The eLSD electromechanical locking differential maintains thrust in extreme situations. This new networked solution brings many other advantages along with it.
February 19, 2019
Florian Tausch was active as a journalist in various industries and countries. Today he heads the editorial team at ZF.
Just as the fun was really getting started, disappointment stole the show. The driver of the SUV had left the road and had dared to take on challenging, steep terrain, but it appeared that the driveline was unable to support the power from the engine with the requisite grip. The vehicle struggled to take on the stony slope, awkwardly creeping up the hill.
Get a grip: surely this is great advice that vehicles with a conventional axle differential would do well to heed. The issue with conventional axle differentials is that if one tire loses traction, the other no longer has any accelerative force on the route. The power literally dissipates due to the spinning wheel. As we have seen, this can occur on country terrain and also during fast cornering or changing lanes.
Controllable locking torque for every driving situation
One crafty solution to this issue is the electronic locking differential (eLSD) from ZF. Unlike purely mechanical differential locking, the eLSD adapts to any actual driving situation via its smart electronics. "Our eLSD reacts to more than just certain input torques or differences in wheel speeds. Rather, it takes a lot of sensor data on driving dynamics into consideration, together with our IBC brake system, in order to provide active, pre-emptive and subtle, nuanced control", explains Sebastian Dendorfer, an application engineer for axle drives at ZF. An important component here is the electromechanical actuator that works on the multidisk package. This makes it possible to continuously and smoothly vary the differential locking torque on the driven axle within a range of 0 to 3000 Nm. If a wheel loses grip, the eLSD transfers the force to another wheel that still has grip with ultimate precision. This is not only noticeable within limit ranges such as those imposed by driving in the country, but also during acceleration or during split mu driving. Split mu driving is driving on a surface that has different coefficients of friction, such as a road that has been partly iced over. "For day-to-day driving, the eLSD can also be used to provide added safety, to prevent fishtailing and to keep the vehicle driving in a stable straight line, for example", states Dendorfer.
When starting up on a track with different friction coefficients, the eLSD ensures optimum power transmission to the wheels.
Intelligent networking for more comfort
The latest generation of the eLSD exemplifies how ZF is producing intelligent mechanical system solutions via digital technology. At its core is a control unit that has just been developed (the ECU) which supports the latest requirements for on-board electronics such as cyber security and modern bus systems such as CAN FD and over- the-air updates via cloud. Compared to its predecessor, the dimensions of its hardware have shrank to half its previous size which means that installing it within the passenger cabin is now far easier.
With a conventional axle differential, the power in this scenario would be lost at the wheel spinning.
"The tight interplay between the eLSD and the brake is more than the sum of its individual parts", explains Dendorfer. The connection with the electronic locking differential means that the vehicle dynamics control receives important information on the actual driving situation which is simply not offered by traditional solutions. In addition to interventions to provide a reaction to events, predictive pilot control strategies can also be implemented. "This provides more traction and prevents oversteering. In addition, it increases stability during lane changes or if the vehicle is pulling a trailer", states the ZF expert.
The eLSD thus provides a big plus in terms of driving dynamics, stability, climbing ability and comfort. Accurately metered force applied to the wheels also ensures that the driver obtains exactly what the SUV driver in our opening story was missing out on: a whole load of fun, even in conditions that truly test the limits.
During fast evasive manoeuvres, it is particularly important that all wheels have the best possible traction.