Patent Publication Number: US-2022234440-A1

Title: Drive system for an electric vehicle and method for operating the drive system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to German Patent Application Serial No. 102021200564.0 filed on Jan. 22, 2021, and German Patent Application Serial No. 102021200941.7, filed on Feb. 2, 2021. The entire disclosures of each of these applications is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a drive system for an electric vehicle with an electric machine, a transmission and two clutches which are arranged parallel to one another in a clutch pack. 
     Furthermore, the invention relates to a method for operating the drive system. 
     BACKGROUND 
     Vehicles are increasingly being converted to purely electric drive. Vehicles of this type will in future be equipped selectively with an open differential or what is known as a TWIN system with two clutches which in each case act on a half axle of the driven axle. 
     DE 11 2017 000 856 T5 has disclosed a solution with two electric machines and one differential. 
     DE 10 2007 008 946 B4 discloses a double clutch in a parallel, symmetrical overall design. The outer disk carriers are connected as torque outputs to the transmission input shafts, with the result that parts of an electric machine, for example a rotor, can be adapted at least on one of the outer disk carriers in order to produce a hybrid drive. In a case of this type, the stator of the electric machine is seated in a stationary manner in the transmission housing. The introduction of torque of the internal combustion engine takes place via a shaft into the main hub of the clutch. 
     For purely electric drive, twin and differential systems are to be capable of being provided with interfaces which are as far as possible identical, with the result that one vehicle platform can be provided with different modules. 
     In addition, electric drive systems are also to be capable of decoupling completely from the drive, in order thus to generate as little power loss as possible in certain operating states. The problem here in the case of double clutch systems is that the wet clutches which are used have a high drag torque in the open state as a result of the high number of disks. In addition, the clutches are used in a low rotational speed range, namely the wheel rotational speed, which provides a poor precondition in the case of ejecting of the oil. 
     Furthermore, the drive system is not to cause any splashing losses in the oil sump, specifically in the decoupled state. 
     In addition, a decoupled drive system should provide drive torque very rapidly again as required. 
     The safe state of the double clutch system (failsafe open) has to be ensured for both rotational directions. The forward direction is to be prioritized here. 
     It is an object of the invention to propose a drive device with a double clutch system, which drive device satisfies the requirements of a purely electric drive. 
     SUMMARY 
     The object is achieved by way of a drive system with an electric machine and two gear stages, formed by gearwheels and clutches, the rotor shaft of the electric machine defining an axial overall length, within which the predominant axial length component of an intermediate shaft which runs parallel to the former with gearwheels and an oil pump and the predominant axial length component of a split output shaft which runs parallel to the other shafts with two clutches run. 
     As a result of the three shafts which run in parallel and the mounting of the oil pump, the entire construction is possible in a highly compact manner completely within the overall length defined by the rotor. 
     It is also advantageous here that the axial extent of the two clutches runs within the overall length of the intermediate shaft with an oil pump. 
     In order to obtain rapidly opened clutches with a sufficiently small residual torque for the failsafe state, the clutches with the clutch actuation are of mirror-symmetrical construction in relation to a mirror plane. 
     The ball ramp is provided with two steps with two regions with a different gradient. As a result, both the air gap between the disks which is necessarily large for a small residual torque and rapid closure of the clutches in the case of re-engaging of the electric machine can be achieved. 
     The compact drive system comprises a common oil system for the electric machine and the clutches, with the oil pump, an oil tank and oil barriers, and inner bores in the housing (as an alternative, in combination with outer lines) for cooling and lubricating the components. 
     An important component is the oil barrier which, installed integrally in the rotating ball ramp part, closes and opens openings in the oil feed, fastened to the stationary ball ramp part, as a rotary slide. 
     Furthermore, the object is achieved by way of a method for operating the drive system for engaging the electric machine, the electric machine first of all being set to a target rotational speed and subsequently the clutches being closed and the oil supply being regulated parallel to this via the oil barriers. 
     In the case of disengaging of the electric machine, the two clutches are opened synchronously and the oil flow is stopped by way of the oil barriers, the rotational speed of the electric machine subsequently being increased, in order to eject oil from the clutches, and the oil pump emptying the oil sump. 
    
    
     
       DRAWINGS 
         FIG. 1  shows a construction of an electric drive system, 
         FIG. 2  shows an enlargement of the region of the clutches, 
         FIG. 3  shows a top view of the electric drive system, 
         FIG. 4  shows a sectional view through an actuator, 
         FIG. 5  shows a section through a ball ramp construction, 
         FIG. 6  shows a view of an oil barrier. 
     
    
    
     DESCRIPTION 
     An electric drive system  1  is shown in  FIG. 1 . The drive system is dominated by an electric machine EM which comprises a rotor shaft  4  in a rotatable manner in a stator (not designated further). The rotor shaft  4  has an axial extent of  4 L. 
     On its output side, the electric machine EM has a transmission G which is distinguished by a gearwheel  7  for the first gear and a gearwheel  8  for the second gear. The transmission G is mounted on an intermediate shaft  6  which is arranged parallel to the rotor shaft  4 . Here, the intermediate shaft  6  lies below the rotor shaft  4  and extends at least partially in the axial extent  4 L of the rotor shaft. The clutches  11   a  and  11   b  are arranged below the transmission G. Here, the clutches are coupled to drive half shafts  3   a  and  3   b  on the output side. 
     The drive half shafts  3   a  and  3   b  also run parallel to the rotor shaft  4  and the intermediate shaft  6 , and lie below the intermediate shaft  6 . 
     Furthermore, an oil pump  5  is mounted on the intermediate shaft  6 . Said oil pump  5  is seated structurally below the rotor shaft  4  of the electric machine EM within the overall length  4 L of the rotor shaft  4 . The intermediate shaft  6  and the oil pump  5  extend over an overall length of  6 L. 
     An actuator  9   a  is shown in a flange-connected manner, whereas the actuator  9   b  is merely indicated. 
     The two clutches  11   a,    11   b  have a common clutch basket  12  which at the same time supports a drive wheel  13  of the second gear stage. Here, the drive wheel  13  is welded to the clutch basket  12 . The drive wheel  13  is mounted in the housing  2  via deep groove ball bearings  50 . 
     All the axles in the drive system  1  run parallel to one another. Here, more than 60% of the intermediate shaft  6  with the oil pump  5  which extends over a length  6 L runs within the extent  4 L of the rotor shaft  4  of the electric machine EM. The two clutches  11   a  and  11   b  lie completely within the extent  6 L, the axial dimensions of the clutch basket  12  corresponding approximately to the length of the intermediate shaft  6 . Therefore, the drive wheel  13  of the second gear stage lies completely below the intermediate shaft  6  within the extent  6 L and partially below the rotor  4  within the extent  4 L. 
     The clutches  11   a,    11   b  are connected firstly to the clutch basket  12 , which is linked to the drive wheel  13 , and secondly to the drive half shafts  3   a ,  3   b,  and represent a hubless design for the drive system  1 . A highly compact construction of the drive system  1  is possible by way of the arrangement. 
     The electric motor EM drives the clutch basket  12  via the two transmission stages with the gearwheel  7  for the first gear and the gearwheel  8  for the second gear. 
     The two clutches  11   a  and  11   b  are activated electro-mechanically via ball ramps  10 . The latter are not configured as identical parts, in order to ensure a sufficiently satisfactory failsafe open function, but rather are of mirror-symmetrical configuration with respect to one another. The clutch ramps to the left and the right of the mirror plane S are therefore not configured with identical parts. As a result, it is possible that opening of the clutches is assisted in the case of the two clutches during forward travel. 
     Two independent actuators  9   a,    9   b  operate the two clutches  11   a,    11   b.    
     The drive system  1  has a common oil system for the electric machine EM, the transmission G, and the clutches  11   a  and  11   b . Said common oil system reduces the complexity of the construction and optimizes the thermal property of the system. Here, a coolant/lubricant has to be used which satisfies the requirement of the clutch for a constant coefficient of friction, the requirement of the electric machine EM with regard to the material compatibility of the machine parts, and the requirements of the bearings and toothing systems. 
       FIG. 2  shows the construction of the clutches  11   a  and  11   b  in detail. A pressure plate  14  is equipped with an integrated return spring  40 . The ball ramp  10  is configured with two steps, in order to ensure the required air gaps even for the case of complete disengagement. 
     The disks of the clutches are provided with a corrugation. In order to open the clutch, a great spacing therefore has to be established between the disks. The clutch ramp has a first very steep course, with the result that a great stroke and therefore a great spacing between the disks can be established by way of a small rotary angle of the rotating ramp part  10   b.  The second region which serves to provide the clutch forces has a lower gradient which has to be actuated by way of a greater rotary angle of the rotating ramp part  10   b  with respect to the stationary ramp part  10   a  for high forces. The maximum possible rotary angle is limited geometrically, since three balls  10   e  are arranged in corresponding ramp grooves. 
     The ball ramp  10  consists of the stationary ramp part  10   a  and a rotating ramp part  10   b,  which is shown in detail in  FIGS. 5 and 6 . 
     An oil barrier  17  is clipped in the rotating ramp part  10   b  and utilizes the rotation of the two ball ramp parts with respect to one another in order to control the oil flow into the clutches  11   a  and  11   b  as required or, in the position of complete disengagement, to interrupt said oil flow completely. The oil barrier  17  is a cylindrical component which acts as a rotary slide. The cylindrical component is connected fixedly to the rotatable ball ramp  10   b  for conjoint rotation, and has a radially extending plane  17   b.  When the ball ramp  10   b  rotates, the cylindrical component rotates and, depending on the position of openings  17   a  in the plane  17   b  of the oil barrier  17 , covers a plurality of openings  16   a  of a perforated plate in the stationary oil feed  16 . 
     An angular contact ball bearing  31  is mounted in the stationary ramp part  10   a.  The outer ring  32  of the angular contact ball bearing  31  can also be integrated into the stationary ramp part  10   a.  Components can be saved as a result. 
     A worm toothing system  22  engages into the rotating ramp part  10   b  into the corresponding external toothing system  10   d  of the rotating ramp part  10   b,  to which the actuator  9  is coupled as drive, which is shown in  FIG. 4 . 
     In the case of forward travel, the rotatable ball ramps  10   b  are supported via the frictional torques on the axial bearings  33  of the clutch actuation in the case of the opening of the clutches. 
     Reaction forces which occur from the vehicle via the drive half shafts  3   a,    3   b  are absorbed via the angular contact ball bearing  31  and the transmission housing  2 . Highly precise guidance of the friction disks and a basic prestress for the actuation are ensured via a compact bearing arrangement  30  between the two drive half shafts  3   a,    3   b.    
       FIG. 3  shows the arrangement as a side view with an oil tank  18  which is installed in the interior and in which oil for the clutches  11   a  and  11   b  is buffer-stored and is fed as required via the oil barriers  17  to the clutches  11   a ,  11   b.    
     The internal oil pump  5  is connected to the intermediate shaft  6  and firstly supplies the electric machine EM on the winding heads with cooling oil via the rotor shaft  4 , in order to stabilize the electrical power. Secondly, the oil pump  5  also supplies the oil tank  18  with cooling oil via inner bores and an oil throttle  19 . The heat in the oil is previously discharged by way of a water/oil heat exchanger  21  which is attached on the housing  2  above the oil tank  18 . 
     The oil pump  5  serves to de-oil the oil sump of the drive system  1  and to convey the oil through the drive system. The oil volumetric flow of the mechanical oil pump  5  is divided into the volumetric flow for the electric machine EM, both for the rotor shaft  4  and for the winding head cooling, and into the volumetric flow for the clutches  11   a,    11   b.    
     In order to transfer the clutches into the completely open state, the drive system has to interact with switchable oil barriers, the oil tank, and. The actuation of the electric machine EM also has to be adapted. 
     In order to completely open the clutch, a procedure takes place which secures the open state and reaches it rapidly. 
     In the case of normal opening of the clutches, only an air play of from 1 to 1.5 mm arises. In order to open the clutch completely, the disks of the clutches have to be separated further from one another, in total approximately from 2 to 3 mm per clutch, and the oil has to be completely ejected, in order to avoid drag losses. 
     The procedure for opening the clutches begins with the request to the drive system to decouple the electric machine EM completely from the drive half shafts  3   a,    3   b.    
     Next, the two clutches  11   a,    11   b  are opened synchronously. This takes place by way of the relief of the return spring  40  on the pressure plate  14 , which leads to the oil barrier  17  on the rotating ramp part  10   b  interrupting the oil flow to the clutches completely by way of rotation of the two ball ramp parts with respect to one another. Here, the openings  16   a  of the oil feed  16  are covered completely by way of the regions between the openings  17   a  of the oil barrier  17 . 
     Subsequently, by way of rotational-speed-regulated starting up of the electric machine EM, the oil sump of the drive system is sucked with the aid of the mechanical oil pump  5  which is coupled to the intermediate shaft  6  into the oil tank  18  which is configured as a deep tank, and the oil is ejected from the clutches  11   a,    11   b  by way of the effect of the elevated rotational speed of the clutch basket  12 . 
     This procedure step can be repeated as required if existing leaks of the oil supply mechanism refill the oil sump. 
     The following procedure is carried out in order to couple the electric machine EM to the drive train  1 : 
     First of all, the rotational speed of the electric machine EM is powered up to the target rotational speed which corresponds approximately to the wheel rotational speed of the vehicle. 
     Subsequently, the clutches  11   a,    11   b  are closed and therefore the release of the oil feed  16  is regulated via the oil barrier  17  in parallel. The regulated closure of the clutches when the electric machine EM and the wheel rotational speed are synchronous avoids negative influences on the longitudinal dynamics of the vehicle. 
     In the case of dynamic connecting operations, the rotational speed of the electric machine EM can also lie above the synchronous rotational speed before engagement, in order to generate an additional acceleration impulse by way of the mass inertia of the rotor of the electric machine. 
     LIST OF DESIGNATIONS 
       1  Drive system 
       2  Housing 
       3  Drive half shaft  3   a,    3   b    
       4  Rotor shaft 
       5  Oil pump 
       6  Intermediate shaft 
       7  First gear 
       8  Second gear 
       9  Actuator  9   a,    9   b    
       10  Ball ramp 
       10   a  Stationary ball ramp part 
       10   b  Rotating ball ramp part 
       10   d  External toothing system 
       10   e  Balls 
       11  Multiple disk clutch  11   a ,  11   b    
       12  Clutch basket 
       13  Drive wheel 
       14  Pressure plate 
       15  Oil distributor, clutch 
       16  Oil feed 
       16   a  Openings, oil feed 
       17  Oil barrier 
       17   a  Openings, oil barrier 
       17   b  Axial plane 
       18  Oil tank 
       19  Oil throttle 
       20  Oil line 
       21  Heat exchanger 
       22  Worm shaft 
       30  Bearing arrangement 
       31  Angular contact ball bearing 
       32  Outer ring 
       33  Radial needle ball bearing 
       40  Return spring 
       50  Deep groove ball bearing