TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The present invention concerns a transmission system for a motor vehicle according to the preamble to claim 1 .

Heavy motor vehicles, such as goods vehicles, tractors and buses, are often equipped with an automated manual transmission, which can be regulated by the vehicle driver via an electronic control device by means of an actuating device in the form of a gear lever or the like. The vehicle driver can, as he desires, order a shifting of gears in the transmission manually by actuating or letting the electronic control device automatically handle the choice of a suitable gear in the transmission. In either case the shifting operations are controlled during a shifting of gears in the transmission by the electronic control device in a preprogrammed manner, and in dependence upon the gears between which shifting is to occur.

An automated manual transmission is equipped with a main gear unit, which comprises a main shaft and a layshaft parallel thereto, and is often also equipped with a range gear unit, which is adjustable to a low-range gear setting and a high-range gear setting. The main shaft is connectable to an input shaft of the transmission via the layshaft, and to an output shaft of the transmission via the range gear unit. The range gear unit brings about a higher gear ratio in the high-range gear setting than in the low-range gear setting. Switching between the low-range gear setting and the high-range gear setting occurs by means of a coupling device incorporated in the range gear unit. A torque interruption is necessary in the range gear unit during shifting between the low-range gear setting and the high-range gear setting in order to enable a synchronization of the rotational speeds of the two coupling elements that must be brought into non-rotatable engagement with one another during a shifting operation.

The term "layshaft" refers in this description and the subsequent claims to a shaft that is designed to transfer torque from an input shaft to a main shaft in a main gear unit.

So-called double clutch gearboxes have been developed in order to prevent torque interruptions. A double clutch gear box has a first input shaft that is connectable to the drive shaft of an engine by means of a first clutch and a second input shaft that is connectable to the drive shaft of the engine by means of a second clutch. A first set of torque transfer paths can be established, via the first input shaft, with mutually different gear ratios between the drive shaft of the engine and the main shaft of the main gear unit, and a second set of torque transfer paths can be established, via the second input shaft, with mutually different gear ratios between the drive shaft of the engine and the main shaft of the main gear unit. The two input shafts are connected alternatingly to the drive shaft of the engine via said first and second clutches, and the two different sets of torque transfer paths can thus be utilized in alternation, one after another. It thus becomes possible to perform stepwise upshifts and downshifts in the main gear unit without torque interruptions, i.e. interruptions in the transfer of torque between the drive shaft of the engine and the output shaft of the transmission.

A double clutch gearbox can be designed with two layshafts, such as is described in, for example, US 2006/0025272 A1 , or with a single layshaft, such as is described in, for example, WO 201 1 /069526 A1 . In order to also be able to avoid torque interruptions during gear shifts that involve a shifting of gears in the range gear unit of a double clutch gearbox, solutions have been developed that make it possible to establish, by means of a layshaft and a bypass shaft connected thereto, a torque transfer path that extends from the one input shaft to the output shaft without passing through the range gear unit. Said bypass shaft thus makes it possible to bypass the range gear unit during a shifting of gears in the range gear unit. When the vehicle is driven in the highest gear of the low-range gear unit, a first input shaft is connected to the drive shaft of the engine and in torque- transferring connection with the output shaft via the range gear unit, while the second input shaft is disengaged from the drive shaft of the engine. When an upshift is to occur from the highest gear in the low-range gear unit to the lowest gear in the high- range gear unit, said second input shaft is brought into torque- transferring connection with the output shaft via the bypass shaft, and this input shaft is subsequently connected to the drive shaft of the engine at the same time as said first input shaft is disengaged from the drive shaft of the engine. In this state, with the range gear unit unloaded, a gear shift occurs in the range gear unit from the low-range gear setting to the high-range gear setting at the same time as a torque transfer path is established from the now disengaged first input shaft to the main shaft that corresponds to the lowest gear in the high-range gear setting. Said first input shaft is subsequently connected to the drive shaft of the engine at the same time as said second input shaft is disengaged from the drive shaft of the engine. The process is reversed in connection with downshifting from the lowest gear in the high-range gear setting to the highest gear in the low-range gear setting.

OBJECT OF THE INVENTION

The object of the present invention is to achieve a transmission system of the type described above with a new and advantageous design. SUMMARY OF THE INVENTION

The foregoing object is achieved according to the present invention by means of a transmission exhibiting the features defined in claim 1 .

The transmission system according to the invention comprises: - a double clutch main gear unit, which comprises:

• two input shafts, which are mutually coaxial,

• a main shaft, and

· one or a plurality of layshafts, wherein each layshaft is equipped with at least one gearwheel that is in engagement with a gearwheel arranged on any of the input shafts and a second gearwheel that is in engagement with a gearwheel arranged on the main shaft,

- a coupling device for alternatingly coupling in said input shafts, - an output shaft,

- a range gear unit, which is adjustable to a low-range gear setting and in a high-range gear setting, wherein the main shaft of the main gear unit is connected to the output shaft via the range gear unit,

- a bypass shaft, by means of which at least one of the input shafts is connectable to the output shaft to establish a torque transfer path from said input shaft to the output shaft without passing through the range gear unit during a shifting of gears in the range gear unit from the low-range gear setting to the high- range gear setting or from the high-range gear setting to the low- range gear setting, whereupon said one or a plurality of layshafts and the bypass shaft are mutually non-coaxial,

- a first gear train for transferring torque from the one input shaft to the bypass shaft, wherein said first gear train comprises a first gearwheel arranged on said input shaft and a second gearwheel arranged on the bypass shaft, and

- a second gear train for transferring torque from the bypass shaft to the output shaft, wherein said second gear train comprises a first gearwheel arranged on the bypass shaft and second gearwheel arranged on the output shaft.

The solution according to the invention makes it possible, in a simple way, to bypass the range gear unit and thus bring it into an unloaded state without necessitating the use of a layshaft from the main gear unit. The drive ratio between the relevant input shaft and the bypass shaft can thus be configured so as to achieve a suitable drive ratio in the torque transfer path by means of which the range gear unit is being bypassed without the need to take into account the suitable drive ratio between a layshaft and input shaft for the various gears in the main gear unit. More specifically, the drive ratio in the torque transfer path by means of which the range gear unit is being bypassed should lie essentially midway between the drive ratio that is obtained in the highest gear in the low-range gear setting and the drive ratio that is obtained in the lowest gear in the high-range gear setting. At such a drive ratio, the bypass shaft will provide an extra gear stage that offers a soft transition between the highest gear in the low-range gear setting and the lowest gear in the high-range gear setting. Using a bypass solution of the type described in WO 201 1 /069526 A1 , wherein the range gear unit is bypassed via a torque transfer path that passes through a layshaft of the main gear unit, it will be difficult in practice to design the transmission system in such a way that said suitable drive ratio is achieved in the bypass torque transfer path, as in this case the drive ratio that is suitable between the input shaft and layshaft must be taken into account.

According to one embodiment of the invention, the transmission system comprises a retarder, which exhibits an input shaft, wherein the bypass shaft is connected or connectable to the input shaft of the retarder in order to thereby enable the establishment of a first torque transfer path to the input shaft of the retarder from the one input shaft via the bypass shaft and a second torque transfer path to the input shaft of the retarder from the output shaft. It thus becomes possible to allow the input shaft of the retarder to be in torque-transferring connection with the one input shaft or with the output shaft, depending on which of these alternatives yields the highest rotational speed of the input shaft of the retarder. The braking power of a retarder increases with increasing rotational speed of the input shaft of the retarder. In a gear stage with a high gear ratio, the former alternative may consequently be the most advantageous, while the latter alternative may be most advantageous in a gear stage with a low gear ratio.

According to another embodiment of the invention, the bypass shaft and the input shaft of the retarder are mutually coaxial, whereupon the bypass shaft is non-rotatably connected with the input shaft of the retarder, or non-rotatably engageable with the input shaft of the retarder by means of a coupling arrangement. The retarder can thus be integrated into the transmission system in a space-saving manner. Other advantageous features of the transmission system according to the invention are described in the dependent claims and description that follow below.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in greater detail below with the help of exemplary embodiments and with reference to the accompanying drawings, which show: Fig. 1 a block diagram of a transmission system according to a first embodiment of the present invention,

Fig. 2 a schematic front view of components included in a transmission system according to Fig. 1 , Fig. 3 a section along the lines A-A in Fig. 2,

Fig. 4 a section along the lines B-B in Fig. 2, Fig. 5a the torque transfer path through the main gear unit and the range gear unit of the transmission system according to Figs. 1 -4 in a gear stage with the highest gear in the low-range gear setting, Fig. 5b the torque transfer path through the main gear unit and the range gear unit of the transmission system according to Figs. 1 -4 in a gear stage with the lowest gear in the high-range gear setting, Fig. 6 a block diagram of a transmission system according to a second embodiment of the present invention,

Fig. 7 a block diagram of a transmission system according to a third embodiment of the present invention,

Fig. 8 a block diagram of a transmission system according to a fourth embodiment of the present invention, and

Fig. 9 a block diagram of a transmission system according to a fifth embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Figs. 1 and 6-9 illustrate, in highly schematic fashion, a transmission system 1 for a motor vehicle according to various embodiments of the present invention. Some of the parts that are included in the transmission system 1 according to the embodiment illustrated in Fig. 1 are shown in greater detail in Figs. 2-5.

The transmission system 1 comprises a double clutch main gear unit 10 and a range gear unit 30. The main gear unit 10 comprises a first input shaft 1 1 and a second input shaft 12, which are mutually parallel and coaxial. The first input shaft 1 1 extends axially through the second input shaft 12, which is thus arranged externally around the first input shaft 1 1 . The transmission system 1 further comprises a coupling device 40 for alternatingly coupling in the two input shafts 1 1 , 12. The input shafts 1 1 , 12 are alternatingly connectable to a drive shaft 2 of a drive engine 3 by means of said coupling device 40. The coupling device 40 comprises a first coupling arrangement 41 , by means of which the first input shaft 1 1 can be brought into torque- transferring connection with and disengaged from the drive shaft 2, and a second coupling arrangement 42, by means of which the second input shaft 12 can be brought into torque-transferring connection with and disengaged from the drive shaft 2. The main gear unit 10 further comprises a first layshaft 13, a second layshaft 14 and a main shaft 15. The two layshafts 13, 14 are mutually parallel and also parallel with the two input shafts 1 1 , 12 and the main shaft 15. In the illustrated example, the main shaft 15 and the first input shaft 1 1 are mutually coaxial. The main shaft 15 is connectable via the layshafts 13, 14 to the two input shafts 1 1 , 12, and the main shaft 15 is connected to an output shaft 5 of the transmission system 1 via a range gear unit 30. The range gear unit 30 is adjustable to a low-range gear setting and a high-range gear setting by means of a coupling device 31 comprised in the range gear unit, which coupling device can be equipped with synchronizing means of a known type. In the illustrated embodiments, the main gear unit 10 and the range gear unit 30 are housed in a common gearbox housing 7, but they could alternatively be housed each in their own gearbox housing. The first input shaft 1 1 is rotatably bearing-mounted in relation to the second input shaft 12, while the second input shaft 12 and the output shaft 5 are rotatably bearing-mounted in relation to the gearbox housing 7 and protrude from opposite ends thereof. The layshafts 13, 14 and the main shaft 15 are also rotatably bearing-mounted in relation to the gearbox housing 7. For the sake of clarity, the bearings by means of which the various shafts 5, 1 1 -15 are rotatably bearing-mounted are not shown in Figs. 1 and 6-9.

At least one of the input shafts 1 1 , 12 is connectable to the output shaft 5 of the transmission by means of a bypass shaft 50 in order to establish a torque transfer path from said input shaft to the output shaft 5 without passing through the range gear unit 30 during a shifting of gears in the range gear unit 30 from the low-range gear setting to the high-range gear setting or from the high-range gear setting to the low-range gear setting. The bypass shaft 50 extends parallel to the layshafts 13, 14 and is arranged in lateral displacement in relation thereto. The bypass shaft 50 is thus non-coaxially arranged in relation to each layshaft 13, 14. The bypass shaft 50 also extends parallel with the main shaft 15 and is arranged in lateral displacement in relation thereto. The bypass shaft 50 is thus also non-coaxially arranged in relation to the main shaft 15. The bypass shaft 50 and the layshafts 13, 14 do not extend in a common plane, and the bypass shaft 50 is consequently indicated by means of a broken line in Figs. 1 and 6-9. The bypass shaft 50 is rotatably bearing-mounted in relation to the gearbox housing 7.

In the illustrated embodiments, it is the aforementioned first input shaft 1 1 that is connectable to the output shaft 5 via the bypass shaft 50. However, as an alternative, the second input shaft 12 could be connectable to the output shaft 5 via the bypass shaft 50. As an additional alternative, both input shafts 1 1 , 12 could be selectively connectable to the output shaft 5 via the bypass shaft 50. The transmission system 1 comprises a first gear train K1 for transferring torque from the one input shaft 1 1 to the bypass shaft 50. Said first gear train K1 comprises a first gearwheel K1 a arranged on said input shaft 1 1 and a second gearwheel K1 b arranged on the bypass shaft 50. The transmission system 1 further comprises a second gear train K2 for transferring torque from the bypass shaft 50 to the output shaft 5. Said second gear train K2 comprises a first gearwheel K2a arranged on the bypass shaft 50 and a second gearwheel K2b arranged on the output shaft 5. In the embodiments illustrated in Figs. 1 , 7, 8 and 9, said first and second gearwheels K1 a, K1 b of said first gear train K1 are in direct engagement with one another, whereupon said first and second gearwheels K2a, K2b of said second gear train K2 are also in direct engagement with one another. As an alternative, the first and second gearwheels K1 a, K1 b in the first gear train K1 could, like the first and second gear wheels K2a, K2b in the second gear train K2, be in torque-transferring connection with one another via one or a plurality of intermediate gearwheels. Fig. 6 illustrates an embodiment in which an intermediate gearwheel K1 c is arranged between the first and second gearwheels K1 a, K1 b in the first gear train K1 , and in which a second intermediate gearwheel K2c is arranged between the first and second gearwheels K2a, K2b in the second gear train K2. Respective intermediate gearwheels K1 c, K2c are rotatably arranged by being rotatably bearing-mounted on a shaft 17, 18 that is non-rotatably connected with the gearbox housing 7, or by being non-rotatably arranged on a shaft that is rotatably bearing- mounted in relation to the gearbox housing 7.

In the illustrated embodiments, the second gearwheel K1 b in the first gear train K1 and the first gearwheel K2a in the second gear train K2 are non-rotatably arranged on the bypass shaft 50, while the first gearwheel K1 a in the first gear train K1 is rotatably bearing-mounted on the input shaft 1 1 and the second gear wheel K2b in the second gear train K2 is rotatably bearing- mounted on the output shaft 5. In this case the first gearwheel K1 a of the first gear train K1 is non-rotatably engageable with the input shaft 1 1 by means of a coupling arrangement 16 arranged on said input shaft, while the second gearwheel K2b in the second gear train K2 is non-rotatably engageable with the output shaft 5 by means of a coupling arrangement 6 arranged on the output shaft. Each respective coupling arrangement 6, 16 can, for example, consist of a conventional synchronizing coupling or claw coupling of a known type, or another suitable type of coupling. Bringing the first gearwheel K1 a in the first gear train K1 into torque-transferring engagement with the input shaft 1 1 and the second gearwheel K2b of the second gear train K2 into torque-transferring engagement with the output shaft 5 by means of the coupling arrangements 6, 16 establishes a torque transfer path from the input shaft 1 1 to the output shaft 5 via the bypass shaft 50.

In the illustrated embodiments, a gearwheel K1 a in the first gear train K1 and a gearwheel K2b in the second gear train K2 are disengageably arranged by means of each their own coupling arrangement 16, 6. However, it would be sufficient to allow only one gearwheel in any of said gear trains K1 , K2 to be disengageably arranged by means of a coupling arrangement. The latter gearwheel could naturally also consist of any of the gearwheels K1 b, K2a arranged on the bypass shaft 50.

Connections between the input shafts 1 1 , 12 and the layshafts 13, 14 and between the layshafts 13, 14 and the main shaft 15 can be established by means of a number of gear trains K3-K9, which define different gear ratios in the main gear unit 10. In the illustrated embodiments, the transmission system 1 comprises: - a third gear train K3 for transferring torque from the first input shaft 1 1 to the first layshaft 13, - a fourth gear train K4 for transferring torque from the first input shaft 1 1 to the second layshaft 14,

- a fifth gear train K5 for transferring torque from the second input shaft 12 to the first layshaft 13,

- a sixth gear train K6 for transferring torque from the second input shaft 12 to the second layshaft 14,

- a seventh gear train K7 for transferring torque from the first layshaft 13 to the main shaft 15,

- an eighth gear train K8 for transferring torque from the second layshaft 14 to the main shaft 15, and

- a ninth gear train K9 for transferring torque from the first layshaft 13 to the main shaft 15.

In the illustrated embodiments, a direct connection can also be established between the first input shaft 1 1 and the main shaft 15 by means of the coupling arrangement 16 arranged on the first input shaft 1 1 .

Said third and fourth gear trains K3, K4 comprise a common first gearwheel K34a that is non-rotatably arranged on the first input shaft 1 1 . The third gear train K3 comprises a second gearwheel K3b that is rotatably bearing-mounted on the first layshaft 13. The latter gearwheel K3b is in engagement with said first gearwheel K34a and is non-rotatably engageable with the first layshaft 13 by means of a first coupling arrangement 19 arranged on the first layshaft. The fourth gear train K4 comprises a second gearwheel K4b that is rotatably bearing-mounted on the second layshaft 14. The latter gearwheel K4b is in engagement with said first gearwheel K34a and is non-rotatably engageable with the second layshaft 14 by means of the coupling arrangement 20 arranged on the second layshaft.

Said fifth and sixth gear trains K5, K6 comprise a common first gearwheel K56a that is non-rotatably arranged on the second input shaft 12. The fifth gear train K5 comprises a second gearwheel K5b that is rotatably bearing-mounted on the first layshaft 13. The latter gearwheel K5b is in engagement with said first gearwheel K56a and is non-rotatably engageable with the first layshaft 13 by means of a second coupling arrangement 21 arranged on the first layshaft. The sixth gear train K6 comprises a second gearwheel K6b that is rotatably bearing-mounted on the second layshaft 14. The latter gearwheel K6b is in engagement with said first gearwheel K56a and is non-rotatably engageable with the second layshaft 14 by means of the coupling arrangement 20 arranged on the second layshaft.

Said seventh gearwheel K7 comprises a first gearwheel K7a that is non-rotatably arranged on the first layshaft 13 and a second gearwheel K7b that is rotatably bearing-mounted on the main shaft 15. The latter gearwheel K7b is in engagement with said first gearwheel K7a and is non-rotatably engageable with the main shaft 15 by means of a coupling arrangement 22 arranged on the main shaft.

Said eighth gear train K8 comprises a first gearwheel K8a that is non-rotatably arranged on the second layshaft 14 and a second gearwheel K8b that is non-rotatably arranged on the main shaft 15 and is in engagement with said first gearwheel K8a. Said ninth gear train K9 comprises a first gearwheel K9a that is non-rotatably arranged on the first layshaft 13, a second gearwheel K9b that is rotatably bearing-mounted on the main shaft 15 and an intermediate third gearwheel K9c. The first and second gearwheels K9a, K9b are in torque-transferring connection with one another via the third gearwheel K9c. The second gearwheel K9b is non-rotatably engageable with the main shaft 15 by means of the coupling arrangement 22 arranged on the main shaft 22. The third gearwheel K9c is rotatably arranged by being non-rotatably arranged on a shaft 23 that is rotatably bearing-mounted in relation to the gearbox housing 7, or by being rotatably bearing-mounted on a shaft that is non-rotatably connected to the gearbox housing 7. The ninth gearwheel K9 forms a reverse gear.

Each respective coupling arrangement 19-22 can, for example, consist of a conventional synchronizing coupling or claw coupling of a known type, or another suitable type of coupling. The transmission system according to the invention could alternatively comprise a main shaft unit with fewer or more gear trains than nine. The transmission system according to the invention could also alternatively comprise a double clutch main gear unit with only one layshaft. In addition, one and the same gearwheel on any of the input shafts 1 1 , 12 could be in engagement with both a gearwheel on the bypass shaft 50 and a gearwheel on one of the layshafts 13, 14.

A first set of torque transfer paths with mutually different gear ratios can be established between the drive shaft 2 of the engine and the main shaft 15 of the main gear unit via the first input shaft 1 1 and the gear trains K3, K4, K7 and K8, and a second set of torque transfer paths with mutually different gear ratios can be established between the drive shaft 2 of the engine and the main shaft 15 via the second input shaft 12 and the gear trains K5, K6, K7 and K8. The two sets of torque transfer paths can be utilized in alternation by alternatingly connecting the two input shafts 1 1 , 12 to the drive shaft 2 of the engine. This makes it possible to perform stepwise upshifting and downshifting in the main gear unit 10 without torque interruptions, i.e. without interruptions in the transfer of torque between the drive shaft 2 of the engine and the output shaft 5 of the transmission.

In the illustrated embodiments, the range gear unit 30 comprises a planetary gear 30 with a sun wheel 33, planet wheels 34 and a ring gear 35. The planet wheels 34 surround the sun wheel 33 and are in engagement therewith, while the ring gear 35 surrounds the planet wheels 34 and is in engagement therewith. The sun wheel 33 is non-rotatably connected to the main shaft 15. The planet wheels 34 are rotatably bearing-mounted in a planet wheel carrier 36, which is non-rotatably connected to the output shaft 5 of the transmission. The ring gear 35 exhibits internal teeth, by means of which the ring gear engages the planet wheels 34. The ring gear 35 is axially displaceable and constitutes a coupling element, by means of which the range gear unit 30 is adjustable to a low-range gear setting and a high- range gear setting.

By being displaced in a first direction, the ring gear 35 can be non-rotatably engaged with a first coupling element 37 that is non-rotatably connected to the gearbox housing 7, as a result of which the ring gear 35 will be prevented from rotating. In this low-range gear setting, the planet wheels 34 rotate together with the planet wheel carrier 36 in relation to the sun wheel 33 and the ring gear 35, whereupon the output shaft 5 rotates at a lower rotational speed than the main shaft 15.

By being displaced in an opposite direction, the ring gear 35 can be non-rotatably engaged with a second coupling element 38 that is non-rotatably connected to the main shaft 15, whereupon the ring gear 35 will come to rotate together with the main shaft 15. In this high-range gear setting, the planet wheels 45 and the planet wheel carrier 36 are prevented from rotating in relation to the sun wheel 33 and the ring gear 35, whereupon the output shaft 5 will consequently rotate with the same rotational speed as the main shaft 15.

As an alternative, the ring gear 35 could be axially fixated and in engagement with a coupling sleeve that surrounds the ring gear and is axially displaceable for a shifting of gears in the range gear unit 30 between said low-range gear setting and high-range gear setting.

The bypass shaft 50 extends past the planetary gear32 radially and external thereto.

Fig. 51 illustrates the torque transfer path V1 through the main gear unit 10 and the range gear unit 30 of the transmission system 1 as per Figs. 1 -4 at a gear stage in the highest gear of in low-range gear setting. In this configuration, the drive shaft 2 of the engine is in torque-transferring connection with the output shaft 5 via the second input shaft 12, the fifth gear train K5, the first layshaft 13, the seventh gear train K7, the main shaft 15, the sun wheel 33, the planet wheels 34 and the planet wheel carrier 36. When an upshift is to occur from this gear stage to the next gear stage, the first gearwheel K1 a in the first gear train K1 is brought into non-rotatable connection with the first input shaft 1 1 and the second gearwheel K2b in the second gear train K2 is brought into non-rotatable connection with the output shaft 5 so that a torque transfer path V2 is established (see Fig. 4) from the first input shaft 1 1 to the output shaft 5 via the first gear train K1 , the bypass shaft 50 and the second gear train K2. The first input shaft 1 1 is subsequently brought into torque-transferring connection with the drive shaft 3 of the engine at the same time as the second input shaft 12 is disengaged from the drive shaft of the engine. The planetary gear 32 is unloaded in this gear stage. When an upshift is to occur from this gear stage to the next gear stage, which corresponds to the lowest gear in the high-range gear setting, a shifting of gears in the range gear unit 30 from the low-range gear setting to the high-range gear setting occurs by means of a displacement of the ring gear 35 so that a torque transfer path V3 (see Fig. 5b) is established from the disengaged second input shaft 12 to the output shaft 5 via the fifth gear train K5, the first layshaft 13, the seventh gear train K7, the main shaft 5 and the planetary gear32. The second input shaft 12 is subsequently brought into torque-transferring connection with the drive shaft 2 of the engine at the same time as the first input shaft 1 1 is disengaged from the drive shaft of the engine. The process is the reverse in connection with a downshift from the lowest gear in the high-range gear setting to the highest gear in the low-range gear setting.

Because the coupling arrangements 41 , 42 are utilized to perform shifts between the aforementioned and sequential gear stages, these shifts can occur without torque interruptions.

The first and second gear trains K1 and K2 are designed in such a way that the gear ratio for the torque transfer path V2 illustrated in Fig. 4, via which the range gear unit 30 is bypassed, lies essentially midway between the gear ratio for the torque transfer path V1 illustrated in Fig. 5a and the gear ratio for the torque transfer path V3 illustrated in Fig. 5b. In the embodiments illustrated in Figs. 7-9, the bypass shaft 50 is connected or connectable to the input shaft 61 of a retarder 60, which constitutes a so-called auxiliary brake. This retarder 60 can be a hydraulic retarder with a rotatable component in the form of an impeller, or an electric retarder with a rotatable component in the form of a rotor. The rotatable component is non-rotatably connected to the input shaft 61 of the retarder. When the rotatable component rotates, the retarder 60 generates a braking power that increases with increasing rotation speed of the rotatable component.

In the embodiments illustrated in Figs. 7 and 8, the bypass shaft 50 and the input shaft 61 of the retarder are mutually coaxial. In the embodiment according to Fig. 7, the bypass shaft 50 is non- rotatably connected to the input shaft 61 of the retarder, whereupon the input shaft 61 of the retarder forms an extension of the bypass shaft 50. In the embodiment according to Fig. 8, the bypass shaft 50 is non-rotatably engageable to the input shaft 61 of the retarder via a coupling arrangement 62. This coupling arrangement can, for example, consist of a conventional synchronizing coupling or friction plate coupling of a known type or another suitable type of coupling.

In the embodiments illustrated in Fig. 9, the bypass shaft 50 and the input shaft 61 of the retarder are in torque-transferring connection with one another via the aforementioned second gear train K2. In this case a gearwheel K2c is non-rotatably arranged on the input shaft 61 of the retarder, whereupon said gearwheel K2c is in engagement with the second gearwheel K2b in the second gear train K2.

In the embodiments illustrated in Figs. 7-9, a first torque transfer path to the input shaft 61 of the retarder from the input shaft 1 1 can be established via the first gear train K1 and the bypass shaft 50, while a second torque transfer path to the input shaft 61 of the retarder from the output shaft 5 can be established via the second gear train K2. When braking action is desired from the retarder 60, the input shaft 61 of the retarder can be brought by means of the coupling arrangement 16 arranged on the input shaft 1 1 and the coupling arrangement 6 arranged on the output shaft 5 into torque-transferring connection with the input shaft 1 1 or with the output shaft 5, depending on which of these alternatives will produce the highest rotational speed of the input shaft 61 at the prevailing gear ratio between the input shaft 1 1 and the output shaft 5, and thus the highest braking power. The transmission system according to the invention is intended in particular for use in a heavy vehicle such as, for example, a bus, a tractor or a goods vehicle. The invention is naturally in no way limited to the foregoing embodiments, but rather numerous possible modifications thereof should be obvious to one skilled in the art, without deviating from the basic idea of the invention as defined in the accompanying claims.