Dual clutch transmission having low gear mesh loss

A power transmission for a motor vehicle In realizing these advantages, a power transmission according to this invention includes an input shaft, an output, and a layshaft disposed parallel to the input shaft. A pair of drive elements includes a first element secured to the input shaft, and a second element journalled on the layshaft and driveably connected to the first element. A first coupler is secured to the layshaft for releasably coupling the second element and the layshaft. A planetary gear unit driveably connecting the layshaft and the output, includies a sun gear secured to the layshaft, a ring gear surrounding the sun gear and fixed against rotation, a carrier driveably connected to the output, and a set of planet pinions meshing with the sun gear and ring gear and rotatably supported on the carrier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to automatic transmissions having a layshaft kinematic arrangement, particularly to such transmissions having input clutches but no torque converter.

2. Description of the Prior Art

Automatic transmissions for transmitting power between an input and an output, either over a continuously variable range of speed ratios or in discrete step changes among speed ratios, have associated with them several sources of parasitic losses, which adversely affect fuel economy. These losses are associated with a torque converter, open hydraulic friction clutches and brakes, hydraulic pump, and gear meshes.

To improve fuel economy in a motor vehicle having an automatic transmission, an automated shift manual (ASM) transmission can be used to eliminate or substantially reduce all of these parasitic losses except gear mesh losses. An ASM transmission generally performs gear ratio changes by first interrupting torque transmitted from the engine to the transmission input, preparing the transmission components associated with the next speed ratio, and then restoring torque. A primary functional feature of ASM transmissions is the need to interrupt power transmitted from the engine to the transmission input shaft before or during each gear ratio change because there is no torque converter or producing a hydrokinetic connection between the engine and transmission input.

Dual clutch layshaft transmissions are essentially two ASM transmissions, one providing odd numbered gears and one providing even numbered gears. Shifts between odd numbered gears and even numbered gears can be accomplished without interrupting power flow. While operating in an odd numbered gear, the couplers can be moved to configure the transmission for the next even numbered gear. Dual clutch transmissions have parasitic losses only slightly higher than ASM transmissions.

Layshaft ASM transmissions offer significant efficiency improvements over traditional automatic transmissions with torque converters. However, ASMs must produce more torque multiplication in the lower gears than would be required of a transmission having a torque converter in order to compensate for the torque multiplication that a torque converter produces at lower speeds. Layshaft ASM transmissions must produce more torque multiplication in the lower gears also to avoid excessive energy into the clutch during launch of a vehicle from a stop condition.

A transmission having a large span usually requires many gear ratios to keep the ratio steps small. Consequently, there are many gears and synchronizer or couplers in large span transmissions.

SUMMARY OF THE INVENTION

This invention reduces the magnitude of gear mesh losses to a much lower magnitude than is present in a conventional ASM transmission. A transmission according to this invention uses only one gear mesh for the high gears, thereby keeping gear mesh loss to a minimum. A planetary gear unit is used to produce additional torque multiplication in the low gears. Furthermore, this transmission reuses some of the pinion-gear meshes to produce multiple gears, resulting in a low number of gears for the number of speed ratios produced.

In one embodiment, the direction of output rotation is opposite that of the input. In a conventional rear wheel drive application, this would require changes to a conventional inter-wheel differential mechanism. However, this requirement is avoided in a second embodiment, in which sprocket wheels and chains replace the pinions and gears.

Because a layshaft ASM transmission must provide a large torque multiplication in first gear, the distance between the main shaft and layshaft, called “center distance,” is usually large and requires a correspondingly large package size. Here, however, much of the torque multiplication is achieved near the output end of the transmission by a planetary gear unit, the other components carry relatively low torque loads, and the center distance is kept small. Therefore, the package size is compact.

Gear ratio changes are accomplished through the use of couplers, such as synchronizers or dog clutches, which mutually driveably connect components operative in each speed ratio. The couplers produce very little drag loss when engaged, and do not require a continuous supply of power to stay engaged.

In realizing these advantages, a power transmission according to this invention includes an input, first input shaft and second input shaft arranged coaxially with a first axis. An output and layshaft are arranged coaxially on a second axis. A first clutch alternately connects and disconnects the input and the first input shaft. A second clutch alternately connects and disconnects the input and the second input shaft. A first torque path driveably connecting the first and second input shafts to the layshaft. A second torque path driveably connects the second input shaft and the output. A third torque path, which includes a planetary gear unit, produces a speed reduction between the layshaft and the output.

In realizing these advantages, a power transmission according to this invention includes an input shaft, an output, and a layshaft disposed parallel to the input shaft. A pair of drive elements includes a first element secured to the input shaft, and a second element journalled on the layshaft and driveably connected to the first element. A first coupler is secured to the layshaft for releasably coupling the second element and the layshaft. A planetary gear unit driveably connecting the layshaft and the output, includies a sun gear secured to the layshaft, a ring gear surrounding the sun gear and fixed against rotation, a carrier driveably connected to the output, and a set of planet pinions meshing with the sun gear and ring gear and rotatably supported on the carrier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now toFIG. 1, a transmission according to the present invention includes an input10for driveably connecting a power source such as an internal combustion engine or electric motor to the transmission, and an output12for driving a load, such as the driven wheels of a motor vehicle, through a powertrain that may include a drive shaft, differential mechanism, and axle shafts.

A first friction clutch14alternately connects and disconnects a first input shaft16as the clutch is engaged and disengaged, respectively. A second friction clutch18alternately connects and disconnects a second input shaft20as the clutch is engaged and disengaged, respectively.

Pinions22,24,26are secured to input shaft16, which is supported for rotation on a transmission case. Pinions28,30, and32are secured to the second input shaft20. Pinion28is in continuous meshing engagement with gear34, which is journalled on output12. Pinion30is in continuous meshing engagement with gear36, which is journalled on layshaft38. Pinion32is in continuous meshing engagement with gear40, which is journalled on layshaft38. Pinion24is in continuous meshing engagement with gear44, which is journalled on layshaft38. Pinion26is in continuous meshing engagement with gear46, which is journalled on a second layshaft48. Reverse pinion22is continuously driveably connected to an idler gear (not shown), which in turn is in continuous meshing engagement with reverse gear42, journalled on layshaft38.

A planetary gear unit50, preferably located near the output end of output12, includes a sun gear52secured to the layshaft38, a ring gear54secured to a transmission case56and held there against rotation, a set of planet pinions58in continuous meshing engagement with sun gear52and ring gear56, and a carrier60driveably connected to the second layshaft48and output12and rotatably supported the planet pinion58. When the transmission being described here is operating in the first, second and third forward gear ratios, and in reverse drive, the planetary gear unit50produces a speed reduction and torque increase at output12relative to the speed of, and torque transmitted by layshaft38. As described here, the planetary gear unit is not included in the torque delivery path for fourth, fifth, sixth and seventh forward gear ratios.

Couplers60,62,64and66are preferably synchronizers of the type used in automotive manual transmissions to connect a gear or pinion to a shaft, after synchronizing the speed of the shaft and that of the pinion or gear. Each coupler may also disconnect the shaft and the associated pinion or gear. An example of such a synchronizer is disclosed in U.S. Pat. No. 4,222,281. Alternatively, each coupler may be a toothed clutch having dogteeth that are engaged with clutch teeth on a gear or pinion. This invention may use couplers in any combination of synchronizers and dog clutches. Each coupler is secured by a hub to the output12or a layshaft38,48. For example, coupler60is secured by hub68to output12for rotation with the output.

In the case where a coupler is a synchronizer, it is provided with a conical surface, which engages mutually with a corresponding conical surface located on a gear or sprocket adjacent the synchronizer. When coupler60is engaging either of its adjacent gears34,36or sprockets82,86, these conical surfaces are forced together into frictional contact, and that frictional engagement synchronizes the speed of the gear or sprocket to that of the output12. Each synchronizer is equipped with a sleeve70supported on the hub68for sliding movement leftward and rightward into engagement with the conical surfaces and dog teeth carried on the adjacent gear and sprocket. When the dog teeth of the sleeve engage those of the gear or sprocket, the layshaft is driveably connected to the gear or sprocket.

In the case where a coupler is a dog clutch, displacement of the sleeve70in opposite axial directions causes mutual engagement of dog teeth formed on the sleeve with dog teeth carried on the gear and sprocket, such that a drive connection is made between the layshaft and the gear or sprocket, but without first synchronizing the rotational speed of the layshaft with the speed of the gear or sprocket.

InFIGS. 1 and 4, the couplers60–66are shown in a neutral position, between the left-hand and right-hand extremities of travel of the connecting sleeve, whose engagement with dog teeth carried on the gear and sprocket completes the drive connection to the output12or layshaft38,48. The hubs of couplers62,64are rotatably secured to layshaft38; the hub of coupler66is rotatably secured to layshaft48; and the hub of coupler60is secured to output12.

Coupler60, located between gears34,36and between sprockets82,86, releasably connects alternately those gears and sprockets to output12, and coupler60may be disengaged from both gears and both sprockets. Coupler62, located between gears36,40and between sprockets86,90releasably connects alternately those gears and sprockets to layshaft38, and coupler62may be disengaged from both gears and both sprockets. Coupler64, located between gears42,44and between sprockets102,94, releasably connects alternately those gears and sprockets to layshaft38, and coupler64may be disengaged from both gears and both sprockets. Coupler66, located between gears44,46and between sprockets94,98, releasably connects alternately those gears and sprockets to layshaft48, and coupler66may be disengaged from both gears and both sprockets.

Operation of the transmission will be discussed next with reference to the positional state of the coupler sleeves and the applied and released state of clutches14and18. The first forward gear ratio is produced by first moving the selector sleeve of coupler62leftward to connect gear36to layshaft38, and then engaging friction clutch18. The speed of gear36is reduced and the torque it transmits is increased relative to those of the input10. A second torque multiplication occurs in the planetary gear unit50, where sun gear52is driven by layshaft38, ring gear54provides a torque reaction, and the carrier60is the output that drives output12. The output12is driven at a torque ratio of 5.321, and the torque ratio produced by planetary gear unit50is about 4.273 using the gear and pinion sizes ofFIG. 2.

The transmission is prepared for an upshift to the second ratio from the first forward ratio by moving the sleeve of coupler64rightward to connect gear44to layshaft38. Disengaging clutch18, engaging clutch14and returning the sleeve of coupler62to the neutral position complete the upshift. These actions produce a first speed reduction due to the engagement of pinion24and gear44, such that layshaft38and sun gear52are driven at a slower speed and higher torque relative to those of input10. An additional speed reduction and torque multiplication occurs at the planetary gear unit50. The output12is driven at a torque ratio of 3.909.

The transmission is prepared for an upshift to the third ratio from the second ratio by moving the sleeve of coupler62rightward to connect gear40to layshaft38. Then disengaging clutch14, engaging clutch18and returning the sleeve of coupler64to the neutral position complete the upshift. These actions produce a first speed reduction due to the engagement of pinion32and gear40, such that layshaft38and sun gear52are driven at a slower speed and higher torque relative to those of input10. An additional speed reduction and torque multiplication occurs at the planetary gear unit50. The output12is driven at a torque ratio of 2.719.

The transmission is prepared for an upshift to the fourth speed ratio by moving the selector sleeve of coupler66rightward to driveably connect gear46to second layshaft48. Then disengaging clutch18, engaging clutch14, and returning the sleeve of coupler62to the neutral position complete the upshift. Layshaft48, carrier60, and output12are driven at a lower speed than that of input10due to the meshing engagement of pinion26and gear46. The output12is driven at a ratio of 1.813.

The transmission is prepared for an upshift to the fifth speed ratio by moving the selector sleeve of coupler60rightward to driveably connect gear36to layshaft38. Then engaging clutch18, disengaging clutch14, and returning the sleeve of coupler66to the neutral position complete the upshift. Output12is driven at a lower speed than that of input10due to the meshing engagement of pinions30and gear36. The output12is driven at a ratio of 1.245.

The transmission is prepared for an upshift to the sixth speed ratio by moving the selector sleeve of coupler66leftward to driveably connect gear44to second layshaft48. Then disengaging clutch18, engaging clutch14, and returning the sleeve of coupler60to the neutral position complete the upshift. Layshaft48, carrier60, and output12are driven at a higher speed than that of input10due to the meshing engagement of pinion24and gear44. The output12is driven at a ratio of 0.915.

The transmission is prepared for an upshift to seventh speed ratio by moving the selector sleeve of coupler60left to driveably connect gear34to output12. Then engaging clutch18, disengaging clutch14, and returning the sleeve of coupler66to the neutral position complete the upshift. Output12is driven at a higher speed than that of input10due to the meshing engagement of pinion28and gear34. The output12is driven at a ratio of 0.730.

The transmission is prepared for a shift from the first forward speed ratio to the reverse drive by disengaging clutch14moving the selector sleeve of coupler62to the neutral position and moving the selector sleeve of coupler64leftward to driveably connect reverse output gear42to layshaft38. Then clutch18is engaged. The first speed reduction occurs due to the drive connection between pinion22and gear42, which is driveably connected through coupler64and layshaft38to sun gear52. A second speed reduction is produced in the planetary gear unit such that output12is driven by carrier60at a slower speed and higher torque than those of input10. The output12is driven at a ratio of −5.298.

Because the direction of rotation of output12is opposite to the direction of input10in the arrangement ofFIG. 1, sprocket wheels70,72and74, secured to input shaft20, replace pinions28,30and32; and sprocket wheels76,78, secured to input shaft16, replace pinions76and78, in the embodiment ofFIG. 4. A chain80driveably connects sprocket70to sprocket wheel82, which journalled on output12. A chain84driveably connects sprocket wheel86, which is journalled on layshaft38, to sprocket72. A chain88driveably connects a sprocket wheel90, which is journalled on layshaft38, to sprocket wheel74. Chain92driveably connects sprocket76to sprocket wheel94, which is journalled on layshaft38. Chain96driveably connects sprocket78to sprocket wheel98, which is journalled on layshaft48. A reverse pinion100is in continuous meshing engagement with a reverse output gear102, journalled on layshaft38.

Each of the forward drive and reverse drive gears of the arrangement ofFIG. 4is produced by operating the clutches14,18and the couplers60–66in the same way as described with reference toFIG. 1.

The planetary gear unit operates with three concentric shafts on the output12axis. The inner and outer shafts12,48, respectively, are directly connected to the output. The middle shaft38is connected to the planetary sun gear52, resulting in torque multiplication at the output. By alternately connecting one of the driven gears to either the middle shaft or the inner shaft, first and fifth gears are produced. Similarly, by alternatively connecting another driven gear to either the middle shaft or the outer shaft, second and sixth gears are produced.