodiment, the actuator 55 is a hydraulically controlled device and the control signal 53 is a hydraulic pressure signal.

[0025] The second pulley 38 is sectioned perpendicular to the second axis 46 to form a second annular groove 62 therebetween. The annular second groove 62 is arranged perpendicular to the second axis 46. The second pulley 38 includes a movable pulley 64 and a fixed pulley 66. The movable pulley 64 moves toward an axis or is moved along the second axis 46 that is opposite the fixed pulley 66. For example, the movable second pulley 64 may be connected to the output member 61 via a spline connection, thereby enabling axial movement of the second movable pulley 64 along the second axis 46. The second fixed pulley 66 is arranged opposite the second movable pulley 64. The second fixed pulley 66 is axially connected to the output member 61 along the second axis 46.As such, the fixed second pulley 66 does not move in the direction of the second axis 46. The movable second pulley 64 and the fixed second pulley 66 each include a second groove surface 68. The second groove surface 68 of the movable second pulley 64 and the fixed second pulley 66 are disposed opposite one another with the second annular groove 62 located therebetween. The second opposite groove surface 68 is preferably an inverted frusto-conical shape such that movement of the second movable pulley 64 toward the second stationary pulley 66 increases an outer pulley diameter of the second annular groove 62.An actuator 65 is arranged with the second pulley 38 to control an axial position of the second movable pulley 64 in response to a driven signal 63, including compressing the second movable pulley 64 toward the second stationary pulley 66. In one embodiment, the actuator 65 is a hydraulically controlled device and the control signal 63 is a hydraulic pressure signal. A ratio between the outer diameter of the first pulley 36 and the outer diameter of the second pulley 38 defines a speed ratio (SR). Other elements, such as clutch assemblies in the form of selectable one-way clutches and the like, may be employed between the variator 30 and other vehicle drive systems and powertrain components and systems.

[0026] Various sensors are suitably positioned to detect and provide signals for operating the CVT 140, including the CVT variator input speed sensor 32 and the CVT variator output speed sensor 34. The input speed sensor 32 may be mounted near the input member 51 to generate an input speed signal 33, and the CVT variator output speed sensor 34 may be mounted near the output member 61 to generate an output speed signal 35.

[0027] The variator speed ratio (SR) is the ratio of the speed of the output member 61 to the speed of the input member 51. Forms of the SR may be employed as a control variable for the CVT 140, including an actual SR and a desired SR. The actual SR indicates a present measured value for the SR and may be determined based on a ratio of the input speed signal 33 and the output speed signal 35. The desired SR may correspond to a commanded, future value for the SR, which may be determined without limitation based on monitored and estimated operating conditions related to an output power command, vehicle speed, engine torque, and the like. The controller 12 controls the CVT 140 to achieve the desired SR by controlling pressure from one or both of the primary pulley 36 and the secondary pulley 38 of the CVT 140.The pressure from one or both of the primary pulley 36 and the secondary pulley 38 of the CVT 140 may be achieved by controlling the drive and driven signals 53, 63 to apply the required pressure to the first and second actuators 55, 65 for the desired SR, wherein the required pressures preferably take the form of a primary pressure command and a secondary pressure command.

[0028] Fig. Figure 3 schematically illustrates an exemplary control system 300 for a vehicle propulsion system according to the present disclosure. System 300 includes a transmission main train 302 in communication with an engine main train 304 via a CAN bus 306. In an exemplary embodiment, transmission main train 302 may include components and be operable in a manner similar to that described in co-pending and assigned U.S. patent application Ser. No. 15 / 596,219, the disclosure of which is incorporated herein in its entirety, to control the gear ratio of a continuously variable transmission.

[0029] In the exemplary embodiment in Fig. 3, the transmission mainline 302 may receive a driver acceleration pedal position signal 308 and a transmission output shaft speed signal 310. A shift point module 312 may determine a control pattern or trajectory of gear ratios, and a sequencing state machine 314 may determine a history of these gear ratios. A clutch shift control module 316, a clutch fill and pressure module 318, and a hardware input / output module 320 may convert these gear ratios into control signals that may be sent to a continuously variable transmission 322, which may then be operated accordingly in response to these signals.

[0030] The clutch fill and pressure control module 316 also responsively determines an amount of torque that may be required to implement a smooth shift. This amount of shift may be generically referred to as a positive torque request (PTR). The clutch fill and pressure control module 316 communicates the PTR to a transfer torque arbitration module 324. The transfer torque arbitration module 324 may arbitrate the PTR and make it available to the CAN bus 306 for access by the engine mainline 304.

[0031] The engine main train 304 may also receive a driver acceleration pedal position signal 308 and a transmission output shaft speed signal 310 in a driver torque request module 326. The driver torque request module 326 may provide a driver requested amount of axle torque, T ax1An axle torque arbitration module 328 may receive the driver-requested axle torque and arbitrate this value based on a number of factors and operating conditions, and further convert the arbitrated value from an axle torque value to an engine torque value based on the gear ratio. A drive torque arbitration module 330 may then further arbitrate the torque request based on a positive torque request received from the transmission mainline 302 over the CAN bus and generate an engine torque request to be provided to an engine torque control module 332.The engine torque control module 332 may then generate a set of command signals that are passed to a set of actuators 334 in the engine, such as, for example, an ignition control module, fuel control module, and the like, without limitation, such that the engine responds in a particular manner to deliver the requested amount of engine torque.

[0032] As explained above, the clutch fill and pressure control module 316 responsively determines an amount of torque that may be required to implement a smooth shift. For example, in the case of a downshift, when the speed ratio is reduced, because the engine and associated driveline are always connected to the vehicle's wheels via the continuously variable transmission, the engine speed must increase. This requires the engine and driveline to accelerate rotationally. Because the engine has rotational inertia, this acceleration consumes an amount of torque, which can generally be referred to as moment of inertia.

[0033] Fig. 4 shows a diagram of signals from a continuously variable transmission undergoing a downshift according to an exemplary embodiment of the present disclosure. A speed ratio signal 400 indicates a decrease in the CVT speed ratio during the downshift. As the CVT speed ratio 400 decreases, the engine speed 402 must increase accordingly. An engine acceleration 404 corresponds to the rate of change of the engine speed 402. Since the engine has rotational inertia, the amount of torque consumed to accelerate the engine and maintain the engine speed 402 can be determined.This problem is represented by the axle torque line 406, which indicates an amount of axle torque that is less than the driver requested axle torque 408 by an amount 410 indicated in line 408, corresponding to the moment of inertia consumed by the acceleration 404 of the engine speed 402.

[0034] In an ideal situation, the amount of torque at the axle corresponds to the driver's requested axle torque 408, resulting in a torque amount at the axle that remains essentially balanced throughout the downshift. This allows for a smooth downshift.

[0035] The axle torque and motor torque have corresponding values ​​in each domain, the axle and the motor. The amount of additional axle torque 410 required to compensate for the motor acceleration has a corresponding value of motor torque 412, which may be referred to as a positive torque request (PTR). The PTR 412 is an amount of motor torque above an amount of motor torque requested from the motor as a result of a driver request for axle torque. The driver requested motor torque 416 plus PTR 412 results in a net motor torque 414 that provides a flat or smooth axle torque 408 in accordance with a driver axle torque request.

[0036] With reference to Fig. 3, it is important to ensure that the amount of torque provided to the axle does not exceed a predetermined amount. The transmission main train 304 typically includes a torque request monitor 336, which serves to monitor the amount of torque requested from the engine through the transmission main train 302 via the CAN bus 306. One purpose of this monitor is to ensure that any errors, corruption, and / or flaws in the transmission main train 302 in generating and providing this torque request to the engine main train 304 are corrected and not propagated. Conventionally, this torque request monitor 336 determines whether the torque request exceeds a predetermined threshold. The predetermined threshold may be based on a torque request amount, resulting in a vehicle acceleration of 1.962 m / s 2above the driver-requested acceleration. While the present disclosure provides exemplary specific predetermined thresholds, it should be understood that the control system and method is not limited to a specific predetermined threshold. If the torque request were to exceed the threshold, then monitor 336 would apply one or more timers. For example, torque request monitor 336 would determine whether a gear ratio change began within 200 milliseconds and, if so, whether the gear ratio change was completed within 900 milliseconds. In a conventional system that generates a torque request that exceeds these conditions, the request for additional torque would be discarded entirely.This would result in the “torque hole,” represented by the dip in axle torque 406, being below the driver requested torque 408, as shown in . Fig. 4. The result is a downshift in which an undesirable lack of power or torque occurs, resulting in discomfort for the driver. In response, the transmission mainline 302 would simply attempt to generate another positive torque request to fill the torque gap; the conventional torque request monitor 336 may again discard this torque if it exceeds a predetermined threshold for more than a predetermined time. Thus, the downshift would not be smooth, and drivability would suffer.

[0037] Another problem with these conventional systems is the inherent communication delay over the CAN bus. This communication delay means that any information relating to a shift or change in the transmission requires time before this shift or change is communicated to the engine mainframe. At that point, it may be too late or no longer relevant to attempt to make an adjustment in the engine mainframe in response. The system simply would not react quickly enough to the changes in the gear ratio. Furthermore, to accommodate this communication delay, the transmission mainframe may be forced to make adjustments to the ratio change strategy so that the engine mainframe may react more quickly to a delay in accepting a transmission torque request. In either situation, the delay and responsiveness remain an issue.

[0038] In contrast to conventional systems and methods, in the exemplary embodiment 300 of Fig. 3, the transmission mainline 302 includes a positive torque request monitor 338 412. The positive torque request monitor 338 412 ensures that the positive torque request never reaches the predetermined threshold, which would otherwise require the application of timers and reset commands. In an exemplary embodiment, the positive torque request monitor 338 412 may include a table of predetermined torque limits for each set of engine accelerations and speed ratios, ensuring that the positive torque request provided to the engine mainline 304 from the transmission mainline 302 via the CAN bus 306 never results in a torque value that would otherwise exceed a predetermined threshold associated with a set of timers.In an alternative exemplary embodiment, the positive torque request monitor 338 412 may include a processor that, in combination with an instruction set, may utilize an equation or algorithm to determine an appropriate predetermined torque limit or limits. The positive torque request monitor 412 may allow a torque request equal to the sum of the driver requested torque and the positive torque request, but not exceeding the sum of the driver requested torque and the predetermined torque limit, corresponding to the engine acceleration and gear ratio, which may be directly communicated to the engine mainline 304 and requires no further control and / or monitoring.In this manner, a torque request that is secured, protected, and rationalized is generated at the transmission main train 302 and, thus, the torque request is never timed, reset, or discarded by the engine main train 304.

[0039] The predetermined torque limits in the table in the positive torque request monitor 338 for the transmission mainline 302 412 may be calculated and / or determined by a calibration process that provides torque limits for each of a plurality of sets of engine accelerations and gear ratios. In an exemplary embodiment, these predetermined torque limits in the positive torque request monitor 338 for the transmission mainline 302 412 may be less than a torque request magnitude that results in a vehicle acceleration of 1.962 m / s 2would exceed the acceleration requested by the driver. With further reference to Fig. 4, the sum of the driver requested torque and the predetermined torque limit from the monitor table results in an engine torque limit 418. In the event that the transmission mainline 302 generates a positive torque request, as indicated at 420, that exceeds the engine torque limit 418, then the table in the monitor would set the torque request to the engine torque limit 418 and not above it. In this example, the axle torque corresponding to the engine torque in Fig. 4, in which the engine torque limit 418 corresponds to an axle torque limit 422, and the axle torque 424 would increase in this example but not exceed the axle torque limit 422. In this way, the exemplary embodiment ensures that enough additional torque (positive torque demand) is provided to enable a smooth downshift, but not so much as to trigger the application of a number of additional protection algorithms that would otherwise negatively impact drivability. Generally, these control algorithms, which might have been generated if only a conventional clutch-to-clutch transmission was contemplated, and not a continuously variable transmission, need not be applied at all.Continuously variable transmissions change gear ratios in a completely different way than clutch-to-clutch transmissions, and can change the gear ratio over a longer period of time, which could easily cause these timers to be exceeded in control monitors designed with clutch-to-clutch transmissions in mind. In contrast, conventional positive torque monitors may allow engine torque to exceed a predetermined limit, but limit the amount of time the torque exceeds the limit. Thus, in conventional systems, engine torque may actually exceed the value indicated at 420 in [number of revolutions per second]. Fig. 4. In fact, conventional torque monitors may not have limited the torque demand to any maximum value at all. In a clutch-to-clutch system, this might be acceptable if the transmission is temporarily decoupled from the engine during a ratio change, as the connecting clutches do not have sufficient torque capacity to transfer excess torque to the axle. In contrast, a continuously variable transmission is unlikely to be decoupled from the engine during a ratio change.

[0040] In the exemplary embodiment in Fig. 3, the positive torque request monitor 338 412 is connected to a first switch 340 in the transmission main train 302 and the positive torque monitor 336 in the engine main train 304 is connected to a second switch 342. In the illustrated configuration of Fig.3, switch 340 bypasses positive torque request monitor 412 in transmission mainline 302, which then requires second switch 342 to connect positive torque monitor 336 in engine mainline 304. To ensure proper torque request control, at least one of torque monitors 338 and 336 must not be bypassed. Preferably, according to this disclosure, first switch 340 ensures that positive torque monitor 336 is used in transmission mainline 302, and second switch 342 bypasses positive torque monitor 336 in engine mainline 304. In this manner, all of the benefits made possible by the exemplary embodiment of the present disclosure are achieved without extensive modification of existing systems to incorporate the features of the present disclosure.

Claims

[1] Vehicle drive system (100) comprising: a prime mover (110) having a prime mover output shaft; a continuously variable transmission (140, 322) having a variator input shaft (51) coupled to the engine output shaft and a variator output shaft (61); a driver torque request module (326) in communication with a driver input and for outputting a driver torque request (416); a main engine train (304) in connection with the drive engine (110); and a transmission main train (302) in communication with the continuously variable transmission (140) and the engine main train (304) in the vehicle drive system (100), the transmission main train (302) comprising: a clutch fill and clutch pressure control module (316) that generates a positive torque request (412); and a positive torque request monitor (338) (412) that limits a torque request from the transmission mainline (302) to the engine mainline (304) to a maximum of a predetermined threshold; wherein the positive torque request (412) is an amount of engine torque above an amount of engine torque requested by the prime mover (110) as a result of a driver torque request (416); wherein the torque request is a sum of the driver torque request (416) and the positive torque request (412); wherein the torque request results in a net engine torque (414) that provides a flat and smooth axle torque (408) in accordance with a driver axle torque request to enable a smooth downshift; and wherein the predetermined threshold prevents the application of additional protection algorithms that would have a negative impact on drivability. [2] The vehicle propulsion system (100) of claim 1, wherein the clutch fill and clutch pressure control module (316) determines the positive torque request (412) based on an engine acceleration (404) and an engine inertia. [3] The vehicle propulsion system (100) of claim 2, wherein the clutch fill and clutch pressure control module (316) determines the positive torque request (412) during a decrease in the speed ratio of the continuously variable transmission (140, 322) such that a net torque (414) at an axle of the vehicle propulsion system (100) matches a driver requested axle torque (408). [4] The vehicle propulsion system (100) of claim 3, wherein the driver requested axle torque (408) corresponds to the driver torque request (326). [5] The vehicle propulsion system (100) of claim 1, wherein the positive torque request monitor (338) (412) limits a torque request from the transmission mainline (302) to the engine mainline (304) to a maximum of a predetermined threshold from a table of predetermined thresholds. [6] The vehicle propulsion system (100) of claim 5, wherein the positive torque request monitor (412) determines the predetermined threshold from the table based on an engine acceleration and a gear ratio of the continuously variable transmission (140, 322).

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