Page 121 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Transmissions and driveline CHAPTER 5.1

automated the clutch so that no pedal was required. At these components can be traced back to the early days of
start-up, they operate as an automatic with the control automotive developments, and in a recognizable combi-
system actuating the clutch to achieve a start from rest nation to the middle of the last century. Yet, it is an area
when the accelerator pedal is depressed. During gear that is still seeing extremely rapid development today.
changes, the clutch is operated in response to movement The success of this combination lies in the simplicity of
of the gear lever. the torque converter as a device that inherently has ideal
Consideration of the mechanics of the automated characteristics to start a vehicle from rest, and the op-
manual systems suggests that it may be difficult for these portunity that epicyclic gear sets provide to give rela-
systems to replace the conventional automatic. The fun- tively easy and controllable changes between ratios.
damental point is that the automated manual systems The controllability of these devices has allowed au-
need to disconnect the drive from the engine to the tomatics to be developed with the good shift quality
transmission in order to achieve a gear change. With necessary to satisfy the driver’s expectations for a gear
conventional automatics only a small reduction in the change. Somehow, drivers of conventional manual-shift
engine power is required to achieve a smooth transition vehicles are always more critical in judging the gear
form one gear to another because of the action of the change of another driver rather than their own where
torque converter. There are, however, twin-clutch designs a misjudged shift can be more easily forgiven. In just the
of transmission, which overcome this limitation by pro- same way, they are more discerning in judging the quality
viding two parallel torque paths through the transmission of an automated gear change and, thus, high standards are
where a gearchange simply switches from one path to required. In the past, these have been virtually impossi-
another and engages one clutch rather than the other. This ble to achieve from automated manual gearboxes. This
can be done without reducing the engine output (a ‘hot situation is, however, changing with the greater use and
shift’). This has been used in the past by large automotive sophistication of electronic controls.
gearboxes, but could be extended to the car market. The downside of an AT in comparison with a manual
In the commercial market, there are a number of gearbox alternative is greater cost, greater weight, larger
manufacturers now producing automated manual trans- size and lower efficiency. It has thus been used most in
missions for trucks. Whereas these developments have larger cars where these penalties are less significant and
needed the driver to indicate the gear selection in the the driveability advantages most appreciated. This may
past, the latest developments have the intelligence to well account for the large proportion of ATs used in the
completely automate the gearchange. In heavy com- USA (approaching 90%) in comparison with Europe
mercial vehicles, this may need to include missing some (around 20%). However, all these disadvantages have
gears, especially when unladen, so the control software acted to maintain the pressure for development of the
required is not trivial. AT leading to modern designs that achieve a greater
number of gear ratios within the same or even a reduced
space envelope.
5.1.4 The AT

The concept of an AT offers considerable advantages to 5.1.4.1 The jatco JF506E –
vehicle drivers since they can be relieved of the burden a state-of-the-art transmission
of selecting the right gear ratio. This burden, both
mental and physical, has become more significant with An example of today’s typical five-speed automatic
increasing traffic congestion. suitable for a medium to large size front-wheel-drive
Any reduction in driver fatigue and increased opportu- vehicle, where packaging and space constraints are
nity for the driver to concentrate on other aspects of severe, is shown as a sectioned view in Fig. 5.1-16. The
vehicle control must contribute to increased safety and input shaft drives first into a three-element torque con-
a reduction in road traffic accidents. There are also verter (top right) directly to a pair of epicyclic gears that
benefits in terms of economy and emissions if an auto- give a four-speed change section. A further fifth-speed
mated system can make a better selection of ratio than change is obtained on the secondary shaft with a final
a non-expert driver does. There are several alternative drive ratio providing the connection to the differential.
solutions to achieve this automation including auto- The output drive shafts can then be taken either side of
mated layshaft transmissions (described above), CVT the differential shown at the bottom of the section. This
(described in the next section) and the ‘conventional’ layout can also be easily extended to give a four-wheel
AT described here. drive output with an additional gear section. The main
The term ‘automatic transmission’ is used to refer to components that make up this transmission are described
a combination of torque converter with a ratio change in more detail below and the combined operation in
section that is based on epicyclic gearsets. The use of Section 5.1.4.4.

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