Every bump a volt with Audi's eROT suspension

A new system under development by Audi promises to enable further improvements in fuel efficiency in its hybrid vehicles by using the vibrations normally absorbed by a car’s suspension system to generate electrical energy. The system, called eROT (electromechanical rotary dampers), will also make the car’s ride more comfortable, the company claims.

The system is intended to replace the hydraulic dampers that have been used in suspension systems for many years. In these, the up-and-down motion of the wheels as they pass over bumps in the road is simply converted into heat as the increasing pressure in the hydraulic cylinders warms up the working fluid.

The eROT system, by contrast, is an active electrical system linked to the car’s 48-volt electrical network, in the same way as the regenerative function of the braking sytem. It absorbs suspension movement via a lever fixed to the wheel carrier mechanism at one end, and to a series of gears at the other. When the lever moves, its motion is transferred, via the gears, to an electric motor configured as a dynamo and connected to the car’s lithium ion battery.

“The recuperation output is 100 to 150 watts on average during testing on German roads – from 3 watts on a freshly paved freeway to 613 watts on a rough secondary road,” Audi claims. “Under customer driving conditions, this corresponds to a CO2 saving of up to three grams per kilometer (4.8g/mile).”

As the system is active suspension rather than passive, as is the case with hydraulic dampers, the characteristics of the rebound stroke (which is what influences the car’s handling) does not depend on the compression stroke (which is what the occupants of the car feel when they go over a bump). Both characteristics are defined by software, so the compression stroke can be softened, increasing comfort, while the rebound is taut. Another advantage, the company adds, is that the electric motors are placed horizontally along the rear axle, rather than vertically as is the case with hydraulic dampers; this means that there is more luggage space.

In current Audi hybrids, the 48V system is a secondary electrical network connected to the primary 12V circuit via a DC converter; but future models will us the 48V system as the primary, directly feeding a mild hybrid drive (mild hybrids use the electric motor to assist the internal combustion engine, but cannot operate in fully-electric mode, unlike strong or full-hybrids). This 48V system will reduce fuel use by around 0.7litres per 100km compared with conventional ICE vehicles, it says.