Page 132 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 5 .1 Transmissions and driveline

giving a zero output speed and hence give IVToperation. allows the belt-rolling radius to be changed such that
The hydrokinetic drives have been described above in a relatively low output speed is obtained when its radius
Section 5.1.4, and, as discussed above, torque converters at the output is large. The output speed can be increased as
are widely used in road going vehicles of all types. this radius is made smaller and the rolling radius on the
Hydrostatic drives also rely on fluid flow to transmit input is increased. As in a conventional V-belt, there is
power but it is the pressure level in the fluid that is sig- a frictional force between the pulley sheaves and the
nificant rather than the flow velocity. A hydrostatic angled belt face that provides the transfer of tractive
transmission comprises a pump unit supplying a motor effort. There must be sufficient normal force between
unit, and both these are of the, so-called, positive dis- themtopreventgross slippingandtheclamping forcesthat
placement type. One at least must be variable capacity, hold the pulley sheaves together also produce a tension
usually this is the pump, and this is used to control the load in the belt. The relative magnitude of these clamping
overall ratio. Hydrostatic drives are very widely used in forces can then also be used to control the belt position and
agricultural and other off-road vehicles but have never overall ratio.
been commercially used in automotive applications. It is Early proposals, in the 1920s (Gott, 1991) relied on
generally the efficiency at low powers and potentially high mechanical linkages to provide the movement of the
noise levels that are the weak points of hydrostatic pulley sheaves and maintain their relative position and
transmissions. However, they are still under development pre-loading. The first commercially produced vehicle
and, even recently, proposals have been made for buses transmission (1958) was the DAF Variomatic and here
and delivery vehicles with energy storage and hence the load was provided by pre-loaded springs in the
a hybrid capability (Ifield website). pulleys, and the ratio change by a centrifugal effect
working against these springs. The belt was fabric
reinforced rubber, and two belts were used in parallel
5.1.5.3 Variable pulley variator designs each driving a different wheel. This design was limited
in its torque capacity to only smaller vehicles, and both
The idea of a variable pulley system is a logical extension of wear and efficiency limitations restricted its broader
a conventional V-belt fixed ratio drive. Fig. 5.1-27 shows application. Simple rubber belt designs have continued
the principle, where there are two pairs of conical pulley to evolve and are still widely used in two-wheel and
sheaves and a fixed length V-belt. One half of each pulley some small city vehicle applications. Most modern
pair is moveable and their movement is synchronized. This variable pulley systems use steel belts to provide the
power capability needed for automotive applications in
a suitably compact package. The design and manufac-
ture of the metal belts provides the heart of modern
Left Right systems. The belt must have sufficient strength and
pulley pulley rigidity to transmit the driving loads and yet also be
sheave sheave
fixed moved in flexible to keep the minimum rolling radius as small as
possible. This has been achieved with two designs that
Input
pulley also have significantly higher efficiency than the earlier
rubber versions.
The first metal belt system was introduced into the
market place in 1987 as the Transmatic, by Van Doorne’s
Transmissie (VDT) of the Netherlands. This trans-
mission is the successor to the Variomatic but shares
little more than the variable conical pulley concept with
its predecessor. This is still the only automotive CVT in
quantity production despite the recent launch of vehicles
with both an alternative belt design (Audi) and a traction
Output drive (Nissan). Over 3 million VDT units have been
pulley
supplied for car use and current production of the belt is
Right Left
pulley pulley 500 000 units per year. Ford, Fiat, Honda, Mitsubishi,
sheave sheave Nissan, Rover, Subaru, Toyota and Volvo have used it.
fixed moved Recent vehicles to use the this system have engine ca-
out
pacities in the 2-L range, representing a significant ad-
Low output High output vance on the early small and super-mini (sub-compact)
speed speed
use. The capability for operating in extremes was shown
Fig. 5.1-27 Variable pulley drive concept. by the successful demonstration of a version fitted in

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