Park-lock or park-brake?
The days of the good old, mechanically operated handbrake seem to be coming to an end in new, modern cars. Electromechanical actuation offers numerous advantages and an expanded range of functions. Incorrect operation is also avoided, since it is usually opened automatically. In vehicles with automatic transmissions, park-lock is usually installed, which are increasingly also being operated electro-mechanically or electro-hydraulically. The main reasons for this are flexibility in interior design and ease of use.
For a long time there were either manual or step automatic transmissions and the latter with a mechanical park-lock. With the introduction of every new transmission technology, the need for park-lock was re-examined. For automated manual transmissions, there were even different versions with and without a park-lock for different markets. In the case of continuously variable transmissions and dual clutch transmissions, park-lock were also installed and subsequently automated (park-by-wire). The majority of registered full electric vehicles do not have a park-lock, although traditional vehicle manufacturers usually provide one in their electric vehicles.
In addition to the actual function of securing the vehicle against rolling away when parking, park-lock and park-brake have other functions and tasks. First of all, both systems offer redundancy for each other. A park-lock also fulfills tasks of theft protection. And a park-brake ensures a legally required emergency braking function, as it is also given with the conventional handbrake.
But there are some differences in the way they work. A park-lock always acts on the primarily driven axle, a park-brake is usually installed on the rear axle. The park-lock is installed before the differential, so it only works when both wheels have traction against the surface they are sitting on. The park-brake, on the other hand, acts directly on the individual wheels of the respective axle. The park-lock is integrated into the transmission and requires installation space there. Parts of the park-lock also increase the rotating masses in the drive train. A park-lock in a transmission or a drive unit thus leads to a greater space requirement, higher weight and lower efficiency.
Required redundancies or emergency actuation systems increase the disadvantages of park-lock in terms of installation space and weight. There are different types of park-brakes. Systems with a single actuator for both wheel brakes and corresponding cables or systems in which a separate actuator is installed on each of the brake calipers. In the latter case, the non-suspended masses increase accordingly. A system-related influence on the efficiency is limited to the energy required for the actuation and diagnostic functions of the electric park-brake, i.e. almost exclusively when the park-brake is applied or released.
In view of the range discussion, I would avoid using a park-lock on electric drive axles and provide an adequate electric parking brake. The automated actuation and the usual comfort functions result in additional added value. And the integration of the electric parking brake control into the electronic immobilizer system also provides effective theft protection. We should resign developing and using park-locks, a significant number of electric vehicles are available on the market without park-lock. A park-brake is required in any case and can be designed and automated in such a way that a park-lock is unnecessary. Thanks to the efficiency gain, this also leads to an improved CO2 balance.
Burkhard Pollak, February 2021
Es gibt nichts Gutes, außer: Man tut es Erich Kästner
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