Transmission with overdrive

A motor vehicle transmission includes a torque converter and planetary gear arrangement. Clutches are provided for connecting the torque converter with the gear arrangement. The clutches are arranged so that the direct drive transmission gear can be engaged either through the torque converter or partially bypassing the torque converter.

BACKGROUND OF THE INVENTION 
This invention relates to hydrodynamic-mechanical transmissions for motor 
vehicles wherein there is provided a torque converter which is selectively 
connected to a planetary gear arrangement with hydraulically actuable 
clutch members to provide the various transmission speeds. 
In particular, the invention relates to such transmissions wherein the 
planetary gear arrangement includes a first sun gear which is connectable 
to the torque converter output turbine rotor by a first clutch and a 
second sun gear connectable to the turbine rotor by a second clutch. A 
planetary gear carrier is connectable by a third clutch to the input pump 
rotor of the torque converter. Mounted on the planetary gear carrier is a 
first planetary gear connected to the second sun gear and a ring gear. A 
second planetary gear is mounted on the same planetary carrier and 
connected between the first sun gear and the first planetary gear. 
A transmission of this general type is described in published German Patent 
Application No. 1,625,124. In the transmission described in that 
reference, the first forward speed is obtained by actuating the first 
clutch to engage the first sun gear to the torque converter output turbine 
rotor and holding the planetary carrier using a one way clutch or brake. 
Reverse speed is similarly obtained by locking the planetary carrier in 
position with a brake and engaging the second clutch to connect the second 
sun gear to the output rotor of the torque converter. Second gear is 
obtained by actuating the first clutch and locking a second brake which 
fixes the second sun gear in position. In order to obtain a direct drive 
for third gear, the first and third clutches are engaged which locks the 
planetary gear system into a single rotating set of gears. A fourth gear 
may be obtained by engaging the third clutch and the second brake, so that 
power is provided directly from the torque converter input to the 
planetary carrier which rotates around the locked second sun gear. 
When the third clutch is engaged, there is provided a direct mechanical 
connection between the engine and the planetary gear system bypassing the 
torque converter, which usually operates with some slippage and consequent 
inefficiency. The planetary gear carrier is directly connected to the 
input pump rotor which is mounted on the engine drive shaft. Thus, in the 
fourth speed, or overdrive, the torque converter is completely bypassed. 
Against that, in third speed, there takes place a power branching because 
part of the power is transmitted to the planetary gear through the torque 
converter over the first clutch and the remaining power over the third 
clutch bypassing the torque converter. 
While elimination of the torque converter from the transmission drive train 
eliminates losses on account of slippage and increases overall efficiency, 
the direct mechanical connection of the drive shaft to the engine causes a 
direct transmission of engine torque irregularities to the drive wheels of 
the vehicle. Especially at low engine and vehicle speeds, the engine 
torque irregularities can cause a perceptible impairment of riding 
comfort. 
It is an object of the present invention to provide a hydrodynamic 
mechanical transmission of the known type, but having improved riding 
comfort, particularly at the lower speed range of third gear. 
SUMMARY OF THE INVENTION 
The invention is an improvement in a motor vehicle transmission having a 
torque converter and a planetary gear arrangement, the torque converter 
including a pump rotor connected to a transmission input shaft and a 
turbine rotor. The gear arrangement includes a first sun gear connectable 
to the turbine rotor by a first clutch, a second sun gear connectable to 
the turbine rotor by a second clutch and a planetary carrier connectable 
to the pump rotor by a third clutch, bypassing the torque converter. A 
first planetary gear is mounted on the carrier and engages the second sun 
gear. A second planetary gear is mounted on the carrier and engages the 
first sun gear and the first planetary gear. An ring gear is connected to 
an output shaft and engages the first planetary gear. In accordance with 
the invention, the gears and clutches are arranged to provide direct drive 
transmission of torque by simultaneous engagement of the first and second 
clutches, or the first and third clutches, or the first, second, and third 
clutches, the torque bypassing the torque converter when the third clutch 
is engaged. 
In a preferred embodiment, there are provided control means responsive to 
vehicle or engine speed for engaging the first and second clutches at 
lower direct drive speeds and for engaging the third clutch, in addition 
to or in place of the second clutch, when a predetermined speed is 
exceeded. A one way clutch is advantageously provided between the third 
clutch and the planetary gear carrier. 
In accordance with the invention, when the first and second clutches are 
engaged, in the direct drive speed, engine torque is transmitted to the 
output shaft by way of the torque converter and the planetary gear 
arrangement. As a result, the torque converter will cushion irregularities 
in the torque provided to the vehicle wheels. This operating arrangement 
is preferable for lower speeds in the direct drive gear. When the third 
clutch is engaged, in addition to or instead of the second clutch, power 
is transmitted partially past the torque converter by a straight 
mechanical connection of the input shaft to the planetary gear carrier. 
This arrangement is more efficient and preferable at higher engine speed 
where there is unlikely to be any irregularity in engine output torque. A 
transmission wherein the first and second clutches are engaged during the 
lower range of engine or vehicle speeds of the direct drive gear can 
provide improved passenger comfort. Higher efficiency can be obtained by 
engaging the third clutch when a predetermined speed, for example 50 or 60 
km/hr, is exceeded. Thus, operation of the transmission using the torque 
converter for the lower speed range, and partially bypassing the torque 
converter in the higher speed range improves riding comfort in the lower 
speed range and increases fuel efficiency in the higher speed range. 
The arrangement of the invention can also provide a one way clutch, which 
releases itself in the drag mode, arranged between the third clutch and 
the planetary carrier. This one way clutch provides additional fuel 
savings, since in fourth speed the connection between the engine shaft and 
the wheels is interrupted when the engine is not driving the wheels, so 
that the engine can run at an idle speed rather than at a higher 
rotational speed which results from a direct connection to the wheels. In 
order to provide the coasting mode in the overdrive speed, it is necessary 
to keep the second clutch engaged for all third speed operation so that 
disengagement of the one way clutch does not result in uncontrolled gear 
oscillations in the planetary gear system when the vehicle operates in the 
drag mode at the higher speed range of third gear. 
For a better understanding of the present invention, together with other 
and further objects, reference is made to the following description, taken 
in conjunction with the accompanying drawing, and its scope will be 
pointed out in the appended claims.

DESCRIPTION OF THE INVENTION 
The transmission shown in the drawing includes a hydraulic torque converter 
1, and a planetary gear arrangement 2. The torque converter has an input 
pump rotor 4 which is directly connected to the engine shaft 3. The 
turbine output rotor 5 is connected to a hollow turbine shaft 8 which is 
connected to the planetary gear arrangement by clutches 9 and 10 as will 
be described. The torque converter also has a stator 6 which is connected 
to the housing 25 by a one way clutch 7. 
The planetary gear arrangement 2 includes a first sun gear 11 which is 
connected to output turbine shaft 8 by a first clutch 9 and a second sun 
gear 12 which is connected to shaft 8 by clutch 10. A first planetary gear 
13 is connected between the second sun gear 12 and ring gear 15. Ring gear 
15 is connected to the output drive shaft 16 and is also provided with 
outer gear teeth 17 for engaging a parking lock, which is not shown. 
A second planetary gear 14 is arranged in connection with the first sun 
gear and the first planetary gear 13. Planetary gears 13 and 14 are 
mounted on a planetary gear carrier 18. Carrier 18 is connected to the 
transmission housing 25 by a one way clutch 19 and connected by a central 
shaft 20 and clutch 21 directly to pump rotor 4 of torque converter 1, and 
consequently to engine shaft 3. Between the planetary carrier 18 and third 
clutch 21, there may be provided a one way clutch 24 which acts as a 
coaster, establishing a power transmitting connection only when the engine 
shaft is providing torque to the vehicle wheels, and disengaging when the 
vehicle wheels are supplying power to the engine shaft. First and second 
brakes 22 and 23, which are in the form of band brakes, are provided for 
acting on an arm 26 of carrier 18 and on a part 27 surrounding clutch 10, 
and connected to the second sun gear. 
By actuation of the control members comprising clutches and brakes which 
are provided in the illustrated transmission, it is possible to obtain 
four forward speeds and one reverse speed. The settings of the control 
elements for each speed are as follows: 
______________________________________ 
CONTROL ELEMENT 
9 10 21 22 23 19 24 
______________________________________ 
Speed 1 X (X) X 
2 X X 
3a X X 
3b X (X) X X 
4 X X X 
R X X (X) 
______________________________________ 
As noted in the Table, first gear is obtained by actuation of clutch 9 
which connects sun gear 11 to the turbine rotor of torque converter 1. One 
way clutch 19 prevents rotation of planetary gear carrier 18. Alternately, 
brake 22 may be used to prevent rotation of gear carrier 18. With the 
planetary gear carrier 18 held by the one way clutch 18, torque is 
transmitted from sun gear 11 through planetary gears 14 and 13 to ring 
gear 15 and thence to drive shaft 16. Additional setting of brake 22 
provides a bidirectional lock of planetary gear carrier 18 and therefore 
enables transmission of torque in both directions permitting engine 
braking. 
To obtain second gear, brake 22 is released and brake 23, which is 
connected to sun gear 12 is connected. One way clutch 19 is also released 
permitting rotation of planetary gear carrier 18 around the fixed sun gear 
12 in response to torque provided to sun gear 11. 
Two control methods are available for providing a third or direct drive 
gear. Clutches 9 and 10 can both be engaged, with brakes 22 and 23 
disengaged, so that both sun gears and consequently the entire planetary 
gear arrangement, including output ring gear 15, rotate at the same 
angular velocity as the drive shaft 8 which is connected to turbine rotor 
5 of the torque converter 1. An alternate way of connecting the direct 
drive is to engage third clutch 21 to connected planetary carrier 18 
directly to engine shaft 3, bypassing torque converter 1. First clutch 9 
must also be engaged for this mode of operation, and second clutch 10 can 
be engaged or disengaged. When the first, second, and third clutches are 
all engaged, there exist two branches of the power train, a first branch 
through the torque converter by shaft 8 and clutches 9 and 10, and a 
second branch bypassing the torque converter and connected by central 
shaft 20 to the planetary carrier 18. If one way clutch 24 is provided for 
disengaging the connection of shaft 20 to clutch 21 when the vehicle is 
coasting in third gear, it is necessary to leave clutch 10 engaged when 
clutch 21 is engaged to prevent uncontrolled oscillations of the planetary 
gear system when the vehicle is coasting. 
A fourth or overdrive speed is provided by releasing the first and second 
clutches 9 and 10, and engaging the third clutch 21 and the brake 23 which 
locks the second sun gear 12 in position. Torque transmission in this gear 
bypasses torque converter 1 by clutch 21 and one way clutch 24 and 
proceeds along the central shaft 20 to the planetary gear carrier 18. 
Since sun gear 12 is fixed in angular position, the rotational speed of 
shaft 20 is stepped up on ring gear 15, and output shaft 16 rotates at a 
higher speed than engine shaft 3. In the drag mode of fourth speed, one 
way clutch 24 disengages and permits the engine to run at idle speed while 
the vehicle coasts. 
Reverse gear is obtained by engaging second clutch 10 and first brake 22, 
fixing the planetary gear carrier and causing reverse rotation of first 
planetary gear 13 and output ring gear 15. 
In accordance with the invention, the third or direct drive speed of the 
transmission is obtained by engaging first and second clutches 9 and 10 
for the lower speed range of third gear. At a predetermined intermediate 
speed, for example, 50 or 60 km/hr, third clutch 21 is additionally 
engaged. Operation in the low speed range therefore provides power 
transmission to the drive shaft by way of the torque converter, which 
tends to smooth out irregularities in the output torque of the engine. In 
the higher speed ranges of third gear, after engagement of clutch 21, the 
torque converter is partially bypassed resulting in a higher operating 
efficiency for the transmission. Second clutch 10 may be released when 
clutch 21 is engaged unless one way clutch 24 is provided between clutch 
21 and planetary gear carrier 18, in which case clutch 10 should be kept 
engaged to prevent uncontrolled gear oscillations in the drag mode of 
third gear. Fourth speed is operated as an overdrive and completely 
bypasses torque converter 6 by clutch 21. One way clutch 24 enables free 
wheel coasting of the vehicle to increase efficiency in overdrive. 
While there has been described what is believed to be the preferred 
embodiment of the invention, those skilled in the art will recognize that 
other and further modifications may be made thereto without departing from 
the spirit of the invention, and it is intended to claim all such 
embodiments as wall within the true scope of the invention.