Electric work vehicle having a hydrostatic drive train

Abstract

The invention relates to a work vehicle comprising an electrical power source 10, a hydrostatic drivetrain, a switching device 20, and a control unit 25. The hydrostatic drivetrain comprises an electric motor 9 powered by the electrical power source 10, a hydrostatic pump 11 driven by the electric motor 9, the hydrostatic pump 11 having a continuously variable pump displacement, drive wheels 8F, 8R, and at least one hydrostatic motor 13F, 13R with a fixed motor displacement. The switching device 20 is configured to selectively switch the drive displacement between at least one slow-moving drive and one fast-moving drive of the work vehicle. The control unit 25 is configured to drive the rotational speed of the electric motor 9 in response to a switching of the drive displacement.

Description

Technical field

[0001] The invention relates to an electric work vehicle driven in movement by a hydrostatic transmission drive train, also called a hydrostatic drive train. Technological background

[0002] Work vehicles, particularly load handling equipment, are generally equipped with a powertrain comprising a diesel-type internal combustion engine coupled to a hydrostatic transmission. Such a hydrostatic transmission may in particular comprise a continuously variable displacement hydrostatic pump and one or more fixed displacement hydrostatic motors.

[0003] On a work vehicle equipped with four-wheel drive, a hydrostatic driveline is capable of developing significant tractive effort in a four-wheel drive configuration, and of enabling the work vehicle to achieve high forward speeds in a two-wheel drive configuration.

[0004] However, switching from a four-wheel drive configuration to a two-wheel drive configuration, and vice versa, cannot be carried out while the work vehicle is moving. Indeed, switching from one configuration to the other generates a sudden variation in the flow rate of hydraulic fluid which cannot be controlled either by a variation in the displacement of the hydrostatic pump, or by a variation in the speed of the internal combustion engine, the responsiveness of the hydrostatic pump and the internal combustion engine not being sufficient. Switching between the four-wheel drive and two-wheel drive configurations is therefore carried out when the work vehicle is stationary.

[0005] Document US 2012 / 023924 A1 discloses a work vehicle according to the preamble of claim 1. Summary

[0006] An idea underlying the invention is to provide a work vehicle equipped with a hydrostatic drive train of simple arrangement, capable of developing a significant tractive effort while allowing the work vehicle to reach high forward speeds.

[0007] According to one embodiment, the invention provides a work vehicle comprising: an electrical power source; a hydrostatic drive train, the hydrostatic drive train comprising an electric motor supplied with electrical energy by the electrical energy source, a hydrostatic pump driven by the electric motor, at least two drive units, each drive unit having at least two drive wheels and at least one hydrostatic motor for rotational driving of the at least two drive wheels, a drive displacement of the drive unit being equal to the displacement of the at least one hydrostatic motor supplied with hydraulic fluid by the hydrostatic pump, a total drive displacement being equal to a sum of the drive displacements of the two drive units, at least one of the two drive units being configured as a variable drive unit having a variable drive displacement,and a switching device configured to switch the variable engine displacement(s) selectively between at least one slow-moving drive in which the total engine displacement is equal to a first value, and one fast-moving drive in which the total engine displacement is equal to a second value, the first value being strictly greater than the second value, and a control unit configured to control a rotational speed of the electric motor such that, in response to a switching of the engine displacement from the fast-moving drive to the slow-moving drive, increase the rotational speed of said electric motor such that the hydrostatic pump provides a flow rate of hydraulic fluid required to respond to an increase in the total engine displacement,and in response to switching the engine displacement from slow motion drive to fast motion drive, reducing the rotational speed of said electric motor such that the hydrostatic pump absorbs excess hydraulic fluid flow resulting from a decrease in the total engine displacement.

[0008] Thanks to these features, the particularly responsive electric motor speed control makes it possible to control the variation in hydraulic fluid flow resulting from the variation in the total engine displacement when switching the engine displacement. The work vehicle can thus switch smoothly and while moving between hydrostatic configurations developing high tractive forces but low forward speeds, and hydrostatic configurations with a lower total engine displacement, developing little tractive force but allowing high forward speeds to be achieved.

[0009] According to one embodiment, the hydrostatic pump is an axial piston volumetric pump or a radial piston volumetric pump.

[0010] According to one embodiment, the hydrostatic pump has a fixed displacement.

[0011] According to one embodiment, the hydrostatic pump has a continuously variable displacement.

[0012] According to one embodiment, the hydrostatic motors are axial piston motors or radial piston motors.

[0013] According to one embodiment, the work vehicle further comprises a forward speed sensor configured to measure a forward speed of the work vehicle, and the control unit is configured to control the switching device so as to switch the driving displacement from the slow travel drive to the fast travel drive when the travel speed is greater than or equal to a switching speed, and from the fast travel drive to the slow travel drive when the travel speed is less than the switching speed.

[0014] The control unit thus automatically controls the switching of the engine displacement in order to optimize the performance of the hydrostatic drive train of the work vehicle.

[0015] According to one embodiment, the control unit comprises a memory and the switching speed has been previously determined by digital simulation or by workshop testing and stored in the memory of said control unit.

[0016] According to one embodiment, the work vehicle further comprises a travel requirement sensor configured to measure a travel requirement, and the switching speed is a function of the forward speed of the work vehicle and the travel requirement.

[0017] Thus, when the work vehicle is positioned on sloping or uneven ground, moving forward against a heavy load or transporting a heavy load, the tractive effort applied to the drive wheels increases compared to the same unladen movement on level ground. Determining the switching speed according to the forward speed of the work vehicle and the travel requirement thus makes it possible to optimize the performance of the hydrostatic driveline according to the ground conditions and handling operations.

[0018] According to one embodiment, the switching device comprises a switching control allowing an operator to manually switch the driving displacement.

[0019] According to one embodiment, the two drive units comprise a front drive unit comprising two front drive wheels and at least one front hydrostatic motor, and a drive unit comprising two rear drive wheels and at least one rear hydrostatic motor, the front and rear hydrostatic motors being connected to each other in parallel.

[0020] According to one embodiment, at least one of the front drive unit and the rear drive unit comprises a transmission axle equipped with a pair of drive wheels and a hydrostatic motor coupled to the two drive wheels.

[0021] According to one embodiment, the switching device comprises a set of valves configured to selectively allow the supply of hydraulic fluid to a said variable drive unit by the hydrostatic pump for the displacement drive and to prevent the supply of hydraulic fluid to said variable drive unit by the hydrostatic pump for the rapid displacement drive.

[0022] In such an embodiment, when the valve assembly prevents the supply of hydraulic fluid to said variable drive unit, no hydrostatic motor of the variable drive unit is supplied with hydraulic fluid by the hydrostatic pump. As a result, the driving displacement that the variable drive unit functionally contributes to the total engine displacement is zero, even though the displacement of the hydrostatic motor(s) not supplied with hydraulic fluid would not be physically zero. Thus, the driving displacement that the variable drive unit functionally contributes to the total engine displacement is variable at least due to the valve assembly; it may even be variable only due to the valve assembly, if the or each hydrostatic motor of the variable drive unit is of constant displacement.

[0023] According to one embodiment, one of the rear drive unit and the front drive unit is configured as a variable drive unit, and wherein the slow travel drive is four-wheel drive, front and rear, and the fast travel drive is two-wheel drive, front or rear.

[0024] According to one embodiment, said or each hydrostatic motor of a variable drive unit is of constant displacement.

[0025] According to one embodiment, a said variable drive unit comprises a dual-displacement hydrostatic motor, and the switching device is configured to selectively switch the dual-displacement hydrostatic motor into a first displacement for fast travel drive and into a second displacement strictly greater than the first displacement for slow travel drive.

[0026] Thus, the engine displacement that the variable drive unit functionally contributes to the total engine displacement is variable at least due to the switching of the dual-displacement hydrostatic motor between the first displacement and the second displacement, i.e. due to the physical change in the displacement of the dual-displacement hydrostatic motor.

[0027] According to one embodiment, the dual-displacement hydrostatic motor does not have a continuously variable displacement. Brief description of the figures

[0028] The invention will be better understood, and other objects, details, characteristics and advantages thereof will appear more clearly during the following description of several particular embodiments of the invention, given solely for illustrative and non-limiting purposes, with reference to the accompanying drawings. [ Fig. 1 ] There Figure 1is a schematic view of a work vehicle according to one embodiment of the invention. Fig. 2 ] There Figure 2 is a functional schematic representation of a work vehicle in a drive mode, according to a first embodiment. Fig. 3 ] There Figure 3 is a schematic representation analogous to that of the Figure 2 , according to the first embodiment. [ Fig. 4 ] There Figure 4 is a functional schematic representation of the control unit according to one embodiment. Fig. 5 ] There Figure 5 is a graphical representation of the variation of the total engine displacement as a function of the forward speed of the work vehicle of the Figure 2 . [ Fig. 6 ] There Figure 6 is a graphical representation of the tractive effort applied to the drive wheels as a function of the working vehicle's flat ground travel speed. Figure 5 . [ Fig. 7 ] There Figure 7is a schematic representation analogous to that of the Figure 2 , according to a second embodiment. [ Fig. 8 ] There figure 8 is a schematic representation analogous to that of the Figure 7 , according to a third embodiment. Description of the embodiments

[0029] The following embodiments are described in relation to an electric work vehicle. This vehicle may be an all-terrain or off-road electric powered truck, such as an electric vertical mast forklift, an electric articulated chassis forklift, an electric load handling truck, for example with a telescopic arm, a rotating telescopic forklift or an electric compact loader. This list is not exhaustive.

[0030] In reference to the Figure 1, the work vehicle 1 is an electric forklift with a vertical mast. Such a vehicle comprises a chassis 2 surmounted by a driver's cab 3 inside which an operator can sit.

[0031] The chassis 2 is equipped with a ground engagement structure S comprising two driving axles, front 7F and rear 7R, located one after the other in the direction of travel D of the work vehicle 1. The front driving axle 7F and the rear driving axle 7R each comprise two driving wheels, denoted 8F and 8R respectively. The driving while moving the work vehicle is enabled by the rotation of the driving wheels 8F and / or 8R in contact with the ground S.

[0032] The work vehicle 1 also comprises a lifting mast 4 articulated at the front of the chassis 2. The lifting mast 4 is generally equipped with forks 5 mounted so as to move up and down along the lifting mast 4. In particular, the lifting mast 4 is coupled to the chassis 2 by a first pivot connection P with a horizontal pivot axis extending transversely to the direction of movement D of the work vehicle, parallel to the axis of rotation of the drive wheels 8F, 8R. At least one actuator 6, such as a hydraulic cylinder, arranged between the chassis 2 and the lifting mast 4 enables the lifting mast 4 to be driven to pivot about the pivot axis P in the direction of an inclination of the lifting mast 4 towards the front or rear of the chassis 2.

[0033] In reference to the figures 2 And 3 , the work vehicle is described according to a first embodiment.

[0034] The work vehicle 1 is equipped with a hydrostatic drive train comprising an electric motor 9 supplied with electrical energy by an electrical energy source 10, a hydrostatic pump 11 driven in rotation by the electric motor 9, and two drive units, namely a front drive unit 12F provided with the two front drive wheels 8F and a rear drive unit 12R provided with the two rear drive wheels 8R.

[0035] The electrical energy source 10 may consist of a modular assembly of electrochemical accumulator batteries and / or energy storage capacitors.

[0036] To drive the hydrostatic pump 11 in rotation, the output shaft of the electric motor 9 is coupled to the drive shaft of the hydrostatic pump 11, for example by means of a keyed or splined connection system.

[0037] Advantageously, the electric motor 9 is dedicated solely to the rotational drive of the hydrostatic pump 11, with the exception of a booster pump 15 which supplies hydraulic fluid to a booster circuit described later. The rotational speed of the electric motor 9 can thus be accelerated or slowed down to vary only the flow rate of hydraulic fluid delivered by the hydrostatic pump 11.

[0038] In the described embodiment, the hydrostatic pump 11 has a continuously variable displacement D p, for example controlled hydraulically, hydromechanically or electrically. Alternatively, the hydrostatic pump has a fixed displacement.

[0039] Advantageously, the hydrostatic pump 11 is a volumetric pump with pistons, axial (barrel or straight axis pump, or connecting rod or broken axis pump) or radial.

[0040] The displacement of the hydrostatic pump 11 is for example controlled by a displacement regulating device (not shown) configured to control the displacement as a function of the rotation speed of the electric motor 9. Such regulation of the displacement of the hydrostatic pump 11 as a function of the engine speed is said to be of the “automotive” type; it can be carried out by hydraulic or electrohydraulic control of the displacement, preferably proportional.

[0041] The hydrostatic pump 11 is provided with a discharge port 11a and a suction port 11b.

[0042] Each drive unit, front 12F and rear 12R respectively, further comprises a hydrostatic motor, front 13F and rear 13R respectively. The hydrostatic motor front 13F, rear 13R respectively, enables the rotational drive of a front 8F, rear 8R respectively drive wheel.

[0043] The 13F front hydrostatic motors are fixed displacement D m (f)< and the 13R rear hydrostatic motors are fixed displacement D m (r)< .

[0044] The front hydrostatic motors 13F, respectively rear 13R, each have an inlet port 13Fa, respectively 13Ra, and an outlet port 13Fb, respectively 13Rb.

[0045] Advantageously, the front 13F and rear 13R hydrostatic motors are axial piston motors (barrel or straight axis motors) or radial piston motors.

[0046] The hydrostatic pump 11 and the front 13F and rear 13R hydrostatic motors operate in a closed circuit. In other words, the hydraulic fluid is first delivered by the hydrostatic pump 11 to the front 13F and rear 13R hydrostatic motors via high-pressure fluid connections in the hydraulic circuitry of the hydrostatic drive train. The hydraulic fluid is then returned to the suction side of the hydrostatic pump 11 via low-pressure fluid connections.

[0047] Advantageously, the hydraulic fluid is a mineral or synthetic oil obtained by refining petroleum.

[0048] More specifically, the hydraulic circuitry of the hydrostatic drive train comprises a high-pressure main line 14a and a low-pressure main line 14b. The high-pressure main line 14a branches into four high-pressure branches 141Fa, 141Ra. The high-pressure main line 14a and the two high-pressure branches 141Fa, respectively 141Ra, connect the discharge port 11a of the hydrostatic pump 11 to the inlet ports 13Fa, respectively 13Ra, of the front hydrostatic motors 13F, respectively rear 13R. The low-pressure main line 14b also branches into four low-pressure branches 141Fb, 141Rb. The two low-pressure branches 141Fb, respectively 141Rb, and the low-pressure main line 14b connect the outlet ports 13Fb, respectively 13Rb, of the front hydrostatic motors 13F, respectively rear 13R to the suction port 11b of the hydrostatic pump 11.Thus, each of the front 13F and rear 13R hydrostatic motors is connected in parallel to the hydrostatic pump 11.

[0049] The main pipes 14a, 14b and their branches 141Fa, 141Ra, 141Fb, 141Rb are, for example, steel pipes or flexible pipelines.

[0050] The hydrostatic drive train further comprises, in a manner known per se, a booster pump 15 coupled to the hydrostatic pump 11 and, therefore, driven by the electric motor 9. The booster pump 15 draws hydraulic fluid from a reservoir 16 through a strainer 17 and delivers it, via booster valves 18, into one of the low-pressure fluid connections 14b, 141Fb or 141Rb to compensate for internal leaks in the closed hydraulic circuit of the hydrostatic drive train. The supply pressure of the booster circuit is limited by the pressure limiter 19.

[0051] The work vehicle 1 further comprises a switching device 20. In the embodiment described, the switching device 20 comprises two on-off, two-position, three-way controlled solenoid valves 21, each controlled by a solenoid 23. Such solenoid valves comprise a fixed, hollow body within which a movable slide valve moves. Orifices drilled in the body make it possible to establish the desired connections or closures between the different ways to control the flow of the hydraulic fluid.

[0052] The solenoid valves 21 are positioned on the high pressure branches 141Ra, i.e. between the discharge port 11a of the hydrostatic pump 11 and the inlet ports 13Ra of the rear hydrostatic motors 13R.

[0053] Each solenoid valve 21 has a first position I, illustrated in the Figure 2, allowing the flow of hydraulic fluid between the hydrostatic pump 11 and the rear hydrostatic motors 13R. The first position I corresponds, for example, to the position of the movable slide in the absence of magnetic excitation of the solenoid 23 under the effect of the mechanical reaction of a spring 22. The activation of the solenoid 23 allows the change of position of the solenoid valve 21, from the first position I to a second position II, illustrated in the Figure 3 , preventing the flow of hydraulic fluid between the hydrostatic pump 11 and the rear hydrostatic motors 13R.

[0054] In other words, when the solenoid valves 21 are in their first position I, the rear hydrostatic motors 13R are supplied with hydraulic fluid by the hydrostatic pump 11; the driving displacement of the rear drive unit 12R is equal to 2 D m (r) < . On the other hand, when the solenoid valves 21 are in their second position II, the supply of hydraulic fluid to said rear hydrostatic motors 13R is cut off; the driving displacement of the rear drive unit 12R is zero. In this embodiment, the rear drive unit 12R is therefore configured as a variable drive unit having a variable driving displacement.

[0055] According to an alternative embodiment not described, the solenoid valves 21 could be positioned on the high pressure branches 141Fa, i.e. between the discharge port 11a of the hydrostatic pump 11 and the inlet ports 13Fa of the front hydrostatic motors 13F.

[0056] The switching device 20 may further comprise a switching control 24, for example electrically connected to the solenoid valves 21, to allow an operator to manually switch the drive mode of the work vehicle 1. On the figures 2 And 3 , the switching control 24 is connected to a control unit 25, described later, which controls the solenoid valves 21.

[0057] The switching device 20 thus allows the selective switching of the driving displacement of the rear drive unit 13R between: a four-wheel drive slow-moving drive 8F, 8R, in which the solenoid valves 21 are in their first position I so that the two front hydrostatic motors 13F, each of fixed displacement D m (f)< , and the two rear hydrostatic motors 13R, each of fixed displacement D m (r)< , are supplied with hydraulic fluid by the hydrostatic pump 11; and a two-wheel drive fast-moving drive 8F, in which the solenoid valves 21 are in the second position II so that only the two front hydrostatic motors 13F, each of fixed displacement D m (r)< , are supplied with hydraulic fluid by the hydrostatic pump 11. By defining the total engine displacement D Σ as the sum of the engine displacements of the front 12F and rear 12R drive units supplied with hydraulic fluid by the hydrostatic pump 11: the total engine displacement D Σ of the four-wheel drive slow-moving drive 8F, 8R is equal to a maximum value D Σ,max of total engine displacement D Σ such that D Σ,max = 2 x D m (f)< + 2 x D m (r)< ; and the total engine displacement D Σ of the two-wheel drive fast-moving drive 8F is equal to a minimum value D Σ,min of total engine displacement D Σ such that D Σ,min = 2 x D m (f)< .

[0058] Thus, switching the engine displacement of the rear drive unit 13R between a four-wheel drive slow travel drive 8F, 8R and a two-wheel drive fast travel drive 8F of the work vehicle 1 correspondingly generates a variation of the total engine displacement between a minimum value D Σ,min and a maximum value D Σ,max respectively.

[0059] In reference to the Figure 4, the work vehicle 1 further comprises a control unit 25. The control unit 25 is an electronic and / or computer unit which can be implemented in different forms, in a unitary or distributed manner, by means of hardware and / or software components, associated with a memory. Usable hardware components are specific integrated circuits ASIC, programmable logic networks FPGA or microprocessors. Software components can be described in different programming languages, for example C, C++, Java or VHDL. This list is not exhaustive. The memory stores data and computer instructions. The computer instructions are executed by the hardware and / or software components to enable the control unit 25 to carry out the operations or steps described later.

[0060] The control unit 25 is configured to, on the one hand, control the switching device 20 so as to switch the driving displacement of the variable rear drive unit 12R in movement as a function of a switching speed VC and, on the other hand, control the rotation speed of the electric motor 9 in response to a switching of said driving displacement, or even in response to a variation of the total engine displacement D Σ .

[0061] The switching speed VC may have been previously determined by numerical simulation or by workshop testing, and stored in the memory of the control unit 25.

[0062] Advantageously, the control unit 25 determines the switching speed VC as a function of the forward speed V of the work vehicle 1 and the forward travel requirement BD specified by the operator. Indeed, when the work vehicle 1 is placed on sloping or uneven ground, advances against a heavy load or transports a heavy load, the tractive effort at the drive wheels 8F, 8R increases compared to the same movement when empty on flat ground. Determining the switching speed VC as a function of the forward speed V and the travel requirement BD makes it possible to optimize the performance of the hydrostatic drive train, taking into account the ground conditions and the handling operations.

[0063] The forward speed V of the work vehicle 1 relative to the ground S is measured by means of a forward speed sensor 26, arranged for example between one of the hydrostatic motors 13F, 13R and the drive wheel 8F, 8R which it drives in rotation. The forward speed sensor 26 is connected by wired or wireless connection to the control unit 25.

[0064] The travel requirement BD can be specified by the operator by means of an accelerator pedal 27 installed in the driver's cab 3 of the work vehicle 1. The accelerator pedal 27 is equipped with a position sensor 28. For example, the position sensor 28 is configured to measure the degree of depression of the accelerator pedal 27, or to measure the pressure exerted by the operator on the accelerator pedal 27. The position sensor 28 is connected by wired or wireless connection to the control unit 25 to which it transmits the data it acquires. The control unit correlates the position data of the accelerator pedal 27 with a travel requirement BD of the work vehicle 1, for example a need for torque to be applied to the drive wheels 8F and / or 8R, a need for forward speed of the work vehicle 1 or a need for power to overcome the resistance to travel of the work vehicle 1.

[0065] In reference to the figures 2 to 4 , the switching control 24 of the switching device 20 is connected, by wired or wireless connection, to the control unit 25, which controls the manual switching of the engine displacement specified by the operator.

[0066] In relation to the figures 5 And 6 , the method of controlling the switching device 20 and the electric motor 9 by the control unit 25 is now described.

[0067] There Figure 5 illustrates a variation profile of the total engine displacement D Σ as a function of the forward speed V of the work vehicle 1.

[0068] There Figure 6is a graphical representation of the tractive effort 29 applied to the drive wheels 8F, 8R as a function of the forward speed V of the work vehicle 1 on flat ground S. Curve 30 represents the tractive effort 29 provided by the hydrostatic drive train when the work vehicle 1 is driven in slow motion, i.e. with four drive wheels 8F, 8R. Curve 31 represents this same tractive effort 29 when the work vehicle 1 is driven in fast motion with two drive wheels 8F. In the acceleration phase

[0069] Work vehicle 1 is initially stationary. The operator presses the accelerator pedal: the tractive effort 29 developed by the hydrostatic driveline is at its maximum, as seen in curve 30 of the Figure 6 . The work vehicle 1 is driven in slow motion at forward speed V.

[0070] As long as the forward speed V is strictly lower than the switching speed VC (V < VC ), the control unit 25 controls the switching device 20 so that the work vehicle 1 remains driven in slow motion, i.e. with four-wheel drive 8F, 8R. In the forward speed range V considered, i.e. between zero forward speed and the switching speed VC (0 ≤ V < VC ), the four-wheel drive 8F, 8R of the work vehicle 1 allows the hydrostatic drive train to develop the greatest tractive forces.

[0071] As previously stated, when the work vehicle 1 is driven in four-wheel drive 8F, 8R, the total engine displacement D Σ is equal to a maximum value D Σ,max since all the hydrostatic motors, namely the front 13F and rear 13R hydrostatic motors of the front 12F and rear 12R drive units are supplied with hydraulic fluid by the hydrostatic pump 11.

[0072] When the work vehicle 1 reaches a forward speed V equal to the switching speed VC (V = VC ), the control unit 25 controls the switching device 20 so as to switch the driving displacement of the rear drive unit 12R to pass from the four-wheel drive slow travel drive 8F, 8R to the two-wheel drive fast travel drive 8F in which only the front hydrostatic motors 13F are supplied with hydraulic fluid by the hydrostatic pump 11. The supply of hydraulic fluid to the two rear hydrostatic motors 13R by the hydrostatic pump 11 being cut off, the total engine displacement D Σ drops abruptly to a minimum value D Σ,min, i.e. a variation of the total engine displacement D Σ of D Σ , max − D Σ , min = 2 × D m r .

[0073] To compensate for this sudden variation in the total engine displacement D Σ from its maximum value D Σ,max to its minimum value D Σ,min , the control unit 25 controls the electric motor 9 to reduce its rotation speed so as to absorb the excess flow of hydraulic fluid resulting from the disconnection of the rear hydrostatic motors 13R.

[0074] Beyond the speed of the switching speed VC (V ≥ VC ), the control unit 25 controls the switching device so that the working vehicle 1 is driven in fast two-wheel drive 8F. The reduction of the total engine displacement D Σ allows the working vehicle 1 to accelerate to reach the maximum forward speed V max . Such a speed could not be achieved if the four-wheel drive 8F, 8R were selected, as can be seen by comparing the curves 30 and 31 on the Figure 6 . In the deceleration phase

[0075] The working vehicle 1 is driven in movement at a forward speed V greater than or equal to the switching speed VC (V ≥ VC ), i.e. with two-wheel drive 8F. As explained previously, the total engine displacement D Σ is equal to its minimum value D Σ,min since only the front hydrostatic motors 13F of the front drive unit 12F are supplied with hydraulic fluid by the hydrostatic pump 11.

[0076] Then, the work vehicle 1 decelerates, for example because the operator brakes or releases the accelerator pedal 27. The forward speed V of the work vehicle 1 decreases until it is equal to the switching speed VC (V = VC ). The control unit 25 then controls the switching device 20 so as to switch the drive displacement of the rear drive unit 12F from the slow travel drive to the fast travel drive in which all the hydrostatic motors 13F, 13R are supplied with hydraulic fluid by the hydrostatic pump 11. The total engine displacement D Σ then increases abruptly from its minimum value D Σ,min to its maximum value D Σ,max .

[0077] To compensate for this sudden variation in the total engine displacement D Σ , the control unit 25 controls the electric motor 9 to increase its rotation speed so as to provide the required hydraulic fluid flow resulting from the connection of the rear hydrostatic motors 13F to the hydrostatic pump 11.

[0078] In reference to the Figure 6, for an example of dimensioning, a work vehicle 1 with a mass of 5 tonnes equipped with front 8F and rear 8R drive wheels with a radius of 0.3 to 0.5 metres driven in rotation by a hydrostatic drive train operating under a working pressure of 450 bars, consisting of a hydrostatic pump with a displacement varying between 0 cc and 70 cc connected to two front 13F and rear 13R hydrostatic motors with a displacement of 688 cc / rev and 398 cc / rev, can, in a four-wheel drive configuration, develop a tractive effort of 3100 daN, but reach a maximum forward speed V max of 14 km / h; while this same vehicle, in a two-wheel drive configuration, can only develop a tractive effort of 1700 daN, but can reach a maximum speed of 24 km / h.

[0079] There Figure 7shows a work vehicle 701 according to a second embodiment. The representation is simplified; in particular, the hydraulic components of the feed circuit are not repeated. Elements similar or identical to those of the first embodiment, in particular illustrated in figures 2 And 3 , carry the same reference figure increased by 700.

[0080] The front drive unit 712F (respectively rear 712R) comprises a front transmission axle 732F (respectively rear 732R) equipped with a pair of front drive wheels 708F (respectively rear 708R), and a front hydrostatic motor 713F (respectively rear 713R) coupled to the front transmission axle 732F (respectively rear 732R). The front hydrostatic motors 732F and rear 732R are connected in parallel.

[0081] The 13F front hydrostatic motor has a fixed displacement D m (f)< and the 13R rear hydrostatic motor has a fixed displacement D m (r)< .

[0082] In this second embodiment, the switching device 720 comprises a single solenoid valve 721, interposed between the discharge port of the hydrostatic pump 711 and the inlet port of the rear hydrostatic motor 713R.

[0083] The slow-moving drive is four-wheel drive 708F, 708R, the solenoid valve 721 being in the first position I so that all the hydrostatic motors, front 713F and rear 713R, are supplied with hydraulic fluid by the hydrostatic pump 711. The total engine displacement D Σ is then equal to its maximum value D Σ,max such that D Σ,max = D m (f)< + D m (r)< .

[0084] The fast moving drive is two-wheel drive 708F, the solenoid valve 721 being in the second position II so that only the front hydrostatic motor 713F is supplied with hydraulic fluid by the hydrostatic pump 711. The total engine displacement D Σ is then equal to its minimum value D m such that D Σ,min = D m (f) < .

[0085] The control of the switching device 720 and the rotational speed of the electric motor 709 by the control unit 725 is identical to that previously described. The control unit 725 controls, on the one hand, the switching device 720 so as to switch the driving displacement of the variable rear drive unit 712R in displacement as a function of a switching speed VC and controls, on the other hand, the speed of the engine 709 in response to a switching of said driving displacement, generating a variation of the total engine displacement D Σ between its maximum value D Σ,max and its minimum value D Σ,min .

[0086] The same observations as those set out for the first embodiment apply, it being specified that the variation in the total engine displacement D Σ corresponds here to the displacement D m (r)< of the rear hydrostatic engine 713R since D Σ , max − D Σ , min = D m r .

[0087] There figure 8shows a work vehicle 801 according to a third embodiment. The representation is simplified; in particular, the hydraulic components of the feeding circuit are not repeated. Elements similar or identical to those of the first embodiment, in particular illustrated in figures 2 And 3 , carry the same reference figure increased by 800.

[0088] The hydrostatic drivetrain is structurally identical to that of the first embodiment illustrated in figures 2 And 3 , but the front 813F and rear 813R hydrostatic motors are dual displacement.

[0089] The switching device 820 comprises specific displacement regulators 833F, 833R configured to selectively switch the front hydrostatic motors 813F, respectively rear 813R, into a first displacement D m1 (f)< , respectively D m1 (r)< , for the fast movement drive, and into a second displacement D m2 (r)< , respectively D m2 (f)< , for the slow movement drive of the work vehicle 1. The first displacement value D m1 (f)< , D m1 (r)< is strictly less than the second displacement value D m2 (f)< , D m2 (r)< .

[0090] In this embodiment, the front 812F and rear 812R drive units are variable drive units. The driving displacement of the front 812F, respectively rear 812R drive unit; varies between a minimum value of 2 x D m1 (r)< , respectively 2 x D m1 (r)< , and a maximum value of 2 x D m2 (f)< , respectively 2 x D m2 (r)< .

[0091] The slow travel drive is a four-wheel drive 808F, 808R in which all hydrostatic motors, front 813F and rear 813R, are supplied with hydraulic fluid by the hydrostatic pump 811 in their second displacement D m2 (f)< , D m2 (r)< . The total engine displacement D Σ is then equal to its maximum value D Σ,max such that D Σ,max = 2 x D m2 (f)< + 2 x D m2 (r)< .

[0092] The fast travel drive is also a four-wheel drive travel drive 808F, 808R, but in which all hydrostatic motors, front 813F and rear 813R, are supplied with hydraulic fluid by the hydrostatic pump 811 in their first displacement D m1 (f)< , D m1 (r)< . The total engine displacement D Σ is then equal to its minimum value D Σ,min such that D Σ , min = 2 × D m 1 f + 2 × D m 1 r .

[0093] The control of the switching device 820 and the rotational speed of the electric motor 809 by the control unit 825 is similar to that previously described. The control unit 825 controls the displacement regulators 833F, 833R so as to simultaneously switch the variable drive displacements of the front 812F and rear 812R drive units so that all the hydrostatic motors operate: in their first cylinder capacity D m1 (f)< , D m1 (r)< when the forward speed V of the work vehicle 1 is strictly lower than the switching speed VC (V < VC ); or in their second cylinder capacity D m2 (f)< , D m2 (r)< when the forward speed V of the work vehicle 1 is greater than or equal to the switching speed VC (VC ≤ V).

[0094] The control unit 825 also controls the speed of the engine 809 in response to a switching of the variable engine displacements. The variation of the rotational speed of the engine 809 makes it possible to control the corresponding variation of the total engine displacement D Σ between its maximum value D Σ,max and its minimum value D Σ,min . In this embodiment, the variation of the total engine displacement D Σ is equal to D Σ , max − D Σ , min = 2 × D m 2 f − D m 1 f + D m 2 r − D m 1 r .

[0095] Although the invention has been described in connection with several particular embodiments, it is obvious that it is in no way limited thereto and that it includes all technical equivalents of the means described as well as their combinations if these fall within the scope of the invention, which is defined by the claims.

[0096] The use of the verb "comprise", "comprise" or "include" and its conjugated forms does not exclude the presence of other elements or other steps than those set out in a claim.

[0097] In the claims, any reference sign in parentheses cannot be interpreted as a limitation of the claim.

Claims

1. Working vehicle (1, 701, 801) comprising: - an electrical energy source (10, 710, 810); - a hydrostatic kinematic chain, the hydrostatic kinematic chain comprising an electric motor (9, 709, 809) supplied with electrical energy by the electrical energy source (10, 710, 810), a hydrostatic pump (11, 711, 811) driven by the electric motor, at least two drive units (12F, 12R, 712F, 712R, 812F, 812R), each drive unit comprising at least two drive wheels (8F, 8R, 708F, 708R, 808F, 808R), and at least one hydrostatic motor (13F, 13R, 713F, 713R, 813F, 813R) enabling a rotation of the at least two drive wheels, a drive displacement of the drive unit being equal to the displacement (Dm, Dm1, Dm2) of the at least one hydrostatic motor supplied with hydraulic fluid by said hydrostatic pump, a total motor displacement (DΣ) being equal to a sum of the drive displacements of the two drive units, characterised in that at least one of the two drive units being configured as a variable drive unit having a variable drive displacement and in that a switching device (20, 720, 820) configured to switch the variable drive displacements selectively between at least one slow movement in which the total motor displacement is equal to a first value (DΣ,max), and a rapid movement of the working vehicle in which the total motor displacement is equal to a second value (DΣ,min), the first value (DΣ,max) being strictly greater than the second value (DΣ,min); and in that - a control unit (25, 725, 825) configured to control a rotation speed of the electric motor, such that, in response to a switching of the drive displacement to pass from the rapid movement to the slow movement, increasing the rotation speed of said electric motor, such that the hydrostatic pump provides a flow rate of hydraulic fluid required to respond to an increase of the total motor displacement, and in response to the switching of the drive displacement to pass from the slow movement to the rapid movement, reducing the rotation speed of said electric motor, such that the hydrostatic pump absorbs an excess flow rate of hydraulic fluid resulting from a decrease of the total motor displacement.

2. Working vehicle according to claim 1, further comprising a forward speed sensor (26) configured to measure a forward speed (V) of the working vehicle, and in which the control unit is configured to control the switching device, so as to switch the drive displacement to pass from the slow movement to the rapid movement, when the forward speed is greater than or equal to a switching speed (Vc), and from the rapid movement to the slow movement, when the forward speed is less than the switching speed (Vc).

3. Working vehicle according to claim 2, further comprising a movement need sensor (28) configured to measure a movement need (BD), and in which said switching speed (Vc) is according to the forward speed of the working vehicle and of the movement need.

4. Working vehicle according to any one of claims 1 to 3, wherein the switching device comprises a switching control (24) making it possible for an operator to manually switch the drive displacement.

5. Working vehicle according to any one of claims 1 to 4, wherein the two drive units comprise a front drive unit (12F, 712F, 812F) comprising two front drive wheels (8F, 708F, 808F) and at least one front hydrostatic motor (13F, 713F, 813F), and a rear drive unit (12R, 712R, 812R) comprising two rear drive wheels (8R, 708R, 808R) and at least one rear hydrostatic motor (13R, 713R, 813R), the front and rear hydrostatic motors being connected to one another in parallel.

6. Working vehicle according to claim 5, wherein at least one from among the front drive unit (712F) and the rear drive unit (712R) comprises a transmission bridge (732F, 732R) equipped with a pair of drive wheels (708F, 708R) and a hydrostatic motor (713F, 713R) coupled with the two drive wheels.

7. Working vehicle according to any one of claims 1 to 6, wherein the switching device comprises a set of valves (21, 721, 821) configured to selectively enable the supply of hydraulic fluid from a said variable drive unit by the hydrostatic pump for the slow movement and to prevent the supply of hydraulic fluid from said variable drive unit by the hydrostatic pump for the rapid movement.

8. Working vehicle according to claim 7, combined with claim 5 or 6, wherein one from among the rear drive unit and the front drive unit is configured as a variable drive unit and wherein the slow movement is with four drive wheels, front and rear, and the rapid movement is with two drive wheels, front or rear.

9. Working vehicle according to any one of claims 1 to 8, wherein said or each hydrostatic motor of a said variable drive unit is with constant displacement.

10. Working vehicle according to any one of claims 1 to 9, wherein a said variable drive unit comprises a double displacement hydrostatic motor, and wherein the switching device is configured to selectively switch the double displacement hydrostatic motor in a first displacement for the rapid movement and in a second displacement strictly greater than the first displacement for the slow movement.

11. Working vehicle according to claim 10, wherein the double displacement hydrostatic motor does not have a continuously variable displacement.

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