Abstract:
A CVT transmission including a start-up element, a variator, and a direct-shift stage for switching between a first operating range (low) and a second operating range (high). A maximum transmission ratio of the second operating range (high) corresponds to a minimum transmission ratio of the first operating range (low). The transmission ratio ranges are arranged so that the marginal variator transmission ratio ranges, which constitute low-efficiency ranges, are not used.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is the U.S. national phase application under 35 U.S.C. §371 of International Application Serial No. PCT/DE2014/200661, having an international filing date of 28 Nov. 2014, and designating the United States, which claims priority based upon German Patent Application No. DE 10 2013 225 264.1, filed on 9 Dec. 2013, the entire contents of each of which applications are hereby incorporated by reference herein to the same extent as if fully rewritten. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a CVT transmission having a start-up element, a variator, and having a direct-shift stage for switching between the first operating range (low) and a second operating range (high). In addition, the present invention relates to a method for operating such a CVT transmission. 
         [0004]    2. Description of the Related Art 
         [0005]    The term CVT refers to a stepless transmission; the letters CVT stand for continuously variable transmission. To increase the transmission ratio range of a stepless transmission—its gear ratio spread—it is known from European published unexamined application EP 2 275 709 A1 to position a planetary gear set after the stepless transmission. The controllable planetary gear set enables two-range shifting and shifting into reverse. In addition, it is known from German published application DE 102 61 900 A1 to provide a multi-range CVT with fixed engageable gears, for example for moving off or for top speed; however, when these fixed transmission ratios are in operation the variator is uncoupled. Consequently, there is only one stepless range; stepless operation is not possible in all driving ranges. 
         [0006]    An object of the present invention is to simplify the operation of a CVT transmission having a start-up element, a variator, and having a direct-shift stage for switching between a first operating range (low) and a second operating range (high). 
       SUMMARY OF THE INVENTION 
       [0007]    The above-identified object is fulfilled in the case of a CVT transmission having a start-up element, a variator, and having a direct-shift stage for switching between a first operating range (low) and a second operating range (high), by letting a maximum transmission ratio of the second operating range (high) correspond to a minimum transmission ratio of the first operating range (low). With conventional dual-range CVT transmissions, a relatively large overlap of the two transmission ratio ranges is necessary to enable convenient implementation of switching between ranges. However, such a large range overlap is achieved at the expense of an efficiency-optimized variator ratio spread in variator operation. By synchronizing the maximum transmission ratio of the second operating range with the minimum transmission ratio of the first operating range according to the present invention, it is possible to improve the efficiency of the CVT transmission having the direct-shift stage. 
         [0008]    The object stated above is fulfilled alternatively or additionally in the case of a CVT transmission having a start-up element, a variator, and having a direct-shift stage for switching between a first operating range (low) and a second operating range (high), by letting a minimum variator ratio of the second operating range (high) be greater than a minimum variator ratio of the first operating range (low). 
         [0009]    According to one aspect of the invention, the transmission ratio ranges are designed so that the marginal variator ranges, which constitute low-efficiency ranges, are not used. According to another aspect of the invention, the marginal variator range of the second operating range (high) is reduced more severely than the marginal variator range of the first operating range (low). In so doing, consideration is given to the fact that the second operating range (high) is normally used for a larger proportion of the running time than the first operating range (low). 
         [0010]    The object stated above is fulfilled alternatively or additionally in the case of a CVT transmission having a start-up element, a variator, and having a direct-shift stage for switching between a first operating range (low) and a second operating range (high), by letting a closing valve device be operatively connected to a variator fluid system in such a way that the variator fluid system is closed fluid-tight by the closing valve device when the direct-shift stage is used for driving with the variator uncoupled. That protects the variator, in particular the variator fluid system, in a simple manner against running empty. The present invention can also relate to a variator fluid system having such a closing valve device. The closing valve device preferably includes a valve that has no or almost no leakage, for example a seated valve. 
         [0011]    The object stated above is fulfilled alternatively or additionally in the case of a CVT transmission having a start-up element, a variator, and having a direct-shift stage for switching between a first operating range (low) and a second operating range (high), by letting a clutch of the direct-shift stage, in particular a jaw clutch, be designed as a normally open clutch. That makes it possible to realize in a simple way a safety function against drive train locking in the event of a system error. To achieve the safety function, the jaw clutch of the direct-shift stage can be designed, for example, as a normally open clutch. Alternatively or additionally, a start-up clutch can be used as a control element against drive train locking when switching to the direct-shift stage, in particular at a time when the variator and the direct-shift stage are included in the power flow simultaneously. 
         [0012]    Another preferred exemplary embodiment of the CVT transmission is characterized in that the direct-shift stage which bridges the variator is tied directly to the drive. The direct connection of the direct-shift stage to the drive enables the direct-shift stage to be used advantageously independently of the start-up element. The direct-shaft stage can be connected, for example, to a gear that is used in conventional CVT drive trains to drive a hydraulic pump. Such a gear is therefore also referred to as a pump gear. If the input drive includes a combustion machine or internal combustion engine, then the direct-shift stage that bridges the variator is driven directly by the combustion machine or internal combustion engine. Because of the direct connection of the direct-shift stage to the input drive, the direct-shaft stage is preferably used within the framework of the present invention exclusively in the driving operation of a motor vehicle equipped with the CVT drive train. 
         [0013]    Another preferred exemplary embodiment of the CVT transmission is characterized in that the direct-shift stage that bridges the variator is connected to a crankshaft with a torsional vibration damper interposed. A torque of the input drive, in particular of the combustion machine or internal combustion engine, is delivered by means of the crankshaft. The torsional vibration damper serves to uncouple unwanted torsional vibrations which occur during operation of the input drive, in particular the combustion machine or internal combustion engine, from the CVT drive train. That prevents unwanted damage to the CVT drive train caused by rotational non-uniformities. 
         [0014]    Another preferred exemplary embodiment of the CVT transmission is characterized in that a sub-transmission is positioned between the variator and the differential. The sub-transmission is, for example, a step-down gear. The sub-transmission is preferably positioned between the variator output and the differential. The direct-shift stage, on the other hand, is preferably positioned between the start-up element and the variator input. 
         [0015]    Another preferred exemplary embodiment of the CVT transmission is characterized in that the sub-transmission is implemented as a dual-range transmission, in particular as a planetary transmission. The dual-range transmission makes driving operation possible, for example, in the first range, which is also referred to as the low range, and in the second range, which is also referred to as the high range. In the first range, it is possible, for example, to drive with a higher transmission ratio than in the second range. Furthermore, the dual-range transmission in the form of a planetary transmission advantageously make it possible to produce a reverse gear. 
         [0016]    In a method for operating a CVT transmission having a start-up element, a variator, and having a direct-shift stage for switching between a first operating range (low) and a second operating range (high), in particular a CVT transmission described earlier, the object stated above is fulfilled alternatively or additionally by the fact that marginal variator ranges of the operating ranges (low and high) are not used. The efficiency of the CVT transmission is poorer in the marginal variator ranges. The efficiency of the CVT transmission can be improved in a simple way by not using the marginal variator ranges. The marginal variator ranges, which are not used according to the method according to the invention, are preferably lower end regions of transmission ratio characteristics, which serve to depict the two operating ranges low and high in a transmission ratio characteristic map. In such a transmission ratio characteristic map, the variator ratio is plotted, for example, on an x-axis. The transmission ratio is then plotted advantageously on the y-axis of the transmission ratio characteristic map. 
         [0017]    The object stated above is fulfilled alternatively or additionally in a method for operating a CVT transmission having a start-up element, a variator, and having a direct-shift stage for switching between a first operating range (low) and a second operating range (high), in particular a CVT transmission described earlier, in particular in a method described earlier, by the fact that in a quick reset of the transmission ratio the system jumps directly from one operating range into the other operating range, without using the direct-shift stage and without resetting completely steplessly. That makes it possible to improve the driving comfort during operation of the CVT transmission. 
         [0018]    The object stated above is fulfilled alternatively or additionally in a method described above by letting the start-up element, in particular a starting clutch, be used as a control element against drive train locking when switching to the direct-shift stage. In that case, the start-up element, in particular the starting clutch, is used in particular at a time when the variator and the direct-shift stage are included in the power stream simultaneously. 
         [0019]    The object stated above is fulfilled alternatively or additionally in a method described above by letting the start-up element, in particular a starting clutch, be used in a slipping mode, in order to accelerate the stationary variator before the direct-shift stage is uncoupled and the power stream is again routed to the variator. That makes it possible to further improve the driving comfort of the CVT transmission. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Additional advantages, features, and details of the present invention can be seen from the following description, in which various exemplary embodiments are described in detail with reference to the drawings. The drawing figures show the following: 
           [0021]      FIG. 1  is a simplified representation of a CVT drive train according to the present invention in a longitudinal section; 
           [0022]      FIG. 2  shows the CVT drivetrain drive train of  FIG. 1  in cross section; and 
           [0023]      FIG. 3  is a transmission ratio characteristic map of the CVT drive train shown in  FIGS. 1 and 2  according to a first exemplary embodiment of a method according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]      FIGS. 1 and 2  show different views of a simplified representation of a CVT drive train  1  according to the present invention. The CVT drive train  1  includes an input drive  3 . The input drive is, for example, a combustion machine, which is also referred to as an internal combustion engine when used in a motor vehicle. The CVT drive train  1  is used in motor vehicles. 
         [0025]    A start-up element  5  makes it possible to move the motor vehicle off. A torque is transmitted from the input drive  3  to a start-up output part  6  through the start-up element  5 . The start-up output part  6  is connected to a variator input of a variator  10  through a gear stage having a gear  8  and a gear  9 . 
         [0026]    The variator  10  includes a conical disk set  11  on the drive side and a conical disk set  12  on the output side. The two conical disk sets  11 ,  12  are coupled with each other by an endless torque-transmitting means  13 , which is only shown generally. The endless torque-transmitting means  13  can be, for example, a special chain. 
         [0027]    By means of the two conical disk sets  11  and  12 , the transmission ratio between the input drive  3  and an output  15  can be varied continuously. The output  15  includes at least one driven wheel (not shown). 
         [0028]    Normally, the output  15  is operatively connected to at least two driven vehicle wheels. An equalizing transmission, also referred to as a differential  16 , serves to distribute the provided torque to the two driven vehicle wheels. The differential  16  includes a spur gear  18 . 
         [0029]    The spur gear  18  of the differential  16  meshes with a sub-transmission output gear  19  of a sub-transmission  20 . The sub-transmission  20  is assigned to a variator output on the output-side conical disk set  12 . 
         [0030]    A torsional vibration damper  22  is operatively connected with the input drive  3  of the CVT drive train  1 . The torsional vibration damper  22  is positioned between the input drive  3  and the start-up element  5 . The start-up element  5  is implemented as a starting clutch  24 . The starting clutch  24  is a wet-running multi-plate clutch. 
         [0031]    An input part  25  of the torsional vibration damper  22  is non-rotatably connected to a crankshaft of the input drive  3 . An output part  26  of the torsional vibration damper  22  represents, on the one hand, an input of the starting clutch  24 . On the other hand, the output part  26  of the torsional vibration damper  22  is non-rotatably connected to a gear  28 . The gear  28  serves, for example, to drive a pump (not shown). The gear  28  is therefore also referred to as a pump gear. However, the gear  28  can also serve to drive a different or an additional vehicle component. 
         [0032]    According to one aspect of the present invention, a direct-shift stage  30  that is switchable with the aid of a switching device  29  is operatively connected to the gear  28 . An arrow  31  indicates that the direct-shift stage  30  serves to bridge the variator  10 . As indicated by the arrow  31 , with the aid of the switching device  29 , the direct-shift stage  30  can provide a direct coupling of the gear  28  to the spur gear  18  of the differential  16 . With the aid of the direct-shift stage  30 , the input drive  3  can be connected as a drive through the torsional vibration damper  22  to the output drive  15 , independently of the start-up element  5 , past the variator  10 , to the differential  16 . 
         [0033]    In  FIG. 2 , the axis of rotation  33  of the crankshaft runs perpendicular to the plane of the drawing. A circle  34  indicates a starter ring gear which is non-rotatably connected to the crankshaft. A radially inner circle represents the gear  8  shown in  FIG. 1 . Another circle represents the gear  28 , also referred to as a pump gear. Gear  8  meshes with gear  9 , which represents the variator input. Gear  9  is operatively connected to the drive-side conical disk set  11 , which is likewise represented in  FIG. 2  as a circle. A circle  12  represents the output-side conical disk set. The sub-transmission gear  19  meshes with the spur gear  18 , which is likewise indicated by a circle. 
         [0034]    The circles in  FIG. 2  make the front-transverse construction clear. In  FIG. 2 , the direct-shift stage is positioned below the axis  33  of the crankshaft and in the direction of the spur gear  18  of the differential  16 . Front-transverse construction means that the input drive  3 , in particular the internal combustion engine, and the transmission, here the variator  10  and the sub-transmission  20 , are positioned next to each other in the transverse direction of the vehicle, for example in front of or above a front axle. 
         [0035]    In  FIGS. 1 and 2 , the sub-transmission  20  is implemented as a planetary transmission having two planetary gear sets and two plate assemblies. The sub-transmission  20  in the form of a planetary transmission makes it possible to switch between a first range low and a second range high. Furthermore, the sub-transmission  20  serves to provide a reverse gear R. 
         [0036]      FIG. 3  shows a transmission ratio characteristic map for the CVT drive train  1  shown in  FIGS. 1 and 2 . The transmission ratio characteristic map is designed as a Cartesian coordinate diagram having an x-axis  51  and a y-axis  52 . A variator transmission ratio is plotted on the x-axis  51 . A sub-transmission transmission ratio is plotted on the y-axis  52 . 
         [0037]    An upper characteristic curve  53  serves to represent the first operating range, which is also referred to as the low range. A lower characteristic curve  54  serves to represent the second operating range, which is also referred to as the high range. The low range  53  begins at a variator transmission ratio of about 0.65 and a sub-transmission ratio of about 4.7. The high range begins at a variator transmission ratio of about 0.57 and a sub-transmission ratio of about 2.1. 
         [0038]    A line  55  running parallel to the x-axis  51  indicates that the direct-shift stage, which is also referred to as the constant stage, is used to switch between the ranges  53  and  54  while the sub-transmission ratio is always the same. In  FIG. 2 , the sub-transmission ratio of about 4.6 is used, which exists at the lower end of the characteristic curve  53  and at the upper end of the characteristic curve  54 . Thus, the line  55  connects the upper end of characteristic curve  54  with the lower end of characteristic curve  53 . That enables the available variator transmission ratio spread to be utilized optimally. 
         [0039]    Dashed lines at the beginning and the lower ends of characteristic curves  53  and  54  indicate that the transmission ratio ranges are laid out according to one aspect of the invention so that the marginal variator transmission ratio ranges at the lower ends of characteristic curves  53  and  54  are not used. Here, the transmission ratios are laid out according to another aspect so that in the case of the second operating range or transmission ratio range, which is also referred to as the high range, the marginal range at the lower end of characteristic curve  54  is reduced more severely than the marginal range at the lower end of characteristic curve  53 . 
         [0040]    According to another aspect of the invention, when driving in the direct-shift stage  30  and with uncoupled, non-rotating variator  10 , a valve switching system is provided to protect the variator  10  or a variator fluid system against running empty. The valve switching system preferably includes a valve that has no or almost no leakage. 
         [0041]    According to another aspect of the invention, when a quick reset is desired by a driver of a motor vehicle equipped with the CVT drive train, the system jumps directly from one range into the other range, without using the direct-shift stage and without resetting completely steplessly. 
         [0042]    According to another aspect of the invention, the switching device  29  of the direct-shift stage  30  is designed as a claw clutch. This claw clutch is equipped with a normally-open function to represent a safety function against drive train locking in the event of a system error. Normally open means that the claw clutch is normally disengaged, and is closed or engaged actively. 
         [0043]    According to another aspect of the invention, the start-up element  5 , in particular the starting clutch  24 , is used as a control element against drive train locking when switching to the direct-shift stage  30 , namely in particular at a time when the variator  10  and the direct-shift stage  30  are included in the power stream simultaneously. 
         [0044]    According to another aspect of the invention, the start-up element  5 , in particular the starting clutch  24 , is used in a slipping mode, in order to accelerate the stationary variator  10  before the direct-shift stage  30  is uncoupled and the power stream is again routed to the variator  10 .