Electric work vehicle featuring a hydrostatic drivetrain

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 a 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. Figure for the abstract: Fig. 2

Description

Title of the invention: Electric work vehicle comprising a hydrostatic drive train Technical field

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

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

[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, the change 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. The switching between the four-wheel drive and two-wheel drive configurations is therefore carried out when the work vehicle is stationary. Summary

[0005] 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.

[0006] According to one embodiment, the invention provides a work vehicle comprising: - a source of electrical energy; - 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 comprising at least two drive wheels and at least one hydrostatic motor for rotating 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 at least one variable drive displacement selectively between at least a slow-moving drive in which the total engine displacement is equal to a first value, and a 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 rotation speed of the electric motor so that, in response to a switching of the engine displacement from fast travel drive to slow travel drive, increasing the rotational speed of said electric motor such that the hydrostatic pump provides a flow rate of hydraulic fluid required to accommodate 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.

[0007] Thanks to these characteristics, the control of the rotation speed of the electric motor - particularly responsive - makes it possible to control the variation in the flow rate of hydraulic fluid resulting from the variation in the total engine displacement when switching the driving displacement. The work vehicle can thus switch smoothly and while moving between hydrostatic configurations developing significant tractive forces but low forward speeds, and hydrostatic configurations with a lower total engine displacement, developing little tractive force but making it possible to achieve high forward speeds.

[0008] According to one embodiment, the hydrostatic pump is a volumetric pump with axial pistons or a volumetric pump with radial pistons.

[0009] According to one embodiment, the hydrostatic pump has a fixed displacement.

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

[0011] According to one embodiment, the hydrostatic motors are axial piston motors or radial piston motors.

[0012] 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 to change from slow travel drive to fast travel drive when the travel speed is greater than or equal to a switching speed, and fast moving drive to slow moving drive when the moving speed is lower than the switching speed.

[0013] Thus, the control unit automatically controls the switching of the engine displacement in order to optimize the performance of the hydrostatic drive train of the work vehicle.

[0014] 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.

[0015] 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.

[0016] Thus, when the work vehicle is placed on sloping or uneven ground, moves forward against a heavy load or transports a heavy load, the tractive force applied to the drive wheels increases compared to the same movement when empty on flat ground. Determining the switching speed as a function of the forward speed of the work vehicle and the travel requirement thus makes it possible to optimize the performance of the hydrostatic drive train as a function of the ground conditions and the handling operations.

[0017] According to one embodiment, the switching device comprises a switching control allowing an operator to manually switch the driving cylinder capacity.

[0018] 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.

[0019] According to one embodiment, at least one of the front drive unit and The rear drive unit has a transmission axle equipped with a pair of drive wheels and a hydrostatic motor coupled to both drive wheels.

[0020] 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.

[0021] 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.

[0022] According to one embodiment, said or each hydrostatic motor of a variable drive unit has a constant displacement.

[0023] 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-moving drive and into a second displacement strictly greater than the first displacement for slow-moving drive.

[0024] According to one embodiment, the dual-displacement hydrostatic motor does not have a continuously variable displacement. Brief description of the figures

[0025] The invention will be better understood, and other aims, 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 appended drawings.

[0026] [Fig-1] [Fig.l] is a schematic view of a work vehicle according to a mode of realization of the invention.

[0027] [Fig.2] [Fig.2] is a functional schematic representation of a work vehicle in a drive mode, according to a first embodiment.

[0028] [Fig.3] [Fig.3] is a schematic representation similar to that of [Fig.2], according to the first embodiment.

[0029] [Fig.4] [Fig.4] is a functional schematic representation of the control unit according to one embodiment.

[0030] [Fig.5] [Fig.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 [Fig.2].

[0031] [Fig.6] [Fig.6] is a graphical representation of the tractive effort applied to the drive wheels as a function of the flat ground travel speed of the work vehicle of [Fig.5].

[0032] [Fig.7] [Fig.7] is a schematic representation similar to that of [Fig.2], according to a second embodiment.

[0033] [Fig.8] [Fig.8] is a schematic representation similar to that of [Fig.7], according to a third embodiment. Description of the embodiments

[0034] 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 even an electric compact loader. This list is not exhaustive.

[0035] With reference to [Fig.l], 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.

[0036] 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 movement D of the work vehicle 1. The front driving axle 7F and the rear driving axle 7R each comprise two driving wheels, denoted respectively 8F and 8R. 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.

[0037] 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 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 allows 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.

[0038] With reference to Figures 2 and 3, the work vehicle is described according to a first embodiment.

[0039] The work vehicle 1 is equipped with a hydrostatic drive train comprising an electric motor 9 supplied with electrical energy by a source electric power unit 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.

[0040] The electrical energy source 10 may consist of a modular assembly of electrochemical accumulator batteries and / or energy storage capacitors.

[0041] 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.

[0042] Advantageously, the electric motor 9 is dedicated solely to driving the hydrostatic pump 11 in rotation, with the exception of a booster pump 15 which supplies hydraulic fluid to a booster circuit described later. The rotation 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.

[0043] In the embodiment described, the hydrostatic pump 11 has a continuously variable displacement Dp, for example controlled hydraulically, hydromechanically or electrically. Alternatively, the hydrostatic pump has a fixed displacement.

[0044] 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.

[0045] 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 called “automotive” type; it can be carried out by hydraulic or electrohydraulic control of the displacement, preferably proportional.

[0046] The hydrostatic pump 11 is provided with a discharge port 11a and a suction port 11b.

[0047] 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, allows the rotational drive of a front 8F, rear 8R respectively drive wheel.

[0048] The 13F front hydrostatic motors have a fixed displacement Dm(f) and the 13R rear hydrostatic motors have a fixed displacement Dm(r).

[0049] The front hydrostatic motors 13F, respectively rear 13R, each comprise an inlet orifice 13Fa, respectively 13Ra, and an outlet orifice 13Fb, respectively 13Rb.

[0050] Advantageously, the front 13F and rear 13R hydrostatic motors are axial piston motors (barrel or straight axis motors) or radial piston motors.

[0051] 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 of the hydraulic circuitry of the hydrostatic drive train. The hydraulic fluid is then redirected to the suction of the hydrostatic pump 11 via low-pressure fluid connections.

[0052] Advantageously, the hydraulic fluid is a mineral or synthetic oil obtained by refining petroleum.

[0053] 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. .

[0054] The main pipes 14a, 14b and their branches 141Fa, 141Ra, 141Fb, 141Rb are for example steel pipes or flexible conduits.

[0055] 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.

[0056] 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 body hollowed out inside which a movable slide moves. Holes drilled in the body allow the desired connections or closures to be established between the different paths to control the flow of hydraulic fluid.

[0057] 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.

[0058] Each solenoid valve 21 has a first position I, illustrated in [Fig. 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 [Fig. 3], preventing the flow of hydraulic fluid between the hydrostatic pump 11 and the rear hydrostatic motors 13R.

[0059] 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 Dm(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.

[0060] 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 orifice 11a of the hydrostatic pump 11 and the inlet orifices 13Fa of the front hydrostatic motors 13F.

[0061] 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. In FIGS. 2 and 3, the switching control 24 is connected to a control unit 25, described later, which controls the solenoid valves 21.

[0062] The switching device 20 thus allows the selective switching of the driving cylinder capacity of the rear drive unit 13R between: - a four-wheel drive slow motion 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 Dm®, and the two hydro motors rear static 13R, each with fixed displacement Dm(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 Dm(r), are supplied with hydraulic fluid by the hydrostatic pump 11.

[0063] By defining the total engine displacement Dz 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 Dz of the four-wheel drive slow motion drive 8F, 8R is equal to a maximum value Dz>max of total engine displacement Dz such that Dz>max = 2 x Dm(f) + 2 x Dm(r); and - the total engine displacement Dz of the 8F two-wheel drive fast travel drive is equal to a minimum value Dy min of the total engine displacement Dz such that Dy min = 2 x Dm®.

[0064] 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 correlatively generates a variation in the total engine displacement between respectively a minimum value Dy min and a maximum value Dz>max.

[0065] With reference to [Fig. 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.

[0066] 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 displacement 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 Dz.

[0067] The switching speed Vc may have been previously determined by if digital mulation or by workshop testing, and stored in the memory of the control unit 25.

[0068] 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 force 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.

[0069] 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.

[0070] The displacement 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 that it acquires. The control unit correlates the position data of the accelerator pedal 27 with a displacement requirement BD of the work vehicle 1, for example a torque requirement to be applied to the drive wheels 8F and / or 8R, a forward speed requirement of the work vehicle 1 or a power requirement to overcome the resistance to displacement of the work vehicle 1.

[0071] With reference to 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.

[0072] In relation to 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.

[0073] [Fig.5] illustrates a variation profile of the total engine displacement Dz as a function of the forward speed V of the work vehicle 1.

[0074] [Fig.6] is a graphical representation of the tractive force 29 applied to the drive wheels 8F, 8R as a function of the forward speed V of the vehicle. work 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-wheel drive 8F, 8R. Curve 31 represents this same tractive effort 29 when the work vehicle 1 is driven in fast motion with two-wheel drive 8F.

[0075] In acceleration phase

[0076] The work vehicle 1 is initially stationary. The operator presses the accelerator pedal: the tractive force 29 developed by the hydrostatic drive train is maximum, as visible on curve 30 of [Fig.6]. The work vehicle 1 is driven in slow motion at the forward speed V.

[0077] 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 movement, 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 movement drive of the work vehicle 1 allows the hydrostatic drive train to develop the greatest tractive forces.

[0078] As previously explained, when the work vehicle 1 is driven in four-wheel drive 8F, 8R, the total engine displacement Dz is equal to a maximum value Dz>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.

[0079] 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 Dz drops abruptly to a minimum value Dz min, i.e. a variation of the total engine displacement Dz of

[0080] Dz>max - Dz>min = 2 x Dm«.

[0081] To compensate for this sudden variation in the total engine displacement Dz from its maximum value Dz>max to its minimum value Dz>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.

[0082] Beyond the speed of the switching speed Vc (V > Vc), the control unit 25 controls the switching device so that the work vehicle 1 is driven in fast two-wheel drive 8F. The reduction in the total engine displacement Dz allows the work vehicle 1 to accelerate to reach the maximum forward speed Vmax. Such a speed could not be achieved if the four-wheel drive 8F, 8R were selected, as can be seen by comparing curves 30 and 31 in [Fig.6].

[0083] In deceleration phase

[0084] The work 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 drive wheels 8F. As explained previously, the total engine displacement Dz is equal to its minimum value Dy min since only the front hydrostatic motors 13F of the front drive unit 12F are supplied with hydraulic fluid by the hydrostatic pump 11.

[0085] 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 driving displacement of the rear drive unit 12F to change 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 Dz then increases abruptly from its minimum value Dy min to its maximum value Dz>max.

[0086] To compensate for this sudden variation in the total engine displacement Dz, 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.

[0087] With reference to [Fig.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 Vmax of 14 km / h; while the 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.

[0088] [Fig.7] shows a work vehicle 701 according to a second 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, particularly illustrated in Figures 2 and 3, bear the same reference number increased by 700.

[0089] 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 mounted in parallel.

[0090] The 13F front hydrostatic motor has a fixed displacement Dm® and the 13R rear hydrostatic motor has a fixed displacement Dm®.

[0091] 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.

[0092] 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 Dz is then equal to its maximum value Dz>max such that Dz>max = Dm® + Dm®.

[0093] 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 Dz is then equal to its minimum value Dm such that Dz>min = Dm®.

[0094] 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 Dz between its maximum value Dz>max and its minimum value Dïjnm.

[0095] The same observations as those set out for the first embodiment apply, it being specified that the variation in the total engine displacement Dz corresponds here to the displacement Dm® of the rear hydrostatic motor 713R since

[0096] Dz>max - Dz>min = Dm®.

[0097] [Fig.8] shows 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, particularly illustrated in Figures 2 and 3, bear the same reference number increased by 800.

[0098] The hydrostatic drive train 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.

[0099] 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 Dmi®, respectively Dmi®, for driving in fast movement, and into a second displacement Dm2®, respectively Dm2(f), for driving in slow movement of the work vehicle 1. The first displacement value Dmi(f), Dmi® is strictly less than the second displacement value Dm2®, Dm2®.

[0100] 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 Dmi(r), respectively 2 x Dmi(r), and a maximum value of 2 x Dm2®, respectively 2 x Dm2(r).

[0101] The slow travel drive is a four-wheel drive travel drive 808F, 8O8R in which all the hydrostatic motors, front 813F and rear 813R, are supplied with hydraulic fluid by the hydrostatic pump 811 in their second displacement Dm2®, Dm2®. The total engine displacement Dz is then equal to its maximum value Dz>max such that Dz>max = 2 x Dm2® + 2 x Dm2®.

[0102] The fast travel drive is also a four-wheel drive travel drive 808F, 8O8R, but in which all the hydrostatic motors, front 813F and rear 813R, are supplied with hydraulic fluid by the hydrostatic pump 811 in their first displacement Dmi®, Dmi®. The total engine displacement Dz is then equal to its minimum value D2jnm such that Dz>min = 2 x Dmi® + 2 x Dml®.

[0103] 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 Dmi(f), Dm[® when the forward speed V of the work vehicle 1 is strictly lower than the switching speed Vc (V < Vc ) ; or - in their second displacement Dm2®, Dm2® when the forward speed V of the work vehicle 1 is greater than or equal to the switching speed Vc (Vc < V).

[0104] The control unit 825 also controls the speed of the engine 809 in response to a switching of the variable engine displacements. The variation in the rotational speed of the engine 809 makes it possible to control the corresponding variation in the total engine displacement Dz between its maximum value Dz>max and its minimum value D2jnm. In this embodiment, the variation in the total engine displacement Dz is equal to

[0105] Dz>max - Dz>min = 2 x [(Dm2® - Dml®) + (Dm2® - Dml®)].

[0106] Although the invention has been described in connection with several particular embodiments, it is quite obvious that it is in no way limited thereto and that it includes all the technical equivalents of the means described as well as their combinations if these fall within the scope of the invention.

[0107] 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.

[0108] In the claims, any reference sign in parentheses cannot be interpreted as a limitation of the claim.

Claims

Claims

1. Work vehicle (1, 701, 801) comprising: - a source of electrical energy (10, 710, 810); - a hydrostatic drive train, the hydrostatic drive train 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, 8O8R) and at least one hydrostatic motor (13F, 13R, 713F, 713R, 813F, 813R) allowing rotational drive of the at least two drive wheels, a driving displacement of the drive unit being equal to the displacement (Dm, DmH D m2) of the at least one hydrostatic motor supplied with hydraulic fluid by said hydrostatic pump, a total engine displacement (Dz) being equal to a sum of the engine displacements of the two drive units, at least one of the two drive units being configured as a variable drive unit having a variable engine displacement and a switching device (20, 720, 820) configured to switch the variable engine displacement(s) selectively between at least one slow motion drive in which the total engine displacement is equal to a first value (Dz>max), and a fast moving drive of the work vehicle in which the total engine displacement is equal to a second value (Dz>min), the first value (Dz>max) being strictly greater than the second value (Dz>min); and - a control unit (25, 725, 825) configured to control a rotational speed of the electric motor such that, in response to a switching of the driving displacement from fast displacement drive to slow displacement drive, increase the rotational speed of said electric motor so that the hydrostatic pump provides a flow rate of hydraulic fluid required to meet an increase in the total engine displacement, and in response to the switching of the driving displacement from the slow-moving drive to the fast-moving drive, reducing the rotational speed of said electric motor so that the hydrostatic pump absorbs an excess flow of hydraulic fluid resulting from a reduction in the total engine displacement.

2. The work vehicle of claim 1, further comprising a forward speed sensor (26) configured to measure a forward speed (V) of the work vehicle, and wherein the control unit is configured to drive the switching device so as to switch the engine displacement from the slow travel drive to the fast travel drive when the forward speed is greater than or equal to a switching speed (Vc), and from the fast travel drive to the slow travel drive when the forward speed is less than the switching speed (Vc).

3. A work vehicle according to claim 2, further comprising a travel requirement sensor (28) configured to measure a travel requirement (BD), and wherein said switching speed (Vc) is a function of the forward speed of the work vehicle and the travel requirement.

4. A work vehicle according to one of claims 1 to 3, wherein the switching device comprises a switching control (24) allowing an operator to manually switch the engine displacement.

5. A work vehicle according to one of claims 1 to 4, wherein the two drive units comprise a front drive unit (12F, 712F, 812F) having 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) having 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 each other in parallel.

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

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

8. A work vehicle according to claim 7 taken in combination with claim 5 or 6, wherein 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.

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

10. A work vehicle according to one of claims 1 to 9, wherein a said variable drive unit comprises a dual-displacement hydrostatic motor, and wherein 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.

11. A work vehicle according to claim 10, wherein the dual displacement hydrostatic engine does not have a continuously variable displacement.

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