Patent Publication Number: US-2015068831-A1

Title: Drive unit for a hybrid vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage of International Application No. PCT/EP2012/053388 filed Feb. 29, 2012 which claims the benefit of and priority to German Application No. DE 102011018861.4 filed Apr. 28, 2011. The entire disclosure of each of the above applications is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a drive unit for a hybrid vehicle which has a front axle and a rear axle, having an internal combustion engine, which is arranged in the region of the front axle, for driving at least the rear axle, and an electric machine for driving the front axle. 
     BACKGROUND 
     Hybrid drive trains for motor vehicles which use both an internal combustion engine and an electric machine as drive source are distinguished by a high degree of efficiency and the possibility to save fuel. A drive train of a vehicle having an internal combustion engine and rear axle drive can be extended to produce a hybrid drive train with all wheel drive, by the front axle being driven by way of an electric machine. This can take place in principle via a front axle differential which has an input member for receiving a drive power output from the electric motor and two output members for coupling to a respective wheel of the front axle. Depending on the application, a front axle differential of this type can be configured as a bevel gear differential or as a planetary gear differential. If, in particular, the internal combustion engine is configured as a front-mounted longitudinal engine, that is to say a crankshaft of the internal combustion engine is oriented parallel to the vehicle longitudinal axis, the only accommodation options in practice for the front axle differential and the electric machine exist laterally next to the internal combustion engine as viewed in the direction of the vehicle longitudinal axis. On account of the limited installation space, only an electric machine of small dimensions with a correspondingly low power output can be used. This restricts the driving performance and the fuel efficiency of the hybrid vehicle. 
     SUMMARY 
     It is an object of the invention to make an effective drive of the front axle by way of an electric machine possible in a hybrid vehicle having an internal combustion engine which is arranged in the region of the front axle. 
     The object is achieved by way of a drive unit for a hybrid vehicle which has a front axle and a rear axle, having an internal combustion engine, which is arranged in the region of the front axle, for driving at least the rear axle, an electric machine for driving the front axle, and a front axle differential which has an input member for receiving a drive power output from the electric machine, and two output members for coupling to a respective wheel of the front axle, the electric machine and the front axle differential being arranged along the front axle on opposed sides of the internal combustion engine, and the electric machine being coupled in drive terms to the input member of the front axle differential via a connecting shaft which is guided through an oil sump of the internal combustion engine. 
     In accordance with the invention, the electric machine and the front axle differential are arranged along the front axle on opposed sides of the internal combustion engine, and the electric machine is coupled in drive terms to the input member of the front axle differential via a connecting shaft which is guided through an oil sump of the internal combustion engine. 
     By virtue of the fact that the electric machine and the front axle differential are arranged on different sides of the internal combustion engine, more space is available overall for accommodating the electric machine, with the result that an electric machine with a correspondingly higher power output can be used. The components which are required for the electromechanical driving of the front axle are therefore divided up to the accommodation space which is available on both sides of the internal combustion engine, which results in improved space utilization overall. On account of the coupling of the electric machine and the front axle differential by means of the connecting shaft which is guided through the oil sump, complicated repositioning or reconstruction of the internal combustion engine is not necessary. Overall, the invention therefore makes simple integration possible of an electromechanical front axle drive into an existing motor vehicle with a front-mounted longitudinal internal combustion engine and rear axle drive. 
     Developments of the invention are specified in the dependent claims, the description and the appended drawings. 
     The connecting shaft is preferably guided through a passage which is provided in the oil sump. In this way, complicated seals and undesired splash effects are avoided. Depending on the application, the passage can be a channel-like leadthrough through the oil sump or else a cutout which is open toward one side of the oil sump, for example the underside. 
     The connecting shaft can be configured as a hollow shaft, through which a lateral shaft is guided which extends from one of the output members of the front axle differential to the associated wheel of the front axle. The connecting shaft can therefore be arranged coaxially with respect to the front axle differential, which results in a particularly space-saving overall arrangement. 
     Furthermore, the drive unit can comprise a clutch for selectively disengaging the electric machine from the input member of the front axle differential, in order to disengage the electric machine and/or a part of an associated step-down gear mechanism from the remaining drive if required, for example if a predefined vehicle speed is exceeded. As a result, for example, drag losses and an excessively high rotational speed of the electric machine can be avoided. A separating clutch of this type is preferably arranged between the electric machine and the connecting shaft or between the connecting shaft and the front axle differential. As an alternative to this, a separating clutch can be provided between one of the two output members of the front axle differential and the associated front wheel, in particular on a lateral shaft which extends from the relevant output member to the associated wheel of the front axle, as a result of which decoupling of the electric machine from the remaining drive can likewise be brought about. Depending on the application, the clutch can be active in a frictional or positively locking manner. 
     In accordance with one embodiment of the invention, a drive shaft of the electric machine is arranged coaxially with respect to the connecting shaft and is configured, in particular, as a hollow shaft. This makes a particularly space-saving construction of the drive unit possible, it being possible for one of the lateral shafts to be guided through the drive shaft. The drive shaft can also be configured in one piece with the connecting shaft. 
     Furthermore, the drive unit can comprise an, in particular two-stage, step-down gear mechanism which is configured for stepping down a rotation of the input member of the front axle differential relative to a rotor of the electric machine. This makes the use of an electric machine with a comparatively high rotational speed possible, which electric machine can be of relatively small dimensions for a predefined power output. 
     The step-down gear mechanism can comprise at least one planetary gear mechanism which is, in particular, coaxial with respect to the connecting shaft and comprises a first member for coupling to the electric machine, a second member for coupling to the input member of the front axle differential and a third, fixed member. In relation to the installation space, a planetary gear mechanism makes a relatively great transmission ratio and the transmission of comparatively high torques possible. It is also advantageous that the input and the output lie coaxially with respect to one another in a planetary gear mechanism. In accordance with one refinement of the invention, the planetary gear mechanism comprises a fixed internal gear, for example an internal gear which is pressed into a housing, a planetary carrier with planetary gears which are mounted movably thereon, and a sun gear, the drive preferably taking place via the sun gear and the output preferably taking place via the planetary carrier. 
     The planetary gear mechanism is preferably arranged on that side of the internal combustion engine, on which the front axle differential is arranged. More space for accommodating the electric machine therefore remains on the opposite side of the internal combustion engine. 
     The second member of the planetary gear mechanism can be configured in one piece with the input member of the front axle differential. For example, a planetary carrier of the planetary gear mechanism can be integrated into a differential cage of the front axle differential. The required installation space can be reduced further by way of a single-piece configuration of this kind of the second member of the planetary gear mechanism with the input member of the front axle differential. 
     In accordance with one refinement of the invention, the planetary gear mechanism comprises a sun gear, an internal gear and a planetary carrier with a plurality of rotatably mounted planetary gears, the planetary gears having a first toothing section which meshes with the sun gear and a second toothing section which meshes with the internal gear, the first toothing section having a greater diameter than the second toothing section. A planetary gear mechanism of this type is also called a “multi-step planetary gear mechanism”. In principle, instead of a greater diameter, the first toothing section could also have a greater number of teeth than the second toothing section. In a refinement of this type, the connecting shaft can drive the larger planetary gears via the sun gear, whereas the smaller planetary gears are supported on the fixed internal gear. This therefore results in two step-down stages which are arranged one behind another, with the result that overall a greater step-down ratio can be achieved than in the case of a single planetary gear mechanism and an electric machine which correspondingly rotates more rapidly can be used. 
     In accordance with a further embodiment of the invention, the step-down gear mechanism comprises two single-stage planetary gear mechanisms which are arranged on opposed sides of the internal combustion engine and form respective step-down gear mechanisms which are connected one behind another. In this refinement, the step-down gear mechanism is divided up along the front axle to the two sides of the internal combustion engine, which can be advantageous with regard to space utilization in certain application situations. 
     In accordance with a further embodiment of the invention, a drive shaft of the electric machine is offset in parallel relative to the connecting shaft, an offset drive forming a step-down gear mechanism for coupling the drive shaft to the connecting shaft. The use of an offset drive or offset gear mechanism results in the possibility of positioning the electric machine at a favorable location in the front engine compartment. 
     In particular, the offset drive can comprise a spur gear set with at least one input spur gear which is coupled or can be coupled to the electric machine and an output spur gear which is coupled or can be coupled to the connecting shaft. Here, the transmission ratio of a step-down gear mechanism of this type results from the difference in the diameters or numbers of teeth of the input spur gear with respect to the output spur gear. In particular, the offset drive can form a first step-down stage which is followed by a second step-down stage in the form of a planetary gear mechanism. 
     The input spur gear and the output spur gear of the offset drive preferably mesh with at least one intermediate spur gear, in order to make a sufficiently great spacing between the lateral shaft and the drive shaft of the electric machine possible. In principle, this spacing could also be bridged by means of a chain drive or belt drive. 
     Furthermore, the drive unit can comprise a shiftable clutch, in order to couple the output spur gear of the offset drive selectively to the connecting shaft. For example, an output spur gear of the offset drive can be coupled selectively to the connecting shaft which is guided through the oil sump to the front axle differential, for example by means of a slider sleeve. As an alternative to this, a switchable clutch can be provided between a drive shaft of the electric machine and the input spur gear of the offset drive or between the connecting shaft and the input member of the front axle differential. The electric machine can be decoupled from the front axle as required by way of the shiftable clutch. 
     In accordance with a further embodiment of the invention, a drive shaft of the electric machine is oriented at an angle which differs from 180° with respect to the connecting shaft, an angle drive forming a step-down gear mechanism for coupling the drive shaft to the connecting shaft. For example, the drive shaft of the electric machine can be oriented at right angles to the connecting shaft. The angle drive can be configured, in particular, as a hypoid drive. For example, this makes an arrangement of the electric machine possible in the longitudinal direction parallel to an internal combustion engine which is configured as a front-mounted longitudinal engine or an arrangement of the electric machine with a vertically oriented axis, which can be advantageous in certain application situations. 
    
    
     
       DRAWINGS 
       The invention will be described in the following text by way of example with reference to the drawings, in which: 
         FIG. 1  is a sectional illustration of a drive unit in accordance with a first embodiment of the invention, 
         FIG. 2  is a sectional illustration of a drive unit in accordance with a second embodiment of the invention, 
         FIG. 3  is a sectional illustration of a drive unit in accordance with a third embodiment of the invention, and 
         FIG. 4  is a sectional illustration of a drive unit in accordance with a fourth embodiment of the invention. 
     
    
    
     DESCRIPTION 
     The drive unit, illustrated in  FIG. 1 , for a hybrid vehicle comprises an internal combustion engine  11  which is configured as a front-mounted longitudinal engine which is arranged in the region of a front axle  12  of the vehicle and drives a rear axle of the vehicle via a drive train (not illustrated). An oil sump  13  is provided on the underside of the internal combustion engine  11 . 
     Furthermore, the drive unit comprises an electric machine  15  with a stator  17  and a rotor  19  which can be rotated with respect to the stator  17  and is seated fixedly on a drive shaft  20  of the electric machine  15  so as to rotate with it. 
     A front axle differential  23  has a differential cage  25 , on which bevel gears  27  are mounted rotatably which mesh with bevel gears  29  which are coupled in drive terms to a left-hand lateral shaft  30  and a right-hand lateral shaft  31 . The left-hand lateral shaft  30  and the right-hand lateral shaft  31  are coupled to a respective wheel (not illustrated) of the front axle  12 , with the result that a drive power output which is introduced into the differential cage  25  can be transmitted with the rotational speed equalization to the two front wheels. The drive shaft  20  of the electric machine  15  is coupled via a step-down gear mechanism  21 A and a connecting shaft  33  to the differential cage  25  of the front axle differential  23 , in order accordingly to transmit an electric drive power output to the wheels of the front axle  12 . 
     As is apparent from  FIG. 1 , the electric machine  15  and the front axle differential  23  are arranged on opposed sides of the internal combustion engine  11  as viewed along the front axle  12 , the connecting shaft  33  being guided through a passage  35  which is provided in the oil sump  13 . Like the drive shaft  20 , the connecting shaft  33  is likewise configured as a hollow shaft, the right-hand lateral shaft  31  being guided through the hollow connecting shaft  33  and the hollow drive shaft  20 . In the embodiment which is illustrated in  FIG. 1 , the front axle differential  23 , the connecting shaft  33  and the electric machine  15  are therefore arranged coaxially. 
     The step-down gear mechanism  21 A is capable of stepping down a rotation of the differential cage  25  of the front axle differential  23  relative to the rotor  19  of the electric machine  15 . In accordance with  FIG. 1 , the step-down gear mechanism  21 A has two stages and is divided into two single-stage planetary gear mechanisms  40 ,  41  which are arranged along the front axle  12  on opposed sides of the internal combustion engine  11 . The first, electric machine-side planetary gear mechanism  40  is flange-connected directly onto an end side of the electric machine  15 , the input taking place via a sun gear  45  which is formed directly on the drive shaft  20  of the electric machine  15 . Planetary gears  49  which are mounted rotatably on a planetary carrier  47  mesh with the sun gear  45  and with a fixed internal gear  50 , the planetary carrier  47 , via which the output takes place, being coupled fixedly to the connecting shaft  33  so as to rotate with it. In the region of the front axle differential  23 , a sun gear  60  of the second, differential-side planetary gear mechanism  41  is formed directly at one end of the connecting shaft  33  and meshes with planetary gears  61  which are mounted rotatably on a planetary carrier  65  and roll on a fixed internal gear  63 . The planetary carrier  65  is configured in one piece with the differential cage  25  of the front axle differential  23  and represents the output of the second planetary gear mechanism  41 . As a result, a rotational movement of the differential cage  25  relative to the rotor  19  of the electric machine  15  is stepped down by means of the two planetary gear mechanisms  40 ,  41  in two step-down stages which are connected one behind another. 
       FIGS. 2 to 4  illustrate alternative embodiments of drive units in accordance with the invention, identical components being provided with the same designations as in the drive unit in accordance with  FIG. 1 . In the illustrations in accordance with  FIGS. 3 and 4 , the internal combustion engine and the oil sump are omitted for the sake of simplification. 
     In the refinement in accordance with  FIG. 2 , instead of two single-stage planetary gear mechanisms, the step-down gear mechanism  21 B comprises a single multi-step planetary gear mechanism  71 . The multi-step planetary gear mechanism  71  has a sun gear  73  which is formed directly on the connecting shaft  33 , a fixed internal gear  78  and a planetary carrier  75  with a plurality of rotatably mounted planetary gears  77 , the planetary gears  77  having a first toothing section  80  which meshes with the sun gear  73  and a second toothing section  81  which meshes with the internal gear  78 . The first toothing section  80  has a greater diameter than the second toothing section  81 , with the result that, in the case of an input via the sun gear  73  and an output by the planetary carrier  75 , the multi-step planetary gear mechanism  71  provides a two-stage step-down transmission. Therefore, one planetary gear mechanism on the side of the electric machine  15  can in principle be omitted. In this embodiment, the drive shaft  20  of the electric machine  15  can be configured integrally, as illustrated, with the connecting shaft  33 . 
     In the embodiment, illustrated in  FIG. 3 , of a drive unit in accordance with the invention, the drive shaft  20  of the electric machine  15  is offset in parallel relative to the connecting shaft  33 , an offset drive  90  in the form of a spur gear set being provided for coupling the drive shaft  20  to the connecting shaft  33 . 
     The offset drive  90  comprises an input spur gear  91  which is formed on the drive shaft  20 , an intermediate spur gear  92  which meshes with the former, and an output spur gear  93  which meshes with the intermediate spur gear  92  and is coupled in drive terms to the connecting shaft  33 . The offset drive  90  therefore forms a step-down stage of a step-down gear mechanism  21 C, a single-stage differential-side planetary gear mechanism  41  forming a second step-down stage. A shiftable clutch  95  makes it possible to couple the output spur gear  93  of the offset drive  90  selectively to the connecting shaft  33 . As a result, the electric machine  15  can be disengaged from the front axle differential  23  as required. 
     A separating clutch such as the shiftable clutch  95  can in principle also be provided in the remaining embodiments which are described. 
     In the embodiment, illustrated in  FIG. 4 , of a drive unit in accordance with the invention, the drive shaft  20  of the electric machine  15  is oriented at a right angle to the connecting shaft  33 , a hypoid drive  97  being provided for coupling the drive shaft  20  to the connecting shaft  33 . Like the offset drive  90  of the embodiment in accordance with  FIG. 3 , the hypoid drive  97  forms a step-down stage of a step-down gear mechanism  21 D, whereas a second step-down stage is provided by a differential-side single-stage planetary gear mechanism  41 . 
     In all the embodiments which are described, the step-down gear mechanism  21 A,  21 B,  21 C,  21 D is of two-stage configuration and thus makes a relatively high transmission ratio possible, with the result that the electric machine  15  can be operated at a comparatively high rotational speed and can therefore be of correspondingly small dimensions with a predefined power output. 
     LIST OF REFERENCE SIGNS 
     
         
           11  Internal combustion engine 
           12  Front axle 
           13  Oil sump 
           15  Electric machine 
           17  Stator 
           19  Rotor 
           20  Drive shaft 
           21 A,  21 B,  21 C,  21 D Step-down gear mechanism 
           23  Front axle differential 
           25  Differential cage 
           27  Bevel gear 
           29  Bevel gear 
           30  Left-hand lateral shaft 
           31  Right-hand lateral shaft 
           33  Connecting shaft 
           35  Passage 
           40  First electric machine-side planetary gear mechanism 
           41  Second differential-side planetary gear mechanism 
           45  Sun gear of the first planetary gear mechanism 
           47  Planetary carrier of the first planetary gear mechanism 
           49  Planetary gear of the first planetary gear mechanism 
           50  Internal gear of the first planetary gear mechanism 
           60  Sun gear of the second planetary gear mechanism 
           61  Planetary gear of the second planetary gear mechanism 
           63  Internal gear of the second planetary gear mechanism 
           65  Planetary carrier of the second planetary gear mechanism 
           71  Multi-step planetary gear mechanism 
           73  Sun gear of the multi-step planetary gear mechanism 
           75  Planetary carrier of the multi-step planetary gear mechanism 
           77  Planetary gear of the multi-step planetary gear mechanism 
         Internal gear of the multi-step planetary gear mechanism 
           80  First toothing section 
           81  Second toothing section 
           90  Offset drive 
           91  Input spur gear 
           92  Intermediate spur gear 
           93  Output spur gear 
           95  Shiftable clutch 
           97  Hypoid drive