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Hydrostatic Torque Vectoring

With new and especially with superior powertrain concept, the active distribution of torque between the wheels of an axle is increasingly brought forward to compensation via brake intervention. The vehicle's response is improved insofar as the torque at a wheel under consideration is not reduced but strategically increased. Ideally the sum of the torque available for the traction should remain constant, and not reduced by active brake intervention. The function of "ESP" can thus be realized in a more elegant and effective way.
The objective of torque vectoring is an improvement to driving agility and driving pleasure while retaining high travel stability and travel comfort.
Important function objectives are:
  • Reduction in understeering during acceleration, especially at high friction coefficient
  • Improvement in turning response
  • Increase in yaw damping at high speed of travel
  • Influence of load change reaction
  • Increase in travel stability without loss of dynamics (inverted ESP)
To increase agility and driving pleasure, the starting torque is electronically controlled and freely distributed between the rear wheels in a certain area. The distribution of the drive torque at the rear axle from the wheel on the inside of the bend to the wheel on the outside of the bend creates torque that supports the cornering manoeuvre.
Small numbers of the current torque vectoring systems with multi-disc brake or clutch are used in a few special models with sporty images. The biggest challenges here are precise torque control in various different travel and environmental states, as well as the actuation mechanism of the discs.
The alternative solution from hofer: hydrostatic torque vectoring
With the hydrostatic torque vectoring unit, the braking torque is not created by a multi-disc brake but by an axial piston pump with adjustable swash plate. The use of a hydrostatic pump has the following advantages:
  • The system works largely wear-free (all friction contact points are hydrodynamically lubricated).
  • The oil in which the heat is generated is immediately pumped further, and cold oil is sucked in. The pumping of the oil achieves a constant and uniform distribution of heat across the entire unit.
  • Since no attention needs to be paid to special friction properties, normal differential transmission oil can be used.
  • The adjustable swash plate enables no-load losses to be kept far lower than with a multi-disc brake. This improves the overall degree of efficiency of the vehicle and reduces fuel consumption.
  • The reaction time of the hydraulic system is better. This makes full use of the potential offered by torque vectoring.
  • Faulty cases are verified at less expense.
  • Control takes place electrically. The valves are directly controlled here, thus reducing the expense of controlling and increasing precision as compared to the multi-disc brake solution.
Analysis of existing disc-based systems and ongoing simulations at hofer that are based on them have resulted in the following evaluation matrix.
Evaluation Matrix 
hydrostatic torque vectoring (hofer)
Response behaviour / Dynamics
The high displacement of the pump unit and low displacement of the regulator piston result in very fast response behaviour. The pressure in the system builds up very fast, because an uncompressible liquid is involved and an acceleration of masses is not necessary.
There is no fill phase (idle stroke).
The torque at the pump is directly proportional to the measured amounts of pressure at the pump exit and of the angle of the swash plate.
Direct temperature dependence is thus eliminated.
The repeatability of the adjusted torque is only significantly influenced by the accuracy of the sensors.
With the pressure sensor a closed-loop system is possible, where the measured pressure is directly further processed. A temperature sensor is not necessary for the control.
Adjustment accuracy
The repeatability of the adjusted torque is only significantly influenced by the accuracy of the sensors.
The repeatability is only influenced by two sensors.
Reaction to faults
The hydraulics can be mounted redundantly. Faults in the adjustment mechanism or at the pressure limitation valve can be intercepted too, because both can be used to influence the torque at the torque vectoring unit (if the piston got stuck, for instance, this would cause a continual volume flow, but if the pressure limitation valve is opened the torque remains low nevertheless). Only if the two systems fail simultaneously will a fault arise that cannot be intercepted.
For system reasons each torque vectoring unit has a "limit-rpm difference". If the current rpm difference approaches this "limit-rpm difference" in an ever narrower curve, for system reasons the torque vector is slowly and steadily reduced, because the pump rpm decreases and as a result the pressure cannot be retained.
Safety requirement ISO 15504
Avoiding unwanted torque adjustment:
The torque is created from the product of tangential deviation and pump pressure. The hydraulics is designed in such a way that the swash plate and the pressure limitation valve can be controlled independently of each other. Two valves thus have to be switched in order to create torque. A redundant system is therefore integrated.
Avoiding adjustment to higher torque:
The swash plate is limited in its maximum angle of adjustment by a mechanical arrester. Because of its design the pressure limitation valve has a maximum pressure. If this pressure is reached, oil is discharged. If the pressure control valve is faulty, the pressure sensor can detect this increase. In this case the swash plate can be reset.
Fast opening to zero torque:
The reduction of pressure as a result of the pressure control valve being opened takes place within roughly 15 ms.
Activation of the swash plate and the pressure control valve via two independent valves means that a redundant system is already integrated. If required, the pressure valve can also be secured with a downstream cut-off switch valve.
Robustness /
The pump unit has been used in industry for several years. The principle here has been much tested and is technologically mature.
The technologies for manufacturing the pump are also mature. In industry, these pumps are used with a durability of 30,000 hours.
The planetary gearing is also a technology that has already been used in very high numbers, e.g. in automatic transmissions.
Inter-axle and inter-wheel differential
The inter-axle and inter-wheel differential function is guaranteed. With the right mounting of the hydraulics, both sides of the torque vectoring rear axle can be controlled simultaneously. If the starting torque is less than 2600Nm, a 100% inter-axle and inter-wheel differential function can be realized.
If slippage occurs, the system reacts automatically, removing the torque from the skidding wheel and transferring it to the wheel that has purchase on the road surface. No separate control expense is required for this.
Wear and tear
The existing friction surfaces are hydrodynamically lubricated. In operation, hardly any wear and tear occurs at all.
The selection of twin friction surfaces and the use of hypoid oil also means that wear and tear in the starting procedure is hardly significant, and extreme situations are also survived without damage (emergency running properties).
the oil is not polluted by abraded particles.
Heat balance
The continual movement of the oil enables optimal distribution of the heat in the entire oil volume and then on to the housing.
The heat capacity of the entire oil volume is optimally utilized.
If required, the design of the oil channels as a cooling section is also possible.
No-load losses
The no-load losses are dependent on the pressure loss from the hydraulic cables and the volume flow.
By adjusting the swash plate, the volume flow can be lowered, enabling the no-load losses to be minimized to almost zero.
The adjustment of the swash plate can be done in no-load state, independently of the position when the torque vectoring unit is activated.
Conditions do not influence each other.
Even within an extreme handling course, the proportion when torque vectoring is activated lies below 25%. The reduction of no-load losses plays a large part in the overall consumption of the vehicle.
Oil balance
For the hydrostatic unit, the existing hypoid oil as used in the differential transmission can be employed.
A separate oil chamber is unnecessary.
Installation space
The installation space for the planetary gearing is also used for the axial piston pump. The adjustment unit is also integrated into this installation space.
The slide-valve gear-box can be integrated into the housing.
Several components are no longer required in comparison to the disc solution (discs, power-pack, etc.). The extra components are simple ones that can be produced at low cost.
From a technological as well as an economic point of view, hydrostatic torque vectoring from hofer offers clear advantages over disc-based systems.

© hofer 2007