Page 130 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 5 .1 Transmissions and driveline

some hysteresis on this action so that, once engaged, it in terms of driving comfort as the more conventional ATs
does not drop out too quickly. The lock-up clutch is ac- that were described above. In addition, they have the
tivated by a pressure-modulating type of solenoid to give ability to vary the engine speed independently of vehicle
a smooth and steady transition between states. The lock- speed. This brings the opportunity to optimize the engine
up feature is normally only available in 4th and 5th gears operating point for any required output conditions and
but can be extended to both 2nd and 3rd gears if a high can offer either best economy or best power for acceler-
fluid temperature is detected, since this will eliminate ation. Fig. 5.1-26 shows a typical petrol engine torque/
any heat input via the torque converter losses. speed map with contours of brake specific fuel con-
The ATCU intelligence can be used in a similar way sumption, and lines of constant power superimposed.
for the provision of this additional lock-up action. Mon- Selection of the minimum fuel consumption for succes-
itoring of system responses allows detection of other sively increasing output power gives the ‘economy line’
failure states, and this can lead to action to protect the that is also shown. This indicates the most efficient point
transmission, but still allow the vehicle to be driven, even of engine operation for any output conditions and a CVTis
with a reduced performance as described in Section required in order to enable these engine conditions to be
5.1.4.5. There are also low-temperature conditions that matched to the vehicle output speed. A study of power
can be detected from the oil temperature sensor and levels used in typical vehicle operation indicates that the
actions taken to try to encourage a quicker warm-up, majority of time only low powers are required and that
both for the engine and transmission. For example, 5th operation will take place predominantly along the low
gear may be inhibited at very low temperatures and the speed section of the economy line. This also indicates the
normal torque converter lock-up inhibited even at need for a wide ratio spread in order to provide a good
moderate temperatures. These features are all designed overdrive ratio that allows the low engine speeds to
to enhance the overall system performance as well as be used.
refining the contribution mode by the transmission There are, in addition, performance benefits that can be
itself. obtained using a CVT since for ultimate acceleration the
engine can be operated at its maximum power when
5.1.5 Continuously variable required. In a vehicle with a discrete ratio transmission,
the engine can only develop maximum power once for any
transmissions ratio and there will be an interruption in power trans-
mitted when the ratios are changed. Porsche showed
The overwhelming majority of transmissions in road (Kraxner et al., 1999) that a CVT vehicle would out ac-
going vehicles is either manual or conventional automatic celerate the same vehicle with a conventional manual
in design. These transmissions use meshing gears that transmission. It was further emphasized that it would only
give discrete ratio steps between engine and the vehicle be the skilled drivers who could achieve the best from
speed. However, alternative designs exist that can a manual transmission anyway.
transmit power and simultaneously give a stepless change Such transmissions have been under development
of ratio; in other words a CVT. Strictly speaking, a CVT is since the early days of automobiles, but have only re-
a transmission that will allow an input to output ratio to cently begun to establish a place in the commercial
change, continuously without any steps, in a range be- market. Why is this, if there are potential benefits in
tween two finite limits. An extension of this idea is terms of driving comfort, economy and performance?
a transmission that also allows a zero output speed to be Different factors have contributed to this unpopularity at
included within this operating range. This can be con- various times, and have included reliability, weight and
sidered as an IVT*. Since all vehicles need to come to cost. However, the main downside is the lower efficiency
rest then, any transmission that inherently has an IVT of all CVTs relative to geared transmissions. Losses in
characteristic is at an advantage. The use of the term geared systems occur as a frictional torque required to
CVT is used here in the more generic sense to include rotate the transmission components. If an ideal speed
both types. ratio is defined for a gear pair then the output torque will
be less than the ideal torque as a result of these losses,
5.1.5.1 The rationale for the CVT but the output speed will be the same as the ideal speed.
However, all CVTs also incur some speed ‘slip’ between
Transmissions that vary ratio continuously can be con- input and output as well as torque losses, and both output
trolled automatically, and hence have the same advantages torque and speed will be less than the ideal. Since gears

* The idea of ‘infinitely’ can be easily accommodated when it is appreciated that trasmission ratios are usually exressed in the form of input to
output as a SPEED ratio, and that an infinite speed ratio is obtained when the output speed is zero.

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