Trucks and buses do not need to set records, but the average speed in hilly and mountainous areas is highly dependent on how quickly and confidently a heavy machine can move not only up but also down.
The rule of a century ago says - get down from the slope in the gear in which you climb it. Even taking into account the high compression ratio, “diesel braking” is ineffective at low revs and often requires an auxiliary motor brake. But in this case, in order to accurately control the deceleration, it is often necessary to apply the service brakes. They warm up, wear out …
There is an effective solution and has long been used - a hydraulic brake retarder. The largest European manufacturer of retarders, the German company Voith, has been manufacturing these devices for cars since 1968. Then the hydraulic brake first appeared on the Setra buses. The principle of operation is simple - the transmission of the car through a multiplier (step-up gear) rotates a hydraulic turbine, which heats the oil due to excess kinetic energy. It, in turn, gives off the heat of the coolant from the engine circuit. Compressed air regulates the amount of oil supplied to the turbine and thereby the braking efficiency. For almost forty years past, only electronic regulation and a diagnostic system have been added to this scheme.
The driver controls the braking using the lever, setting one of five positions. Or it uses an automatic descent speed control system. The retarder's braking torque is two to three times higher than the engine's capabilities, so that it can handle even long-term downhill movements. At the same time, the working brakes of the machine “rest” and are always ready to help.