This is less about the guide and turbine wheel shown than about the beauty of the construction. We first take care of the oil flows that transmit the torque here. We will come to the hydrodynamic torque converter later. But we can already reveal that it is installed between the engine and the automatic transmission, effectively replacing the conventional clutch, at least when starting off.

First of all here, the hydraulic clutch to get used to. It must be strictly distinguished from the hydraulically operated clutch, because it works with friction and usually has a clutch pedal. Ours here is free from almost any outside influence, especially from any opportunity for operation.

You have to imagine the engine on the left and the automatic transmission on the right. It may be a bit confusing that the pump wheel, driven by the motor and completely integrated into the housing, is nevertheless on the right, while the turbine wheel on the left is freely rotatable in the housing and mounted positively on the gearbox input shaft.

What happens when the left fan is connected to the power and switched on and the right one is not? Clearly, after a while the left one will be driven also by the air flow generated by the right one. Please note how unsuitable two identical fans are for this experiment, because the blades are aligned the wrong way round. You can be happy when the right one is two thirds of the speed frequency of the left one.

They should be close to each other and the airflow coming from the left should drive the right wheel in the corresponding direction by setting the wings accordingly. Take a look above to see how differently the blades of the pump and turbine wheel are shaped. Of course, liquid also conveys torque better than air.

The torque converter, as the name suggests, can strengthen the torque (at the expense of the rev), the hydraulic clutch cannot. It is typical of the automatic systems from Mercedes even decades after their introduction. The cars therefore always had one gear more than the competition.

We start with the pump wheel and relatively straight blades in the casing driven at engine speed. The hydraulic oil is moved by this in the direction of rotation and outwards by the centrifugal force. There it meets the blades of the turbine wheel. So far no difference to the hydraulic clutch, except that the blades are particularly aligned and shaped.

Look at the turbine wheel without a guide wheel. It is driven to rotate counterclockwise. The oil coming from the pump wheel disappears into the pockets on the outer edge. The semicircle in the middle can contain oil, but it is not involved in the entire flow process. A little further inside, the oil reappears.

It has now changed its direction due to the very strongly curved blades, so if the rotation of the turbine wheel is disregarded, it flows out again almost in the opposite direction and thus hits the guide wheel. If it were not properly supported, it would rotate backwards. And now the oil flow undergoes another change of direction, namely again in an anti-clockwise direction and thus strikes the pump wheel, which is rotating in the same direction.

Such a double-deflected flow of liquid develops considerable forces. They are not effective against the supported guide wheel, but they are effective against the turbine wheel. This additional force in their direction of rotation acts as a strengthening in the torque. It is all the greater, the more the liquid flow is prevented from flowing freely through to the pump wheel again. And it is precisely this effect that increases as the speed difference becomes larger.

When starting up, such a torque converter can double the torque coming from the engine, which decreases with increasing synchronization of the pump and turbine wheel. In the end, the deflection by the guide wheel is also disruptive to the efficiency. A kind of freewheel (picture above) has provided the possibility that it simply runs along in the direction of rotation. In the first and fifth picture you can see the guide wheel uncut in the middle.

Together with normal variable-speed gearboxes and a small, electrically operated clutch, the result was a semi-automatic. The first gear could be omitted because of the torque multiplication.

As long as the stator is stationary, it is referred to as a converter area, if it rotates with it, it is referred to as the clutch area. In order to keep the losses a little lower in the latter, there is the converter lock-up clutch, in the picture below even as a multi-disc friction clutch with a vibration blocker. If it is fully engaged, it prevents any difference in engine speed between the pump and turbine wheel. In many cases, in modern designs, this part remains closed after starting, even when further gear changes are made.

If you look closely, you can see that the edge of the disc acting on the clutch is reasonably flush with the housing. So an oil pressure builds up in the space behind it, this ensures that the clutch is released, while pressure from the much larger space at the front causes the clutch to close. This can be achieved by the oil flows that flow in and out of the torque converter through a bore in the innermost shaft or between the shafts.

So if the oil gets directly into the rear room in this way and returns through an opening in the front room, the clutch is disengaged. If you turn the oil flow now, the clutch will be engaged. This is done by electrically controllable valves between the oil pump and torque converter, of which there are many more in the actual automatic transmission. Meanwhile, their control is also possible for a certain slip.

Have you ever considered why, despite the absence of a clutch pedal, you still operate the accelerator and brake with one foot? And that also applies to American cars, where hardly anyone would think of switching between automatic and manual transmission. The reason is relatively simple. The aim is to prevent the accelerator pedal and brake from being operated at the same time. Because that brings the hydraulic oil to a boil in a very short time.

The pump wheel whirls oil around and the turbine wheel is prevented from rotating by the brake. One speaks of a temperature increase of up to 10° C per second. There is hardly any other method of damaging your own drive so effectively. The converter oil is already working in a relatively high temperature range. Just keep in mind that there are coolers for this oil that are integrated into the engine's radiator.

The advantage of this method is, of course, that the coolant warms up much faster than the oil of the automatic transmission with normal treatment. Then the energy goes in the opposite direction. But if the cooler can reach temperatures of more than 100° C, you should be aware of how hot the hydraulic oil of the automatic transmission can get. The converter is the hottest point, by the way. Also the more frequent and significantly more complex maintenance is mainly due to the warming.

Of course, if the footbrake is depressed, it also limits the maximum possible engine speed. It is called stall speed and is a maximum of around 2,500 rpm.

All these multi-plate clutches can now also be designed to be slipping, at least in some areas. The lock-up clutch can even be open in overrun mode and when braking. Converters themselves can significantly change the perception of automatic transmissions, inter alia working softly in a comfort sedan, e.g. American-style, strong at saving fuel in a city bus.