Abstract:
A powershift transmission in a motor vehicle is arranged between a prime mover and driven wheels of the motor vehicle for transmission of propulsive power and selection of different gear speed ratios. The powershift transmission includes at least two factional clutches for alternatively engaging at least two input shafts, a main transmission, a range section and an output shaft. A countershaft in the powershift transmission is rotationally connectable to the output shaft in order to establish a by-pass torque path, passing by the range section, and in which propulsive power can be transferred when the range section is gear shifted between different range speed ratios.

Description:
BACKGROUND AND SUMMARY 
       [0001]    The present invention relates to vehicle transmissions, especially for heavy, on- as well as off-road vehicles, and more particularly to dual clutch transmissions with a range section that can be by-passed to allow it to be shifted without interrupting the power transfer. 
         [0002]    The conventional stepped transmission for rear-wheel drive vehicles can be regarded as a robust, compact and cost-effective product. A typical example of such a lay-out is shown in  FIG. 2  in DE10242823 A1. An input shaft is coaxial with a main (output) shaft and parallel with a countershaft. A gearwheel that is rotationally fixed on the input shaft meshes with a gearwheel that is rotationally fixed on the countershaft. Several pairs of meshing gearwheels are then located side by side. In each of these pairs, one gearwheel is coaxial with the countershaft; the other is coaxial with the main shaft. One of these gearwheels is rotationally fixed on its coaxial shaft. The other gearwheel, the loose gearwheel, is free to rotate relative to its coaxial shaft, but can be rotationally locked to the shaft by a mechanical tooth clutch. This construction is used both for manually shifted transmissions as well as for automated mechanically engaged transmissions, AMTs. Because of the mechanical tooth clutches, there will during the shift be an interruption of the power transfer between the engine and the driven wheels of the vehicle. Thus, this type of transmission is not powershifting. 
         [0003]    For heavy road vehicles, e.g., heavy trucks and buses, a large number of gears is required. Often, the conventional stepped transmission construction is extended with a splitter and a range section. The basic principle of this is shown in  FIG. 1  in U.S. Pat. No. 5,351,569. With the splitter, there are two alternate ways of transferring the power from the input shaft to the countershaft. This will double the number of gears. The range section can functionally be regarded as an additional gearbox that is connected in series with the ordinary, main transmission. There are two gears therein; a low range gear with a reduction ratio in the same order of magnitude as the largest reduction ratio of the main transmission, and a high range gear where there is no change in speed from the main transmission to the output shaft of the range section. Thus, a range section also has the ability to double the number of gears of the transmission. 
         [0004]    Hence, with a splitter and range construction of a transmission, a large number of gears, e.g.,  12  or  16 , can be embodied using a limited number of gearwheels. Furthermore, with a range section, large reduction ratios can easily be embodied. Such ratios are required for start-off from rest and low-speed manoeuvres of a heavy vehicle. Still, there will be power transfer interruptions at gear shifts. Most heavy vehicles have turbo-charged diesel engines that have slow dynamic response. Then, the power interruptions at gear shifts will have an unfavourable impact on the performance of the vehicle. This is especially the case at off-road driving. 
         [0005]    Powershifting transmissions are available for heavy road vehicles. Such transmissions are, in general, based on planetary gear trains and wet multi-plate clutches, e.g., as in EP0073102A2. That makes them expensive, and they have considerable power losses. Thereby, they are not suited for vehicles of the future, where fuel consumption will be more important than ever. 
         [0006]    Hence, a cost-efficient transmission with low power losses and no power interruptions at gear shifts would be desirable for heavy on- and off-road vehicles. 
         [0007]    Moreover, it would be of further advantage if it could be similar in size to conventional transmissions for facilitating the installation in the vehicle chassis. 
         [0008]    Dual clutch transmissions are an interesting cross-breed between powershifting planetary transmissions and conventional stepped transmissions with power interruption at gear shifts. In principle, a dual clutch transmission has two input shafts, each connectable with a friction clutch to the output of the engine. Functionally, this is equivalent to having two conventional transmissions in parallel, i.e., two parallel sub-transmissions, and using one at a time for power transfer. The sub-transmission that is not used, idling, for the time being, can have a gear engaged and prepared for a subsequent shift. This shift is carried out by simultaneously disengaging the friction clutch of the previously used sub-transmission and engaging the friction clutch of the previously idling sub-transmission. 
         [0009]    When properly designed, dual clutch transmissions have a potential of providing powershifts at a reasonable production cost and low power losses. This is due to the fact that the rotating parts, i.e., gearwheels, shafts and tooth clutches, are similar to those in conventional stepped transmissions. This, furthermore, enables the use of the same production equipment. So, it makes sense to produce dual clutch transmissions in the same facilities as used for conventional stepped transmissions. 
         [0010]    Dual clutch transmissions for rear wheel drive vehicles often have two separate countershafts, one connected to each input shaft. One example is found in U.S. Pat. No. 5,150,628. These countershafts make the transmission considerably wider than a conventional stepped transmission. That may lead to difficulties in installing the transmission into the vehicle. However, in some dual clutch transmission designs there is only one countershaft, e.g., as in DE923402 and DE3131156A1. On this countershaft there are loose gearwheels arranged that can be rotationally connected to each other and to the countershaft by means of mechanical tooth clutches. In a way, this could be regarded as if the second countershaft is arranged coaxial to the first one. The result will be a power-shiftable dual clutch transmission that is not wider than a corresponding conventional stepped transmission. The number of gears and the corresponding speed reduction ratios are insufficient for heavy severe-duty vehicles, though. 
         [0011]    Some dual-clutch transmission concepts in a so-called winding structure have been presented, for instance U.S. Pat. No. 5,347,879, U.S. Pat. No. 5,592,854, DE10325647A1 and DE10339758 A1. In these, the power is led via four gear meshes in at least one gear, and several gearwheels are used for more than one gear. That will give further reduction of speed. However, this corresponds to just one or two additional gears. These concepts are hence less suited to heavy vehicles. 
         [0012]    For rear wheel drive, DE102005030987A1, DE102005033027A1, DE 102006015661 A1 and EP1624232A1 show dual clutch transmissions that are connected in series with a range section. That makes them suited for heavy vehicle use. Unfortunately, there are shifts between consecutive gears where the power transfer will be interrupted in these designs. 
         [0013]    Two further examples of a dual clutch main transmission being combined with a range section are shown in U.S. Pat. No. 4,966,048 and U.S. Pat. No. 7,204,166B2. These designs comprise two countershafts and will, due to a rear-mounted combined splitter- and range section, have several power interruptions at shifts between consecutive gears. Therefore, they are not favourable from an installation point of view, and not with respect to vehicle performance. 
         [0014]    Two similar designs of dual clutch transmission in combination with a range section are shown in DE102005050067A1 and WO2007/039021A1. Therein, the input and output of the transmission can be connected by a friction clutch. This friction clutch can transfer power when a gear shift takes place. Thereby, power interruption can be avoided at all gear shifts. However, for reasonable sizes of this friction clutch, the power transferred to the driven wheels is very small at shifts between low gears. At the same time, the power dissipated in this friction clutch is large at these gear shifts. Thus, these types of dual clutch transmission would have a limited practical use, especially for on- and off-road vehicles. 
         [0015]    The dual-clutch transmissions in DE923402 and DE3131156A1 could be combined with a range section. That would give a compact transmission with several gears and high reduction ratios. Gear-shifts between consecutive gears would be without power interruption except when the range section is shifted. This would probably be acceptable on most heavy on-road vehicles, but not for, e.g., trucks in hilly applications or articulated haulers. 
         [0016]    U.S. Pat. No. 7,353,724B2 shows in  FIGS. 1 and 3  dual-clutch transmissions where a direct connection between one of the input shafts and the output shaft can transmit power when changing between low, underdrive, gears and high, overdrive, gears. This is not a true range section, though. The number of gears is doubled, but in the low, underdrive, gears the power is transmitted via two gear meshes, only, as in  FIG. 2  in U.S. Pat. No. 6,958,028B2. That limits the practically possible speed reduction. Thereby, these transmissions are not well suited to heavy vehicles. 
         [0017]    A somewhat similar principle is disclosed in U.S. Pat. No. 4,777,837. There, separate gearwheel pairs are provided for intermediate gears between the low and high range gears. This will give a large number of gears and no power interruptions at gear-shifts between consecutive gears. In low range gears, the power is transmitted via three gear meshes, which will enable large reduction ratios. However, the transmission is bulky due to two parallel countershafts. Moreover, the output shaft is not coaxial with the input shaft. That makes the transmission incompatible with most heavy truck designs. The number of components is large, adding costs. 
         [0018]    Further on, U.S. Pat. No. 7,070,534B2 presents a dual clutch transmission  10  with a planetary range section  56  and coaxial input  86  and output  68 . A dual clutch unit  20 ,  22  selectively transfers power to input shafts  90  and  92 . To each of these input shafts  90  and  92  a countershaft,  74  and  76 , respectively, is arranged. From each of these countershafts  74 ,  76  the power can be selectively directed with tooth clutches  80  and  84  to the output  68  in either of two ways. Firstly, the power can be led to the sun gear  58  of the planetary range section  56  via gearwheels  44 ,  46  and  54 ,  46 , respectively. That will give a speed reduction in the planetary range section  56 , thus corresponding to low range gears. Secondly, the power can be led more directly to the output  68  via gearwheels  40 ,  42  and  50 ,  42 , respectively. The planetary range section will then be idling, and high range gears are established. Shifts without power interruption can be carried out between low and high range gears. Unfortunately, the number of rotating components, e.g., gearwheels and tooth clutches, is relatively large in comparison with the number of gears. The large number of gearwheels makes the transmission long, and the two parallel countershafts make it wide and difficult to fit in the vehicle. Furthermore, the idling planetary range section will imply unnecessarily large power losses in high range gears. 
         [0019]    U.S. Pat. No. 6,958,028B2,  FIG. 5 , shows a dual clutch transmission with a planetary range section. This transmission is similar to the one in U.S. Pat. No. 7,070,534B2. The main difference is that both input shafts,  30  and  40 , use the same countershaft  50 , tooth clutch  130 , and gearwheels  122 ,  132  and  132 ,  128  between this countershaft and the planetary range section. Power interruption between low and high range gears is eliminated by a bridge torque path via a separate countershaft  152 . That makes the transmission wide, and it shares the rest of the disadvantages of the one in U.S. Pat. No. 7,070,534B2; many components, long, and high power losses for high range gears. 
         [0020]      FIG. 1  in US2008/0188342A1 shows a dual clutch transmission with one countershaft and a planetary range section. A bridge torque path is formed by a tooth clutch  84  that rotationally locks a loose gearwheel  64  on main shaft  28  to a planet carrier  68  rotationally fixed to output shaft  70 . When power is led in this path, the gearwheels in the planetary range section are idling, and the range section can be shifted between high and low positions. This gives a narrow transmission with high reduction ratios where power interruptions can be avoided at every shift between consecutive gears. Unfortunately, this bridge path embodiment has many drawbacks. Firstly, the tooth clutch  84  is of complex design, making it costly and long. Secondly, the bearing  32  that carries main shaft  28  must be located in front of loose gearwheel  64 . This puts a large part of main shaft  28  behind bearing  32 , which, in turn, increases the misalignments in the range section and tooth clutch  84 . Moreover, the assembly of the transmission is not facilitated by a main shaft having loose gearwheels and tooth clutches on both sides of the housing wall that carries bearing  32 . Thirdly, the addition of parts for the tooth clutch  84  will make the already complex shape of planet carrier  68  even more complex and difficult to produce. DE102007047671A1 shows a similar design that has similar disadvantages. 
         [0021]    It is desirable to present an alternative powershift transmission arrangement where the drawbacks of above mentioned prior art are eliminated. 
         [0022]    It is desirable to present an improved powershift transmission. 
         [0023]    The device according to an aspect of the invention is a powershift transmission in a motor vehicle, where said powershift transmission is arranged between a prime mover and driven wheels of said motor vehicle for transmission of propulsive power and selection of different gear speed ratios, where said powershift transmission comprising at least two frictional clutches for alternatively engaging at least two input shafts, a main transmission, a range section and an output shaft, said main transmission comprising said input shafts, a main shaft and a countershaft, that is parallel to said main shaft, and where said countershaft carries a number of gearwheels that are in mesh with gearwheels that are carried by said main shaft or said at least two input shafts, and where said main shaft is arranged as an output member of said main transmission and integral with or rotationally fixed to an input member of said range section, 
         [0024]    and where an output member arranged in said range section is integral with or rotationally fixed to said output shaft, and where said range section is arranged with at least two alternating torque paths with different range speed ratios that can be established by selective engagement and disengagement of at least one range clutch, characterized in that said countershaft is rotationally connectable to said output shaft in order to establish a by-pass torque path, passing by said range section without putting any load on any of the parts in said range section, and in which propulsive power can be transferred when said range section is gear shifted between said different range speed ratios. 
         [0025]    The benefit with the device is that for heavy, on- and off-road vehicles the invention provides a transmission that i) enables high power transfer to the driven wheels during all shifts between consecutive gears, ii) can provide high reduction ratios, iii) is cost-effective and simple to produce, iv) has low power losses and v) can be easily installed in a vehicle as an alternative to conventional stepped transmissions. 
         [0026]    In another embodiment according to the invention in said by-passing torque path the propulsive power is led between a by-passing gearwheel, which is coaxial with and rotationally connectable to said countershaft, and a by-passing output gearwheel, which is coaxial with and rotationally connectable to said output shaft. 
         [0027]    In a further embodiment according to the invention said by-passing gearwheel and said by-passing output gearwheel are in mesh. 
         [0028]    In another embodiment according to the invention said by -passing gearwheel is arranged on a by-passing shaft that is integral with or rotationally fixed to said countershaft. 
         [0029]    In another embodiment according to the invention said by-passing gearwheel is a loose gearwheel that can be selectively rotationally locked to said by-passing shaft by a by-passing clutch. 
         [0030]    In a further embodiment according to the invention said by-passing output gearwheel is a loose gearwheel that can be selectively rotationally locked to said output shaft by a by-passing clutch. 
         [0031]    In another embodiment according to the invention said by-passing shaft is arranged to drive a power take-off unit. 
         [0032]    In a further embodiment according to the invention said by-passing output gearwheel is located at the same axial position behind said range section as an optional retarder drive gearwheel that can drive an optional retarder auxiliary brake. 
         [0033]    In another embodiment according to the invention said by-passing output gearwheel is arranged to drive a retarder. 
         [0034]    In a further embodiment according to the invention said by-passing clutch in engaged state rotationally locks said countershaft with said by-passing gearwheel or a by-passing shaft that is integral with or rotationally fixed to said by-passing gearwheel. 
         [0035]    In another embodiment according to the invention said range section is of planetary gear design. 
         [0036]    In a further embodiment according to the invention said planetary range section comprising a planet carrier that is integral with or rotationally fixed to said output shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The present invention will be described in greater detail below with reference to the accompanying drawings which, for the purpose of exemplification, shows further preferred embodiments of the invention and also the technical background, and in which: 
           [0038]      FIG. 1  schematically shows a stepped automatic mechanically engaged transmission (AMT) according to known art. 
           [0039]      FIG. 2  schematically shows a stepped automatic mechanically engaged dual clutch transmission (DCT) according to known art. 
           [0040]      FIGS. 3 and 4  schematically shows a stepped automatic mechanically engaged dual clutch transmission (DCT) according to two embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]      FIG. 1  shows schematically a longitudinal section of a stepped automatic mechanically engaged transmission (AMT)  100  according to known art for a heavy road vehicle. The transmission  100  comprises three housing parts; a clutch housing  101 , a main housing  102  and a range housing  106 . In the clutch housing  101  a frictional clutch  110  is arranged, comprising a flywheel  111 , a clutch assembly  112  bolted thereon, a clutch disc  115  with torsional damper  116  and a clutch actuator  117 . The flywheel III is attached to the engine crankshaft (not shown). 
         [0042]    A main transmission  120  is arranged inside the main housing  102 . There are three shafts in the main transmission  120 ; an input shaft  121 , a countershaft  123  and a main shaft  124 . The input shaft  121  and the main shaft  124  are coaxial, and the countershaft  123  is arranged parallel to them. The input shaft  121  is suspended in the clutch housing  101  by an input shaft bearing  125  and in the flywheel  111  by a flywheel pilot bearing  119 . The countershaft  123  is suspended in the housings  101  and  102  by a front countershaft bearing  126  and a rear countershaft bearing  127 , respectively. The main shaft  124  is suspended in the main housing  102  by a main shaft bearing  128  and in the input shaft  121  by a main pilot bearing  129 . 
         [0043]    On the input shaft  121  and the main shaft  124  a number of rotatable, loose gearwheels are arranged. Each of these loose gearwheels is in mesh with a gearwheel that is either fixed on or integral with the countershaft  123 . On the input shaft  121  there is a second primary loose gearwheel  130  that meshes with a second primary gearwheel  131  on the countershaft  123 . To the right of the input shaft  121  in  FIG. 1 , a first primary loose gearwheel  132  is arranged on the main shaft  124 . The first primary loose gearwheel  132  meshes with a first primary gearwheel  133  on the countershaft  123 . Further to the right in  FIG. 1  a second secondary loose gearwheel  134  on the main shaft  124  is in mesh with a second secondary gearwheel  135  that is fixedly attached to the countershaft  123 . Next, a first secondary loose gearwheel  136  On the main shaft  124  is meshing with a first secondary gearwheel  137  that is integral with the countershaft  123 . Finally, a reverse secondary loose gearwheel  138  is arranged on the main shaft  124  close to the main shaft bearing  128 . The reverse secondary loose gearwheel  138  is via a reverse idler gearwheel (not shown) drivingly connected with a reverse secondary gearwheel  139  that is integral with the countershaft  123 . 
         [0044]    A splitter tooth clutch  142  can rotationally lock the input shaft  121  to either the second primary loose gearwheel  130  or the first primary loose gearwheel  132 . That gives two alternate power transfer paths between the input shaft  121  and the countershaft  123 . Two tooth clutches are used for transferring power between the countershaft  123  and the main shaft  124 . A second/third tooth clutch  143  can rotationally lock the main shaft  124  to either the first primary loose gearwheel  132  or the second secondary loose gearwheel  134 . A first/reverse tooth clutch  144  can rotationally lock either the first secondary loose gearwheel  136  or the reverse secondary loose gearwheel  138  to the main shaft  124 . Thereby, four alternate ways for power transfer between the countershaft  123  and the main shaft  124  are provided. 
         [0045]    Inside the range housing  106 , a planetary range section  160  is arranged, comprising a sun gear  161 , an internally cogged ring gear  162 , a plurality of planet gears  163  on a planet carrier  164 , a high range engaging ring  165 , and a low range clutch ring  166 . The sun gear  161  and high range engaging ring  165  are rotationally fixed to or integral with the main shaft  124 . The planet gears  163  mesh with ring gear  162  and sun gear  161 . An output shaft  171  is rotationally fixed to or integral with planet carrier  164 , carried by an output shaft bearing  172 , and rotationally fixed to an output companion flange  173  and an optional retarder drive gearwheel  174  that is in mesh with a gearwheel of a not shown optional retarder auxiliary brake. 
         [0046]    By displacement to the left in  FIG. 1 , the ring gear  162  can be rotationally locked to the high range engaging ring  165 , main shaft  124  and sun gear  161 . Then, the planet gears  163  will be fixed between the ring gear  162  and sun gear  161 . That will give the same rotational speed for the main shaft  124  and output shaft  171 , yielding a high range gear. On the other hand, by displacing the ring gear  162  to the right in  FIG. 1 , it can be rotationally locked to a low range clutch ring  166  that is fixed to the range housing  106 . The ring gear  162  will not rotate, causing the output shaft  171  to rotate slower than the main shaft  124 , hence a low range gear. Thus, said ring gear together with said range engaging ring and said low range clutch ring respectively work as a range clutch. Further, said low range gear and said high range gear can be defined as range speed ratios. 
         [0047]    The automatic or semi-automatic gear shift is carried out by a gear-shift control unit  150 . This gear-shift control unit  150  comprises a gear-shift control housing  151 , a splitter shift fork  152 , a second/third shift fork  153  and a first/reverse shift fork  154 . The splitter shift fork  152  controls the splitter tooth clutch  142 . The second/third shift fork  153  controls the second/third tooth clutch  143 , and the first/reverse shift fork  154  controls the first reverse tooth clutch  144 . A range shift rod and fork  156  that controls the displacement of the ring gear  162  is connectable to, but not part of, the gear-shift control unit  150 . Other configurations are possible; any of the shift forks  152 ,  153  and  154  may or may not be a part of the gear-shift control unit  150 . Furthermore, the gearshift control Unit  150  may be composed of different parts that are arranged at different locations of the transmission  100 . 
         [0048]    The design of the gear-shift control unit  150  will not be described in detail here. Its structural part, the gear-shift control housing  151 , may be machined from a cast blank and it is bolted to the main housing  102 . Therein, microcontrollers, sensors, valves, and actuators may be found. The shift forks  152 ,  153  and  154  may be carried by shift rods that connect to actuators. 
         [0049]    An optional power take-off drive device  180  is located in the lower part of the range housing  106 . Therein, a power take-off drive shaft  181  is rotationally fixed to or integral with the countershaft  123 . The power take-off drive shaft  181  drives a power take-off unit  182  that is bolted to the range housing  106 ; it may also drive a transmission oil pump (not shown). The power take-off unit  182  is not shown in detail, but may comprise housing parts, bearings, shafts, clutches and control parts for engagement and disengagement, Bearings for speed adaption, and output flanges, as is readily known by a person skilled in the art. 
         [0050]    Due to the single clutch  110  and the tooth clutches  142 ,  143  and  144 , there will be a power interruption at every gear shift in the transmission  100 . By using a dual clutch unit and two input shafts, these power interruptions can be eliminated to a far extent. Then, the transmission  100  would be converted to a dual clutch transmission with a range section  160 .  FIG. 2  shows this schematically. In a modified clutch housing  201 , a multiple clutch or a dual frictional clutch  210  is arranged. The dual frictional clutch  210  comprises the flywheel  111 , a torsional damper  216  bolted thereon, and a dual clutch assembly  212  with a first clutch disc set  213  and a second clutch disc set  214 . There is also an actuator device (not shown) to control the dual frictional clutch  210 . 
         [0051]    A main transmission  220  is arranged inside the main housing  102 . There are two input shafts; a first input shaft  221  and a second input shaft  222 . The first input shaft  221  can be rotationally driven by the first clutch disc set  213 . Similarly, the second input shaft  222  can be rotationally driven by the second clutch disc set  214 . Hence, the dual frictional clutch  210  and the input shafts  221  and  222  will in a way replace the splitter tooth clutch  142  in  FIG. 1 . 
         [0052]    First primary gear teeth  232  are rotationally fixed to or integral with the first input shaft  221 . Second primary gear teeth  230  are rotationally fixed to or integral with the second input shaft  222 . A modified main shaft  224  is coaxial with the input shafts  221  and  222 . A modified countershaft  223  is parallel thereto. The second input shaft  222  is suspended in the clutch housing  201  by an input shaft bearing  225 . Between the main shaft  224 , the first input shaft  221  and the second input shaft  222 , four pilot bearings  229  are arranged. Thereby, a thorough but not over-constrained suspension of the main and input shaft is achieved. 
         [0053]    The main shaft  224  carries three loose gearwheels; the second secondary loose gearwheel  134 , the first secondary loose gearwheel  136  and the reverse secondary loose gearwheel  138 . Here, the first and reverse secondary loose gearwheels  136  and  138  can be rotationally locked to the main shaft by the first/reverse tooth clutch  144 , exactly as in the transmission  100  in  FIG. 1 . The second secondary loose gearwheel  134  can be rotationally locked to the main shaft  224  by a second tooth clutch  241 . Moreover, the main shaft  224  can be rotationally locked to the first input shaft  221  by a direct tooth clutch  240 . 
         [0054]    On the countershaft  223 , the second primary gearwheel  131  meshes with the second primary gear teeth  230  of the second input shaft  222 . A primary countershaft loose gearwheel  233  meshes with the first primary gear teeth  232  of the first input shaft  221 . A secondary countershaft loose gearwheel  235  meshes with the second secondary loose gearwheel  134  on the main shaft  224 . Finally, the first secondary gearwheel  137  and the reverse secondary gearwheel  139  function as in the transmission  100  in  FIG. 1 . 
         [0055]    The primary countershaft loose gearwheel  233  and the secondary countershaft loose gearwheel  235  can be rotationally locked by a first countershaft tooth clutch  248 . The secondary countershaft loose gearwheel  235  can be rotationally locked to the countershaft  223  by a second countershaft tooth clutch  249 . The countershaft tooth clutches  248  and  249  are controlled by a first countershaft shift fork  258  and a second countershaft shift fork  259 . The shift forks  258  and  259  are actuated via shift rods (not shown) by a countershaft actuator  257 . 
         [0056]    The main shaft tooth clutches,  240 ,  241  and  144 , are controlled by a gear-shift control unit  250  with shift forks  252 ,  253  and  154 . The direct shift fork  252  and the second shift fork  253  are new compared to the gear-shift control unit  150  in  FIG. 1 . There may be further differences with respect to sensors, electronics and software, etc., between the gear-shift control units  150  and  250 . However, with the countershaft actuator  257 , the gear-shift control housing  151 , or at least its blank, can be common for both gear-shift control units  150  and  250 . 
         [0057]    The shift rods that connect the countershaft shift forks  258  and  259  with the countershaft actuator  257  may require some suspension device in the main housing  102 . In such a case, modifications would be required on the main housing  102 . It would still, however, be possible to use the same main housing in the conventional transmission  100  as in the dual clutch transmission  200 . In order to save costs even further, it may be of advantage to use the same (cast) blank for the main housing, but do the additional machining for the said suspension device for the dual clutch transmission  200 , only. 
         [0058]    Several parts in  FIG. 2  are, or could be, identical to the corresponding parts in  FIG. 1 . Those parts are referred to by the same numbers as in  FIG. 1 , by the initial digit  1 . 
         [0059]    The main transmission  220  is functionally identical or very similar to dual clutch transmissions in DE923402 and DE3131156A1. It has six forward gears where high power transfer to the driven wheels is enabled during shifts between consecutive gears. The combination with range section  160  increases the number of gears and the maximum reduction ratio of the complete transmission  200 , although with power interruption when range section  160  is shifted. 
         [0060]    So, it would be of advantage if the transmission  200  could be modified to eliminate the range shift power interruption. Preferably, such a modification should imply a minimum impact on the advantageous features of the transmission  200  in terms of size, cost, power losses and similarities with the conventional AMT  100 . There are interesting solutions in U.S. Pat. No. 6,958,028B2 ( FIG. 5 ) and U.S. Pat. No. 7,070,534B2, with low and high range power paths in the main transmission. However, that would result in a longer and wider transmission or a lower number of gears. The design in US2008/0188342A1 would be better, however still requiring significant axial space. Moreover, the forward position of the rear main shaft bearing will, as discussed earlier, have a negative impact at the assembly and on misalignments in operation. 
         [0061]    A modified dual clutch transmission  300  according to the invention is shown in  FIG. 3 . There, a rotatable, loose by-passing gearwheel  383  and a by-passing tooth clutch  384  are added to a modified power take-off device  380 . The by-passing gearwheel  383  meshes with a modified retarder drive gearwheel  374  that is rotationally fixed to a modified output shaft  371 . The by-passing tooth clutch  384  can selectively rotationally lock the by-passing gearwheel  383  to a modified by-passing power takeoff drive shaft  381 . A modified countershaft actuator  357  controls the by-passing tooth clutch  383  via a shift rod (not shown) and a by-passing shift fork  385 . 
         [0062]    Then, a by-passing power path can be established from the countershaft  223  to the output shaft  371  via the power take-off drive shaft  381 , by-passing tooth clutch  234 , by-passing gearwheel  383  and retarder drive gearwheel  374 . Thereby, the range section  360  will not be subjected to any torque load, neither on the gear teeth nor on the planet carrier  364 . It can then be shifted between low and high range positions while power is transferred to the driven wheels, hence eliminating the power interruption. Moreover, the planet carrier design is not more complex. 
         [0063]    With the by-passing power path on the rear, down-stream side of the range section  360 , no modifications are required in the main transmission  220 . Thus, the main housing  102  can be identical in transmissions  200  and  300 . This also holds for the external dimensions; since the range section  160  is prepared for an optional auxiliary brake, retarder, the length, from the clutch housing  201  to the companion flange  173  is the same for these transmissions. That facilitates the installation in the vehicle chassis. 
         [0064]    Thus, the result is a transmission with several forward gears that can have; i) gear-shifting between consecutive gears without power interruption, and ii) high reduction ratio. With similar range section  360  as the conventional transmission  100 , and similar construction with gearwheels and tooth clutches in the main transmission  220 , iii) cost advantages will result. The use of just a dual frictional clutch, and tooth clutches for the rest, will result in iv) low power losses, similar to those of the conventional transmission  100 . Furthermore, the use of one countershaft ( 223 ), only, will make the dual clutch transmission  300  just as narrow and v) easy to install in the vehicle as the conventional transmission  100 . 
         [0065]    Non-planetary range sections as in U.S. Pat. No. 4,966,048 or U.S. Pat. No. 7,204,166B2 would be possible, as well as designs with in- and output shafts parallel, non-Coaxial, e.g., EP1624232A1 and U.S. Pat. No. 4,777,837.  FIG. 4  discloses an embodiment of the invention 
         [0066]    where a non-planetary two-speed range is used in a dual clutch transmission  400  and where in- and output shafts are coaxial (as in  FIG. 3 ). The embodiment of  FIG. 4  differs from the embodiment disclosed in  FIG. 3  only in the design of the range gear section. The modified power take-off device  380  can stay intact. 
         [0067]    The non-planetary range section  460  comprises a first range gearwheel  401  rotationally fixed to the output end of the main shaft  224 . Said first range gearwheel is in mesh with a second range gearwheel  402  rotationally fixed to a range gear shaft  403  arranged in parallel to said main shaft and output shaft  471 . A third range gearwheel  404  is also arranged rotationally fixed to said range gear shaft. Said third range gearwheel is in mesh with a rotatable loose fourth range gearwheel  405  arranged on the output shaft  471 . Said fourth range gearwheel can selectively be rotationally fixed to said output shaft by a range tooth clutch  406 , which in  FIG. 4  will be displaced to the right. When the fourth range gear wheel is rotationally fixed to the output shaft and the main shaft not engaged with the output shaft a low range gear ratio is achieved via said first to fourth range gear wheels. The range tooth clutch is also arranged to selectively connect the output end of said main shaft to said Output shaft thereby engaging a 1:1 (high) range gear ratio. A modified range shaft actuator (not shown) controls the range tooth clutch  406  via a shift rod (not shown) and a range shift fork  407 . 
         [0068]    Hence, the dual clutch transmissions  300  and  400  can potentially fulfill the full scope of the invention. Variations from  FIG. 3  or  4  are possible without departing from the scope of the invention: 
         [0069]    In another embodiment of the invention, the retarder drive gearwheel  374  is used for driving the optional retarder as well as for the by-passing power path. 
         [0070]    In a further embodiment of the invention, the by-passing power take-off drive shaft  381  is used for driving the optional power take-off unit  182  as well as for the bypassing power path. 
         [0071]    In yet another embodiment of the invention, the by -passing shift fork  385  is controlled by the same actuator as a shift fork in the main transmission  220 . Furthermore, these two shift forks may be carried by the same shift rod. 
         [0072]    In an alternative embodiment of the invention, the by-passing tooth clutch  384  is substituted by a tooth clutch between the output shaft  371  and retarder drive gearwheel  374  (which has to be rotatably arranged on the output shaft  371 ). Then, a conventional retarder option is no longer feasible. The elimination of power interruptions makes an efficient engine brake an adequate substitute. 
         [0073]    In further embodiments of the invention, the by-passing tooth clutch  384  is located on the other side of the by -passing gearwheel  383 . It could also (if the power take-off function is relocated) be located close the main housing wall  102 w and selectively connect a modified end of the countershaft ( 223 ) with a combined power take-off drive shaft ( 381 ) and by-passing gearwheel ( 383 ). 
         [0074]    In another embodiment of the invention, the power take-off drive shaft  381  is substituted by, or integrated with, an elongated variant of the countershaft  223 . 
         [0075]    The rotating parts in the main transmission  220  and range section  360  or  460  may be modified in order to yield speed ratios and number of gears that better suit the fully power-shifting transmission  300  or  400 . 
         [0076]    Other arrangements and locations of the friction clutches  210  would also be possible, e.g., as in U.S. Pat. No. 5,347,879 or U.S. Pat. No. 4,777,837. 
         [0077]    Moreover, designs with three friction clutches, “triple clutch transmissions”, or more, e.g., DE4226577C1 or US2008/0190228A1, could also be included. Power-shifting transmissions having more than one friction clutch can be referred to as multi-clutch transmissions. 
         [0078]    The tooth clutches  240 ,  241 ,  144 ,  248 ,  249  and  162 , 165 , 166  may or may not be equipped with synchronizing parts in order to facilitate engagement, as is readily known by a person skilled in the art. A central synchronizing device would also be possible, e.g. as 30 in U.S. Pat. No. 5,150,628. 
         [0079]    The invention should not be deemed to be limited to the embodiments described above, but rather a number of further variants and modifications are conceivable within the scope of the following patent claims. 
         [0080]    Below follows further suggested solutions which lies outside the scope of the above described invention: 
         [0081]    With another design of the range section  360 , e.g., as in U.S. Pat. No. 5,351,569 or DE102005030987A1, the by -passing gearwheel  383  and retarder drive gearwheel  374  could be located between the main housing wall  102   w  and planetary gear train  161 ,  162 ,  163 . That would increase the complexity of the planet carrier.