Automatic all-wheel drive

New single-wheel drive solution from Bosch Rexroth switches automatically and at just the right moment between 2-wheel and all-wheel drive

Automatic all-wheel drive can be a misleading term. It associates with a certain nature of a drive train all right. But the technology, loosely referred to as all-wheel drive, is in itself quiet far ranging. An automatic all-wheel drive could in this range from anything that is purely mechanical in nature, or severely complicated with the application of mechatronics. Going beyond, the greatest possible traction in difficult conditions while retaining optimal energy efficiency – often conflicting goals when developing or purchasing single-wheel driven municipal vehicles and work machines.

An area where a lot of research and development is underway, examples being the development of hybrid refuse trucks that have to encounter stop and go conditions, and are a very good candidate for supplementry technologies like start-stop and brake energy regeneration, Bosch Rexroth has introduced HET High Efficiency Traction Control, a new hydrostatic drive solution that combines and meets both requirements.

The energy efficiency of HET is based among other things in the fact that all-wheel drive is only activated when needed – fully automatically with no driver intervention. If in difficult terrain for example a wheel loses its grip with the ground, slip rapidly increases. The Rexroth MCR radial piston motor driving the wheel responds by increasing the oil flow. The rising oil flow causes a momentary and seamless transmission of the torque to the series-connected MCR motor and wheel located on the other axle. By using this “slip activation” of the all-wheel drive the High Efficiency Traction Control provides reliable response to the terrain and saves significant energy compared with fulltime all-wheel drive. In less difficult terrain or on the road the wheels in the driving axle generally have sufficient traction. The radial piston motors on the other axle then simply “free-run.” Only the two permanently driven MCR motors transfer the power from the diesel engine. However, slip occurs not only when a wheel spins. Due to deformation of the rubber tires it is actually always present to a greater or lesser degree. Even this comparatively slight slip will also result in an increasing oil flow by the driven radial piston motors. The increased flow is hydraulically clamped between the driving and free-running radial piston motor, resulting in a significant loss of efficiency for the entire travel drive. HET completely eliminates this efficiency loss. A specially developed valve block ensures that “excess” hydraulic oil caused by natural slip is directed to the low-pressure side of the circuit. In addition, the HET valve block is not located in the main oil flow, thereby minimizing losses even further. Integration of the purge function and other design features make the unit highly compact, while the plug-and-play solution reduces time and money associated with installation and piping. The HET uses no sensors or control electronics. In addition to the newly developed valve block it consists of proven standard components from Rexroth such as the A4VG or A10VG pump as well as MCR motors with constant or selectable displacement volume. This makes the HET a flexible solution for use in a wide variety of vehicle types such as municipal vehicles, mini-dumpers or small front loaders. It is designed to cover the greatest possible number of requirements, including vehicle speed or installation space.