EP4630709A1

EP4630709A1 - Vehicle powertrain, method and vehicle

Info

Publication number: EP4630709A1
Application number: EP23794109.1A
Authority: EP
Inventors: Richard Heindl; Josef Mayer
Original assignee: AGCO International GmbH
Current assignee: AGCO International GmbH
Priority date: 2022-12-08
Filing date: 2023-10-12
Publication date: 2025-10-15
Also published as: WO2024121634A1; GB202218505D0

Abstract

A vehicle powertrain has an electric motor and a hydraulic-mechanical power split transmission. The transmission is operable a first transmission mode in which drive is transmitted via both mechanical and hydrostatic transmission branches with a variable transmission ratio and a second transmission mode in which drive is transmitted only through the mechanical transmission branch at a constant transmission ratio. For vehicle speeds up to a first value V1, the powertrain is operated in a first powertrain mode in which the transmission is operated in the first transmission mode and the rotational speed of the motor does not exceed a first motor speed value N1. For vehicle speeds above V1, the powertrain is operated in a second powertrain mode in which the transmission is operated in the second transmission mode and the motor speed permitted to exceed the first motor speed value N1 up to a maximum motor speed N_max.

Description

TITLE

VEHICLE POWERTRAIN, METHOD AND VEHICLE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

FIELD

[0002] Embodiments of the present disclosure relate generally to a vehicle powertrain including an electric prime mover, especially an electric motor, and a continuously variable transmission and to a method for controlling such a powertrain. The powertrain and method in accordance with the disclosure are adaptable for use with a variety of configurations of commercial and utility vehicles, such as trucks, construction vehicles, plant machinery, and agricultural vehicles including agricultural tractors.

BACKGROUND

[0003] It is known to use an electric motor as a prime mover in vehicles and to use a battery as the source of electric power. The storage of energy in a battery generally requires greater volume and is heavier than an equivalent fuel source for an internal combustion engine, such as a diesel engine. This is a particular issue for larger and utility vehicles, such as tractors, where the prime mover is required to deliver high levels of power and where available space for locating an electrical power storage/supply is limited. Similar considerations apply to electric vehicles where an electric power supply is provided in other ways, such as where an electric power supply is provided by a fuel cell system and the storage is a hydrogen reservoir, for example.

[0004] The primary purpose of a vehicle transmission is to transmit torque from the prime mover to driven ground engaging members, such as wheels or tracks, so that the vehicle can be moved across the ground in a controlled manner. Many different transmission configurations are known which can be adapted to the needs of a specific type of vehicle, for example whether the vehicle is rear axle drive, front axle drive, twin axle drive (e.g., four-wheel drive), front wheel steered, centrally articulated, or track steered. The transmission may also provide drive to front and/or rear power take off (PTO) shafts to allow the vehicle to operate ancillary equipment, e.g., for driving agricultural implements in the case of a tractor.

[0005] In addition to a prime mover and transmission, a typical powertrain may include a flywheel; one or more clutches; a transfer box; and at least one of a front, centre, or rear axle final drive/differential.

[0006] Utility vehicles with infinitely or continuous variable transmissions (CVT) are well known. Modern examples often employ a power-split CVT in which power is divided between a hydrostatic branch with variable ratio and a mechanical branch with a fixed ratio, the power being recombined at, or upstream of, one or more output shafts. Power-split CVT transmissions enable, to an extent at least, the vehicle speed (i.e., the speed of the vehicle across the ground) to be adjusted independently of the rotary speed of the prime mover so that the prime mover can be operated at an efficient rotary speed regardless of the vehicle speed. Examples of or these types of transmissions are described in European Patent Publication EP1273828 A2, published 8^th January 2003, by AGCO GmbH & Co. and German Patent Publication DE 10 2007 021 733 Al, published 13^th November 2008, by AGCO GmbH.

[0007] CVTs generally cover the full range of torque/speed conversion of the vehicle and so must be engineered accordingly.

BRIEF SUMMARY

[0008] Aspects of the invention relate to a vehicle powertrain having a electric prime mover and a hydraulic-mechanical power-split transmission, to a vehicle having such a powertrain, especially a utility vehicle such as an agricultural tractor, and to methods of operating such a powertrain.

[0009] In an aspect of the invention, there is provided a vehicle powertrain including an electric prime mover, especially an electric motor, and a hydrostatic-mechanical power split transmission having a mechanical transmission branch and a hydrostatic transmission branch for driving the vehicle, the transmission being operable in two transmission modes, a first transmission mode in which drive is transmitted via both the mechanical and hydrostatic transmission branches with a variable transmission ratio icvr and a second transmission mode in which drive is transmitted only through the mechanical transmission branch at a constant transmission ratio icvr; and a control system for controlling operation of the electric prime mover and the transmission; wherein the powertrain is configured to be selectively operable in two powertrain modes: a first powertrain mode in which the transmission is operated in the first transmission mode and the rotational speed of the electric prime mover does not exceed a first prime mover speed value Nl, and a second powertrain mode in which the transmission is operated in the second transmission mode and the rotational speed of the electric prime mover is permitted to exceed the first prime mover speed value Nl.

[0010] Since power is only transmitted through the hydrostatic transmission branch at prime mover speeds up to the first prime mover speed value Nl, the components of the hydrostatic branch do not have to be capable of transmitting drive (torque/power) at the higher prime move speeds available in the second powertrain mode and so lower cost and lighter hydrostatic units can be adopted.

[0011] The transmission may comprise an input shaft drivable by the electric prime mover, at least one output drive shaft for driving at least one driven ground engaging member of the vehicle, the mechanical transmission branch configured to transfer drive mechanically from the input shaft to the at least one output drive shaft, the hydrostatic transmission branch having at least one variable displacement hydrostatic pump driven from the input shaft and at least one hydrostatic motor for driving the at least one output drive shaft. The at least one hydrostatic motor may be a variable displacement hydrostatic motor. The hydrostatic pump and the at least one hydrostatic motor are fluidly connected.

[0012] The powertrain may be configured to be operated in the first powertrain mode for vehicle speeds between zero (standstill) and a first vehicle speed value VI and to be operated in the second powertrain mode for vehicle speeds above the first vehicle speed value VI. The first vehicle speed value VI may optionally be about 1/3 of a maximum vehicle speed.

[0013] The control system may comprise at least one controller configured to receive inputs corresponding to vehicle speed and to rotational speed of the electric prime mover and to generate output signals for controlling the speed of the electric prime mover and operation of the transmission.

[0014] The powertrain may be configured to enable the electric prime mover to operate at rotational speeds up to a maximum prime mover speed Nmaxin the second powertrain mode, the first prime mover speed value N1 being lower than the maximum prime mover speed Nmax.

[0015] The electric prime mover may be operable in a range of speeds from a minimum prime mover speed N_min up to the maximum prime mover speed N_max, the powertrain configured such that the electric prime mover is constrained to operate in a first speed range from and including the minimum prime mover speed N_min up to and including the first prime mover speed value N1 in the first powertrain mode and in a second speed range above the first prime mover speed value N1 up to the maximum prime mover speed Nmax in the second powertrain mode.

[0016] The first prime mover speed value N1 may be no more than 70% of the maximum prime mover speed Nmax, or it may be no more than 60% of the maximum prime mover speed Nmax, or it may be no more than 50% of the maximum prime mover speed Nmax, or it may be no more than 40% of the maximum prime mover speed Nmax.

[0017] The electric prime mover may be an AC electric motor and the control system may comprise a Variable Frequency Drive operable to regulate the electrical power supply to the electric motor so as to control the rotational speed and torque of the electric motor.

[0018] The transmission may comprise a planetary gear assembly and be configured as a planetary output coupled type power-split transmission. In this case, the transmission may include a brake arrangement selectively operable to prevent the at least one hydrostatic pump being driven when the powertrain is operating in the second powertrain mode and/or a clutch arrangement selectively operable to drivingly decouple the at least one hydrostatic motor from the mechanical transmission branch when the powertrain is operating in the second powertrain mode. The hydrostatic pump may be driven from a ring gear of the planetary gear assembly and the brake may be operative in the drive line between the ring gear and an input shaft of the hydrostatic pump. [0019] The transmission may comprise a planetary gear assembly and be configured as a planetary input coupled type power-split transmission. In this case, the transmission may include a brake arrangement selectively operable to prevent the at least one hydrostatic motor being driven from the mechanical transmission branch when the powertrain is operating in the second powertrain mode and/or a clutch arrangement selectively operable to drivingly decouple the at least one hydrostatic pump from the input shaft when the powertrain is operating in the second powertrain mode. The at least one hydrostatic motor may be drivingly coupled to a ring gear of the planetary gear assembly and the brake may be operative in the drive line between an output shaft of the at least one hydrostatic motor and the ring gear.

[0020] The input shaft to the transmission may be an output shaft of the electric prime mover.

[0021] In an aspect of the invention, there is provided a vehicle powertrain including an electric prime mover, especially an electric motor, and a hydrostatic-mechanical power split transmission, the transmission comprising an input shaft drivable by the electric prime mover, at least one output drive shaft for driving at least one driven ground engaging member of the vehicle, a mechanical transmission branch configured to transfer drive mechanically from the input shaft to the at least one output drive shaft, a hydrostatic transmission branch having at least one variable displacement hydrostatic pump driven from the input shaft and at least one hydrostatic motor for driving the at least one output drive shaft, the transmission being operable in two transmission modes, a first transmission mode in which drive is transmitted via both the mechanical and hydrostatic transmission branches with a variable transmission ratio icvr and a second transmission mode in which drive is transmitted only through the mechanical transmission branch at a constant transmission ratio icvr; and a control system for controlling operation of the electric prime mover and the transmission; wherein the powertrain is configured to be selectively operable in two powertrain modes: a first powertrain mode in which the transmission is operated in the first transmission mode in which the rotational speed of the electric prime mover is independent of the vehicle speed and is no more than a first prime mover speed value Nl, which first prime mover speed value Nl is less than a maximum prime mover speed N_max, and a second powertrain mode in which the transmission is operated in the second transmission mode in which the rotational speed of the electric prime mover is proportional to the vehicle speed and is permitted to exceed the first prime mover speed value Nl.

[0022] The input shaft to the transmission may be an output shaft of the electric prime mover.

[0023] The electric prime mover may be an AC electric motor and the control system may comprise a Variable Frequency Drive operable to regulate the electrical power supply to the electric motor so as to control the rotational speed and torque of the electric motor.

[0024] In an aspect of the invention, there is provided a vehicle having a powertrain according to either of the previous aspects of the invention set out above. The vehicle may be a utility vehicle and may be an agricultural vehicle such as a tractor.

[0025] In an aspect of the invention, there is provided a method of operating a vehicle powertrain including an electric prime mover, especially an electric motor, and a hydrostaticmechanical power split transmission having a mechanical transmission branch and a hydrostatic transmission branch for driving the vehicle, the transmission being operable in two transmission modes, a first transmission mode in which drive is transmitted via both the mechanical and hydrostatic transmission branches with a variable transmission ratio icvr and a second transmission mode in which drive is transmitted only through the mechanical transmission branch at a constant transmission ratio icvr; and a control system for controlling operation of the electric prime mover and the transmission; wherein the method comprises selectively operating the powertrain in one of a first powertrain mode in which the transmission is operated in the first transmission mode and the rotational speed of the electric prime mover does not exceed a first prime mover speed value Nl, and a second powertrain mode in which the transmission is operated in the second transmission mode and the prime mover is permitted to operate at rotational speeds which exceed the first prime mover speed value Nl.

[0026] Since power is only transmitted through the hydrostatic transmission branch at prime mover speeds up to the first prime mover speed value Nl, the components of the hydrostatic branch do not have to be capable of transmitting drive at the higher prime move speeds available in the second powertrain mode and so lower cost and lighter hydrostatic units can be adopted.

[0027] The transmission may comprise an input shaft drivable by the electric prime mover, at least one output drive shaft for driving at least one driven ground engaging member of the vehicle, the mechanical transmission branch configured to transfer drive mechanically from the input shaft to the at least one output drive shaft, the hydrostatic transmission branch having at least one variable displacement hydrostatic pump driven from the input shaft and at least one hydrostatic motor for driving the at least one output drive shaft. The at least one hydrostatic motor may be a variable displacement hydrostatic motor. The hydrostatic pump and the at least one hydrostatic motor are fluidly connected.

[0028] The method may comprise operating the powertrain in the first powertrain mode for vehicle speeds between zero (standstill) and a first vehicle speed value VI and operating the powertrain in the second powertrain mode for vehicle speeds above the first vehicle speed value VI. The first vehicle speed value VI may optionally about 1/3 of a maximum vehicle speed.

[0029] The powertrain may be configured to enable, by the control system, the electric prime mover to operate at rotational speeds up to a maximum prime mover speed N_max in the second powertrain mode, the first prime mover speed value N1 being lower than the maximum prime mover speed N_max. The first prime mover speed value N1 may be no more than 70% of the maximum prime mover speed Nmax, or it may be no more than 60% of the maximum prime mover speed Nmax, or it may be no more than 50% of the maximum prime mover speed Nmax, or it may be no more than 40% of the maximum prime mover speed Nmax.

[0030] The method may comprise preventing the at least one hydrostatic pump and/or the at least one hydrostatic motor from being driven when the powertrain is operating in the second powertrain mode.

[0031] The electric prime mover may be an AC electric motor and the control system may comprise a Variable Frequency Drive operable to regulate the electrical power supply to the electric motor so as to control the rotational speed and torque of the electric motor. [0032] In an aspect of the invention, there is provided a method of operating a vehicle powertrain including an electric prime mover, especially an electric motor, and a hydrostaticmechanical power split transmission, the transmission comprising an input shaft drivable by the electric prime mover, at least one output drive shaft for driving at least one driven ground engaging member of the vehicle, a mechanical transmission branch configured to transfer drive mechanically from the input shaft to the at least one output drive shaft, a hydrostatic transmission branch having at least one variable displacement hydrostatic pump driven from the input shaft and at least one hydrostatic motor for driving the at least one output drive shaft, the transmission being operable in two transmission modes, a first transmission mode in which drive is transmitted via both the mechanical and hydrostatic transmission branches with a variable transmission ratio icvr and a second transmission mode in which drive is transmitted only through the mechanical transmission branch at a constant transmission ratio icvr; and a control system for controlling operation of the electric prime mover and the transmission; wherein the method comprises selectively operating the powertrain in either of a first powertrain mode in which the transmission is operated in the first transmission mode in which the rotational speed of the electric prime mover is independent of the vehicle speed and not does not exceed a first prime mover speed value Nl, which first prime move speed value N1 is less than a maximum prime mover speed N_max, and a second powertrain mode in which the transmission is operated in the second transmission mode in which the prime mover speed is proportional to the vehicle speed and the prime move speed is permitted to exceed the first prime mover speed value Nl up to the maximum prime mover speed N_max.

[0033] Still other embodiments involve a computer-readable storage medium (e.g., a non-transitory computer-readable storage medium) having processor-executable instructions configured to implement one or more of the techniques presented herein.

[0034] Within the scope of this application it should be understood that the various aspects, embodiments, examples and alternatives set out herein, and individual features thereof may be taken independently or in any possible and compatible combination. Where features are described with reference to a single aspect or embodiment, it should be understood that such features are applicable to all aspects and embodiments unless otherwise stated or where such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0036] FIG. l is a representation of an agricu Itu ra l/uti lity vehicle, in the form of a tractor which may be provided with a powertrain according to an aspect of the present invention;

[0037] FIG. 2 is a schematic representation of an embodiment of a vehicle powertrain configuration according to an aspect of the invention suitable for use in the vehicle of FIG. 1;

[0038] FIGs. 3 and 4 are diagrams illustrating schematically an embodiment of a method of controlling the powertrain of FIG. 2 according to an aspect of the invention;

[0039] FIGs. 5, 6a, 6b and 7 are schematic representations of modified embodiments of the powertrain configuration of FIG. 2;

[0040] FIG. 8a is a simplified representation of the schematic views of the powertrain configurations of FIGs. 5, 6a, 6b, and 7; and

[0041] FIG. 8b is a simplified representation of a further powertrain configuration according to an aspect of the invention suitable for use in the vehicle of FIG. 1.

DETAILED DESCRIPTION

[0042] Illustrations presented herein are not meant to be actual views of any particular vehicle, application system, agricultural implement, component, or system, but are merely idealized representations that are employed to describe embodiments of the disclosure. Additionally, elements common between figures may retain the same numerical designation for convenience and clarity.

[0043] The following description provides specific details of embodiments. However, a person of ordinary skill in the art will understand that the embodiments of the disclosure may be practiced without employing many such specific details. Indeed, the embodiments of the disclosure may be practiced in conjunction with conventional techniques employed in the industry. In addition, the description provided below does not include all the elements that form a complete structure or assembly. Only those process acts and structures necessary to understand the embodiments of the disclosure are described in detail below. Additional conventional acts and structures may be used. The drawings accompanying the application are for illustrative purposes only, and are thus not drawn to scale.

[0044] As used herein, the terms "comprising," "including," "containing," "characterized by," and grammatical equivalents thereof are inclusive or open-ended terms that do not exclude additional, unrecited elements or method steps, but also include the more restrictive terms "consisting of" and "consisting essentially of" and grammatical equivalents thereof.

[0045] As used herein, the singular forms following "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0046] As used herein, the term "may" with respect to a material, structure, feature, or method act indicates that such is contemplated for use in implementation of an embodiment of the disclosure, and such term is used in preference to the more restrictive term "is" so as to avoid any implication that other compatible materials, structures, features, and methods usable in combination therewith should or must be excluded.

[0047] As used herein, the term "configured" refers to a size, shape, material composition, and arrangement of one or more of at least one structure and at least one apparatus facilitating operation of one or more of the structure and the apparatus in a predetermined way.

[0048] As used herein, any relational term, such as "first," "second," "third," etc. is used for clarity and convenience in understanding the disclosure and accompanying drawings, and does not connote or depend on any specific preference or order, except where the context clearly indicates otherwise.

[0049] As used herein, the term "substantially" in reference to a given parameter, property, or condition means and includes to a degree that one skilled in the art would understand that the given parameter, property, or condition is met with a small degree of variance, such as within acceptable manufacturing tolerances. By way of example, depending on the particular parameter, property, or condition that is substantially met, the parameter, property, or condition may be at least 90.0% met, at least 95.0% met, at least 99.0% met, or even at least 99.9% met.

[0050] As used herein, the term "about" used in reference to a given parameter is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the given parameter, as well as variations resulting from manufacturing tolerances, etc.).

[0051] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0052] Embodiments of vehicle powertrains according to an aspect of the invention and embodiments of methods for controlling a vehicle powertrain according to a further aspect of the invention will be described below with reference to a powertrain for use in an agricultural tractor 1, such as that illustrated in FIG. 1. However, it should be understood that powertrains and methods in accordance with the invention as described herein are not limited to use in agricultural tractors but can be adapted for use in a range of other types of vehicle, including (without limitation): trucks and lorries as well as utility vehicles including construction vehicles and plant machinery, such as excavators and the like, and agricultural vehicles other than tractors. The term vehicle should be understood as encompassing self-propelled mobile machines.

[0053] FIG. 1 illustrates an agricultural tractor 1 (hereinafter referred to simply as a tractor) provided with front and rear wheels 2, 3 mounted to front and rear axles 4, 5 which are connected to a chassis 6. The tractor 1 has an operator cab 7 supported on the chassis and/or the rear axle 5.

[0054] Whilst the tractor 1 is illustrated with front and rear wheels 2, 3, the present invention can be adapted for use with vehicles having tracks or other types of driven ground engaging members. In this regard, the term "axle" should be understood as encompassing any suitable arrangement for transferring drive in a lateral direction to drive ground engaging members on opposite sides of the vehicle. [0055] The tractor 1 may have a powertrain 10 as illustrated schematically in FIG. 2. The powertrain as illustrated includes an electrically driven prime mover 20, which may be an electric motor, and a transmission 100 for forwarding power/torque from the electric prime mover 20 to the axles 4, 5 and hence to the front and rear wheels 2, 3 or other driven ground engaging members as may be the case.

[0056] The electric prime mover 20 is supplied with electrical power by a power source 40. Power source 40 may be a battery, typically a rechargeable battery. The battery 40 may be provided as part of a battery unit including further components such as, without limitation: a power electronics unit, interfaces for the high voltage networks, such as a suitable inverter, a brake chopper and a high voltage connector control. Other suitable power electronics components may also be encapsulated within the power source 40 as required. The power source 40 may also include a controller 40a to regulate the supply of energy to the electric prime mover 20 and to adjust the rotational speed of electric prime mover 20. The power source 40 can take other forms depending on the requirements of the electric prime mover 20 such as a fuel cell or an external power line.

[0057] Motive power from the electric prime mover 20 is forwarded to the front and rear axles 4, 5 via the transmission 100. An electronic control system 50 includes at least one controller 50a (which may be part of an ECU) and input/output (I/O) interface(s) which may include a touch screen display or monitor 60a. The control system 50 is operative to control, amongst other things, the electric prime mover 20, the power source 40, and the transmission 100 to meet requirements including a demanded vehicle speed. The demanded vehicle speed may be input via a driver input device (illustrated schematically at 60b) which is operatively connected (wired or wireless) to the control system 50. The driver input device 60b may comprise a foot pedal or a hand control (e.g., a joystick) manually operated by a driver and which provides an input signal to the control system 50 indicative of a demanded vehicle speed. A driver may input a demand for a vehicle speed in other ways. For example, the control system 50 may be configured to enable the driver to select or input a demanded vehicle speed using other input devices such as buttons or via icons on the touch screen monitor 60a or voice control. The control system 50 may be configured such that a driver is able to select various operational modes for the vehicle which may include instructions for predetermined demanded vehicle speeds. Alternatively, the control system 50 may generate a demanded vehicle speed requirement if the vehicle is operated in a fully or partly autonomous mode or a demanded vehicle speed input may be provided remotely by means of a suitable interface (wired or wireless) if the vehicle is remotely controlled, say as part of a leader-follower vehicle configuration.

[0058] In an embodiment, the electric prime mover 20 is an AC electric motor 20 and the power source 40 is a battery. The supply of electrical power to the electric prime mover 20 from the battery is regulated by an inverter in the form of a Variable Frequency Drive 40a which converts the DC power supply from the battery 40 to an AC power supply to the motor windings. The Variable Frequency Drive 40a forms part of the control system 50 and its operation is regulated by the controller 50a to control the rotary speed N and torque of the electric motor 20 for example by varying the frequency and current of the electric power supply to the motor windings, as is well known in the art. Since the mechanical power output by the motor 20 is a function of the speed and torque of the motor, regulating the speed and torque of the motor 20 also regulates its mechanical power output. The Variable Frequency Drive 40a is a particular embodiment of the controller 40a which regulates the supply of electrical power to the electric prime mover 20 to adjust the rotational speed and torque output of the electric prime mover 20. Alternative control arrangements can be adopted depending on the nature of the electric prime mover 20 and the electrical power source 40.

[0059] Drive is transmitted from the electric prime mover 20 to the transmission 100 though a transmission input shaft 102 (which is drivingly coupled to, or may be an extension of, an output shaft of the electric prime mover) and is output from the transmission through front and rear output drive shafts 134, 136, which in this embodiment provide propulsive drive to the front and rear axles 4, 5 to drive the front and rear wheels 2, 3. Alternatively, the front and/or rear output drive shafts 134, 136 may provide propulsive drive to tracks. In alternative embodiments, the transmission may have only one output drive shaft to provide drive to a single axle or may comprise output drive shafts which extend laterally to drive ground engaging members on opposite sides of the vehicle. [0060] In addition to providing tractive drive for the vehicle, the powertrain also has an auxiliary drive system 132 referred to as a power take-off (PTO) system. The PTO system 132 has a PTO shaft 132a (PTO output or stub shaft) which can be drivingly coupled with an input drive shaft on an implement attached to the tractor or other ancillary equipment to provide drive to the implement or ancillary equipment. The PTO shaft 132a is drivingly connectable to the electric prime mover 20, in this case through the transmission input shaft 102, by a PTO clutch 132b. The PTO clutch 132b may be of any suitable type and may be a friction clutch or a positive engagement clutch, such as a dog clutch. The PTO clutch 132b may be fluid or electronically actuated. When the PTO clutch 132b is engaged, drive is transmitted from the electric prime mover via the input shaft 102 to the PTO shaft 132a. When the PTO clutch 132b is disengaged, drive is not transmitted from the electric prime mover 20 to the PTO shaft 132a. The PTO clutch 132b may actuatable under control of the control system 50. A PTO brake 132c may also be provided to enable the PTO shaft 132a and any attached implement to be braked. Where provided, the PTO brake 132c may be of any suitable type and may be a friction or positive engagement brake which may be fluid actuated, for example. Where provided, the PTO brake 132c may actuatable under control of the control system 50. The PTO system 132 may also include a PTO transmission 132d. In an embodiment the PTO transmission 132d provides a fixed drive ratio between the electric prime mover 20 and the PTO shaft 132a. The PTO transmission 132d can be provided at any suitable position in the drive line between the electric prime mover 20 and the PTO shaft 132a or may be omitted if no speed reduction is required between the electric prime mover and the PTO shaft 132a.

[0061] It will be appreciated that drive from the electric prime mover 20 to the transmission 100 and the PTO system 132 can be split in many different ways and that drive to the PTO clutch need not be routed through an input shaft 102 to the transmission 100.

[0062] The transmission 100 is a power-split CVT transmission drive in which drive (power/torque) is divided into two branches or paths, a mechanical branch 110 with a fixed ratio and a hydrostatic branch 120 with a variable ratio, before being recombined at or upstream of the output drive shafts 134, 136. The hydrostatic branch with variable ratio may also be referred to as a variator. In the present case, the transmission 100 is an output coupled planetary transmission (also referred to as an input-side power split or divider planetary power-split transmission) in which the power is split into the mechanical transmission branch 110 and hydrostatic transmission branch 120 by an epicyclic or planetary gear assembly 148 coupled to the input shaft 102. The power is subsequently recombined or summed at a summing shaft 104 upstream of the output drive shafts 134, 136. The summing shaft 104 is drivingly coupled to the front and rear output drive shafts 134, 136 by an intermediary gear 156 rotationally fast with the summing shaft 104 and an offset gear 158 rotationally fast with the front and rear output drive shafts 134, 136 and which is drivingly engaged with the intermediary gear. In other embodiments, the power can be recombined at one or more output drive shafts which functions as a summing shaft.

[0063] In other embodiments, the transmission 100 is an input coupled planetary power-split transmission (also known as a summing planetary power-split transmission) in which a planetary gear assembly is used to recombine the power transmitted through the mechanical and hydrostatic transmission branches, as will be described with reference to FIG. 8b below.

[0064] In the embodiment illustrated, the input shaft 102 may be drivingly coupled to a planetary gear carrier 148a of the planetary gear assembly 148 and the mechanical transmission branch 110 may be driven from the sun gear 148b of the planetary gear assembly 148. As illustrated, the sun gear may be drivingly coupled to the summing shaft 104 by means of a first output drive gear 150 rotationally fast with the sun gear 148b and a second output drive gear 154 rotationally fast with the summing shaft 104.

[0065] In the embodiment illustrated, hydrostatic transmission branch 120 includes a hydrostatic pump 140 which may be driven from a ring gear 148c of the planetary gear assembly 148, for example though a pump gear set 152. The pump 140 is hydraulically connected (not shown in FIG. 2 for clarity) to at least one hydrostatic motor 142, 142a which is drivingly coupled to the summing shaft 104. For low specification transmissions commonly used for low horsepower applications, the hydrostatic branch 120 may have only one hydrostatic motor 142, as indicted in solid lines in FIG. 2. However, for high horsepower applications, two hydrostatic motors 142, 142a both hydraulically connected with the hydrostatic pump 140 can be used. In this case, the second hydrostatic motor 142a may also be drivingly coupled with the summing shaft 104 as illustrated in broken line in FIG. 2. However, other configurations for incorporating two hydrostatic motors 142, 142a are possible, see for example the embodiment of Fig. 7 described below. Indeed, embodiments of the invention may use more than two hydrostatic motors and/or more than one hydrostatic pump 140 in the hydrostatic branch 120. In the following detailed description of the embodiments, references to a hydrostatic motor in the singular should be understood as encompassing more than one hydrostatic motor unless the context requires otherwise.

[0066] In the embodiment shown in FIG. 2, the intermediary and offset gears 156, 158 enable the axes of the front and rear output drive shafts 134, 136 to be offset relative to the hydrostatic motor 142, 142a and also enable a gear reduction. However, in other embodiments it may not be necessary to offset the axes of the front and rear output drive shafts 134, 136 and the intermediary and offset gears 156, 158 could be omitted so that the summing shaft 104 forms, or is directly connected with, the front and rear output drive shafts 134, 136. In still further embodiments, a speed range gear box may be incorporated between a summing shaft 104 and any output drive shaft or shafts 134, 136.

[0067] The hydrostatic pump 140 and the hydrostatic motor 142, 142a are variable displacement hydrostatic units and the control system 50 is able to vary the division of power through the mechanical and hydrostatic branches 110, 120 of the transmission 100 by varying the amount of hydraulic assistance provided by the hydrostatic motor 142, 142a to propel the vehicle. The transmission speed ratio, referred to as the CVT transmission ratio IC T , is the ratio of the rotational speed of the summing shaft 104 to the rotational speed of the input shaft 102 When power is transmitted through both the mechanical branch 110 and the hydrostatic branch 120, the CVT transmission ratio icvris continuously variable depending on what proportion of the power is transmitted through the hydrostatic transmission branch 120. The proportion of power transmitted through the hydrostatic branch 120 and hence the CVT transmission ratio i_cvt is varied by adjustingthe displacements of the hydrostatic pump 140 and the hydrostatic motor 142, 142a. In some embodiments, the CVT transmission ratio ICVT may be expressed as the ratio of the rotational speed of the front and/or rear output drive shafts 134, 136 to the rotational speed of the input shaft 102 or the ratio of the rotational speeds any two rotational components of the transmission, one of which is upstream of the power-split and one downstream.

[0068] The hydrostatic pump 140 and the hydrostatic motor 142, 142a may be any suitable type of variable displacement hydrostatic units. For example, they may be axial piston swash plate or bent axis units in which changing the angle of the swash plate or axis alters the displacement. This angle will be referred to as the operating angle which in this embodiment is set by an adjustment unit 160 electronically connected to and regulated by the vehicle control system 50. Details of the adjustment unit 160 are not described further but an example of a typical adjustment unit is described in International patent application publication No. WO2014/096446, published 26 June 2014, by AGCO International GmbH.

[0069] The displacements of the hydrostatic pump 140 and hydrostatic motor 142, 142a are varied by the adjustment unit 160 under control of the control system 50, which generates suitable control signals, and provides two transmission modes depending on the speed of the vehicle: a. a first transmission mode is provided up to a first vehicle speed value VI, wherein power is transmitted through both the mechanical and hydrostatic transmission branches 110, 120 with a variable CVT transmission ratio icvr; and b. a second transmission mode is provided for vehicle speeds above the first vehicle speed value VI, wherein power is transmitted only through the mechanical transmission branch 110 at a fixed CVT transmission ratio icvr.

[0070] This control process is well known but will be described briefly, In an embodiment, when the vehicle is stationary and the vehicle is in neutral (no drive is selected), the operative angle of the hydrostatic pump 140 is zero (i.e., the pump is in a neutral or idle configuration) and the operative angle of the hydrostatic motor 142, 142a is at its maximum in one direction. Because the operative angle of the pump 140 is zero, no fluid is supplied to the hydrostatic motor 142, 142a even if the pump is rotated and no power is transmitted through the hydrostatic branch 120. As there is no resistance on the ring gear 148c, the sun gear 148b does not turn and no drive is transmitted through the mechanical transmission branch 110 either. [0071] When forward drive is selected and an increase in the vehicle speed demanded, the operative angle of the hydrostatic pump 140 is increased in a first (forward) direction so that the hydrostatic pump 140 begins to pump fluid to the hydrostatic motor 142, 142a which begins to turn the summing shaft 104 and propel the vehicle. When the hydrostatic pump 140 starts to pump fluid, this places a resistance on the ring gear 148c so that the sun gear 148b starts to transmit an equivalent torque through the mechanical transmission branch 110. As the demanded vehicle speed increases, the operative angle of the hydrostatic pump 140 is further increased to increase its output and the operative angle of the hydrostatic motor 142, 142a is decreased, swinging the motor 142, 142a towards a zero operative angle (i.e., towards a neutral or idle configuration). This increases the speed of the hydrostatic motor(s) 142, 142a and sun gear 148b and results in an increasing proportion of the motive power from the prime mover being transmitted through the mechanical transmission branch 110. In this phase for vehicle speeds between standstill and the first vehicle speed value VI, the transmission is in its first mode of operation.

[0072] When the vehicle reaches the first vehicle speed value VI, the hydrostatic pump 140 is at its maximum operative angle in the first direction and the operative angle of the hydraulic motor 142, 142a is zero. This prevents oil flow between the pump and the motor and thus the hydrostatic pump 140 and ring gear 148c are stopped and all power is transmitted through the sun gear 148b and the mechanical branch 110. The transmission 100 is in its fixed ratio for the second mode of operation which may be maintained at all vehicle speeds above the first ground speed value VI.

[0073] When the vehicle speed is reduced, the above procedures are reversed. For drive in a reverse direction, the hydrostatic pump 140 is swung away from the zero operative angle in a second direction opposite to the first.

[0074] The transmission 100 can also be operated in an active standstill mode when a vehicle is standing on an inclined plane so that the vehicle is held stationary without the brakes applied. In the active standstill mode, the operative angle of the hydrostatic pump 140 is zero (i.e., the pump is in a neutral or idle configuration) and the operative angle of the hydrostatic motor 142, 142a is at its maximum in one direction so that no fluid is pumped. If the vehicle attempts to roll down the incline, torque from the wheels is applied to the hydrostatic motor 142, 142a through the output shafts 134, 136. However, the hydrostatic motor 142, 142a is unable to turn due to the pressure of fluid in the hydrostatic transmission and so the vehicle is held stationary. In the event of a fluid leakage in the hydrostatic transmission, the operative angle of the hydrostatic pump 140 is adjusted by the adjustment unit 160 to compensate for the leakage but without generating a flow of fluid sufficient to drive the hydrostatic motor 142, 142a. Thus the vehicle is held stationary with a minimum of load placed on the electric prime mover 20. This is referred to as active standstill as the system is active in maintaining a zero speed at the output shaft or shafts 134, 136.

[0075] A method of operating the powertrain 10 according to an aspect of the present invention will now be described with reference to FIGs. 3 and 4.

[0076] Figure 3 is a graph of the rotary speed N of the electric prime mover (e.g., electric motor) 20 in RPM (vertical axis) against the ground speed of the vehicle V in kph (horizontal axis). Fig. 4 is a graph of CVT transmission ratio icvr (vertical axis) against the ground speed of the vehicle V in kph (horizontal axis). The scale of the horizontal axes in the graphs of FIGs. 3 and 4 are the same to allow a direct comparison.

[0077] The electric prime mover 20 has a nominal minimum speed N_min and a maximum speed Nmax. The minimum and maximum rotational speeds of the prime mover 20 will vary depending on the size and type of the prime mover 20 and the vehicle requirements. In some non-limiting embodiments the minimum prime mover speed N_min may be in the region of 800 to 1200 RPM and the maximum prime mover speed N_max may be in the region of 3000 to 5000 RPM but these values should not be regarded as limiting. The minimum prime mover speed Nmin is indicated by line 161 in FIG. 3 and the maximum engine speed Nmax is indicated by line 162 in FIG. 3.

[0078] It is a known problem with electric vehicle drives that starting the electric motors under full load, i.e. in the run up from zero RPM to minimum prime mover speed Nmin, is critical. If an electric motor is subject to a high toque loading at very low speed, the current in the motor winding rapidly increases and results in thermal overload. To avoid this problem, the load on the prime mover 20 can be kept to a minimum during start-up. The tractor 1 will usually be stationary during start-up and, if on level ground and/or with the brakes applied, the drive can be placed in neutral. In this configuration, the operative angle of the hydrostatic pump 140 is set to zero and the operative angle of the hydrostatic motor 142, 142a is at its maximum so that no drive is transmitted to the summing shaft 104 and the load on the electric prime mover 20 is small. Even if the vehicle is on an incline during start-up and the transmission operated in the active standstill mode as described above, the load on the prime mover 20 is very low as the prime mover 20 has only to turn the hydrostatic pump 142, 142a sufficiently to compensate for leakage at zero oil flow to prevent the vehicle rolling down the incline.

[0079] The method of operating the powertrain 10 in accordance with the invention comprises operating the powertrain in at least two different modes for vehicle speeds V above standstill (V - 0 kph) up the maximum vehicle speed Vmax.

[0080] A first operating mode of the powertrain 10, referred to a first powertrain mode, is applied for demanded vehicle speeds V from standstill V0 (V= 0 kph) to a first vehicle speed value VI. A second operating mode of the powertrain 10, referred to as a second powertrain mode, is applied for demanded vehicle speeds above the first vehicle speed value VI up to the maximum vehicle speed V_max. The first vehicle speed value VI is indicated by line 164 in FIGs. 3 and 4 and the maximum vehicle speed Vmax is indicated by line 166 in FIGs. 3 and 4. It will be appreciated that the first vehicle speed value VI can be selected as appropriate to the prime mover 20 and vehicle requirements. However, in some non-limiting embodiments, the first vehicle speed value VI may be in the range of 10 to 30 kph, or may be in the range of 12 to 18 kph, or may be in the range of 13 to 17 kph, or may be in the range of 14 to 16 kph, or may be in the region of 15 kph. Generally speaking, the first vehicle speed value VI may be in the range of about 20% to 40% of the maximum vehicle speed Vmax, or it may be within the range of about 25% to 35% maximum vehicle speed Vmax. In an embodiment, the first vehicle speed value VI is about 1/3 of the maximum vehicle speed Vmax.

[0081] In the first powertrain operating mode, the transmission is operated in the first transmission mode as described above in which power is transferred from the electric prime mover 20 through both the mechanical and hydrostatic branches 110, 120 of the transmission 100 such that the CVT transmission ratio icvr is variable and the rotary speed N of the prime mover 20 is normally limited to a first prime mover speed value Nl, which is less than the maximum speed Nmax of the prime mover 20. Accordingly, in the first powertrain operating mode, the prime mover 20 will be operated in a range of speeds between the minimum prime mover speed N_min and the first prime mover speed value Nl and the CVT transmission ratio icvr can be varied by adjusting the relative proportions of the power transmitted through the hydrostatic and mechanical transmission branches 110, 120 in the manner described above for the first transmission mode. In the first powertrain mode, the rotational speed N of the electric prime mover 20 is independent of the vehicle speed V.

[0082] The first prime mover speed value Nl is indicated by line 163 in FIG. 3. The preferred range of prime mover speeds N in the first powertrain mode is indicated in FIG. 3 by the area of the graph between lines 163 and 161 for vehicle speeds V between standstill V0 and the first vehicle speed value VI, i.e., to the left of line 164. The CVT transmission ratio icvr is indicated schematically in FIG. 4 by line 170. The inclined section of line 170 for vehicle speeds between standstill V0 and the first vehicle speed value VI as indicated by line 164 represents a variable CVT transmission ratio icvr.

[0083] In the second powertrain mode for vehicle speeds above the first vehicle speed value VI, as illustrated to the right of line 164 in FIGs. 3 and 4, the transmission is operated in the second transmission mode as described above so that power from the electric prime mover 20 is transmitted only through the mechanical transmission branch 110 of the transmission 100 and the CVT transmission ratio icvr is constant. This is indicated by the horizontal portion of line 170 in FIG. 4. In this second powertrain mode in relation to the embodiment described above with reference to FIG. 2, the operative angle of the hydrostatic pump 140 is at a maximum value and the operative angle of the hydrostatic motor 142, 142a is set to zero so that all drive is transferred through the mechanical transmission branch of the transmission 110.

[0084] In the second powertrain mode, the rotational speed N of the prime mover 20 is proportional to the vehicle speed V and the prime mover 20 can be operated up to its maximum prime mover speed N_max, line 162. Usually, the prime mover 20 will be operated at rotational speeds above the first prime mover speed value Nl in the second powertrain mode. In the present embodiment there are no variable or selectable gear/speed ratio systems in the transmission downstream of the summing shaft 104 and so the ratio of prime mover speed N1 to vehicle speed V remains constant in the second powertrain mode. The actual ground speed V of the tractor 1 relative to the prime mover speed N is dependent on the CVT transmission ratio icvr, as well as the ratio of any fixed ratio gearing systems downstream of the transmission, such as the final drives in the front and rear axles, and the size of the wheels. However, as these are all fixed in terms of the overall speed ratio, there is a linear relationship between the speed N of the electric prime mover 20 and the ground speed V in the second powertrain mode as illustrated by the solid line 168 in FIG. 3. In other embodiments, a selectable gear or variable speed ratio system (e.g. a range gearbox) may be provided downstream of the summing shaft 104. However, the vehicle speed V and rotational speed N of the electric motor 20 will be proportional.

[0085] When the tractor 1 is operating In the first powertrain mode, there is no fixed linear relationship between the speed N of the electric prime mover 20 and ground speed V as the CVT transmission ratio IC T is variable. Broken line 168a illustrates how the prime mover speed N might be increased from N_min up to the first prime mover speed value N1 in order to accelerate the tractor 1 up to the first vehicle speed value VI as the CVT transmission ratio icvr gradually changes. This might represent a scenario where the tractor 1 is being accelerated from standstill V0 up to and beyond the first vehicle speed VI, say at the beginning of a journey along a road. However, often a tractor 1 will be operated at speeds below first vehicle speed value VI for long periods of time, say when undertaking field work. In this case, the prime mover 20 may be operated in a range of different ways. For example, the electric prime mover 20 could be set to operate at a constant speed N in the range N_min to N1 and the CVT transmission 100 automatically controlled by the controller 50a to provide a demanded vehicle speed V between standstill V0 and the first vehicle speed VI. In one possible scenario, the driver inputs to the controller 50a a demand for a constant ground speed between standstill V0 and the first vehicle speed VI and the controller 50a is operative to adjust the speed N of the prime mover 20 and the CVT transmission ratio icvrin order to maintain the tractor 1 at the demanded ground speed V in the most efficient way. This may involve operation the electric prime mover 20 at a constant speed N and adjusting the CVT transmission ratio icvrto maintain the demanded vehicle ground speed V or adjusting both the speed N of the prime mover 20 and the CVT transmission ratio ICVT as operating conditions vary.

[0086] In another mode of operation when the PTO system 132 is in operation, the control system 50 may regulate the speed of the electric prime mover 20 so as to provide a desired PTO shaft 132a speed. Thus where the PTO shaft 132a is driven at a fixed ratio from the electric prime mover 20, the electric prime mover is operated at a fixed speed N_pto necessary to deliver a target PTO shaft speed. The target PTO shaft speed may be one of a number of industry standardized PTO shaft speeds such as 540 rpm, 1000 rpm, or 1300 rpm typically used to drive agricultural implements. The arrangement may be configured such that the powertrain can selectively deliver either of at least two such industry standardized PTO shaft speeds through a fixed ratio PTO drive line by regulating the prime mover speed. Typically, a tractor 1 will be operated at ground speeds below the first vehicle speed value VI when the PTO system 132 is in use and the speed N_pt_o of the electric prime mover 20 will be between Nmin and Nl. In this mode of operation, the control system 50 is operative to regulate the CVT transmission ratio icvr as required to obtain a desired vehicle ground speed V at the set prime mover speed N_pto.

[0087] Limiting the speed of the prime mover 20 to a value less than its maximum speed Nmax in the first powertrain mode has the advantage that the transmission is able to provide high torque with high efficiency over varying vehicle speeds from standstill to the first vehicle speed value VI while the electric prime mover 20 is operated at optimum efficiency. In the second powertrain mode, the electric prime mover 20 can be operated at higher speeds at low torque level whilst the powertrain 100 is operated with purely mechanical power transfer. Limitation of the prime mover speed N in the first powertrain mode may be regulated by the control system 50, which generates suitable control signals using any suitable techniques. A significant advantage of the method of operating the powertrain as described is that torque range of electric prime mover 20 is reduced in the first powertrain operating mode. This enables the use of electric motors having a smaller torque capability and which are more dynamic, are less heavy and expensive and which require less cooling.

[0088] It will be appreciated that the first prime mover speed value Nl can be selected as appropriate to the prime mover 20 and vehicle requirements and a value for Nl may be selected that falls within the optimum efficiency area of the electric prime mover 20. In some non-limiting embodiments, the first prime mover speed value N1 may be in the range of 1500 to 2300 RPM. Generally, in non-limiting examples, the first prime mover speed value N1 may be no more than 70% of the maximum prime mover speed N_max, or may be no more than 60% of the maximum prime mover speed N_max, or may be no more than 50% of the maximum prime mover speed Nmax, may be no more than 40% of the maximum prime mover speed Nmax. Generally, the first prime mover speed value N1 should not exceed the maximum possible speed of the hydrostatic pump 140 and the prime mover maximum speed Nmax should not exceed the maximum possible speed of the hydrostatic pump 140 or the hydrostatic motor 142, 142a.

[0089] Whilst the mechanical transmission branch 110 must be capable of bearing full load/torque/input prime mover speed range over the full range of vehicle speeds V, the hydrostatic transmission branch 120 is only operative to transfer torque/power in the first powertrain mode in which the prime mover speed N is limited significantly below the maximum available prime mover speed Nmax. As a consequence, the components in the hydrostatic transmission branch 120, especially the hydrostatic units such as the pump 140 and motor 142, 142a, need only be engineered to bear full torque and speed capability for limited input prime mover speeds of up to Nl. This is in contrast to prior known powertrains with a power split CVT having mechanical and hydrostatic transmission braches in which the hydrostatic transmission branch must be capable of operating over the full input speed range of the prime mover (combustion engine or electric prime mover) at all vehicle speeds. The ability to use hydrostatic units 140, 142, 142a with a lower technical specification reduces the cost and weight of the powertrain 10. Furthermore, since the hydrostatic transmission branch 120 is only operative over a limited range of input prime mover speed, this opens up the possibility of using an electric prime mover 20 with a higher maximum speed Nmax than provided by the prime mover in prior art powertrains for tractors or other similar vehicles. For example, combustion engines used in powertrains with a power-split mechanical and hydrostatic CVT transmissions previously used in tractors would typically have a maximum speed in the region of 2500 RPM, whereas the electric prime mover 20 in the powertrain according to the invention may have a maximum speed Nmax in the region of 4000 or more to enable higher vehicle speeds. [0090] For vehicle speeds above the first vehicle speed value VI when the electric prime mover 20 is operating at speeds above the first prime mover speed value Nl, the hydrostatic transmission branch 120 is operating in an idle condition with no fluid being delivered from the pump 140 to the motor 142, 142a as the power is transferred via the mechanical transmission branch 110. Whilst this idle condition enables hydrostatic units 140, 142, 142a to be used which are not specified (capable) to transfer full power up to the maximum prime mover speed N_max, the hydrostatic units 140, 142, 142a must still be capable of running in the idle condition and the system will generate internal losses, reducing the overall efficiency of the powertrain 10. To overcome these issues, the powertrain 10 according to the invention can be modified to prohibit rotation under pressure of hydrostatic pump 140 and/or the motor 142, 142a when the powertrain is operating in the second powertrain mode. FIGs. 5, 6a, 6b, 7, 8a and 8b illustrate alternative embodiments of a powertrain 10 according to the invention which incorporate such modifications.

[0091] FIG. 5 illustrates a powertrain 10 according to the invention which is substantially identical to that illustrated in FIG. 2 as described above and which may be operated according to the method described above in a first and a second powertrain mode. The powertrain 10 of FIG. 5 differs from that of FIG. 2 only in that a brake 180 is provided which is operative to prevent the hydrostatic pump 140 being driven from the ring gear 148c when the brake is engaged. This prevents the hydrostatic pump 140 being placed under load when the powertrain is operating in the second powertrain mode, reducing internal losses in the hydraulic circuit and so improving the overall efficiency of the powertrain 10. In the embodiment shown, the brake 180 is operative on the ring gear 148c and can be selectively engaged to prevent the ring gear 148c rotating and so prevent drive being transmitted to the hydrostatic pump 140 when the powertrain 10 is being operated in the second powertrain mode. It will be appreciated that the brake 180 need not be operative on the ring gear 148c but could be operative at any point in the drive line between the ring gear 148c and the pump 140. For example, a brake 180 could be provided which is operative on an input shaft 153 to the pump 140 to prevent the pump 140 being driven when the brake is engaged. [0092] The brake 180 may be of any suitable type such as a friction brake, which may be a wet or dry brake, or a positive engagement brake with inter-engaging formations similar to a dog clutch. Operation of the brake 180 may be controlled by the control system 50 issuing control signals so that the brake is actuated to prevent drive to the pump 140 when the powertrain is operative in the second powertrain mode and is released to enable the pump 140 to be driven by the ring gear 148c when the powertrain is operative in the first powertrain mode.

[0093] While the embodiment of FIG. 5 offers an improvement in efficiency by preventing the hydrostatic pump 140 being driven when the powertrain is operative in the second powertrain mode, the hydraulic motor 142, 142a will be rotated at high speeds as it is externally driven by the mechanical transmission branch 120 through the summing shaft 104. Since the motor 142, 142a is at a zero operative angle in the second powertrain mode, no fluid will be pumped as a result of the motor 142, 142a being driven. Nevertheless, this leads to a reduction in the overall efficiency of the powertrain. To overcome this problem, a clutch arrangement can be incorporated in the powertrain 10 to enable the motor 142, 142a to be drivingly decoupled from the summing shaft 104 when the powertrain 10 is operative in the second powertrain mode.

[0094] FIG. 6a illustrates an embodiment in which a clutch 182 is provided which is selectively actuatable to couple/decouple an output shaft of the hydrostatic motor 142 with the summing shaft 104. The clutch 182 is engaged when the powertrain 10 is operative in the first powertrain mode so that the hydrostatic motor 142 is able to drive the summing shaft 104. When the powertrain 10 is operative in the second powertrain mode, the clutch 182 is disengaged so that the hydrostatic motor 142. 142a is not driven by the summing shaft 104. As illustrated in broken lines in FIG. 6a, where the transmission includes two hydrostatic motors 142, 142a, which both drive the summing shaft 104 in the first powertrain mode, a respective clutch 182, and 182a can be provided for each motor 142, 142a.

[0095] FIG. 6b illustrates a further variation in which the hydrostatic transmission branch 120 has two hydrostatic motors 142, 142a for driving the summing shaft 104 in the first powertrain mode and a single clutch 184 is operative to selectively couple or decouple both motors and the summing shaft 104. In the embodiment of FIG. 6b, the output drive gear 154 and the intermediary gear 156 are drivingly interconnected by a housing 184a of the clutch unit 184 so that the gears 154, 156 and the clutch housing 184a rotate together. One or more clutch plates 184b located in the clutch housing 184a are drivingly engaged with the summing shaft 104. When the clutch is engaged, the clutch plates 184b are caused to rotate together clutch housing 184a, the output drive gear 154 and the intermediary gear 156 so that drive can be transmitted through both the mechanical and hydrostatic branches 110, 120 of the transmission 100. When the clutch 184 is disengaged, the summing shaft 104 and hydrostatic motors 142, 142a are decoupled and drive can be transmitted only through the mechanical branch 110 of the transmission. The clutch 184 is engaged in the first powertrain mode and disengaged in the second powertrain mode so that the hydraulic motors 142, 142a are not driven at high speed when the powertrain is operating in the second powertrain mode.

[0096] The clutch or clutches 182, 182a, 184 can be of any suitable type. In some embodiments, the clutch or clutches may be wet clutches and may be operated by a fluid pressure or an electronic actuation system. Operation of the clutches may be regulated by the control system 50.

[0097] In the embodiments of FIGs. 6a and 6b, the hydrostatic motor or motors 142, 142a is/are not rotated at high speeds in the second powertrain mode and so do not have to be specified for operation, at least in an idle condition, with the vehicle driving a maximum speed Vmax-

[0098] The powertrains 10 illustrated in FIGs. 6a and 6b may include a brake 180 for the hydrostatic pump 140 in addition to the clutch arrangements 182, 182a, 184 and are otherwise constructed and operated in a similar manner to the powertrain 10 illustrated in FIGs. 2 and 5 as described above.

[0099] Figure 7 illustrates a further embodiment of a powertrain 10 according the invention which has two hydrostatic motors 142, 142a in the hydrostatic transmission branch 120. The powertrain 10 illustrated in FIG. 7 differs from that shown in FIGs. 2 and 6a in having a different arrangement of gears and shafts on the output side of the transmission. Where components in the powertrain 10 in FIG. 7 are unchanged and perform the same function as

T1 those described above with reference to FIG. 2 or FIG. 6a, they are identified by the same reference numerals and will not be described further.

[0100] In the powertrain 10 shown in FIG. 7, the output drive gear 154 of the mechanical branch 110 is mounted rotationally fast to the rear output drive shaft 136. Drive is forwarded to the front output shaft 134 from the rear output drive shaft 136 via the intermediary gear 156, the offset gear 158 and a torque distribution clutch 186. The intermediary gear 156 is mounted rotationally fast to the rear output drive shaft 136 and the offset gear 158 is drivingly coupled to one side of the torque distribution cutch 186, in this embodiment the clutch plate or plates. The other side of the torque distribution clutch 186, in this embodiment the clutch housing, is rotationally fast with the front output drive shaft 134 so that when the torque distribution clutch 186 is engaged, drive is transmitted between the front and rear output drive shafts 134, 136 and when the torque distribution clutch 186 is disengaged, no drive is transmitted between the front and rear output drive shafts 134, 136.

[0101] A first hydrostatic motor 142 is directly connectable to the rear axle output drive shaft 136 via a first hydrostatic motor clutch 182 to primarily, but not exclusively, drive the rear wheels 3. The second hydraulic motor 142a is connected to the front axle drive shaft 134 to primarily, but not exclusively, drive the front wheels 2 via first and second hydraulic motor gears 188, 190, the torque distribution clutch 186 and a second hydrostatic motor clutch 182a. In this embodiment, the second hydraulic motor gear 190 is rotationally fast with the second side of the torque distribution clutch 186 and the second hydrostatic motor clutch 182 is operative to selectively couple an output shaft of the second hydrostatic motor 142a to the first hydraulic motor gear 188. Drive via hydraulic transmission branch 120 can be provided to the front axle drive shaft 134 by the second hydraulic motor 142a in addition to, or alternatively to, the drive delivered to the front axle drive shaft 134 from the rear axle drive shaft 136 depending on the extent of engagement of the torque distribution clutch 186 and first and second hydrostatic motor clutches 182, 182a.

[0102] The torque distribution clutch 186 is provided to selectively engage and disengage the front axle drive shaft 134 from the rear axle drive shaft 136 or to control the ratio of torque distribution between the two axles. This allows grip to be optimized dependent on the ground conditions. Furthermore, the second hydrostatic motor clutch 182a allows the second hydrostatic motor 142a to be selectively engaged and disengaged from the front axle drive shaft 134. In this transmission layout, the first hydrostatic motor 142 is configured for delivering lower torque but over the full range of vehicle speeds up to the first vehicle speed value VI. The second hydrostatic motor 142a is connected to front axle drive shaft 134 by the hydraulic motor gears 188, 190 which have a high transmission ratio. This allows the second hydrostatic motor 142a to provide higher torque but over a more limited, lower range of vehicle speeds than the first hydrostatic motor 142. At higher vehicle speeds, the second hydrostatic motor 142a may be disconnected from the front drive output shaft 134 by disengaging the second hydrostatic motor clutch 182a. In combination, both hydrostatic motors 142, 142a enable the hydrostatic transmission branch 120 to provide a full transmission output power with variable torque, variable vehicle speed and variable driving direction over a full range of vehicle speeds up to the first vehicle speed value VI.

[0103] By way of further explanation, various drive configurations for the hydrostatic branch are possible within the first powertrain mode depending on the operating positions of clutches 182, 182a, 186: a. with the torque distribution clutch 186 disengaged and the first and second hydrostatic motor clutches 182, 182a engaged, the first hydrostatic motor 142 drives the rear output drive shaft 136 and thereby rear wheels 3, and second hydrostatic motor 142a, drives the front output drive shaft 134 and thereby front wheels 2; b. with the torque distribution clutch 186 disengaged, the first hydrostatic motor clutch 182 engaged and the second hydrostatic motor clutch 182a disengaged, the first hydrostatic motor 142 drives the rear output drive shaft 136 and thereby rear axle and the respective rear wheels 3. The front output drive shaft 134 and thereby front axle and the respective front wheels 2 are not driven; c. with the torque distribution clutch 186 engaged, the first hydrostatic motor clutch 182 engaged and the second hydrostatic motor clutch 182a disengaged, the first hydrostatic motor 142 drives the rear output drive shaft 136 and thereby rear axle and the respective rear wheels 3 as well as the front output drive shaft 134 and thereby front axle and the respective front wheels 2; d. with the torque distribution clutch 186 engaged, the first hydrostatic motor clutch 182 and the second hydrostatic motor clutch 182a both engaged, the first and second hydrostatic motors 142, 142a both drive the front and rear output drive shafts 134, 136 and thereby the front and rear axles and the front and rear wheels. [0104] Drive is transmitted through the mechanical transmission branch 110 to the front output drive shaft 134 whenever the torque distribution clutch 186 is engaged. When operating in the second transmission mode, engagement of the torque distribution clutch 186 can be regulated to vary the distribution of torque to the front axle.

[0105] The powertrain 10 as illustrated in FIG. 7 can be operated in first and second powertrain modes in accordance with the method of the present invention as described above so that the speed N of the electric prime mover 20 is limited to a first prime mover speed value N1 in the first powertrain mode for vehicle ground speeds between standstill and a first vehicle speed value VI but where the speed of the electric prime mover 20 can be increased beyond the first prime mover speed N1 in the second powertrain mode for vehicle speeds above the first vehicle speed value. In the second powertrain mode, the first and second hydrostatic motor clutches 182, 182a can be disengaged so that the hydrostatic motors 142, 142a are not driven by the mechanical transmission branch for electric prime mover speeds above Nl. A brake 180 can be provided which is engaged to stop the hydrostatic pump 140 being driven in the second powertrain mode when the electric prime mover 20 is being operated at prime mover speeds above the first prime mover speed value Nl. In alternative embodiments, the brake 180 and/or the first hydrostatic motor clutch 182 could be omitted. Where the first hydrostatic motor clutch 182a is omitted, the second hydrostatic motor clutch 182a could be disengaged when the powertrain is operated in the second powertrain mode to at least prevent the second hydrostatic motor 142a from being driven at high speeds from the mechanical transmission branch 110.

[0106] As noted above, hydrostatic-mechanical power split transmissions can be configured as planetary output coupled (input-side split) or planetary input coupled (output-side split). In the embodiments described above, the transmission 100 is arranged as planetary output coupled transmission, in which toque is split into the mechanical and hydrostatic transmission branches 110, 120 by the planetary gear assembly 148, the drive being subsequently summed upstream of the transmission output drive shafts 134, 136. In this configuration, the planetary gear assembly 148 is located upstream of the hydrostatic and mechanical branches 110, 120. This type of power split transmission is shown in a simplified scheme in FIG. 8a, wherein the planetary gear assembly 148 is provided at the input shaft 102 to split the power between the mechanical branch 110 and the hydraulic branch 120 (indicated with hydrostatic pump 140 and hydrostatic motor 142). Power from the two braches 110, 120 is summed up at the summing shaft 104 for further connection to at least one output drive shaft 134,136. This simplified scheme corresponds to the transmissions 100 shown in Figures 2, 5, 6a, 6b, and 7. Powertrains 10 having an electric prime mover 20 as the prime mover and a transmission 100 of this type can be operated in the first and second powertrain modes according to the invention as described above. Such powertrains can be provided with a brake 180 that can be engaged to stop the hydrostatic pump 140 being driven when operated in the second powertrain mode and/or a clutch arrangement 182 to prevent the hydrostatic motor 142, 142a being driven when operated in the second powertrain mode.

[0107] The invention is also applicable to powertrains 10 having a power split hydrostatic-mechanical transmission which is planetary input coupled (also referred to as outputside power split). Such a transmission 200 is illustrated in a simplified schematic form in FIG. 8B, in which components which are the same as or which perform the same function as components in the previously described embodiments are provided with the same reference numerals but increased by 100 in each case. In this arrangement, toque is split between the mechanical transmission branch 210 and the hydraulic branch 220 (indicated with hydrostatic pump 240 and a hydrostatic motor 242) by gearing 290 at the input shaft 202 and recombined or summed up via a planetary gear assembly 248 at the summing shaft 204 for further connection to at least one output drive shaft 234, 236. In planetary input coupled hydrostatic-mechanical power split transmissions, the hydraulic motor or motors 242 can be of fixed displacement type. A powertrain having an electric prime mover (e.g., an electric motor) 20 and a planetary input coupled power-split hydrostatic-mechanical transmission 200 can be operated in accordance with the invention in a first and a second powertrain mode as described above, in which the speed N of the electric prime mover is limited to a first electric prime mover speed value N1 below the maximum prime mover speed N_max in a first powertrain mode for vehicle speeds between standstill and a first vehicle speed value VI during which power is transmitted though both the mechanical and hydrostatic transmission ranges with a variable ratio. The speed of the electric prime mover 20 not being so limited when the powertrain is operated in a second powertrain mode for vehicle speeds above the first vehicle speed value VI when power is transmitted only through the mechanical transmission branch 210 of the transmission.

[0108] As illustrated in FIG. 8b, a clutch arrangement 292 may be provided to disconnect the hydrostatic pump 240 from the input shaft 202 when the powertrain is operated in the second powertrain mode and/or a brake arrangement 280 may be provided to brake the hydrostatic motor 242 when the powertrain is operated in the second powertrain mode. This arrangement isolates the hydrostatic components 240, 242 from the mechanical transmission branch 110 when the powertrain is being operated in the second powertrain mode so that they are not under load.

[0109] It will be appreciated that the transmission 200 is shown in FIG. 8b in a simplified schematic form and that the transmission can be adapted in various ways, for example to include two or more hydrostatic motors in a similar manner to the transmissions 100 in FIGs. 2, 5, 6a, 6b and 7 as required in any practical application.

[0110] Where provided, actuation of the various brakes 132c, 180, 280 and clutches 132b 182, 182a, 184 186, 292 in the above-described embodiments may be controlled by the vehicle control system 50. Such brakes and/or clutches may be actuated by external forces, e.g., pressurized fluid such as hydraulic oil, whereby the supply of pressurized is controlled by vehicle control system 50. Alternatively, such brakes 132c, 180, 280 and/or clutches 132b, 182, 182a, 184, 186, 292 may be operated by electric power using e.g., electromagnetic activation. In this case, they may then be electrically connected to power source 40 and control of vehicle control system 50.

[0111] More alternatively, any clutches 132b, 182, 182a, 184, 186, 282 provided may be of centrifugal clutch type characterised in that the disconnection of the clutch is provided if their rotational speed exceeds a predetermined value. The activation speed of such centrifugal clutches must then be determined by considering the ratio of the planetary gear assembly 148 and the subsequent mechanical branch 110 along the gear path. To ensure that malfunction of such centrifugal clutches does not result in excessive load on the hydrostatic motors 142, 142a, rotational speed sensors (not shown) may be provided to monitor the actual speeds of the hydrostatic motors 142,142a. If set values for the maximum rotational speeds of the hydrostatic units are exceeded, the vehicle control system 50 may then limit the speed of the electric prime mover 20 and warn the driver appropriately. The usage of centrifugal clutches may reduce costs as pressurized fluid or current supply to close/open the clutch can be omitted.

[0112] The vehicle control system 50 may have at least one controller 50a configured to generate output signals to control the speed and/or torque of the prime mover 20 (e.g., though a Variable Frequency Drive), the operative angles of the variable displacement hydrostatic pump 140, 240 and motor 142, 142a, 242 via the adjustment unit 160 as well as actuation of any brakes 132c, 180, 280, and clutches 132b, 182, 182a, 184, 186, 292. The vehicle control system 50 may include sensors for detecting vehicle speed V and prime mover speed N which signals can be provided to the controller 50a as inputs.

[0113] The control system 50 can be considered as part of the powertrain to the extent that it is operative to monitor and regulate operation of the powertrain and its various components, although the control system 50 may be part of a larger vehicle control system, which monitors and controls operation of other systems on the vehicle.

[0114] In one embodiment, the controller 50a comprises one or more processors, such as processor 50b, input/output (I/O) interface(s), and memory 50c, all coupled to one or more data busses. The memory 50c may include any one or a combination of volatile memory elements (e.g., random-access memory RAM, such as DRAM, and SRAM, etc.) and non-volatile memory elements (e.g., ROM, hard drive, tape, CDROM, etc.). The memory 50b may store a native operating system, one or more native applications, emulation systems, or emulated applications for any of a variety of operating systems and/or emulated hardware platforms, emulated operating systems, etc. In one embodiment the memory comprises an operating system and powertrain control software. It should be appreciated by one having ordinary skill in the art that in some embodiments, additional or fewer software modules (e.g., combined functionality) may be stored in the memory 50c or additional memory. In some embodiments, a separate storage device may be coupled to the data bus, such as a persistent memory (e.g., optical, magnetic, and/or semiconductor memory and associated drives).

[0115] Electronic communications among the various components of the control system 50 may be achieved over a controller area network (CAN) bus or via a communications medium using other standard or proprietary communication protocols (e.g., RS 232, etc.). Communication may be achieved over a wired medium, wireless medium, or a combination of wired and wireless media.

[0116] The control system 50 may include sensors, indicated schematically at 50d which provide input to the controller 50a. These might include a ground speed sensor to provide a signal indicative of the ground speed V of the vehicle. The sensors might also include a sensor for detecting the rotary speed N of the electric prime mover 20, e.g. the rotary speed of an output shaft of the electric prime mover 20. This might for example detect the rotary speed of the output shaft of the electric prime mover 20 or some other component whose rotary speed is proportional to the speed of the electric prime mover. Alternatively, the controller may be configured to calculate the rotary speed of the electric prime mover 20 based, for example, on the parameters of the electric power supply to the electric prime mover 20. The control system may include a sensor for detecting the rotary speed of the PTO shaft 132a. However where the PTO shaft 132a is driven at a fixed ratio from the electric prime mover 20, the rotary speed of the PTO shaft can be determined from the measured or calculated rotary speed of the electric prime mover 20 or vice versa. A sensor to detect whether the PTO clutch 132b is engaged may also be provided.

[0117] The invention is not limited to the embodiments disclosed herein but should be understood as encompassing variations and modifications falling within the scope of the various aspects of the invention as defined in the appended claims and other equivalent statements of invention set out above. For example, in alternative embodiments, the input shaft 102 may be drivingly coupled to the sun gear 148b of the planetary gear assembly 148 and the mechanical transmission branch coupled to the planetary gear carrier 148a in a planetary output coupled type transmission. In a planetary input coupled transmission, the hydrostatic branch 220 may be drivingly coupled to the ring gear of the planetary gear assembly 248, the mechanical branch 210 drivingly coupled to the sun gear and the summing shaft/drive output shafts 204, 234, 236 drivingly coupled to the planetary gear carrier.

[0118] All references cited herein are incorporated herein in their entireties. If there is a conflict between definitions herein and in an incorporated reference, the definition herein shall control.

Claims

CLAIMS What is claimed is:

1. A vehicle powertrain including an electric prime mover, especially an electric motor, and a hydrostatic-mechanical power split transmission having a mechanical transmission branch and a hydrostatic transmission branch for driving the vehicle, the transmission being operable in two transmission modes, a first transmission mode in which drive is transmitted via both the mechanical and hydrostatic transmission branches with a variable transmission ratio ic T and a second transmission mode in which drive is transmitted only through the mechanical transmission branch at a constant transmission ratio icvr; and a control system for controlling operation of the electric prime mover and the transmission; wherein the powertrain is configured to be operable in two powertrain modes: a first powertrain mode in which the transmission is operated in the first transmission mode and the rotational speed of the electric prime mover does not exceed a first prime mover speed value Nl, and a second powertrain mode in which the transmission is operated in the second transmission mode and the rotational speed of the electric prime mover is permitted to exceed the first prime mover speed value Nl.

2. A vehicle powertrain according to claim 1, wherein the powertrain is configured to enable the electric prime mover to operate at rotational speeds up to a maximum prime mover speed Nmax in the second powertrain mode, the first prime mover speed value Nl being lower than the maximum prime mover speed N_max.

3. A vehicle powertrain according claim 1 or claim 2, wherein the powertrain is configured to be operated in the first powertrain mode for vehicle speeds between zero (standstill) and a first vehicle speed value VI and to be operated in the second powertrain mode for vehicle speeds above the first vehicle speed value VI, wherein the first vehicle speed may optionally be about 1/3 of a maximum vehicle speed.

4. A vehicle powertrain according to any one of the preceding claims, wherein the powertrain comprises an input shaft drivable from the electric prime mover, at least one output drive shaft for driving at least one driven ground engaging member of the vehicle, the mechanical transmission branch configured to transfer drive mechanically from the input shaft to the at least one output drive shaft, the hydrostatic transmission branch having at least one variable displacement hydrostatic pump driven from the input shaft and at least one hydrostatic motor for driving the at least one output drive shaft.

5. A vehicle powertrain according to any one of the preceding claims, wherein the transmission comprises a planetary gear assembly and is configured as a planetary output coupled type power-split transmission.

6. A vehicle powertrain according to claim 5, wherein the transmission includes a brake arrangement selectively operable to prevent the at least one hydrostatic pump being driven from the input shaft when the powertrain is operating in the second powertrain mode and/or a clutch arrangement selectively operable to drivingly decouple the at least one hydrostatic motor from the mechanical transmission branch when the powertrain is operating in the second powertrain mode.

7. A vehicle powertrain according to any one of claims 1 to 4, wherein the transmission comprises a planetary gear assembly and is configured as a planetary input coupled type power-split transmission.

8. A vehicle powertrain according to claim 7, wherein the transmission includes a brake arrangement selectively operable to prevent the at least one hydrostatic motor being driven from the mechanical transmission branch when the powertrain is operating in the second powertrain mode and/or a clutch arrangement selectively operable to drivingly decouple the at least one hydrostatic pump from the input shaft when the powertrain is operating in the second powertrain mode.

9. A vehicle powertrain according to claim 4, or ant one of claims 5 to 8 when dependent on claim 4, wherein the at least one hydrostatic motor is a variable displacement hydrostatic motor.

10. A method of operating a vehicle powertrain including an electric prime mover, especially an electric motor, and a hydrostatic-mechanical power split transmission having a mechanical transmission branch and a hydrostatic transmission branch for driving the vehicle, the transmission being operable in two transmission modes, a first transmission mode in which drive is transmitted via both the mechanical and hydrostatic transmission branches with a variable transmission ratio icvr and a second transmission mode in which drive is transmitted only through the mechanical transmission branch at a constant transmission ratio icvr; and a control system for controlling operation of the electric prime mover and the transmission; wherein the method comprises operating the powertrain in one of a first powertrain mode in which the transmission is operated in the first transmission mode and the rotational speed of the electric prime mover does not exceed a first prime mover speed value Nl, and a second powertrain mode in which the transmission is operated in the second transmission mode and the prime mover is permitted to operate at rotational speeds which exceed the first prime mover speed value Nl.

11. A method according to claim 10, wherein the powertrain is configured to enable the electric prime mover to operate at rotational speeds up to a maximum prime mover speed Nmax in the second powertrain mode, the first prime mover speed value Nl being lower than the maximum prime mover speed N_max.

12. A method according to claim 10 or claim 11, the method comprising operating the powertrain in the first powertrain mode for vehicle speeds between zero (standstill) and a first vehicle speed value VI and operating the powertrain in the second powertrain mode for vehicle speeds above the first vehicle speed value VI, wherein the first vehicle speed is optionally about 1/3 of a maximum vehicle speed.

13. A method according to any one of claims 10 to 12, wherein the powertrain comprises an input shaft drivable from the electric prime mover, at least one output drive shaft for driving at least one driven ground engaging member of the vehicle, the mechanical transmission branch configured to transfer drive mechanically from the input shaft to the at least one output drive shaft, the hydrostatic transmission branch having at least one variable displacement hydrostatic pump driven from the input shaft and at least one hydrostatic motor for driving the at least one output drive shaft.

14 A method according to claim 13, the method comprising preventing the at least one hydrostatic pump and/or the at least one hydrostatic motor from being driven when the powertrain is operating in the second powertrain mode.

15. A vehicle, especially a utility vehicle, having a powertrain according to any one of claims 1 to 9 and/or which is operated according to the method of any one of claims 10 to 13.