The present invention relates to a hydraulically actuated continuously variable transmission, as defined in the preamble of the claim 1 hereinafter. Such a transmission is generally known in the art, for example from the publication of the European patent application EP 1 482 215 A1 or the international applications WO 2007/141323 A1 or WO 2013/097880 A1 , and is widely applied in the driveline of passenger cars and other vehicles.

The known continuously variable transmission includes a primary or driving pulley and a secondary or driven pulley, as well as flexible drive element that is wrapped around and in friction contact with these pulleys. Each such pulley comprises two (frusto-)conical discs arranged on a shaft, whereof at least one disc is axially moveable and can be urged towards the disc under the influence of a hydraulic pressure exerted in a hydraulic cylinder of that pulley. The flexible drive element comes in several types such as a metal push belt, a metal drive chain or a fibre- reinforced rubber pull belt. In the typical vehicle application of the transmission, the driving pulley is connected to -and rotationally driven by- a prime mover of the vehicle, such as an electric motor or an internal combustion engine, and the driven pulley is connected to -and rotationally drives- the driven wheels thereof.

During operation of the continuously variable transmission the flexible drive element is clamped between the two discs of each pulley by exerting a respective hydraulic pressure in a respective one of the pulley cylinders. A rotational speed and an accompanying torque can then be transmitted from the one pulley to the other one pulley by means of friction between the flexible drive element and the pulleys. Also by these respective cylinder pressures, more in particular by the ratio between the clamping forces that are respectively exerted thereby on the flexible drive element at each pulley, a radius of curvature of the flexible drive element at each pulley, i.e. between the discs thereof, is controlled. In turn, these radii of curvature determine a speed ratio of the transmission, which speed ratio can be controlled to an arbitrary value within a speed ratio range that is determined by the transmission design, through the appropriate setting of the respective cylinder pressures. Hereto, i.e. for generating and controlling the respective cylinder pressures, the transmission further includes an electro-hydraulic control system.

The known electro-hydraulic control system includes at least a reservoir for hydraulic fluid, a pump for supplying pressurised hydraulic fluid from the reservoir to other parts of the control system and one or more hydraulic valves for controlling a hydraulic pressure in and/or a flow of hydraulic fluid towards or away from such other parts of the control system. The control system further includes several hydraulic users such as the pulley cylinders and the friction contact between the flexible drive element and the pulleys that requires hydraulic fluid for lubricating and/or cooling such transmission parts during operation.

In a conventional vehicle, the pump of the control system is typically driven by the internal combustion engine and thus supplies pressurised hydraulic fluid to the other parts of the control system at all times during operation of the engine. However, in a hybrid or an electric vehicle, the pump is typically electrically driven by its own electric motor, such that the supply of pressurised hydraulic fluid is, or at least can be made available also when the prime mover of the vehicle is stopped. In such hybrid or electric vehicle, the overall operating efficiency thereof is typically considered of prime importance. For such specific applications thereof, the present invention aims to provide a novel continuously variable transmission that provides an improved power efficiency with respect to the state of the art.

According to the present invention the aforementioned aim is achieved with the novel continuously variable transmission according to the claim 1 hereinafter. The novel transmission is provided with two electrically driven pumps, whereof a first pump is arranged to be able to supply hydraulic fluid to at least one of the pulley cylinders via at least a first hydraulic line and whereof a second pump is arranged to be able to supply hydraulic fluid to the friction contact between the flexible drive element and the pulleys via at least a second hydraulic line. This novel transmission favourably allows the electro-hydraulic control system thereof to operate more efficiently than the known electro-hydraulic control system. In particular in the novel transmission, the two pumps can each operate at a distinct pressure level, which respective pump (discharge) pressures are adapted to the pressure requirement of the hydraulic user that is respectively supplied with hydraulic fluid by the respective pump. In this respect it is noted that during operation the pulley cylinders require a relatively small flow of hydraulic fluid on average and/or require a flow only intermittently, however, at a relatively high pressure level, whereas the said friction contact typically requires a flow of hydraulic fluid essentially continuously, but at a relatively low pressure level. By separating the supply of hydraulic fluid to these two hydraulic users of the continuously variable transmission, it can be favourably avoided that the pump of the control system supplies the flow of hydraulic fluid required by the said friction contact at the pressure required by the at least one pulley cylinder. Moreover, in particular the first pump can be favourably stopped altogether when the at least one pulley cylinder does not require any flow of hydraulic fluid, e.g. when the transmission is at rest.

In a favourable embodiment of the continuously variable transmission according to the present invention, the electric motor of the first pump is powered at relatively high voltage and the electric motor of the second pump is powered at relatively low voltage. Hereby, the pumps are optimally matched to the pressure and flow requirements thereof posed by the respective hydraulic users. More in particular, the first pump can be powered by the main battery, i.e. runs of the main electric network of the hybrid or electric vehicle that also powers the electric motor prime mover thereof and the second pump can be powered by the secondary battery, i.e. runs of the secondary or auxiliary electric circuit that also powers secondary/auxiliary functions thereof such as dashboard functions, radio, cabin lights, etc. The main electric network typically carries a voltage of 48 Volt or more in hybrid vehicles up to 300-400 Volt in full, i.e. pure (battery) electric vehicles, whereas the secondary electric network carries a lower voltage, typically of 12 Volt.

In a more detailed embodiment of the continuously variable transmission according to the present invention, it is provided with a third electrically driven pump that is, or at least can be, hydraulically connected to the respective other one of the pulley cylinders via, at least, a third hydraulic line. This third pump too can be favourably powered by the main battery of the hybrid or electric vehicle.

In a cost-effective embodiment of the transmission according to the present invention, the first pump is hydraulically connected to both pulley cylinders, such that the third pump can be omitted. In this case, one of the pressure levels in the pulley cylinders, i.e. one of the cylinder pressures, is preferably regulated (e.g. in dependence on a difference between a required cylinder pressure and a measured cylinder pressure) by the controlled activation, i.e. the controlled electric drive of the first pump through the electronic part of the control system, i.e. favourably without applying a (hydraulic) pressure control valve for this purpose. The respective other one of the cylinder pressures then being derived from the said one cylinder pressure, by means of a respective cylinder pressure control valve that is placed -in relation to the flow of hydraulic fluid- between the first pump and the said other one pulley cylinder. Alternatively, both cylinder pressures are respectively regulated by a respective cylinder pressure control valve. The cylinder pressures then being derived from the pump pressure of the first pump, which pump pressure then is regulated by the controlled activation of the first pump, e.g. in dependence on a difference between a required pump pressure and a measured pump pressure. Alternatively, a further, i.e. pump pressure control valve can be applied to regulate the pump pressure of the first pump, in which case the said controlled activation of the first pump can be favourably related to a discharge flow of that pump pressure control valve. This latter discharge flow can either be returned directly to the reservoir, but preferably is supplied to the friction contact between the flexible drive element and the pulleys to favourably alleviate the second pump.

The afore-mentioned pressure control valves are as such operated by the electronic part of the electro-hydraulic control system in a well-known manner in relation to the difference between a respectively required pressure and a respectively measured pressure, either indirectly under influence of a respective pilot pressure or favourably directly by a respective electric actuator such as a solenoid.

The two or three pumps of the present invention can in principle favourably, i.e. cost-effectively be driven in common by one and the same electric motor. In this case, The respective pump pressures of these commonly driven pumps then being regulated by respective pump pressure control valves. Nevertheless, for an optimal adaptation of the hydraulic pressure and flow generated by a respective pump to the hydraulic pressure and flow required by a hydraulic user that is respectively hydraulically connected to and/or supplied with hydraulic fluid by such respective pump, that pump is preferably driven independently from the other pump or pumps. Hereto each such independently driven pump is provided with its own electric motor. In this case, the pump pressure of such independently driven pump is preferably regulated (e.g. in dependence on a difference between a required pump pressure and a measured pump pressure) by the controlled activation, i.e. the controlled electric drive of that pump through the electronic part of the control system, i.e. favourably without applying a pump pressure control valve for this purpose. More preferably, such regulated pump pressure corresponds to the pressure requirement of the hydraulic user that is respectively hydraulically connected to and/or supplied with hydraulic fluid by that pump. For example, the cylinder pressures can be regulated by the controlled electric drive of the first pump and the third pump respectively.

In another cost-effective embodiment in a pure electric vehicle of the continuously variable transmission according to the present invention, at least one of the two or three pumps of the present invention is electrically driven by the electric motor that also serves as the prime mover of that electric vehicle. In this case, the said at least one pump is the pump that is hydraulically connected and/or supplies hydraulic fluid to the pulley cylinder or cylinders (i.e. corresponds to the first pump or to the third pump mentioned hereinabove). The other pump or pumps is/are each provided with and/or are driveable by a further electric motor that, preferably, is dedicated to driving only that other pump. This particular arrangement of the control system is possible, because the cylinder pressures are mainly required when the prime mover electric machine is activated, i.e. either is driving or is driven by (i.e. is regeneratively braking) the vehicle. Of course, in such arrangement of the control system, the hydraulic pressure generated by the said at least one pump cannot be regulated by the controlled activation of the prime mover electric machine, instead a pump pressure control valve must be applied for this purpose.

In another more detailed embodiment of the continuously variable transmission according to the present invention, a hydraulic switch, i.e. an on/off valve is provided between the first or the third hydraulic line and the second hydraulic line, which hydraulic switch is designed and arranged to at least allow hydraulic fluid to pass from the second hydraulic line into the first or third hydraulic line respectively. This hydraulic switch can be a passive one-way check valve that opens (only) when the pressure level in the second hydraulic line is higher than the pressure level in the first or third hydraulic line respectively. By this hydraulic switch, the first or third hydraulic line respectively -and thus also the pulley cylinder(s) respectively hydraulically connected thereto- can be favourably kept pressurised by the second pump when the first pump is stopped to conserve energy, e.g. when the transmission is transmitting minimal or no torque. This hydraulic switch is thus particularly useful also in the above-discussed embodiment of the continuously variable transmission, wherein the first pump and/or the third pump is/are driven by the prime mover of the electric vehicle that can be at standstill or even can be running in reverse.

Preferably according to the present invention, a hydraulic accumulator is, at least selectively, hydraulically connected to the first or the third hydraulic line. The accumulator favourably reduces the flow of hydraulic fluid that is respectively maximally required from the first pump and/or from the third pump, by releasing hydraulic fluid that is stored therein when the required flow is high, e.g. when the speed ratio of the transmission is changed. Alternatively or additionally, the accumulator can be used to favourably keep the first or third hydraulic line respectively -and thus also the pulley cylinder(s) respectively connected thereto- pressurised when the first pump is stopped. If desired or necessary, e.g. if the first or the third hydraulic line respectively is connected directly to a pulley cylinder and not via a cylinder pressure control valve, a hydraulic switch is provided (also) between the accumulator and the first or third hydraulic line respectively. This latter hydraulic switch can be a passive one-way check valve that opens (only) when the pressure level in the accumulator is higher than the pressure level in the first or the third hydraulic line respectively, but is preferably actively controllable by the electronic part of the control system. In the latter case, the filling and emptying of the accumulator can be controlled in relation to other operating parameters than such pressure difference alone, e.g. preventing a filling of the accumulator during an emergency stop of the vehicle when a rapid pressure increase to a high pressure level is required in the first or third hydraulic line respectively.

In addition to the said friction contact between the flexible drive element and the pulleys, typically also other components of the vehicle driveline, such as shaft bearings, require a flow of hydraulic fluid for their lubrication and/or cooling. In this case, it is preferable according to the present invention, to provide a further hydraulic line between the second pump and these other driveline components that is at least partly arranged in parallel with the second hydraulic line. Namely, in this latter arrangement of the control system of the novel continuously variable transmission:

- an oil filter can be provided in a section of the second hydraulic line that is arranged in parallel with the said further hydraulic line, with the flow of hydraulic fluid required by the said other driveline components favourably bypassing such filter and thus avoiding a pressure drop created by such filter, while the said friction contact is favourably supplied with filtered hydraulic fluid, or

- a (hydraulic) flow control or switch valve can be provided in a section of the second hydraulic line that is arranged in parallel with the said further hydraulic line, favourably allowing the flow of hydraulic fluid to the said friction contact either to be switched-off completely, e.g. when the transmission is at rest, to be controlled, e.g. dependence on a friction heat generated in such contact, or both.

This latter flow control or switch valve is preferably operable, i.e. switchable, under influence of the hydraulic pressure in the first or third hydraulic line respectively, in another hydraulic line, or in a hydraulic user that is connected to and/or supplied with hydraulic fluid from such first or third hydraulic line respectively. After all, these hydraulic pressures are indicative of whether the transmission is in operation or at rest, i.e. whether the flexible drive element thereof is rotating and/or transmitting torque or not. Moreover, in this case and at least when applied in combination with the said hydraulic switch between the first or third hydraulic line and the second hydraulic line, the flow control or switch valve is preferably opened only when the hydraulic pressure in the first or third hydraulic line respectively exceeds the pump pressure of the second pump. Alternatively, the flow control or switch is opened by the hydraulic pressure of a hydraulic user that is held at ambient pressure when the transmission is at rest, such as a hydraulically operable clutch in a hybrid vehicle.

Preferably, a heat exchange device is arranged between the second pump and the second hydraulic line and -if present- the said further hydraulic line, such that the entire flow of hydraulic fluid generated by the second pump passes through it.

The continuously variable transmission according to the present invention is explained in more detail hereinafter by means of non-limiting, illustrative embodiments thereof and with reference to the drawing, in which:

- figure 1 is a schematic representation of the functional arrangement of the main components of a specific type hybrid powertrain provided with a transmission system including a continuously variable transmission;

- figure 2 shows an embodiment of a novel electro-hydraulic control system of the transmission system, in particular of the continuously variable transmission thereof;

- figure 3 shows another embodiment of a novel electro-hydraulic control system of the transmission system, in particular of the continuously variable transmission thereof

Figure 1 shows a hybrid powertrain for a vehicle such as a passenger car. In such shown functional arrangement thereof, the hybrid powertrain comprises an internal combustion engine, i.e. ICE 1 , with a crankshaft 1 1 , an electric machine, i.e. EM 2, with a rotor shaft 21 , two driven wheels 3 with wheel shafts 31 and with a transmission system 4 there between. The known transmission system 4 comprises a continuously variable transmission 9 and a first differential gearing 5.

The continuously variable transmission 9 is provided with an input shaft 91 that is drivingly, i.e. rotationally connected to -and in the embodiment of the figure 1 rotates as a unit with- the ICE 1 and with an output shaft 92 that is drivingly connected to the first differential gearing 5 via a set of two gear wheels 96, 100. The continuously variable transmission 9 can vary a speed ratio between an input shaft 91 and an output shaft 92 thereof within a continuous range of speed ratios.

In the illustrative embodiment thereof in figure 1 , the continuously variable transmission 9 is in the form of two rotatable, variable pulleys 93 and 94, each associated with (i.e. mounted on and possibly partly formed integral with) a respective one of the said input and output shafts 91 , 92, and a the flexible drive element 95 that is wrapped around the pulleys 93 and 94 for drivingly connecting these. The pulley 93 on the input shaft 91 that is directly, albeit possibly via a set of gear wheels, connected to the ICE 1 (or other type of prime mover of the vehicle such as a prime mover electric machine; not illustrated) is typically denoted as a primary pulley 93, whereas the pulley

94 on the output shaft 92 that is connected to the ICE 1 via the flexible drive element

95 is typically denoted as a secondary pulley 94. Each respective pulley 93, 94 is known to comprise two (frusto-)conical discs, whereof at least one disc is axially moveable on the respective shaft 91 , 92 and can be urged towards the other disc under the influence of a respective, i.e. primary or secondary hydraulic pressure exerted in a respective hydraulic cylinder of each pulley 93, 94, i.e. the primary hydraulic cylinder and secondary hydraulic cylinder respectively.

During operation of the continuously variable transmission 9, the flexible drive element 95 is clamped between the two discs of each pulley 93, 94 as a result of the primary pressure and the secondary pressure respectively. A rotational speed and an accompanying torque can then be transmitted between pulleys 93, 94 by the flexible drive element 95 by means of friction between. These primary and secondary pressures, in particular the ratio between the clamping forces that are respectively exerted thereby on the flexible drive element 95 at the pulleys 93, 94, also determine a radius of curvature of the flexible drive element 95 at each pulley, 93, 94, i.e. between the discs thereof. In turn, these radii of curvature determine the said speed ratio of the continuously variable transmission 9. Hereto, i.e. for generating and controlling the primary and secondary pressures, the continuously variable transmission 9 further includes an electro-hydraulic control system, whereof two possible embodiments are illustrated in figures 2 and 3.

The first differential gearing 5 is provided with three rotatable members 51 , 54, 53 that are respectively directly, or at least most directly, connected to the output shaft 92 of the continuously variable transmission 9, the rotor shaft 21 of the EM 2 and the wheel shafts 31 of the driven wheels 3. The first differential gearing 5 balances the torque levels acting on its rotatable members 51 , 54, 53, based on the rotational speed ratios provided there between.

In the illustrative embodiment thereof in figure 1 , the first differential gearing 5 is in the form of a planetary gearing 5 provided with a central sun gear 51 that is in meshing contact with one or more planet gears 52, which planet gears 52 are rotatably carried by a planet carrier 53 arranged coaxially rotatable with the sun gear 51 , and with a ring gear 54 that is in meshing contact with the planet gears 52 and that is also arranged coaxially rotatable with the sun gear 51 . A bridging clutch 55 is provided as part of planetary gearing 5, between the planet carrier 53 and sun gear 51 thereof. This bridging clutch 55 can be closed to internally lock the planetary gearing 5 such that the sun gear 51 , the planet carrier 53 and the ring gear 54 thereof rotate as a unit. The sun gear 51 of the planetary gearing 5 is coupled to the crankshaft 1 1 of the ICE 1 via a second clutch 8, an auxiliary gear 100 on an auxiliary shaft 101 , an output gear 96 on the output shaft 92 meshing with that auxiliary gear 100 and the continuously variable transmission 9 itself.

The second clutch 8 can be closed to drivingly connecting, i.e. to couple the ICE 1 and the continuously variable transmission 9 to the planetary gearing 5, or can be opened to decouple, i.e. to isolate the ICE 1 and the continuously variable transmission 9 from the rest of the hybrid powertrain. The ring gear 54 of the planetary gearing 5 is coupled to a pinion gear 22 on the rotor shaft 21 of the EM 2 via an idler gear 23 and the planet carrier 53 of the planetary gearing 5 is coupled to the driven wheels 3 via a final reduction gearing 7 including a further differential gearing 71 . The final reduction gearing 7 provides a speed reduction between the ICE 1 and/or the EM 2 and the driven wheels 3, while the further differential gearing 71 thereof allows the two driven wheels 3 to each rotate at a respective rotational speed, as is common knowledge in the art.

The transmission system 4 is provided with a brake or park lock 6 that can be engaged to lock, i.e. to prevent rotation of the final reduction gearing 7, in which case the ICE 1 can drive the EM 2, in particular to charge a battery 24 of the vehicle, or the EM 2 can drive the ICE 1 , in particular to start it, without simultaneously driving and/or rotating the driven wheels 3 of the vehicle. When the park lock 6 is released, the EM 2 can drive the vehicle while drawing electric power from the battery 24, possibly supported by the ICE 1 . Instead of the park lock 6 it is of course also possible to (automatically) engage the vehicle wheel brakes to charge the battery 24 without simultaneously driving the vehicle.

Further technical details of this particular type of hybrid powertrain, as well as the specific benefits and operations thereof, are described in the Dutch patent application NL-1042199.

It is noted that even though the present invention is thus illustrated in relation to the hybrid powertrain, it can equally be applied in the driveline of a full electric vehicle that is not equipped with the ICE 1.

Figures 2 and 3 each a show possible embodiment of an electro-hydraulic control system of the novel transmission system 4, in particular of the continuously variable transmission 9 thereof, and thus illustrate certain technical aspects of the present invention in relation thereto. In either such embodiment thereof the control system is provided with two electrically driven pumps 201 , 205.

A first electrically driven pump 201 supplies, or at least can supply, hydraulic fluid to at least one of the primary hydraulic cylinder 202 and the secondary hydraulic cylinder 203, wherein the primary pressure (figures 2, 3: “Prim press.”) and the secondary pressure (figures 2, 3:“Sec. press.”) are respectively exerted, via at least a first hydraulic line 204 for the supply of pressurised hydraulic fluid to such at least one of the hydraulic cylinders 202; 203. The respective, i.e. primary and secondary pressures in the hydraulic cylinders 202, 203 are controlled by means of a respective, i.e. primary and secondary cylinder pressure control valves 217, 218. In the illustrated embodiments of the control system, these cylinder pressure control valves 217, 218 are operated favourably directly by means of an electric actuator (figures 2, 3:“DESC - NH”).

The hydraulic pressure generated by the first pump 201 , i.e. the pressure in the first hydraulic line 204 is controlled towards a desired level by the controlled activation, of an electric motor EM that is part of, or at least is driving that first pump 201 . Alternatively, but not illustrated, the pressure in the first hydraulic line 204 can be controlled by of one of the cylinder pressure control valves 217, 218, in which case the pump pressure corresponds to the respective, i.e. primary or secondary cylinder pressure. Moreover, a further pressure control valve, i.e. a pump pressure control valve can be included in the control system (not illustrated) to control the pressure in the first hydraulic line 204.

A second electrically driven pump 205 supplies, or at least can supply, hydraulic fluid to a friction contact 216 between the flexible drive element 95 and the pulleys 93, 94 via at least a second hydraulic line 206 for lubricating and cooling such friction contact 216 (figures 2, 3: “Var. lubr./cooling”). In the thus illustrated arrangement thereof, the two pumps 201 , 205 can each operate at a distinct pressure level, which respective pump pressures are adapted to the pressure requirement of the hydraulic user, such as the hydraulic cylinders 202, 203 or the said friction contact 216, that is respectively supplied with hydraulic fluid by the respective pump 201 , 205. In the illustrated embodiments of the control system, the hydraulic pressure generated by this second pump 205, i.e. the pressure in the second hydraulic line 206 is controlled towards a desired level by the controlled activation of an electric motor EM included in or at least driving this second pump 205. However, a yet further (pump) pressure control valve can be included in the control system (not illustrated) for this purpose.

In the illustrated embodiments of the control system, both hydraulic cylinders 202, 203 are supplied with (pressurized) hydraulic fluid by the first pump 201 , however, a third electrically driven pump can be included in the control system (not illustrated), such that each hydraulic cylinder 202, 203 is supplied with (pressurized) hydraulic fluid by a respective one of such first and third pumps.

The two pumps 201 , 205 can in principle be driven commonly by one and the same electric motor EM in a cost-effective embodiment of the control system. However, to minimise power loss, these two pumps 201 , 205 each preferably include or are at least driven by a respective electric motor EM. In the latter case, the electric motor EM of the first pump 201 is preferably powered at a relatively high voltage, while the electric motor EM of the second pump 205 is preferably powered at a relatively low voltage. If included, the said third pump is preferably provided with a respective electric motor EM as well, preferably powered at the same, i.e. relatively high voltage as the first pump 201.

In addition to the friction contact 216 between the flexible drive element 95 and the pulleys 93, 94, typically also other components 208 of the transmission system 4, such as shaft bearings, require a flow of hydraulic fluid for their lubrication and/or cooling (figures 2, 3:“Tr. lubr./cooling”). To this end, a further hydraulic line 209 is provided between the second pump 256 and these other transmission components 208, for example (and as illustrated in figures 2 and 3) branching-off of the second hydraulic line 206. An oil filter 210 is then preferably provided in a section of the second hydraulic line 206 extending in parallel with (i.e. is provided beyond the point of branching-off of) the further hydraulic line 209. Hereby only the part of the flow of hydraulic fluid generated by the second pump 205 that is directed to the said friction contact 216 passes through such oil filter 210, removing suspended particulates from it that could otherwise accelerate a mechanical wear of/in the said friction contact 216, while minimizing a loss of pressure towards the said other transmission components

208. Moreover, a heat exchange device 215 is preferably provided between the second pump 205 and both the second hydraulic line 206 and the further hydraulic line

209, such that the entire flow of hydraulic fluid generated by the second pump 205 passes through it. The heat exchange device 215 is preferably capable of both cooling and heating the hydraulic fluid passing through it.

As illustrated in figure 3, the control system can be provided with a check-valve 207 between the second hydraulic line 206 and the first hydraulic line 204 that is arranged to allow hydraulic fluid to pass from the second hydraulic line 206 into the first hydraulic line 204 when the pressure in the second hydraulic line 206 is higher than the pressure in the first hydraulic line 204. By such check-valve 207, the first hydraulic line 204 is kept pressurised by the second pump 205 when the first pump 201 is stopped, e.g. to save energy when continuously variable transmission 9 is not in operation, i.e. is not transmitting torque. This allows the hydraulic cylinders 202, 203 to remain pressurised and thus to prevent an undesired slipping of the flexible drive element 95 relative to the pulleys 93, 94. The inclusion of this check-valve 207 in the control system is thus particularly beneficial in the full electric vehicle, especially so if the first pump 201 is driven by the prime mover electric machine thereof as mentioned hereinabove.

Moreover, the control system can be provided with a switch valve 21 1 in a section of the second hydraulic line 206 extending in parallel with the further hydraulic line 209. This switch valve 21 1 allows a flow of hydraulic fluid to the said friction contact 216 to be controlled, e.g. dependence on a friction heat generated therein. By means of this switch valve 21 1 , the flow of hydraulic fluid to the said friction contact 216 can be switched-off completely, e.g. when the continuously variable transmission 9 is at rest, can be regulated, e.g. dependence on a friction heat generated in such contact 216, or both.

This latter switch valve 21 1 is preferably operable, i.e. is switched or regulated, under the influence of the hydraulic pressure in the first hydraulic line 204 or (as illustrated in figure 3) in a hydraulic user 213 that is connected to and/or supplied with hydraulic fluid from such first hydraulic line 204. After all, this latter hydraulic pressure is indicative of whether the continuously variable transmission 9 is in operation or at rest, i.e. whether the flexible drive element thereof is rotating and/or transmitting torque or not. Moreover, if the switch valve 21 1 is applied in combination with the afore mentioned check-valve 207 the first hydraulic line 204 and the second hydraulic line 206, the said hydraulic pressure for operating the switch valve 21 1 must either be higher than the pump pressure of the second pump 205, or must be taken from a hydraulic user that is held at a reduced pressure relative to this latter pump pressure, e.g. at ambient pressure, when the continuously variable transmission 9 is at rest, such as a hydraulically operable clutch 213 of the hybrid vehicle. Otherwise, the switch valve 21 1 either will not close when the first pump 201 is switched-off, or will never open. In this respect it is noted that such switching-behaviour of the switch valve 21 1 is typically determined by a spring 214 that works to close the switch valve 21 1 against the said hydraulic pressure that works to open the switch valve 21 1.

The present invention, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all the features of the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as non-binding examples of the respective features. The claimed features can be applied separately in a given product or a given process, as the case may be, but it is generally also possible to apply any combination of two or more of such features therein.

The invention is not limited to the embodiments and/or the examples that are explicitly mentioned herein, but also encompasses variants, modifications and practical applications thereof that lie within reach of the person skilled in the relevant art.