Hard braking can damage your car

Most drivers have at least some idea of ​​how the brakes work on their cars. With this knowledge and experience, many drivers can also estimate the braking distance of their car. The situation changes when the language comes up on truck braking systems - because these not only have to meet completely different requirements, they also function fundamentally differently.

Hydraulic brake in the car

It is known that the hydraulic service brake in an automobile works by pressing the brake pedal. More specifically: The pressure exerted by the foot on the brake pedal is distributed to all four wheels via the line system connected to it and the brake fluid contained therein.

In between there are now all kinds of electronic helpers such as ABS, ESP, EBV, etc., which increase safety when braking. In order to increase the braking force, modern vehicles are equipped with a brake booster. With such a hydraulic service brake, cars can usually be decelerated without any problems, because in road traffic either light permanent braking or short-term hard braking is the rule.

But if a steep mountain pass has to be mastered with a fully loaded car and trailer, which requires constant heavy braking, a vehicle brake can already reach its load limit. The same applies to sporting use on a racetrack. Hydraulic automobile brakes, as they are installed ex works, are not fully suitable for this. You can find out more about how a car brake works in our article "How does a car brake actually work?"

Pneumatic brake in the truck

Even this brief description of the brakes in cars indicates the fundamental problem. Because: Is there a brake in the car? Not Designed for continuous load when loaded, a truck must be equipped for this. In addition, given the moving mass of a fully loaded truck, it is no longer sufficient to merely increase the driver's foot strength hydraulically. For this reason, vehicles with a total weight of 7.5 t or more, i.e. heavy trucks, tractor units and buses, have a pneumatic brake as a so-called external power brake system.

A pneumatic brake system basically consists of a system of pressure generator (compressor), pressure regulator, compressed air tanks, compressed air lines and membrane cylinders (brake cylinders).
In contrast to a hydraulic brake in a car, the pedal force generated by the driver in a truck is transmitted to the brakes by the compressed air generated by the compressor and not by the brake fluid that is permanently in the system. The driver's foot force does not act directly on the force-transmitting medium, but on a brake valve.

The compressed air requested by the driver using the foot pedal flows through a line system to the individual wheels, where a membrane cylinder moves the friction linings. The driver controls the required braking force via the pedal travel. If the pneumatic brake enables heavy vehicles to be braked safely and permanently without a decrease in the braking force (fading), this type of brake is not suitable for permanent deceleration. Because the service brake of a truck would burn if it were to decelerate for a long time and would ultimately fail, the legislator here prescribes a further, independent brake system (third brake).

Continuous braking system as a third brake

This permanent brake, which is mandatory for heavy trucks, tractor units and buses, is an auxiliary brake that is intended to relieve the service brake on long downhill stretches, for example. It should guarantee permanent, wear-free deceleration without decreasing its braking performance.

According to the law, an endurance brake must be able to keep the speed of a fully loaded vehicle constant at a driving speed of 30 km / h and a gradient of 7% - however, it is not intended to bring the vehicle to a standstill by its deceleration performance. A basic distinction is made between two systems for the retarder:

  • Engine braking systems: There are two types of engine brake, on the one hand the engine brake system with exhaust flap and on the other hand the engine brake system with exhaust flap and constant throttle. Both systems have an exhaust flap in the exhaust line, which reduces or almost closes the exhaust line by means of a foot-operated solenoid valve. This means that the energy generated by the engine in the 4th work cycle (exhaust cycle) is used for braking. At the same time, the fuel supply to the engine is cut off.

    The braking power of the exhaust flap can be significantly increased by installing a constant throttle, which acts as a central functional part of the so-called decompression system. The constant throttle opens after each compression, thus allowing the air compressed in the 2nd stroke to escape into the exhaust line without pressure in the 3rd stroke. In summary: In the case of engine braking systems, the vehicle brakes because it has to move the engine.
  • Retarder systems: The retarder systems are also divided into two different designs. The hydrodynamic retarder (flow brake) is connected to the transmission and brakes through the resistance of a liquid medium. The rotor and stator are usually mounted in a hydraulic block. As a result of the rotary movement of the rotor blades, the oil flows against the stationary stator blades, braking the vehicle due to the flow resistance. The greater the amount of oil, the stronger the braking effect.

    An electrodynamic retarder (eddy current brake) does not work with the resistance of a liquid medium, but uses the resistance of a preselectable electromagnetic field. The braking torque of the electrohydraulic retarder depends on the size of the rotor coils. The electrodynamic retarder can be coupled with a system for braking energy recovery, with which the kinetic energy of the vehicle can be converted directly into storable electrical energy.

This rough outline already shows that truck brakes differ considerably from brakes in cars. For a safe coexistence in traffic!