Abstract:
Automated multi-stage gearbox, includes a basic gearbox with a number of gears and a two-stage split gearing coupled in front of and in series with the basic gearbox. Shifting is effected by compressed-air cylinders which are controlled by a control computer ( 45 ) which is programmed to put the split gearing in a neutral position in the event of an input signal representing zero throttle.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a multi-stage gearbox for motor vehicles, comprising an input shaft mounted rotatably in a casing, at least one intermediate shaft which is mounted in the casing and has at least one gear wheel in engagement with a gear wheel on the input shaft, a main shaft, mounted rotatably in the casing, with gear wheels which engage with gear wheels on the intermediate shaft, at least one gear wheel in each pair of mutually engaging gear wheels on the intermediate shaft and the main shaft being mounted rotatably on its shaft and lockable on its shaft by engaging means, and also operating means which interact with the engaging means and are controlled by a control unit depending on signals fed into the control unit representing various engine and vehicle data which comprise at least engine speed, vehicle speed and throttle pedal position. 
   DESCRIPTION OF THE RELATED ART 
   Automatic gearboxes of this type, what are known as automated multi-stage gearboxes, have become increasingly common in heavy-duty vehicles as microcomputer technology has developed further and made it possible to use a control computer and a number of actuators, for example servo motors, to precision-regulate engine speed, engagement and disengagement of a disk clutch between the engine and the gearbox and also the engaging means of the gearbox in relation to one another, so that soft shifting is always obtained at the correct engine speed. The advantage of this type of automatic gearbox compared with a conventional automatic gearbox constructed with planetary gear stages and with a hydrodynamic torque converter on the input side is, especially as far as use in heavy-duty vehicles is concerned, that on the one hand it is simpler and more robust and can be produced at a considerably lower cost than the conventional automatic gearbox, and on the other hand it has greater efficiency, which means that lower fuel consumption is possible. 
   The automatic gearbox constructed from planetary gears usually has one-way engaging means between the planetary gear stages, which, when the engine is driving in the automatic transmission position, lock for torque transmission from the engine to the driving wheels but, when torque transmission takes place in the opposite direction, that is to say with zero throttle and the vehicle in motion, disengage and allow the vehicle to roll freely without engine braking, which results in lower fuel consumption by utilizing the motive energy of the vehicle than if the engine remains engaged and brakes. It has previously been possible to achieve the corresponding freewheel function in previously known automated multi-stage gearboxes only by manual disengagement of the disk clutch between the engine and the gearbox. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to produce an automated multi-stage gearbox of the type indicated in the introduction, which is made in such a manner that an automatic freewheel function corresponding to that in the conventional automatic gearbox with planetary gear stages and overrunning clutches can be obtained. 
   According to the invention, this is achieved by virtue of the fact that the control unit is arranged so as, in. the event of input signals representing zero throttle, to give an output signal to put a currently engaged synchronized gear in the neutral position, and so as, when the throttle is subsequently opened (i.e., a positive throttle value), to give the operating means a signal to engage the synchronized gear when the engine has reached a speed suitable for the synchronizing procedure. 
   By virtue of putting a synchronized gear in its neutral position, the drive line is uncoupled, so that the vehicle can roll freely without the braking effect which is otherwise obtained through friction losses in the engine. In this way, a freewheel function is therefore obtained without the use of special overrunning clutches. 
   In a preferred embodiment of the gearbox according to the invention, use is made, for the freewheel function, of a synchronized split gearing which is coupled in front of and in series with a basic gearbox and has at least two rotatably mounted gear wheels which are simultaneously disengageable and alternatively lockable relative to the input shaft by engaging and synchronizing means controlled by said operating means and each engage with a gear wheel on the intermediate shaft. In this connection, the control unit is arranged so as, in the event of input signals representing zero throttle and engine brale switched off, to disengage the gear wheel currently locked relative to the input shaft by putting said engaging and synchronizing means in the neutral position. By utilizing a synchronized split gearing for the freewheel function, use can be made of an unsynchronized basic gearbox, which further simplifies, and reduces the cost of, the gearbox as a whole. 
   In a development of the gearbox according to the invention, in order to prevent a possible engine stoppage at the same time as the freewheel function is activated resulting in various servos, for example the steering servo and the brake servo, ceasing to function by virtue of the fact that the servo pumps stop simultaneously with the engine, the input shaft is connected to an automated disk clutch controlled by the control unit, the control unit being arranged so as—on receiving input signals representing zero engine speed and a given vehicle speed at the same time as said synchronized gear is disengaged—to give a signal first to disengage the disk clutch, then to engage the synchronized gear and lastly to engage the disk clutch. When the disk clutch couples the gearbox to the engine, the engine and servo pumps coupled thereto are driven by the vehicle, so that the servo functions are maintained. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in greater detail with reference to illustrative embodiments shown in the accompanying drawings, in which 
       FIG. 1  shows a diagrammatic representation of an internal combustion engine with an adjoining clutch and gearbox, and 
       FIG. 2  shows the clutch and the gearbox in  FIG. 1  on enlarged scale. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , reference number  1  designates a six-cylinder internal combustion engine, for example a diesel engine, the crankshaft  2  of which is coupled to a single-plate dry disk clutch which is designated generally by reference number  3  and is enclosed in a clutch case  4 . The crankshaft  2  is connected non-rotatably to the clutch housing  5  of the clutch  3 , while its disk plate  6  is connected non-rotatably to an input shaft  7  which is mounted rotatably in the casing  8  of a gearbox designated generally by reference number  9 . A main shaft  10  and an intermediate shaft  11  are also mounted rotatably in the casing  8 . 
   As can be seen most clearly from  FIG. 2 , a gear wheel  12  is mounted rotatably on the input shaft  7  and is lockable on the shaft by means of an engaging sleeve  13  which is provided with synchronizing means and is mounted non-rotatably but axially displaceably on a hub  14  connected non-rotatably to the input shaft  7 . By means of the engaging sleeve  13 , a gear wheel  15  mounted rotatably on the main shaft  10  is also lockable relative to the input shaft  7 . With the engaging sleeve  13  in a central position, both gear wheels  12  and  15  are disengaged from their shafts  7  and, respectively,  10 . The gear wheels  12  and  15  engage with gear wheels  16  and, respectively,  17  which are connected non-rotatably to the intermediate shaft  11 . Arranged in a rotationally fixed manner on the intermediate shaft  11  are further gear wheels  18 ,  19  and  20  which engage with gear wheels  21 ,  22  and, respectively,  23  which are mounted rotatably on the main shaft  10  and are lockable on the main shaft by means of engaging sleeves  24  and, respectively,  25  which, in the illustrative embodiment shown, do not have synchronizing arrangements. A further gear wheel  28  is mounted rotatably on the main shaft  10  and engages with an intermediate gear wheel  30  which is mounted rotatably on a separate shaft  29  and engages in turn with the intermediate shaft gear wheel  20 . The gear wheel  28  is lockable on its shaft by means of an engaging sleeve  26 . 
   The gear wheel pairs  12 ,  16  and  15 ,  17  and also the engaging sleeve  13  form a split gearing with a low gear stage LS and a high gear stage HS. The gear wheel pair  15 ,  17  also forms, together with the gear wheel pairs  21 ,  18 ,  22 ,  19 ,  23 ,  20  and  28 ,  30 , a basic gearbox with four forward gears and one reverse gear. Arranged in a rotationally fixed manner on the output end of the main shaft is a gear wheel  31  which forms the sun gear in a two-stage range gear of the planetary type designated by reference number  32 , the planet wheel carrier  33  of which is connected in a rotationally fixed manner to a shaft  34  which forms the output shaft of the gearbox. The planet wheels  35  of the range gear  32  engage with a ring gear  36  which, by means of an engaging sleeve  37 , is lockable relative to the gearbox casing  8  for low range LR and relative to the planet wheel carrier  33  for high range HR. The engaging sleeves  13 ,  24 ,  25 ,  26  and  37  are displaceable as shown by the arrows in  FIG. 2 , the gear stages shown next to the arrows being obtained. The displacement is brought about by servo devices  40 ,  41 ,  42 ,  43  and  44  which are indicated diagrammatically in FIG.  2  and may be pneumatically operated piston/cylinder arrangements of the type used in a gearbox of the type described above, which is marketed under the name Geartronic®. 
   The servo devices  40 ,  41 ,  42 ,  43  and  44  are controlled by an electronic control unit  45  (FIG.  1 ), comprising a microcomputer, depending on signals fed into the control unit representing the various engine and vehicle data which comprise at least engine speed, vehicle speed, throttle pedal position and, in this case, engine brake on/off, when an electronic gear selector  46  coupled to the control unit  45  is in its automatic transmission position. When the selector is in the position for manual shifting, shifting is effected via the gear selector  46  at the command of the driver. The control unit  45  also controls fuel injection, that is to say the engine speed, depending on the throttle pedal position, and also the air supply to a pneumatic piston/cylinder arrangement  47 , by means of which the clutch  3  is disengaged. 
   According to the invention, the control unit  45  is programmed so that the freewheel function is activated when the driver or the cruise control no longer requests either any fuel (i.e., a zero throttle value) or activation of any engine brake, for example an exhaust gas pressure regulator or a compression brake. This is effected by the control unit  45  first controlling the engine speed, so that no torque is transmitted between the input shaft  7  and the main shaft  10  of the gearbox. The control unit  45  then gives a signal to the servo device  40  to put the engaging sleeve  13  in neutral position, after which the engine is controlled to idling speed. The drive line is then uncoupled, and the vehicle can roll freely. By disengaging a synchronized split gearing, the freewheel function is obtained by disengaging one and the same gear irrespective of which gear is engaged in the basic gearbox. 
   The control unit  45  is programmed to deactivate the freewheel function when the driver requests fuel with the throttle pedal or the cruise control or requests engine braking by, for example, activating an exhaust gas brake or compression brake. In this connection, the control unit first regulates the engine speed in toward a speed which makes synchronization possible, and then the engaging sleeve  13  is displaced into its previous engagement position. The drive line is then coupled together, and driving or engine braking is possible again. 
   Also programmed into the control unit  45  is a safety function in the event that the engine should stop when the freewheel function is activated, which involves the control unit  45  then giving a signal first to disengage the clutch  3  between the engine and the gearbox, and then to displace the engaging sleeve  13  located in the neutral position into its previous engagement position. After engagement of the synchronized gear, the control unit  45  gives a signal to reengage the clutch  3 . When the clutch  3  is engaged, the engine is driven by the vehicle and any auxiliary sets, such as servo pumps and engine brakes, normally driven by the engine are then driven by the vehicle via the engine. 
   The invention has been described above with reference to a preferred embodiment, in which the freewheel function is achieved by disengaging one and the same gear, namely a synchronized split gearing, irrespective of which gear is engaged when free-rolling of the vehicle is desired. Within the scope of the invention, it is of course possible to conceive of disengaging the currently engaged synchronized gear in a synchronized gearbox without split gearing.