Hybrid Braking Systems
A braking system made up of a combination of pneumatic and hydraulic systems represents a further possibility. It is most frequently used in lightweight commercial vehicles of a maximum permissible weight of approx. 6 t to 12 t.
The figure shows the functional diagram of an externally loaded, dual-circuit, dual-line brake system with hydraulic transmission unit.
When compared to a purely pneumatic system, it becomes obvious that a hybrid braking system generally includes the identical components.
The wheel brakes in this case, are however actuated hydraulically. This requires the use of so-called preloaded cylinders. These cylinders convert the pneumatic pressure into hydraulic pressure which in turn creates the clamping forces at the wheel brakes.
Advantages of the pneumatic braking system with hydraulic transmission unit are:
The packaging volume available at the rim for the wheel brakes is sharply limited in lightweight commercial vehicles as a result of the small rim diameters,
Due to the fact that heavy vehicles, particularly when driven with a trailer, require brake pressures higher than that which can be achieved using vacuum-boosted brake system in association with a realistic layout (diaphragm diameter, foot force, legal regulations), a power brake system is used,
Lightweight trucks can be coupled to trailers with a maximum load of more than 3.5 t. Trailers weighing higher than 3.5 t cannot be equipped with an overrun brake.
Disadvantages of an air brake system with hydraulic transmission system are:
The second braking system with its additional components means additional weight and thus a reduction of the payload,
The additional components require additional maintenance expense,
New developments allow the use of compressed-air disk brakes at all axles, also in lightweight trucks. In this way, the additional expense for hydraulic components of the braking system need not be applied.