The picture shows a primary retarder for heavy trucks, to which would actually fit better the secondary retarders located behind the transmission. Here one decelerates then directly the engine and has a good reason. Because this deceleration is not the only task of this retarder.

Imagine a truck in front with a fully loaded semitrailer. Let it start with one of the modern, especially high-torque engines, then you would come to mind the clutch as particularly claimed component at this moment and this is where there is a further application of this retarder.

You can compare it confidently in its function and effect with the (old) hydraulic clutch. When starting up it's the one paddle wheel (impeller - red) connected to the engine and the other (turbine - turquoise) detached from the housing to the transmission input. Even when the clutch is engaged now fully and quickly, the torque is transmitted nevertheless relatively soft. All components in the power flow are protected.

And how is again a retarder of the hydraulic clutch? While the paddle wheel (turbine wheel) being associated with the transmission seizing the brakes. This is made possible by the free-wheeling over which it is connected with the transmission input.

It drives thus only if it is faster than this. So again created a simple retarder, the is to dose well by controlling the oil inflow. The valve above is a kind of float, raised by the oil level and by low pressure sucked down provides for pressure equalization of air. In the oil reservoir, however, it is desired under pressure. Thus, the inflow/outflow can be infinitely controlled from the driver's seat. 06/12