Abstract:
The invention relates to a hydraulic system for actuating a belt-driven conical-pulley transmission with a variably adjustable transmission ratio, of a vehicle, having at least one hydraulic energy source and having a moment sensor that is supplied with working medium by a pump flow of the hydraulic energy source. 
     The invention is distinguished in that a disconnecting valve device is connected between the hydraulic energy source and the moment sensor, which makes it possible to connect or disconnect an additional pump flow of the hydraulic energy source, depending on need.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a hydraulic system for actuating a belt-driven conical-pulley transmission with a variably adjustable transmission ratio, of a vehicle, having at least one hydraulic energy source and having a moment sensor that is supplied with working medium by a pump flow of the hydraulic energy source. 
         [0002]    The object of the invention is to create a clutch control device according to the preamble of claim  1 , with which losses that occur in operating the hydraulic energy source can be reduced. 
       SUMMARY OF THE INVENTION 
       [0003]    The problem is solved in a hydraulic system for actuating a belt-driven conical-pulley transmission with a variably adjustable transmission ratio, of a vehicle, having at least one hydraulic energy source and having a moment sensor that is supplied with working medium by a pump flow of the hydraulic energy source, by connecting a disconnecting valve device between the hydraulic energy source and the moment sensor that makes it possible to connect or disconnect an additional pump flow of the hydraulic energy source, if necessary. The first-named pump flow is also referred to as the first pump flow. The other pump flow is also referred to as the second pump flow. Independent of this identification of the pump flows, however, the hydraulic energy source can include more than two pump flows. The pump flows can be realized through a single pump or through a plurality of pumps. Preferably, the pump flows are provided by a single pump. If the second pump flow is connected to the first pump flow, then both pump flows are conveyed together to the moment sensor. If the second pump flow is disconnected, then the second pump flow is conveyed into a working medium tank, so that only the first pump flow is conveyed to the moment sensor. The invention furnishes a sensible connection layout of a double-flow pump. That makes it possible to uncouple a pump flow from the system pressure in certain operating states. On the other hand, the two pump flows provide for an adequate transport volume in critical situations. Among other things, that provides the advantage that unnecessary losses in the transmission can be reduced. 
         [0004]    A preferred exemplary embodiment of the hydraulic system is characterized in that a check valve is connected between the two pump flows in such a way that the first pump flow is severed from the second pump flow as soon as the second pump flow is disconnected. That prevents the first pump flow from being partially conveyed into the tank when the second pump flow is disconnected. 
         [0005]    Another preferred exemplary embodiment of the hydraulic system is characterized in that the disconnecting valve device is connected via a control pressure line to a return line that comes from the moment sensor. The disconnecting valve device is actuated by way of the control pressure line, depending on the pressure in the return line. Preferably, a clutch cooling valve device is connected into the return line between the moment sensor and the control pressure line with which excess working medium passes from the clutch cooling valve device to a clutch cooling device, for example by way of a jet pump. 
         [0006]    Another preferred exemplary embodiment of the hydraulic system is characterized in that a pressure conversion valve device is connected into the control pressure line. The pressure conversion valve device enables the actuating force at the disconnecting valve device to be increased significantly. 
         [0007]    Another preferred exemplary embodiment of the hydraulic system is characterized in that the disconnecting valve device is executed as a 2/2 directional valve with an open position and a closed position, into which the 2/2 directional valve is pre-tensioned. When the disconnecting valve device is in the open position, the second pump flow is conveyed into the tank; that is, it is disconnected. When the disconnecting valve device is in the closed position, the second pump flow is connected to the first pump flow. Both pump flows are then conveyed together to the moment sensor. The pre-tensioning is realized for example with the aid of a pre-tensioning spring device. 
         [0008]    Another preferred exemplary embodiment of the hydraulic system is characterized in that the disconnecting valve device is connected via another control pressure line to a connecting valve device, which is connected ahead of the moment sensor. The connecting valve device, which is preferably connected between the pressure regulating valve device and the moment sensor, works together with the pre-tensioning of the disconnecting valve device, in order to quickly switch the disconnecting valve device to its closed position when necessary. 
         [0009]    Another preferred exemplary embodiment of the hydraulic system is characterized in that the connecting valve device is executed as a 3/2 directional valve with an open position and a closed position, into which the 3/2 directional valve is pre-tensioned. When the connecting valve device is in the open position, the other control pressure line is pressurized with the pressure ahead of the moment sensor. When the connecting valve device is in the closed position, the other control pressure line is relieved into the tank. 
         [0010]    Another preferred exemplary embodiment of the hydraulic system is characterized in that the connecting valve device is actuated with the pressure ahead of the moment sensor. If the pressure ahead of the moment sensor rises above a specified value, the connecting valve device is opened and the disconnecting valve device is closed, in order to connect the second pump flow again. 
         [0011]    Another preferred exemplary embodiment of the hydraulic system is characterized in that the disconnecting valve device is pressurizable via the other control pressure line and the connecting valve device with the pressure ahead of the moment sensor. That enables the second pump flow to be connected quickly when necessary. 
         [0012]    Another preferred exemplary embodiment of the hydraulic system is characterized in that a pressure regulating valve device is connected between the hydraulic energy source and the moment sensor. The pressure regulating valve device, which is also referred to as a pressure holding valve device, provides for a desired system pressure to be maintained ahead of the moment sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    The structure, operation, and advantages of the present invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings in which: 
           [0014]      FIG. 1 : a detail of a hydraulic circuit diagram of a hydraulic system according to the invention, according to a first exemplary embodiment, having a disconnecting valve device and a connecting valve device, and 
           [0015]      FIG. 2 : a detail of a hydraulic circuit diagram similar to that in  FIG. 1  according to a second exemplary embodiment, having an additional pressure conversion valve device. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]      FIGS. 1 and 2  depict hydraulic systems  1  and  51 , respectively, with the aid of symbols conventionally used in hydraulics for a hydraulic circuit diagram. In the hydraulic circuit diagram, a tank with hydraulic medium is designated at each of various places by a symbol with the reference label  5 . The hydraulic medium contained in the tank is preferably hydraulic oil, also referred to as oil. Hydraulic system  1 ,  51  serves to control a belt-driven conical-pulley transmission, which is situated in the power train of a motor vehicle. Such belt-driven conical-pulley transmissions are also referred to as CVT transmissions (CVT, continuously variable transmission). This is a stepless vehicle transmission, which makes great driving comfort possible through jerk-free changing of the transmission ratio. 
         [0017]    The two hydraulic systems  1 ,  51  are so similar that the same reference labels are used to designate like parts. In contrast to a conventional hydraulic system, hydraulic systems  1 ,  51  include two pump flows  2 ,  3 , which are provided by a hydraulic energy source  4 . Hydraulic energy source  4  is preferably a hydraulic pump which conveys the two pump flows  2 ,  3  out of tank  5  into a connecting line  6 . From connecting line  6  comes another connecting line  7 , through which hydraulic system  1 ,  51  is connected to other consumers, which are not shown for the sake of clarity. The other consumers are for example disk sets of the belt-driven conical-pulley transmission, clutches, cooling devices, etc., which act in combination through a plurality of valve devices. 
         [0018]    The two pump flows  2 ,  3  are connected via connecting line  6  to a moment sensor  10 , which ensures that when the transmission is in operation there is constantly sufficient clamping pressure applied to transmit torque between the pulleys and a corresponding encircling element of the belt-driven conical-pulley transmission, in particular depending on the torque applied to the belt-driven conical-pulley transmission. The pressure ahead of moment sensor  10  is identified as p MF . The flow rate of hydraulic medium through or after moment sensor  10  is identified as Q nMF . Connected ahead of moment sensor  10  is a pilot valve device  12 , which is also referred to as a pressure regulating valve device, and which serves to keep the pressure p MF  ahead of moment sensor  10  to a minimum pressure level of 6 to 6.5 bar, for example. Pressure regulating device  12  is actuatable through pressure return lines  14 ,  15 . 
         [0019]    Medium sensor  10  is connected through a return line  18  to an input line  20 , which in turn is connected to the inputs of pump flows  2 ,  3 . Connected to input line  20  ahead of pump flows  2 ,  3  is a pump injector  21 , which serves to improve the suction performance. Situated in return line  18  and connected after moment sensor  10  is a clutch cooling valve device  24 , which serves to convey surplus hydraulic medium from return line  18  through a jet pump  25  to a clutch cooling device (not shown). Return line  18  is therefore also referred to as the cooler return line. 
         [0020]    Clutch cooling valve device  24  is connected after a pressure holding valve device  28 , which serves to maintain a desired minimum pressure in return line  18 . Hydraulic medium is discharged from return line  18  into tank  5  through a discharge line  30 . A substitute restrictor  31  is situated in discharge line  30 . In reality there are numerous restrictors situated in return line  18  between the output of moment sensor  10  and pressure holding valve device  28 , for example for pulley cooling, for oiling and so on. These restrictors (not shown) are combined in substitute restrictor  31  for the sake of simplification. 
         [0021]    Disconnection of second pump flow  3  is only possible if moment sensor  10  is supplied with adequate flow volume. Assuming the critical conditions in regard to temperature and gap height for a certain flow volume Q nMF  through moment sensor  10 , a maximum possible pressure p MF  exists ahead of moment sensor  10 . Second pump flow  3  can only be disconnected if first pump flow  2  is conveying sufficient volume to build up the necessary pressure p MF  ahead of moment sensor  10 . 
         [0022]    In conjunction with the present invention, a threshold value for Q nMF  of about 3 liters per minute has proven to be especially advantageous. Starting at a flow volume Q nMF  of about 3 liters per minute, according to an aspect of the present invention second pump flow  3  should be disconnected, so that for large portions of driving operation one pump flow conveys or is conveyed into the tank, and thus unwanted losses are minimized. It has also been found in conjunction with the present invention that with this flow volume a pressure p MF  of only about 30 bar can be built up ahead of moment sensor  10 . According to another aspect of the invention, this pressure affects the circuit according to the invention. 
         [0023]    The area between the output of moment sensor  10  and pressure holding valve device  28  has proven to be especially well suited for obtaining information about the flow volume Q nMF  via moment sensor  10 . If the pressure p MF  ahead of moment sensor  10  is less than 1.8 bar, then pressure holding valve device  28  is closed, and only the flow volume Q nMF  which is flowing through moment sensor  10  determines the pressure in this area. This pressure can be determined with the aid of a restrictor formula. A flow volume of 2 liters per minute results in a pressure p R  of about 0.5 bar in return line  18  after moment sensor  10  or after clutch cooling valve device  24 . When this pressure is reached, then according to an essential aspect of the invention a disconnecting valve device  33  is supposed to switch second pump flow  3  in the direction of tank  5 . 
         [0024]    Disconnecting valve device  33  is connected to the outputs of the two pump flows  2 ,  3  of hydraulic energy source  4  through a disconnecting line  34 . A change-back valve device  35  is connected into disconnecting line  34  between the outputs of the two pump flows  2 ,  3 . Disconnecting valve device  33  is designed as a 2/2 directional valve that has a closed position in which a connection between disconnecting line  34  and tank  5  is interrupted. Disconnecting valve device  33  is pre-tensioned by a spring  36  in its closed position depicted in  FIGS. 1 and 2 . In an open position (not shown), disconnecting valve device  33  connects disconnecting line  34  to tank  5 . 
         [0025]    Disconnecting valve device  33  is actuated by way of a control pressure line  37 , through which disconnecting valve device  33  is pressurized with the pressure p R  behind moment sensor  10  or behind clutch cooling valve device  24 . The control pressure provided via control pressure line  37  counteracts the pre-tensioning force of spring  36 . Above a certain pressure in return line  18 , which acts on disconnecting valve device  33  through control pressure line  37 , disconnecting valve device  33  or a corresponding valve piston in disconnecting valve device  33  moves from the depicted closed position into the open position; that is, disconnecting valve device  33  begins to open. That causes part of the flow volume of second pump flow  3  to be conveyed in the direction of the tank. 
         [0026]    The remaining portion of the flow volume of second pump flow  3 , on the other hand, continues to be conveyed through disconnecting line  34  to moment sensor  10 . At disconnecting valve device  33  an equilibrium develops between the force from the pressure p R  in return line  18  or control pressure line  37  and the pre-tensioning force of spring  36 . If the volumetric flow rises further, then at some time the flow volume of first pump flow  2  is sufficient to supply the additional consumers via connecting line  7 , and in addition to also transport the desired 3 liters per minute through moment sensor  10  that are necessary to switch disconnecting valve device  33 . At that moment second pump flow  3  is transporting entirely in the direction of tank  5 , and check valve  35  closes. The pressure against which second pump flow  3  must work when transporting then collapses suddenly, and the losses of hydraulic energy source  4  are greatly reduced. 
         [0027]    In most cases a flow volume of 3 liters per minute is sufficient for moment sensor  10  to function. However, at higher torques and consequently higher p MF  levels the volume flow requirement is greater. That need can be met by adding second pump flow  3  back in above a certain pressure level of p MF . According to another aspect of the invention, such connecting of second pump flow  3  is achieved by means of a connecting valve device  40 . 
         [0028]    It has been found in connection with the present invention that for the design of the circuit according to the invention the switching point of pressure holding valve device  28  is relevant at around 5.9 liters per minute. From that point on the pressure p R  no longer has any restriction behavior but remains nearly constant, since pressure holding valve device  28  operates as an absolute pressure valve. The ratio of areas between the effective area of pressure p MF  and of pressure p R  at disconnecting valve device  33  is chosen according to another aspect of the invention so that at a break point the maximum p MF  causes second pump flow  3  to be connected. If the pressure exceeds a switch-on threshold, then disconnecting valve device  33  is switched back to its closed position again, so that the flow volume of both pump flows  2 ,  3  is available. 
         [0029]    It has also been found in connection with the present invention that a direct action of the pressure p MF  on the disconnecting valve device could result in even a very low pressure p MF  being sufficient to reconnect the second pump flow  3 . It would be desirable, however, to reconnect second pump flow  3  only at higher pressures. At the same time, however, it is important to ensure that the second pump flow is reconnected in any case before the maximum possible pressure p MF  is reached. By adding the second pump flow only when pressure becomes greater, it is possible to reduce the losses in particular when traveling at high velocity, that is, at high rotational speeds or high flow volumes. 
         [0030]    For this purpose a connecting valve device  40  is provided, which is designed as a 3/2 directional valve. Connecting valve device  40  is pre-tensioned in its closed position by a spring  41 , and is connected to the input of moment sensor  10  via a moment sensor pressure line  42 , so that the pressure p MF  prevails in moment sensor pressure line  42 . In the closed position of connecting valve device  40  depicted in  FIGS. 1 and 2 , another control pressure line  38 , which comes from disconnecting valve device  33 , is connected through a tank line  43  to tank  5 , so that the other control pressure line  38  is relieved into tank  5 . A connection between the other control pressure line  38  and the moment sensor pressure line  42  is interrupted in the closed position by connecting valve device  40 . 
         [0031]    In its closed position (not shown), connecting valve device  45  connects the moment sensor pressure line  42  to the other control pressure line  38 , so that the pressure p MF  ahead of the moment sensor acts on disconnecting valve device  33  through the moment sensor pressure line  42  and the other control pressure line  38 . Connecting valve device  40  is pressurized against the pre-tensioning force of spring  41  with the pressure p MF  through a moment sensor control pressure line  44  . When the pressure p MF  rises above a specified value, then connecting valve device  40  opens, so that disconnecting valve device  33  is closed, since the control pressure provided via the other control pressure line  38  works together with the pre-tensioning force of spring  36 . 
         [0032]    In the hydraulic system  1  depicted in  FIG. 1  there can be a problem in that the actuating force for disconnecting valve device  33  through control pressure line  37  is rather small; that is, the pressure for a switching action is about 0.5 bar. A possible result of that is that an unwanted effect of flow forces at disconnecting valve device  33  can no longer be ignored. 
         [0033]    As a remedial measure, in  FIG. 2  a pressure conversion valve device  60  is provided in control pressure line  37  in hydraulic system  51 . Pressure conversion valve device  60  can be pressurized at an elevated pressure through a setting pressure line  62 . Pressure conversion valve device  60  is actuated by the pressure in control pressure line  37  through a pressure return line  64 . Pressure conversion valve device  60  serves to elevate the pressure in return line  18  or in control pressure line  37 , and thus to raise the actuating force at disconnecting valve device  33  significantly. 
         [0034]    Although particular embodiments of the present invention have been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit of the present invention. It is therefore intended to encompass within the appended claims all such changes and modifications that fall within the scope of the present invention.