09-08-2012, 01:11 PM
Automatic Gearbox – VT1F
Automatic Gearbox.pdf (Size: 1.41 MB / Downloads: 119)
Introduction
The stepless shifting pattern of the transmission provides a very comfortable drive, as well as
having full vehicle performance, available at any time.
The advantages of using an automatic transmission of this type are:
- Low engine revolutions at constant speeds.
- Improved emission control/fuel consumption.
- Low noise, vibration and harshness levels.
- Smooth acceleration.
- Flexible driving on mountain roads.
Conventional Automatic Transmission vs. Continuously Variable Transmission.
In the figure below the variograms of a hand shifted or normal automatic transmission and the
CVT are compared with each other. With the conventional automatic transmission, maximum
6 ratio’s (gears) are available, but mostly even less. When the transmission upshifts, it has to
follow the bold or dotted lines of the first image depending of the amount of throttle. With the
CVT, the whole range of ratios between the Low and the Overdrive lines is available for
shifting as shown in the second image.
The shift point of both transmissions is related to the amount of throttle. As more throttle is
applied, the transmission upshifts at a higher engine speed. With the conventional
transmission it is clear that the engine speed drops back when shifting to a higher gear. This is
not the case with the CVT. The CVT will upshift at a constant engine speed, by moving the
pulleys as explained in the next part. However, alternative shifting strategies are also possible.
This can help to create a quicker acceptance by new CVT drivers.
Shift Positions.
Possible shift positions for the VT1F are Park, Reverse, Neutral, Drive and Sport/Manual
position (other shifter lay outs are also possible of course, for instance with an L position).
In the Drive position, the car will move forwards in standard CVT mode. The engine speed
will be limited to approximately 4500 rpm unless kick down is used by fully depressing the
throttle pedal when the engine speed will rise to approximately 5500 rpm. Of course other
calibrations are also possible.
In the Manual/Sport position, the car will move forwards in CVT sport mode. The engine
speed will be limited to approximately 5000 rpm unless kick down is used by fully depressing
the throttle pedal when the engine speed will rise to approximately 5500 rpm. Again, other
calibrations are also possible.
Functionality in Park and Neutral
The engine can only be started in Neutral or Park, as with any automatic transmission.
A spring and cone operated pawl mechanically locks the secondary pulley when the selector
lever is moved to the Park position. If the lever is set in the Park position when the vehicle
has a higher speed, the pawl will rattle without engaging Park. It will not engage until the
vehicle speed drops below approximately 5 km/h.
Normal Operation
The operation of the transmission, during driving, has no comparison with that of a
conventional automatic transmission. All kinds of tunings are possible. A possible tuning is
that if the accelerator pedal is depressed sharply, the engine rpm will rise considerably more
than in relation to the speed of the vehicle. This functionality is normal for a CVT but may
surprise drivers not familiar with this type of transmission. Other tunings / variograms are
also possible, for example tunings that are closer to that of a conventional automatic
transmission.
Transmission Communication
The VT1F has electronic components to control the gear ratios, the clamping force on the belt
and the clutch pressure. The location of the components that form the steptronic transmission
vary depending upon vehicle installation.
There are different options for the position of electronic components in the transmission. For
example, the control methods associated with the transmission can be run as part of the
software in the ECU. The CVT software in the ECU receives inputs from the main sensors of
this system, communicates with the gearbox interface unit (GIU) to control the transmission,
accepts driver inputs and provides information to the driver via the instrument cluster. With
this example the software is located in the ECU, while the drivers are in the Gearbox Interface
Unit (GIU). It is also possible to put both the software and the drivers in a TCU.
Towing of the Vehicle
The VT1F transmission must not be towed. This is because there is no oil pressure in the
primary pulley unless the engine is running and belt slip would therefore occur. Recovery
must be conducted with the front wheels lifted clear of the ground (suspended tow).
With a manual transmission it still is possible to start the car by pushing or towing. This is not
possible with the CVT because there will be no oil pressure (as the engine and the pump are
not running) meaning that both clutches are disengaged and that there is no connection
between the engine and wheels.