Abstract:
A drive train for a motor vehicle including a main engine, an auxiliary engine, and a parallel shaft transmission. The transmission includes: at least one input shaft connected to the main engine by a clutch device, at least one output shaft including a drive pinion that engages with a differential ring gear, a countershaft that is not coaxial with the input and output shafts, and a plurality of synchronizers configured to connect the input shaft to the output shaft selectively in rotation. The countershaft is fitted with a drive wheel connected to the auxiliary engine and that is permanently rotatably linked to the input shaft.

Description:
BACKGROUND 
     The invention relates to the field of motor vehicle drivetrains and in particular to the field of drivetrains having a main engine, an auxiliary engine and a parallel shaft transmission. 
     In this field, patent application WO 2005/065 976 (EATON CORPORATION) has described a transmission comprising a solid primary shaft and a hollow primary shaft which are coaxial and connected to a first engine by a double clutch. The hollow primary shaft comprises splines rotationally driven by a rotor belonging to a second engine. The transmission ratios are established by dog-clutch synchromeshes. The sequences for shifting from one transmission ratio to the next involve two steps. A first step in which the dog-clutch synchromesh of the next transmission ratio is engaged, then a second step in which the engine torque is switched from one primary shaft to the other primary shaft. This allows the shift to be performed under torque. In a transmission such as this, the size of the second engine is combined with the length of the transmission. The reverse-gear, second-gear and fourth-gear ratios are activated by the solid primary shaft. When one of these ratios is engaged, an additional clutch allows the second engine to be connected to the first engine. The additional clutch is of a size which further increases the length of the transmission. 
     BRIEF SUMMARY 
     The invention proposes a drivetrain having a main engine and an auxiliary engine, which overcomes the above disadvantages especially in that the auxiliary engine can be connected either to the main engine or to the wheels of the vehicle, or to both simultaneously, and this can be done for all the transmission ratios and without the need for an additional clutch. 
     According to one embodiment of the invention, the drivetrain for a motor vehicle is equipped with a main engine, with an auxiliary engine and with a parallel shaft transmission. The transmission comprises at least one primary shaft connected to the main engine by a clutch device, at least one secondary shaft equipped with a drive pinion meshing with a differential ring gear, an intermediate shaft that is not coaxial with the primary and secondary shafts, and a plurality of synchromeshes capable selectively of causing a primary shaft to rotate as one with a secondary shaft. The intermediate shaft is equipped with a drive wheel connected to the auxiliary engine and permanently connected in terms of rotation to the primary shaft or shafts. 
     If there is just one primary shaft, then the drive wheel is connected to this primary shaft. If there are several primary shafts connected to the main engine by a clutch device, then the drive wheel is rotationally connected to each of the primary shafts. 
     It will be appreciated that, in a drivetrain such as this, the auxiliary engine can be connected to the main engine, and not to the wheels of the vehicle, when no synchromesh is forming a connection between one of the primary shafts and one of the secondary shafts, and when the clutch device is establishing a connection between the main engine and one of the primary shafts. The auxiliary engine can also drive the wheels of the vehicle in all the transmission ratios, without being connected to the main engine. To do that, no clutch makes a connection between the main engine and one of the primary shafts, and at least one of the synchromeshes makes a connection between one of the primary shafts and one of the secondary shafts. The auxiliary engine may also drive the wheels of the vehicle in all the transmission ratios while at the same time also being connected to the main engine. In this drivetrain there is no need to actuate a clutch additional to the clutch or clutches used for driving the primary shaft or shafts. 
     In an alternative form of embodiment, the drivetrain comprises a computer-controlled transmission equipped with at least one motorized actuator for controlling the synchromeshes and/or the clutch device. 
     Advantageously, the transmission comprises a main primary shaft and an auxiliary primary shaft each selectively connected to the main engine by the clutch device, the drive wheel being fixedly mounted on the intermediate shaft, which intermediate shaft is equipped with a pair of fixed step-down pinions, one of them meshing with a fixed pinion belonging to the main primary shaft, and the other meshing with a fixed pinion belonging to the auxiliary primary shaft. 
     Advantageously, the clutch device has a main engaged position in which the main primary shaft is synchronized with the main engine, a neutral position in which no primary shaft is connected to the main engine, and an auxiliary engaged position in which the auxiliary primary shaft is synchronized with the main engine. 
     Advantageously, the intermediate shaft is equipped with an intermediate reverse-gear pinion in mesh with a reverse-gear pinion mounted on the secondary shaft, a reverse-gear synchromesh being mounted either on the intermediate shaft or on the secondary shaft. The rotational speed of the drive wheel may be greater than the rotational speed of the intermediate reverse-gear pinion. 
     Advantageously, the drivetrain comprises means of attachment of the auxiliary engine. The drive wheel and a rotor belonging to the auxiliary engine are connected by a chain. The rotational speed of a rotor of the auxiliary engine may be greater than the rotational speed of the primary shaft or shafts. 
     Advantageously, the main engine is a combustion engine and the auxiliary engine is a generator capable of converting mechanical energy into some other form of energy, particularly electrical energy. 
     According to one alternative form of embodiment, the transmission comprises a double dog-clutch synchromesh mounted on one of the secondary shafts, contributing to the reverse-gear ratios and to the first-gear ratio, a freewheel being interposed between a sliding gear of said synchromesh and a first-gear idler pinion, the first-gear pinion being the closest pinion on the secondary shaft to the drive pinion. 
     According to another alternative form of embodiment, the transmission comprises a double synchromesh and a single synchromesh of the friction cone type mounted on one of the primary shafts, said double friction-type synchromesh contributing to at least three forward-gear ratios. 
     According to one particular embodiment of the invention, the transmission has five forward-gear ratios. The double friction-type synchromesh and the double dog-clutch synchromesh are actuated by a first motorized actuator with two different selection positions. The single friction-type synchromesh engages a second-gear pinion situated on the same side of said single synchromesh as the clutch device, said single synchromesh and the clutch device being actuated by a second motorized actuator. 
     According to one particular embodiment of the invention, The transmission has six forward-gear ratios. The double friction-type synchromesh and the double dog-clutch synchromesh are actuated by a first motorized actuator with two different selection positions. The single friction-type synchromesh engages a second-gear pinion situated on the opposite side of said single synchromesh with respect to the clutch device, said single synchromesh and the clutch device being actuated by a second motorized actuator with one and the same selection position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will become apparent from reading the detailed description of a number of embodiments taken by way of nonlimiting examples and illustrated by the attached drawings in which: 
         FIG. 1  is a longitudinal section on I-I of  FIG. 2 , of a first embodiment of a six-speed hybrid computer-controlled transmission; 
         FIG. 2  is a partial cross section of the six-speed hybrid computer-controlled transmission showing the system for controlling the forks; and 
         FIG. 3  is a longitudinal section through a second embodiment of a five-speed hybrid computer-controlled transmission. 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIG. 1 , one embodiment of a vehicle drivetrain comprises a transmission equipped with a main casing  1  and with a clutch casing  2 . A clutch device  3  connects a main primary shaft  4  to a crankshaft  5  of a combustion engine, not depicted, via a flywheel  6 , depicted in outline in  FIG. 1 . A secondary shaft  7  is equipped with a drive pinion  8  meshing with a differential ring gear  9  and is thus connected permanently to the wheels of the vehicle. 
     The main primary shaft  4  extends over the entire length of the transmission, from one side of the transmission containing the clutch device  3  to an end region  10  containing a plurality of synchromeshes  11 . The transmission also comprises an intermediate shaft  12 , parallel to and not coaxial with the main primary shaft  4  and the secondary shaft  7 . 
     The main primary shaft  4  comprises, in succession, from left to right in  FIG. 1 , a ball bearing  13 , an idler pinion  14  for the second transmission ratio, which for simplification will be termed the “second-gear pinion”  14 , a single synchromesh body  15  mounted on splines, an idler pinion  16  for the fourth and sixth transmission ratios which for simplicity will be termed the “fourth- and sixth-gear pinion”  16  mounted on a ring  16   a , a double synchromesh body  17  mounted on splines, an idler pinion  18  for the third or fifth transmission ratios, which for simplicity will be termed the “third- and fifth-gear pinion” mounted on a ring, a reverse-gear tooth set  19 , a first-gear tooth set  20 , a first step-down pinion  21  mounted on splines, an auxiliary primary shaft  22  in the form of a sleeve mounted on two needle bearings  22   a , a sleeve  23  mounted on splines, and an external bell housing  24  enclosing the clutch device  3  and mounted such that it is free to rotate. 
     The auxiliary primary shaft  22  receives, in succession, from left to right in  FIG. 1 , a second step-down pinion  25  secured by splines to the sleeve  22 , a ball bearing  26  and splines for driving the clutch device  3 . The auxiliary primary shaft  22  is coaxial with the main primary shaft  4  with respect to which it can rotate on the bearings  22   a.    
     The clutch device  3  comprises a main multiple-disk assembly  27  connecting the external bell housing  24  to the main primary shaft  4  and an auxiliary multiple-disk assembly  28  driving the auxiliary primary shaft  22 . The two multiple-disk assemblies  27  and  28  are coaxial. The auxiliary multiple-disk assembly  28  is axially offset slightly with respect to the main multiple-disk assembly  27 , on the auxiliary primary shaft  22  side. 
     The main multiple-disk assembly  27  comprises a plurality of external disks which are connected in terms of rotation to the external bell housing  24  by pegs collaborating with a slot formed in a skirt  24   a  of the external bell housing  24 . The main multiple-disk assembly  27  also comprises a plurality of internal disks, interposed with the plurality of external disks and which rotate as one with a main piston  29  by virtue of pegs on each of the disks of the plurality of internal disks, collaborating with slots. The two pluralities of disks of the main disk assembly  27  are capable of translational movement between the skirt  24   a  of the external bell housing  24  and a corresponding cylindrical part  29   a  of the main piston  29 . The external bell housing  24  comprises an axial thrust bearing, not depicted, that prevents the plurality of external disks from moving to the left in  FIG. 1 . The main piston  29  comprises an axial thrust bearing  30  situated to the right of the main multiple-disk assembly  27  and capable of pressing the main multiple-disk assembly  27  against the axial thrust bearing of the external bell housing  24 . 
     The main piston  29 , capable of moving axially, rotates as one with a main internal bell housing  31  secured to the sleeve  23  and driving the main primary shaft  4 . A needle thrust bearing  32  is positioned axially between the external bell housing  24  and the main internal bell housing  31 . A main assistance device  33  is positioned axially between the main internal bell housing  31  and the main piston  29 . 
     The auxiliary multiple-disk assembly  28  is made up of a plurality of external disks and of a plurality of internal disks, interposed with one another. The external disks are secured to an auxiliary external bell housing  34  surrounding the assistance device  33  and the main multiple-disk assembly  27 . The auxiliary bell housing  34  is rigidly attached to the main external bell housing  24 . The internal disks rotate as one with an auxiliary piston  35 . The two pluralities of auxiliary disks are capable of translational movement along the axis of the clutch device  3  by virtue of slots formed in skirt parts  34   a  of the auxiliary external bell housing  34  and  35   a  of the auxiliary piston  35 . 
     The auxiliary external bell housing  34  comprises an axial thrust bearing, not depicted, located to the right of the auxiliary multiple-disk assembly  28 . The auxiliary piston  35  comprises an axial thrust bearing  36  allowing the auxiliary multiple-disk assembly  28  to be pressed toward the thrust bearing of the auxiliary bell housing  34 . The auxiliary piston  35  rotates as one with an auxiliary internal bell housing  37  by virtue of auxiliary assistance devices  38 . An axial needle thrust bearing  40  is positioned axially between the main piston  29  and the auxiliary piston  35 . The auxiliary piston  35  has operating fingers  39  extending axially and passing through the auxiliary internal bell housing  37 . The operating fingers  39  can be actuated by an operating fork F 2   b  of the clutch device  3  via a ball bearing  39   a . The auxiliary internal bell housing  37  drives the auxiliary primary shaft  22  via splines. 
     The way in which the clutch device  3  works will now be described. The assembly comprising the clutch operating fork F 2   b , the auxiliary piston  35 , and the main piston  29  forms an assembly capable of axial movement and compressed by the pressing device  33   a.    
     When the clutch operating fork F 2   b  is not actuated, the pressing device  33   a  keeps the main  29  and auxiliary  35  pistons in the left-most position in  FIG. 1 . The main multiple-disk assembly  27  is in the compressed state and the auxiliary multiple-disk assembly  28  is in the uncompressed state. This position is a main position of engagement of the clutch device  3 . In this position, the engine is connected to the main primary shaft  4 . In this position, there is grip between the main external bell housing  24  and the main internal bell housing  31 , thus driving the main primary shaft  4 . 
     When the clutch operating fork F 2   b  is shifted to the right in  FIG. 1 , the two pistons  29  and  35  together compress the pressing device  33   a  and allow the external and internal disks of the main multiple-disk assembly  27  to separate from one another without thereby further compressing the auxiliary multiple-disk assembly  28 . This position of the fork F 2   b  corresponds to a neutral position of the clutch device  3  in which position the engine is connected neither to the main primary shaft  4  nor to the auxiliary primary shaft  22 . 
     When the clutch operating fork F 2   b  is moved further to the right in  FIG. 1 , the main multiple-disk assembly  27  continues to be in the free state and the auxiliary multiple-disk assembly  28  is compressed. This position constitutes an auxiliary position of engagement of the clutch device  3  in which position the engine is connected to the auxiliary primary shaft  22 . 
     The pinions of the other shafts of the transmission will now be described. The secondary shaft  7  comprises, from left to right in  FIG. 1 , a ball bearing  41 , a fixed second-gear pinion  42 , a spacer ring  42   a , a fixed fourth- and sixth-gear pinion  43 , a spacer ring  43   a , a fixed third- and fifth-gear pinion  44  bearing axially against a shoulder  7   a  of the secondary shaft  7 . The secondary shaft  7  comprises, in succession, from left to right from the shoulder  7   a , a reverse-gear idler pinion  45 , a dog-clutch synchromesh body  46  mounted on splines, a first-gear idler pinion  47  bearing axially against the drive pinion B. The right-hand end of the secondary shaft  7  is mounted such that it can rotate on a roller bearing  48 . 
     The transmission also comprises an intermediate shaft  12  mounted such that it can rotate on two bearings  49  and  50  situated at each of its ends. The end located on the same side as the clutch device  3  is mounted such that it can rotate on the ball bearing  50  fixed into the clutch casing  2 . An attached support  51  comprises a part  52  fixed to the main casing  1  and a part  53  projecting radially into the casing  1 . The projecting part  53  accepts the bearing  49 . The part  52  is fixed to the main casing  1  by fastening means comprising screws  54 . 
     Because of the existence of the attached support  51 , it will be appreciated that the synchromesh  17  can easily be housed in the main casing  1  in spite of the fact that the maximum radial dimension of the synchromesh  17  extends beyond the end of the intermediate shaft  12 . The pinions  45  and  47  and the synchromesh  46  corresponding to the reverse-gear and first-gear ratios are transferred onto the secondary shaft  7 , in order to allow the intermediate shaft  12  to be brought closer still to the main primary shaft  4 . 
     Because of the means of fastening the attached support  51  to the main casing  1 , the intermediate shaft  12  is immobilized irrespective of the direction of the radial loadings applied to it. The intermediate shaft  12  comprises, in succession, from left to right in  FIG. 1 , an intermediate reverse-gear pinion  55  mounted freely on the intermediate shaft  12 , a drive wheel  56 , a first step-down pinion  57  and a second step-down pinion  58 . The drive wheel  56  and the two step-down pinions  57  and  58  are as one with the intermediate shaft  12 . The drive wheel  56  collaborates with a chain  59  connected to a rotor of an auxiliary engine  71  of the vehicle, visible in  FIG. 2 . 
     The second step-down pinion  58  has a lower number of teeth than the first step-down pinion  57 . When the clutch device  3  is in the main position of engagement, the main shaft  4  has a rotational speed identical to that of the crankshaft  5 . The idler pinion  18  can drive the vehicle in third gear. When the clutch device  3  is in the auxiliary position of engagement, the auxiliary primary shaft  22  has a rotational speed identical to that of the crankshaft  5 . The main primary shaft  4  is driven at a rotational speed higher than that of the crankshaft  5 , and the idler pinion  18  can drive the vehicle in fifth gear. 
     The single synchromesh  15  and the double synchromesh  17  are of the friction cone type like those described, for example, in French patent application FR-A-2 821 652 to which reference may be made. 
     The first-gear idler pinion  47  is equipped with a freewheel  47   a , as described in patent application EP 1 273 825 (RENAULT). 
     The double dog-clutch synchromesh  46  is operated by a fork F 1   b . The double friction-type synchromesh  17  is operated by a fork Fla. The single friction-type synchromesh  15  is operated by a fork F 2   a  and the clutch device  3  is operated by the fork F 2   b.    
     The second-gear idler pinion  14  meshes with the fixed second-gear pinion  42  and, with the single synchromesh  15 , constitutes a second-gear module. The idler pinion  16  and the fixed pinion  43  together with part of the double synchromesh  17  form a fourth- and sixth-gear module. The idler pinion  18  and the fixed pinion  44 , together with the other part of the double friction-type synchromesh  17 , constitute a third- and fifth-gear module. The tooth set  19  meshes with the intermediate reverse-gear pinion  55 , which also meshes with the reverse-gear idler pinion  45 . The tooth set  19 , the intermediate reverse-gear pinion  55  and the reverse-gear idler pinion  45  together with part of the double synchromesh  46  constitute a reverse-gear module. The tooth set  20  meshes with the first-gear idler pinion  47  and, with the other part of the synchromesh  46 , constitutes a first-gear module. The first step-down pinions  21  and  57  mesh with one another as do the second step-down pinions  25  and  58 . All the transmission modules are located within a mechanical assembly delimited, at one end, by the main casing  1  and, at the other, by the wall  2   a  of the clutch casing  2 . The ball bearings  13  and  41  are fixed into the main casing  1 . The roller bearing  48  and the ball bearings  26  and  50  are fixed into the wall  2   a  of the clutch casing  2 . 
       FIG. 2  shows a control system  70  for operating the drivetrain and the differential ring gear  9 , the axis  4   a  of the main  4  and auxiliary  22  primary shafts and of the clutch device  3 , the axis  12   a  of the intermediate shaft  12 , the axis  7   a  of the secondary shaft  7 . The drivetrain comprises an auxiliary engine  71  connected to the intermediate shaft  12  by the chain  59 . The auxiliary engine  71  is an electrical machine combining the functions of starter motor, alternator, and drive engine. 
     The control system  70  comprises a first motorized actuator  72  capable of pivoting a first selector unit  73  about an axis  73   a  running transversely to the shafts of the transmission. The first selector unit  73  is equipped with a first shift finger  74  and with a second shift finger  75 . The first actuator  72  is equipped with a selector device  76  capable of moving the first selector unit  73  translationally between a first selection position illustrated in  FIG. 2 , in which first position the first shift finger  74  is collaborating with a first fork drive rod  77 , and a second selection position, not depicted in  FIG. 2 , in which second position the second shift finger  75  is collaborating with a second fork drive rod  78 . The first fork drive rod  77  drives the fork F 1   b  to actuate the dog-clutch synchromesh  46 . The second fork drive rod  78  drives the fork F 1   a  to actuate the double synchromesh  17 . 
     The control system  70  also comprises a second motorized actuator  79  capable of causing a second selector unit  80  to pivot about a transverse axis  80   a . The second selector unit  80  is equipped with a shift finger  81  collaborating with a fork drive rod  82  connected, on the one hand, to the drive fork F 2   a  of the single synchromesh  15  and, on the other hand, to the fork F 2   b  that actuates the clutch device  3 . 
     The first actuator  72  drives the forks F 1   a  and F 1   b  alternatively with two different selection positions. The second actuator  79  drives the forks F 2   a  or F 2   b  alternatively. 
     The way in which the transmission works will now be described. The second-gear idler pinion  14  meshes with the fixed second-gear pinion  42 . The fourth- and sixth-gear idler pinion  16  meshes with the corresponding fixed pinion  43 . The third- and fifth-gear idler pinion  18  meshes with the corresponding fixed pinion  44 . The reverse-gear tooth set  19  meshes with the intermediate reverse-gear pinion  55 , which also meshes with the reverse-gear idler pinion  45 . The first-gear tooth set  20  meshes with the corresponding idler pinion  47 . 
     Engaging a first-gear or reverse-gear ratio entails first of all actuating the clutch fork F 2   b  so as to bring the clutch device  3  into a neutral configuration. The operation then entails actuating the fork F 1   b  toward the corresponding idler pinion  16  or  18 , then returning the fork F 2   b  into the main position of engagement in which the main multiple-disk assembly  27  has its disks engaged with one another. 
     The shift between first and second gear is done by directly engaging the single synchromesh  15  by moving the fork F 2   a  toward the second-gear pinion  14 , that is to say to the left in  FIG. 1 . This engagement occurs while the fork F 1   b  remains in the first-gear-engaged position. The rotational speed of the secondary shaft  7  is dictated by the second-gear synchromesh  15 . The freewheel  47   a  allows the first-gear idler pinion  47  to have a rotational speed lower than that of the secondary shaft  7 . The shift between first and second gear is effected under torque. 
     When second gear is engaged and the vehicle speed increases, the gearshift to be prepared for is no longer a shift from second gear to first gear but becomes a shift from second gear to a third- or fourth-gear ratio. A computer, not depicted, instructs the first actuator  72  to return the fork F 1   b  to the neutral position and then to change selection position so as to be ready to operate the fork F 1   a.    
     The shift between second and third or fourth gear is done by directly engaging the fork F 1   a  toward the corresponding pinion  16  or  18  at the same time as returning the fork F 2   a  to a neutral position. Making the movement whereby the fork F 1   a  is engaged coincide with the movement whereby the fork F 2   a  disengages second gear allows a shift to be performed under torque without the need to use the clutch device  3 . 
     The shift between third gear and fourth gear is performed with a brief interruption in the transmission of torque. The first actuator  72  moves the fork F 1   b  to the left in  FIG. 1  and the double synchromesh  17  switches from a configuration in which third gear is engaged into a neutral configuration followed immediately by a configuration in which fourth gear is engaged. 
     The shift from third gear to fifth gear is performed by leaving the fork F 1   a  engaged with the pinion  18  and moving the fork F 2   b  to the right in  FIG. 1 . The clutch device  3  switches from a main position of engagement to an auxiliary position of engagement via a transient neutral position. The shift from third to fifth is also a shift with a brief interruption in the transmission of torque. 
     The same is true of the shift between fourth and sixth gears. The step-down pinions  57  and  58  allow a doubling-up of the ratios established by the double synchromesh  17 , acting only on the clutch device  3 . 
     The shift between fourth and fifth gear is performed by making the movement of the fork F 1   a  by the first actuator  72  from the pinion  16  to the pinion  18  via a neutral position coincide with the movement of the fork F 2   b  by the second actuator  79 . Having the two gearshift phases of the clutch device  3  and of the double synchromesh  17  coincide with one another means that a shift from fourth gear to fifth gear can be obtained also with a brief interruption in the transmission of torque. 
     The shift between fifth and sixth gears is performed by moving only the fork F 1   a  and takes place with a brief interruption in the transmission of torque. 
     In the transmission, the shifts between any two forward-gear ratios are performed either under torque, in the case of gear ratios lower than or equal to third gear, or with a brief interruption in the transmission of torque. The difference between two transmission ratios from third gear upward is smaller than the difference between two transmission ratios from third gear downward. The gearshift times between two brief transmission ratios are practically imperceptible to the driver whose vehicle behaves almost exactly as if it were equipped with a transmission that shifts gear under torque for all these gear ratios. The time during which the transmission of torque is interrupted when shifting gear is a few hundred milliseconds, or even less than 100 milliseconds. 
     The behavior of the transmission connected, on the one hand, to a combustion engine via the crankshaft  5  and, on the other hand, to the auxiliary engine  71  via the chain  59  will now be described. When the combustion engine and the vehicle are stationary, the start-up phase is effected by positioning the fork F 2   b  into the auxiliary position of engagement, the other forks of the transmission being in their neutral positions. In this configuration, the secondary shaft  7  is not driven by any pinion and the engine torque is passed, with the auxiliary engine  71  acting as a starter motor, to the second step-down pinion  58 , to the auxiliary primary shaft  22 , and then to the combustion engine, allowing the latter to start. 
     Conversely, when the vehicle is stationary and combustion engine is running, the engine torque is transmitted either via the auxiliary primary shaft  22  or via the main primary shaft  4  to the auxiliary engine  71  acting as an alternator which can then recharge the vehicle batteries. When first or second gear is engaged, the combustion engine and the auxiliary engine  71  contribute to providing mechanical power to the main primary shaft  4 . 
     When the vehicle is operating in an urban environment, the combustion engine can be switched off and the auxiliary engine  71  alone drives the main primary shaft  4  via the first step-down pinions  21  and  57 . Shifting between reverse gear and the first four gear ratios takes place as described hereinabove. 
     When the vehicle is running on the open road, the auxiliary engine  71  and the combustion engine together contribute to driving the vehicle. When the combustion engine is being used at an engine speed at which its energy efficiency is mediocre, the auxiliary engine  71  provides mechanical power. Conversely, when the vehicle is running down hill, a computer can configure the auxiliary engine  71  so that most of the engine braking is performed by the auxiliary engine  71  and the mechanical energy of the vehicle is converted into electrical energy. 
       FIG. 3  illustrates another embodiment of the invention in the form of a hybrid transmission with five forward-gear ratios. This embodiment again includes all the structural features already described in conjunction with  FIGS. 1 and 2 , except for the features associated with the second-gear, fourth-gear and sixth-gear modules and the way in which these modules are actuated. Parts which are identical or similar bear the same references as in  FIGS. 1 and 2 . 
     Only those parts which differ and which correspond to the left-hand part of  FIG. 3  will now be described. The main primary shaft  4  comprises, from left to right, the bearing  13 , a single friction-cone synchromesh  100  and a second- and fourth-gear idler pinion  101  collaborating, to its left, with a single synchromesh sliding gear  100  and, to its right, with a double synchromesh sliding gear  17 . The single synchromesh  100  is actuated by a fork F′ 2   a  driven by the second actuator  79  ( FIG. 2 ). The secondary shaft  7  is equipped, from left to right in the figure, with the ball bearing  41 , then with a spacer piece, and a pinion  102 , mounted on splines of the secondary shaft  7 . The remainder of the transmission, illustrated in  FIG. 3 , is identical to the previous embodiment described with reference to  FIGS. 1 and 2 . 
     The pinion  101  meshes with the idler pinion  101 . The second actuator  79  is equipped with a selector unit that has two opposing shift fingers that contribute toward pushing two fork drive rods that move along one and the same axis parallel to the shafts of the transmission. When the selector unit of the second actuator  79  rotates in one direction, one of the shift fingers drives a fork drive rod which pushes the fork F 2   b  to the right in  FIG. 3 . When that same selector unit rotates in the other direction, the other shift finger pulls another drive rod which pulls the fork F′ 2   a  also to the right in  FIG. 3 . 
     Those aspects of the operation of this transmission which differ from the operation of the transmission already described will now be described. The shift between first gear and second gear occurs under torque by moving the fork F′ 2   a  to the right in  FIG. 3  while the fork F 1   b  remains engaged. When the engine speed is such that the transmission needs to be placed in a configuration that prepares it for a shift from second to third gear, the fork F 1   b  is returned to the neutral position and the first actuator  72  changes selection position so as to be ready to move the fork F 1   a  to the right in  FIG. 3 . 
     During the shift between second gear and third or fourth gear, the second actuator  79  returns the fork F′ 2   a  to the neutral position at the same time as the fork F 1   a  is engaged either toward the third-gear idler pinion  18  or to the left in  FIG. 3  to synchronize the idler pinion  101 . Having these two changes occurring simultaneously means a shift from second gear to third or fourth gear can be performed under torque. The shift from third gear to fourth gear occurs with a brief interruption in the transmission of torque, and the same is true of the shift between third gear and fifth gear. 
     As in the previous embodiment, the shift between fourth gear and fifth gear involves synchronizing the movement of the fork F 1   a  and of the fork F 2   b  so that the moment that torque is interrupted as a result of the double friction-type synchromesh coincides with the interruption due to the clutch device  3 . In this embodiment, the same idler pinion  101  can be synchronized with the main primary shaft  4  either by the action of the first actuator  72  or by the action of the second actuator  79 .