Automatic transmission for a motor vehicle

An automatic transmission for a motor vehicle comprises a power transmitting system divided into two gear-change units with respect to an input shaft for receiving the power from the engine, the power being transmitted to an output shaft through the two units, each of having respective counter shafts. The counter shaft of one of the units is provided thereon with a first gear stage meshed with a first drive gear fixedly supported on the input shaft. Control of the engine braking at the first-gear stage is accomplished by the engagement or disengagement of the lock up clutch.

BACKGROUND OF THE INVENTION 
The present invention relates to automatic transmission for a motor 
vehicle. 
When in first gear or when driving at low speed, releasing the accelerator 
pedal causes engine braking to act strongly and uncomfortable drive 
feeling. Many measures have been proposed to overcome this defect, as 
disclosed in Japanese Patent Laid-Open Appln. No. 75844/1985. 
When the vehicle with the conventional automatic transmission is running at 
a low speed on a steep declining road, it becomes difficult to fully 
control the vehicle speed by only the engine brake set at only low gear 
ratio. That is, much use of the brakes is made, and fade tends to occur. 
Another possible countermeasure is to couple a lock up clutch with an 
one-way clutch in the transmission. However, this increases an entire 
length of the transmission. 
As still another measure, the same applicant has previously proposed the 
automatic transmission having two parallel counter shafts. These counter 
shafts are respectively provided with first- and second-gear stages and 
third- and fourth-gear stages. Space of a reverse gear unit in this 
transmission is used for a fourth gear. Therefore engine braking may be 
selectively utilized without increasing the entire length of the 
transmission. 
In this transmission, however, these still remains the problems relating to 
strength and durability of transmission elements due to inertial force. 
These problems in the prior art will be described more fully hereinafter 
referring to drawings. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the circumstances described 
above. It is an object of the present invention to provide an automatic 
transmission characterized by the following features. Engine braking can 
be utilized through a first-gear stage. This feature can be acquired 
without increasing the entire length of the transmission. Moreover, the 
relative inertial force arising in the servomechanism at the time of 
shifting between forward and reverse can be reduced. 
This and other objects of the present invention have been achieved by a 
provision of the automatic transmission wherein driving power of an engine 
through an input shaft of the transmission is transmitted to a driving 
power transmitting system divided into two gear-changing units. From these 
two gear-changing units, transmission of the driving power to a single 
output shaft is accomplished by the system. The automatic transmission 
provides the following four elements on a counter shaft of each unit; A 
first counter gear is meshed with a first drive gear fixedly supported on 
the input shaft. A counter output gear is coupled to the first counter 
gear supported on the counter shaft of a first-gear transmission clutch, a 
lock up clutch, and a one-way clutch. A counter reverse gear is supported 
on the same counter shaft and coupled through idler gears with a reverse 
input gear fixedly supported on the output shaft. A synchromechanism is 
provided for selectively coupling the lock up clutch and the counter 
reverse gear to the counter shaft. 
Control of the engine braking at the first-gear stage is accomplished by an 
engagement or a disengagement of the lock up clutch. 
As described above, the power transmitting system of the transmission is 
divided into the two gear change units on opposite (left and right) sides 
of the input shaft. In one unit, the lock up clutch coupled to the output 
shaft for the first-gear stage and a reverse mechanism are provided. In 
the other unit, the mechanisms of the second-gear and succeeding gear 
stages are provided. By this arrangement, a structural balance is 
maintained by distribution of the gear-change stages into two sub-systems. 
For this reason, the entire length of the automatic transmission becomes 
short. Moreover, the engine braking by the first gear stage can be 
utilized when necessary. At the same time, the relative inertial force in 
a synchromechanism may be reduced when shifting between forward and 
reverse and therefore reduces an impact due to shifting. Therefore 
strength and durability are improved. 
These and other objects and features of the present invention will become 
understood from the following description with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 3, power outputted by an engine (not shown) is 
transmitted through a torque converter a to an input shaft b of the 
transmission. A counter shaft is provided parallel to the input shaft b. A 
third drive gear d is provided on the input shaft b. A third driven gear e 
is meshed with the third drive gear d. The power is transmitted from the 
third driven gear e to the counter shaft c through a third speed 
transmission clutch f. Thus, a third-gear stage is attained. At the same 
time, the power is transmitted by a second speed transmission clutch g 
provided on the input shaft b to a second drive gear h. The power is 
further transmitted by a second driven gear i meshed with the second drive 
gear h to the counter shaft c. Thus a second-gear stage is attained. 
Similarly, the power is transmitted by a fourth-gear transmission clutch j 
provided on the input shaft b to a fourth drive gear k. The power is 
further transmitted by a fourth driven gear l and a synchromechanism m to 
the counter shaft c. Thus a fourth gear stage is attained. At the same 
time, the power is transmitted by a reverse drive gear n rotating with the 
fourth drive gear k and a reverse idler gear p to a reverse driven gear q. 
The power is further transmitted by the synchromechanism m to the counter 
shaft c. Thus a reverse-gear stage is attained. 
Furthermore, the power is transmitted through a first-gear transmission 
clutch r provided on the input shaft b to a first ("low") drive gear s. 
The power is further transmitted via a first driven gear t and a one-way 
clutch u to the counter shaft c. Thus a first-gear stage is attained. 
By transmitting the power by the first driven gear t and the one-way clutch 
u to the counter shaft c as described above, an aforementioned undesirable 
engine braking action as felt by the driver at the time of the first-gear 
stage driving or the low-gear driving is prevented. Additional mechanical 
elements in FIG. 3 are a parking gear v, a parking pawl w and a 
servo-valve x for selectively changing over the synchromechanism m. 
However, even when the vehicle with the automatic transmission is running 
at low speed on a relatively steep downhill, it becomes difficult to 
sufficiently control the vehicle speed by setting the gear ratio for 
engine braking. That is, when the vehicle is being driven on a steep and 
relatively long downhill road having many sharp curves, the brakes are 
used extensively, and fade tends to occur. For this reason, one 
conceivable measure is the additional coupling of a lock up clutch to the 
one-way clutch u aforementioned. By engaging or disengaging the lock up 
clutch, the engine braking is controlled by the first-gear stage. In this 
case, the entire length of the automatic transmission will be increased 
because of the installation of this lock up clutch. 
However, this measure has a problem in such cases as the use of the above 
described automatic transmission with an engine mounted lengthwise. In 
this case, an increase in the entire length of the automatic transmission 
must be avoided because of securing narrow floor space near the driver's 
seat. 
Accordingly, the applicant has previously proposed an automatic 
transmission as disclosed in the specification of Japanese Pat. Appln. No. 
133560/1988. In this transmission, as shown in FIG. 4, two parallel 
counter shafts C and Ca are used. These counter shafts C, Ca are provided 
respectively with first- and second-gear stages and third and fourth-gear 
stages. In this transmission, the space of the reverse gear unit is used 
in the case of fourth gear stage. Therefore the functional capability of 
selectively utilizing the engine braking without lengthening the entire 
length of the automatic transmission can be added. In this transmission, 
however, the first-gear lock up clutch A installed on one of the counter 
shafts C is always matched with a clutch r for the first gear to operate 
cooperatively therewith. For this reason, the counter reverse gear Q 
(corresponding to a reverse drive gear q) is to function in combination 
with the third driven gear e and the fourth driven gear l fixed to the 
other counter shaft Ca and producing a relatively high gear ratio. Then, 
in order to bring the reverse gear ratio to a predetermined value, a 
desired gear ratio must be obtained in a mechanism between the counter 
shaft Ca and the drive shaft B to which power is transmitted from the 
counter shaft Ca. On the other hand, the shifting from drive (forward 
gears) to reverse (reverse gear) gives rise to an inertial force. In order 
to reduce this inertial force, the provision of a shifting clutch for the 
counter shaft Ca is optimal. However, this counter shaft Ca and the 
counter reverse gear Q associated therewith rotate in opposite directions 
for forward gears. For this reason, the relative rotational speed in the 
synchromechanism at the time of shifting is high. Consequently, a problem 
relating to the strength and durability of these parts occurs. 
In view of the above described circumstances the present invention seeks to 
provide an automatic transmission characterized by the following features. 
Engine braking can be effected to when necessary with the transmission in 
its first gear stage. This feature can be achieved without increasing the 
entire length of the transmission. Moreover, inertial force arising in the 
synchromechanism at the time of shifting between forward gears and reverse 
gear can be reduced. 
The above described problems encountered in the prior art regarding the 
automatic transmissions have been overcome by the present invention which 
will now be described in detail with respect to a preferred embodiment 
thereof. 
Referring to FIGS. 1 and 2, power from an engine (not shown) is transmitted 
via a torque converter 2 to an input shaft 1 of an automatic transmission. 
Parallel to the input shaft 1 and on opposite sides thereof are provided a 
first counter shaft 3 and a second counter shaft 4. The input shaft 1 
fixedly supports a first/second drive gear 7, a third drive gear 5, and a 
fourth drive gear 6. 
On the first counter shaft 3 is fixedly supported a second counter gear 8 
meshed with the first/second drive gear 7. A third counter gear 9 meshed 
with the third drive gear 5 is rotatably supported on the first counter 
shaft 3. The third counter gear 9 is coupled by a coaxial third-gear 
transmission clutch 10 (a hydraulic multi-plate clutch) to a coaxial 
counter output gear 21 to transmit the power thereto. Furthermore, a 
fourth counter gear 11 meshed with the fourth drive gear 6 is rotatably 
and coaxially supported on the first counter shaft 3. The fourth counter 
gear 11 is coupled by a coaxial fourth-gear transmission clutch 12 (the 
hydraulic multi-plate clutch) to the counter output gear 21 to transmit 
motive power thereto. In addition, a second-gear transmission clutch 20 
(the hydraulic multi-plate clutch) is disposed coaxially on one side of 
the output gear 21. Through the clutch 20, the output gear 21 and the 
first counter shaft 3 are selectively coupled and uncoupled. 
On the second counter shaft 4, a first counter gear 13 meshed with the 
first/second drive gear 7 is rotatably supported. The first counter gear 
13 is coupled by a first-gear transmission clutch 14 (the hydraulic 
multi-plate clutch) to the second counter shaft 4 to transmit the power 
thereto. 
A synchromechanism 22 is further disposed coaxially on the second counter 
shaft 4. On one side of the synchromechanism 22, a coaxial counter output 
gear 15 is coupled thereto by a one-way clutch 16 and a lock up clutch 17 
(the hydraulic multi-plate clutch). When a selector 22a of the 
synchromechanism 22 is selectively shifted to the side of the counter 
output gear 15, the power is transmitted from the second counter shaft 4 
through either of the one-way clutch 16 or the lock up clutch 17 to the 
counter output gear 15. 
At a position lower than the input shaft 1 and parallel to the first and 
second counter shafts 3 and 4 is disposed an output shaft 18. The output 
shaft 18 transmits the power through a differential mechanism and a final 
speed-reduction device (both not shown) to at least one driving axle of 
the vehicle. A drive input gear 19 meshed with the counter output gears 15 
and 21 is fixedly supported on the output shaft 18. 
On the second counter shaft 4, on the other side of the synchromechanism 
22, a counter reverse gear 23 is rotatably supported. When the selector 
22a of the synchromechanism 22 is selectively shifted to the side of the 
counter reverse gear 23, the power is transmitted from the second counter 
shaft 4, through the counter reverse gear 23, first and second idler gears 
24a and 24b fixedly supported on an idler shaft 24, and a reverse input 
gear 25 fixedly supported on the output shaft 18, to this output shaft 18. 
The synchromechanism 22 is actuated by a servo-valve 26. 
The automatic transmission of the mechanical type described above according 
to the present invention operates in the following manner. 
The power entering into the transmission via the torque converter 2 drives 
the input shaft 1. The power thus transmitted to the input shaft 1 is 
transmitted further through the drive gears 5, 6, and 7 fixed to the input 
shaft 1 to drive the driven gears 8, 9, 11, and 13 provided on the first 
and second counter shafts 3 and 4. 
First, when starting, the vehicle the first-gear transmission clutch 14 is 
engaged. Then, as the engine speed increases, the transmitted torque 
increases. Thus, the power being transmitted to the first counter gear 13 
through the torque converter 2, the input shaft 1 and the first/second 
drive gear 7 fixedly supported on this input shaft 1 is transmitted to the 
second counter shaft 4. Here, the power is transmitted through the 
synchromechanism 22 and the one-way clutch 16 to drive the counter output 
gear 15. The power is further transmitted through the drive input gear 19 
meshed with this counter output gear 15 and thus through the output shaft 
18 to the driving axle (not shown) of the vehicle. The vehicle is thus 
driven forward by the first gear. 
In this case, if an accelerator pedal is released, rotational speed of the 
second counter shaft 4 will decrease. Then the one-way clutch 16 will 
operate to make a free-wheeling rotation. Then, even if the rotational 
speed of the counter output gear 15 is relatively higher than that of the 
second counter shaft 4, an engine braking will not be transmitted to the 
output shaft 18. 
When, under these operational conditions, an application of the engine 
braking is desired, the lock up clutch 17 is engaged. As a consequence, 
the second counter shaft 4 and the counter output gear 15 are coupled via 
the synchromechanism 22. Thus the engine braking with the transmission by 
the first gear stage is realized. 
In shifting between forward and reverse, the transmission operation passes 
through neutral. For this reason, the first-gear transmission clutch 14 is 
temporarily released. The second counter shaft 4 is thereby in a free 
state relative to the engine and then, hydraulic pressure is applied to 
the servo-valve 26. The synchromechanism 22 thereby operates to couple the 
second counter shaft 4 and the counter reverse gear 23. In this case, 
shifting is performed only by the second counter shaft 4 and the 
first-gear transmission clutch 14. Therefore the inertial force is small. 
The impact force occurring during shifting can thereby be reduced. 
The states of engagement of the gear-change elements at each of the 
gear-change stages of the above described automatic transmission are 
indicated in the following Tabled 1. In Table 1, a symbol "O" indicates 
engagement, and a symbol .DELTA. indicates that engagement is made only 
when necessary at the time of the engine braking. 
TABLE 
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Engagem't Element 
Clutch 14 
Clutch 20 
Clutch 10 
Clutch 12 
gear 
for 1st- 
for 2nd- 
for 3rd- 
for 4th- 
Lock 
One-way 
change 
gear speed 
gear speed 
gear speed 
gear speed 
clutch 
clutch 
stage 
change 
change 
change 
change 
17 16 
__________________________________________________________________________ 
1st O .DELTA. 
O 
2nd O 
3rd O 
4th O 
Rev. 
O 
__________________________________________________________________________ 
The automatic transmission described above and operation according to the 
present invention have the following features of merit. 
The power from the engine through the input shaft is divided into two 
systems. The first counter gear is supported on the counter shaft of one 
of these systems. To the first counter gear, the counter output gear is 
coupled via the first-gear transmission clutch, the lock up clutch, and 
the one-way clutch. At the same time, the lock up clutch and the counter 
reverse gear can be selectively coupled to the counter shaft by the 
synchromechanism. Therefore, without increasing the size of the automatic 
transmission, unpleasant feeling of the engine braking when the 
accelerator pedal is released during driving in the first gear can be 
prevented. Furthermore, during descent on the steep downhill road, a 
desired engine braking action can be obtained by engaging the lock up 
clutch. In addition, the load imposed by inertial force with respect to 
the synchromechanism at the time of shifting between forward and reverse 
can be reduced. Thus the automatic transmission of the present invention 
is advantageous with respect to both strength and endurance. 
While the presently preferred embodiment of the present invention have been 
shown and described, it is to be understood that these disclosures are for 
the purpose of illustration and that various changes and modifications may 
be made without departing from the scope of the invention as set forth in 
the appended claims.