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Audi RS Q8
Audi RS Q8
( Source: Audi Media Center)

POWERTRAIN Hardware meets software: from classic mechanical systems to a fully interlinked high-end control unit

| Editor: Jochen Schwab

Today, thanks to smart interlinking by the Electronic Chassis Platform (ECP), innovative chassis systems such as electromechanical Active Roll Stabilization (eAWS), the predictive active suspension and Dynamic All-wheel Steering (DAS) are able to unfold their full potential.

In the Audi e-tron, the integrated Brake Control System (iBRS) emphasizes the fact that efficiency will become the third variable in chassis development alongside ride comfort and sportiness. As a high-tech control unit, the future vehicle dynamics computer can simultaneously actuate up to 90 components.

On the road toward an integrated vehicle dynamics brain

Audi consistently drives the idea of integrating chassis and powertrain technology. In the future, an integrated vehicle dynamics processor will handle longitudinal and lateral dynamics control as well as energy and powertrain management: recuperating energy during a braking event, determining the damper compression rate within milliseconds and keeping the vehicle precisely on track – all practically at the same time. The development objective for future chassis generations is clear: besides an even greater spread between sportiness and comfort, the integration of efficiency technologies will play a central role.

Vision central vehicle dynamics computer
Vision central vehicle dynamics computer
(Source: Audi Media Center)

The future vehicle dynamics processor will be centrally controlling nearly all functions in longitudinal, lateral and vertical dynamics: chassis, powertrain and recuperation functions, and be clearly more powerful than today’s ECP, for instance. It will operate about ten times as fast as current systems and be able to control up to 90 system participants – compared to about 20 in the case of the current ECP. New as well will be its modular usability for diverse types of powertrains, in other words, ICE-powered, hybrid or electric vehicles, as well as front, all-wheel or rear-wheel drive systems for electric models. As a result, the central vehicle dynamics computer, in addition to car-to-x functions, will enable function-on-demand features as well. Its precise data computation will also execute the requirements of advanced driver assistance functions.

Chassis technology development at Audi emphasizes intensified interlinking of individual mechatronic chassis components and vehicle functions using smart electronic control. The Electronic Chassis Platform (ECP) made its debut in the Audi Q7 in 2015. Today, it interlinks the individual component assemblies of the chassis systems in Audi’s mid-size, full-size and luxury models. Due to this smart ECP interlinking of chassis technologies that have undergone continuous further development such as permanent quattro all-wheel drive, the adaptive air suspension and Dynamic All-wheel Steering, Audi has forged a successful link between superior ride comfort and high-level driving dynamics.

Interlinking as an enabler – previously unknown flexibility, from comfortable to sporty

A perfect example that illustrates the enormous effort Audi invests in the technical design of the chassis is the electric roll stabilization used in the Audi SQ7 and SQ8. Due to smart interlinking, the system can unfold its full potential. Body roll of the full-size SUV during cornering and load changes is reduced to a minimum. In addition, it provides the Audi full-size SUVs with amazingly high lateral dynamics capabilities resulting in an impressive driving experience for the driver.

Sebastian Kirch, Audi TechTalk
Sebastian Kirch, Audi TechTalk
(Source: Audi Media Center)

In fast cornering situations, due to the electronic adjustment of the stabilizer bar, body roll is reduced because the stabilizer, within milliseconds, smoothly lifts the vehicle side that is on the outside of a corner against the centrifugal forces with a moment of up to 1,200 Newton meters. This makes higher cornering speeds possible and clearly reduces load change reactions as well. During straight-line driving, for instance on uneven road surfaces, a planetary gear system disconnects the two halves of the stabilizer, which enhances ride comfort. As a central control unit, the Electronic Chassis Platform also matches information from other chassis technologies in the SQ7 and SQ8, such as the all-wheel steering system, the air suspension and the quattro sport differential. The driver experiences this close collaboration of the Audi chassis systems in the form of high handling precision and agility.

Another chassis highlight with a decidedly comfort-enhancing function is used in the Audi A8. Here the active suspension operates: a fully active, electromechanically operated suspension system. Per wheel, it has one electric motor that is supplied by the 48-volt primary electrical system. The control signals for the active suspension are sent by the Electronic Chassis Platform every five milliseconds. A belt drive and compact strain wave gearing convert the torque of the electric motor to 1,100 Nm and transfer it to a steel torque tube. From the end of the torsion bar, the force reaches the chassis via a lever and a coupling rod. At the front axle, it acts on the air spring strut of the adaptive air suspension and on the rear axle, on the transverse control arm (wishbone).

In this way, every wheel of the Audi A8 can be separately subjected to or relieved of additional loads and adapted to the respective roadway. As a result, it actively controls the position of the body in any driving situation. Due to the flexibility of the active suspension, driving characteristics are expanded to an all-new range. When the driver selects dynamic mode in the Audi drive select system, the car becomes sportier: it firmly turns into corners, the roll angles are only about half as big compared to the normal suspension, and the body hardly dips during braking events. In comfort mode, however, it smoothly floats across surface irregularities of any kind. In order to calm the bodywork, the active suspension constantly – adapted to the respective driving situation – supplies energy to or removes energy from the body. Thus, drivers and passengers are practically “disconnected” from mechanical powertrain and driving effects.

Braking is a winner – the integrated Brake Control System in the Audi e-tron

The integrated Brake Control System (iBRS) of the Audi e-tron models illustrates the increasing intertwinement of chassis and powertrain technology. As a result, efficiency becomes the third objective of chassis development alongside comfort and sportiness.

The recuperation system, for instance, contributes up to 30 percent to the range of the electric SUV. The iBRS includes the two electric motors as well as the hydraulically integrated braking system in this process and is the first to combine three different types of recuperation: manual overrun recuperation using paddle shifters, automatic overrun recuperation using the predictive efficiency assistant and brake recuperation with a smooth transition between electric and hydraulic deceleration. The Audi e-tron recuperates up to 0.3 g exclusively via the electric motors without using the conventional brake – this is the case in more than 90 percent of all deceleration events. As a result, practically all normal braking maneuvers are energetically fed back into the battery.

Drivers can select the level of overrun recuperation in the Audi e-tron in three stages using the paddle shifters. On the lowest level, the car coasts without additional drag torque when the driver’s foot is lifted off the accelerator pedal. On the highest level, the electric SUV noticeably reduces speed – the driver can decelerate and accelerate strictly via the accelerator pedal, resulting in the so-called one-pedal feel. The brake pedal does not have to be used in this case of deceleration. The wheel brakes only come into play below the threshold of 10 km/h, when the brake pedal actuation exceeds 0.3 g or when the battery is fully charged and thus no brake recuperation is possible.

Due to a new electrohydraulic actuation concept, which Audi uses in an electrically propelled production automobile as the first manufacturer to do so worldwide, the driver can no longer perceive the transition from electrical brake recuperation via the electric traction motors to the mechanical braking effect via the hydraulically operated conventional friction brake. This “brake blending” results in an effectively variable pedal feel with a clearly defined, constant pressure point, just like in a vehicle with a conventional internal combustion engine and hydraulic wheel brakes. The brake pedal is not connected to the hydraulic system, the transition from the motor brake through the electric motors to the conventional brake is smooth and can no longer be felt by the driver’s foot.

This is enabled by a complex electrohydraulic system: a hydraulic piston in the compact brake module generates additional pressure and braking force that complements the recuperation torque. In an automated emergency braking event, only 150 milliseconds elapse between the time deceleration is initiated and the maximum brake pressure is applied between the linings and discs. Depending on the driving situation, the electrohydraulically integrated Brake Control System decides whether the Audi e-tron will decelerate by means of the electric motors, the wheel brake or a combination of both – electrically and individually on each axle. With this system the electric SUV makes specific use of its maximum recuperation potential.

Interlinking is emphasized in the integrated Brake Control System as well, with the iBRS being supported by the efficiency assistant that is provided as standard equipment. The system recognizes the traffic environment and route using radar sensors, camera images, navigation data and car-to-x information. As soon as it would make sense for the driver to lift his foot from the right pedal, respective information will be provided in the Audi virtual cockpit. In interaction with optional adaptive cruise assist, the efficiency assistant can also predictively decelerate and accelerate the electric SUV.

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