 Introduction 2
The 'retarder' is referred to as the third brake, also called 'flow brake'. It is installed somewhere at the entrance to the gearbox and brakes the engine or the entire vehicle either directly ('primary') or via the gearbox output
('secondary'). It is very similar to a hydraulic clutch in which the turbine wheel is fixed. It is operated with oil or, more recently, with cooling water.
And then there is the flap in the exhaust, which acts as an engine brake when closed. Despite the additions that have become more complex, the principle is still the same: the engine cannot get rid of its exhaust gases. The
congestion in the exhaust ducts slows it down considerably. The last two systems are not actually intended as emergency brakes, but are intended to support the service brake and thus prevent it from overheating.
No, we're still not at the electronics stage. And to top it off, you could show how finely the ratio of front to rear axle brakes is usually changed depending on the axle loads. This also applies to a somewhat lesser extent to the
assignment of the trailer to the towing vehicle. The trailer control valve in the towing vehicle is responsible for this, for example.
And to finally confirm what control processes pneumatics are capable of without electronics, we cite the dual-circuit brake valve, which is directly connected to the brake pedal. Here, very fine gradations must be available,
especially for partial braking. Because nothing is more detrimental to the flow of traffic than a brake that, for example, grips more tightly than the driver intended.
At the end of this chapter, a short but rather basic discussion of the legal regulations. First of all, these regulate the normal driving conditions, e.g. that the parking brake on a truck is only released when there is sufficient
pressure for the service brake. In principle, this means full (auxiliary) braking effect even with the permissible total weight.
Of course, it shouldn't take too long for this braking effect to be achieved after pressing the pedal, after all, a truck traveling at 80 km/h also covers 22 m in one second. For cars, it is generally assumed that it takes 0.2 s,
while for trucks the legal limit is 0.6 s, but that doesn't mean that any manufacturer makes full use of this.
But you can't determine that with a stopwatch. For example, Wabco has the Conformity Test Unit, a test device including software that of course also performs other tasks in addition to this. Likewise,
the prescribed minimum deceleration of 4 m/s2 is nowhere near what a modern truck must achieve, as it would result in a braking distance of around 60 m at 80 km/h.
But it goes even further, because both the fine gradation on the brake pedal in its effect on the service brake and the gradients at which the continuous brake must still hold a fully loaded truck or tractor-trailer on a hill are
specified. Even how quickly the pressure in the supply line must drop in the event of a defect in the brake line and a full braking is prescribed.
The CTU checks an abstract threshold time, i.e. the time between activation of the pedal and a certain pressure increase behavior on the wheel brake cylinder. The least favourably located one is selected, for example the
one on the longest air line. In vehicles with trailers, this basic test also includes the pressure behavior on the yellow clutch head.
That was the pneumatic part. The signals transmitted to the trailer are of course also evaluated in the electrical part. The breaking off of coupling heads is even simulated and the pressure drop at the red clutch head is
checked. Communication with the trailer can also be simulated and the correct function of the towing vehicle control can be determined.
Conversely, all of these functions can also be carried out with the trailer or semi-trailer. From an electrical point of view, the test goes so far that, for example, the capacity utilization of the data bus and the response times
within the systems are also recorded. As always, such test units are also needed for calibration, for example if a new pressure sensor has been installed somewhere.
The test device itself is an add-on, i.e. a normal (Windows) computer is required. Together with the expansion packages, it contains such interesting things as an actuator for the dual-circuit brake valve and of course the
sensor system for the pressure on the diaphragm cylinder.
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