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It's quite possible, that drivers of modern trucks don't even notice the presence of a retarder at all. Previously, they had to be consciously operated by a seperate switch, now they switch themselves on with a light tap
on the brake pedal. Thus it's job also becomes obvious, with reference to to the permissible gross weight, to relieve the, not stable enough, foot-brake during prolonged use.
Retarder saves at certain speed range brake pads.
The retarder also supports the engine-brake. It is found, as a primary version, on the drive shaft, and as a secondary version, on the gearbox output shaft (see picture). Because it is not seperatly, as an independent
block mounted behind the gearbox, but rather integrated into it, the above shown secondary retarder is also known as an 'intarder' (in-board and same lubrication source as the gearbox). The primary retarder brakes
particularly well at high engine RPMs which is, e.g., important for the very often low speed operation of city-busses. As opposed, the secondary retarder relates directly to the wheel speed, and is thus, more suitable for
It converts, when required, kinetic energy into heat. One could compare the hydraulically operated retarder with a torque-converter without a guide wheel, whose turbine-wheel is solidly connected with the casing.
Thereby, it's the response time which is important. Through a special valve working at certain intervals, the retarder is partly or completely flooded , depending on the necessity, the more it floods, the greater the
braking effect. To ensure that it operates quickly, seen on the right of the picture is a large reservoir of pressurised gearbox-oil waiting to be used.