Automatic transmission 3
The story of the automatic transmission does not end with the two transmissions in the previous chapters. All of these have the disadvantage of being stepped in principle, the automatic converter of course only when the
lock-up clutch is engaged.
The diagram above shows the so-called tensile force hyperbola. This is the curve that is based on the five tensile force curves for the individual gears.It shows the tensile force that would be possible if the number of gears
increased to infinity. You can see in the light gray areas what is wasted by the stepping of a transmission.
Quite apart from the fact that switching to a higher gear is rarely just done, with an automatic transmission it is at least accompanied by taking off the accelerator. So it's not just the gray areas that cost efficiency, but also the
actions surrounding the gas change.
| You can no longer determine the current speed based on the engine speed. |
A large number of Asians, for example, with their more leisurely driving style due to greater speed restrictions, have stuck with the continuously variable automatic transmission. The engine works for much longer in a lower
speed range, which obviously irritates drivers who are used to changing gears. They also align their activities much more closely with the engine noise when driving.
kfz-tech.de/PME90
Here is the basic principle of the continuously variable automatic system. Both the driving and the driven wheel consist of two conical belt pulleys which can be displaced axially in relation to each other. Their effective
diameter is large closer together and small further apart. A pair of belt pulleys provides a distance, with the other, a spring is enough to compress it.
kfz-tech.de/PME91
It probably started with this Variomatic from DAF, in which at the bottom the shaft of a centrifugal clutch arrives at the crankshaft and its torque is distributed to the two sides. The controlled, transmitted negative pressure of
the engine acts on a spring-loaded membrane on the left and right, which determines the distance between the belt pulleys and thus the effective diameter.
Springs act on the two halves of the bevel gears further back. There is no compensation; it is achieved by slip on the belts, which actually results in a certain locking effect. In the housing at the bottom center there is a ring
gear on the left and right for both belt pulleys. The entire unit can be displaced from the outside using a lever so that one is effective for forward travel or the other for reverse travel.
| The DAF was as fast forward as it was backwards. The Dutch used this for special races in
scrapyards./td> |
kfz-tech.de/PME92
As an example of a more modern application of the continuously variable transmission, we choose the Multitronic from Audi. The belt has long since given way to a metallic thrust link belt. Its crossbars result in significantly
more friction, especially since the pressure on the conical belt pulleys can also be changed by various factors such as the torque to be transmitted.
kfz-tech.de/PME93
If this is large, the pressure reduces the efficiency. This means that much more is possible than the maximum 100 Nm that can be achieved with DAF. After protests, however, they decided to adjust artificial gear ratios,
which completely contradicts the nature of a continuously variable automatic transmission and probably marked the end of development.
kfz-tech.de/PME94
The continuously variable automatic transmission has had a real triumph in the two-wheel sector; the lower the torque to be transmitted, the more popular. But we turn to another area, that of tractors, where there is a great
need for easy-to-use transmission technology.
kfz-tech.de/PME95
Coming from the engine on the left, the torque is transferred to the planetary gear carrier on the right via a torsion damper. There it branches via the ring gear onto the hydro-axial piston pump at the top (hydraulic drive) and
via the sun gear to the axle drive at the bottom right. This is connected to an axial piston motor (hydraulic downforce). There is a line connection between the hydraulic pump and the hydraulic motor.
kfz-tech.de/PME96
When starting, only very little torque is transmitted at low rpm via the mechanical branch and a very large amount of torque is transmitted via the hydraulic part. This is done by continuously tilting of the axial pistons.
kfz-tech.de/PME97
The higher the required torque, the more the tilting of the drive increases.
kfz-tech.de/PME98
Faster travel with little torque via the hydrostatics can be achieved by pivoting the pump part into the zero position. This is where the significantly lower efficiency of the mechanical connection comes into its own.
kfz-tech.de/PME99
By tilting the drive engine in the other direction, you can even shift smoothly into the reverse gear. There are hybrid systems in which an electric motor is installed instead of the hydraulic motor.
kfz-tech.de/YME47
kfz-tech.de/YME43
| 
