M-Differential Lock
M-Differential Lock
The technical innovations of the variable M-differential lock developed by BMW M in cooperation with GKN Viscodrive GmbH is fitted in the E46 M3.
Variable M-differential Lock
To date, torque-sensing limited slip differentials with a constant basic locking torque have been used in M vehicles. The differential lock value for current M vehicles is 25%. However, if traction is very low, e.g. on snow, the advantages with this differential lock concept are limited due to the restricted support torque.
A variable differential lock is used for the first time in the E46 M3. Compared to the conventional torque-sensing differential lock, the variable M-differential lock is capable of providing traction advantages even under these extreme conditions.
When there is a speed difference between both wheels, a shear pump located on the ring gear side generates a pressure. A working piston transmits this controlled pressure on the basis of the differential speed of the drive wheels to the multi-disc clutches so that the drive torque is transmitted to the wheel with the most "grip".
The pump pressure and locking power increase as the speed difference between the two wheels increase. If the differential speed between both wheels decreases, the pump pressure is also reduced and the locking power diminishes.
The Pump Unit
The pump unit is sealed (cannot be dismantled) and is filled with approx. 46 grams of high viscosity silicone oil.
As a result of the speed difference between the drive wheels, shear forces occur in the silicone oil in the pump unit between a channel filled with silicone oil and a pump disc located above it.
These shear forces generate a pressure that is dependent on the differential speed of the drive pinions.
The pressure is built up by a piston (max. 38 bar) that presses the multi-disc clutches together achieving a variable differential lock effect dependent on the differential speed.
Principle of Operation
The conveyor belt model illustrates the functional principle of the shear pump. As the conveyor belt moves, the viscous friction in the laterally sealed shear channel conveys the fluid in the direction of movement.
By transferring the operation principle to a rotary system, the conveyor belt becomes a displacement disc driven by the Hub. The shear channel is then a circular groove in the control disk. The control disk is located in the pump housing.
Transfer ports that are connected to outlet holes in the pump housing are provided at the ends of the shear channel.
Silicone oil is conveyed through the outlet holes from the equalization chamber (fluid reservoir) into the pressure chamber. Piston exerts a pressure on the friction discs.
The arrows shown to the right represent the flow of silicone oil between the displacement disc and the control disc. The silicone oil is transferred from the reservoir on the outside of the pump housing to the pressure chamber between displacement discs and the piston.
The arrangement of the pump elements and specific routing of the silicone oil in a gap between the piston and displacement disc ensures the pressure generated acts on the piston.