ACTIVE ENGINE MOUNT SYSTEM IN THE NEW AUDI S8

Audi has tted an active, mechatronically-controlled engine mount system in its new S8 model. This allows

the fuel-saving cylinder management system to deactivate four of the V8 engines cylinders across a broad

engine speed and load range, without compromising on comfort for the occupants of the car.

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DEVELOPMENT AcOUSTIcS | NVH

Acoustics | NVH

ACOUSTIC CHALLENGES

The new Audi S8 uses a five-point mounting principle, , with the objec-tives of positioning the drivetrain in the vehicle and resisting driveline torque. For premium-category cars, insulation from disturbing engine vibration and the damping of drivetrain vibration caused by excitation from the road surface are especially important.

A new feature of the Audi S8 is cylin-der cut-off when not required, known to Audi as so-called cylinder on demand. Depending on the power requirement, four of the eight cylinders of the new 4.0-l V8 TFSI engine are deactivated. The aim of this measure is for the customer to benefit from the maximum possible reduction in fuel consumption. When four of the eight cylinders are deactivated but the demand for torque and power does not change, the remaining cylinders have a different operating point. The raised load point displaces the operating point to regions in which specific fuel con-sumption is lower. It is this effect that leads to a definite reduction in fuel con-sumption. The change in firing order during four-cylinder operation naturally gives rise to increased excitation. This more marked low-frequency excitation represents a challenge when developing drivetrain mounts; this will be discussed in more detail later.

The same four cylinders are always deactivated, namely two in each cylinder

bank. The cylinders for deactivation have been ingeniously chosen to retain a uni-form firing sequence equivalent to that of a conventional four-cylinder engine, though as would be expected the four-cylinder engine runs with greater vibra-tion than would be encountered from a V8 engine.

As is well known, there are eight igni-tion sparks on every two revolutions of an eight-cylinder engine. As the crank-shaft rotates, these ignition impulses are superimposed, resulting in a relatively well-balanced total impulse signal of the fourth engine order. This in turn ensures smooth engine running that is scarcely noticeable inside the car. If four of the cylinders are then deactivated, every sec-ond ignition spark is eliminated and the mutual equalisation of the ignition im -pulses is reduced, since the inertial forces in the engine remain the same. As a result the individual ignition impulses become more noticeable. This is felt as second engine order vibration typical of a four-cylinder engine, but of course not typical of a V8 and undesirable for this class of car, . The new Audi S8 can be described as having two engines in a single car, and these must be designed to take acous-tic demands and freedom from vibration into account for the complete vehicle.

Drivetrain vibration and noise are transmitted to the interior of a vehicle along various paths. Part of this input (e.g. engine noise and noise at the exhaust tailpipe) reaches the drivers ear by an airborne path; another part is con-veyed through the structure, that is to say directly via the engine mounts and the body to the interior, . In order to reap the full potential benefit of cylinder on demand, it is essential for the engine to operate in the V4 mode even at the lowest possible engine speeds and at high loads. It is desirable for the cus-tomer, regardless of the actual driving situation, not to notice that the engine is operating in the slightly rougher V4 mode. Since conventional technical solu-tions provide only limited scope for tun-ing, the Audi S8 has been provided with several active systems, including ANC (active noise control) and the new active engine mounts.

ACTIVE ENGINE MOUNT SYSTEM

The engine mounts have always exerted a major influence on in-car refinement. A

AUTHORS

DIPL.-ING. STEPHAN RMLING is Project Manager for Active Engine

Mount Systems at the Audi AG in Ingolstadt (Germany).

DIPL.-ING. (FH) STEFANVOLLMANN is Test Engineer for Active Engine

Mount Systems at the Audi AG in Ingolstadt (Germany).

DIPL.-ING. TORSTEN KOLKHORST is Head of the Engine/Gearbox Mountings Development at the

Audi AG in Ingolstadt (Germany).

Five-point mounting in the Audi S8

01I2013 Volume 74 35

Acoustics | NVH

distinction has to be made between acoustic properties (dynamic stiffness, hardening of rubber) and freedom from vibration (static stiffness, damping behaviour); there is normally a conflict of objectives between them. Audi cur-rently distinguishes between three types of engine mount: : hydraulic engine mounts : switchable hydraulic engine mounts : active hydraulic engine mounts.These three types of mount are installed according to the cars require-ment profile and as a means of main-taining Audis premium claim for its cars. The hydraulic and switchable hydraulic mounts have been state-of-the-art at Audi for some time, and are installed on all model lines with a longitudinal engine.

CONSTRUCTION OF THE ACTIVE ENGINE MOUNT

Audi defines hydraulic or switchable hydraulic engine mounts as passive bearings. An active engine mount, in contrast, generates a force inside the mount that acts through the mounts support spring. In this way a counter-force can be developed to act in the opposite direction to engine vibration and compensate for it. The aim is to eliminate vibration directly in its trans-mission path. This is in contrast to an

active so-called tuned mass damper, which is a separate component attached at a suitable point on the body, to which it transmits its own oscillations.

According to the state-of-the-art, vari-ous types of actuator are known and can function within the mount: : in solenoids, a metal plate is actuated

by alternating field, with return spring; a permanent magnet is not necessary; the change in the size of the air gap makes the action non-linear

: oscillating coil actuators have a mov-ing alternating-field coil and perma-nent magnet field; highly linear force path, low moving masses, movable coil connection; strong technical con-struction familiar from loudspeakers

: linear electromagnetic actuators have moving permanent magnet in alternat-ing field coil; linearity, high moving mass, high inductivity makes force build-up more difficult

: in piezo actuators the piezo element expands when a voltage is applied, and can be stacked (inline arrange-ment) for use as an actuator; high line-arity, high forces generated at very low amplitudes

: in magneto-rheological (MR) hydraulic mounts the viscosity of the MR fluid changes, and with it the dynamic characteristics of the mount; cannot actively generate a counter-force.

For the Audi S8 installation a mount with an oscillating coil actuator was cho-

Orders of engine vibration in the V8 and V4 modes (schematic view)

Acoustic transmission paths in the car

DEVELOPMENT AcOUSTIcS | NVH

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sen. As with a hydraulic engine mount, this mount consists of a supporting body with a fluid chamber beneath it. The fluid can flow through the external damping channel into the lower equalis-ing space, the size of which is limited by the bellows. The upper fluid chamber has a rubber diaphragm as a barrier. The actuator is attached to this diaphragm, and consists of a permanent magnet sur-rounded by two iron armatures. This creates an enclosed magnetic field of high field strength. The coil moves within this magnetic field; it is attached to the diaphragm ring by way of the coil car-rier. If an electric current flows through the coil, the field of the permanent mag-net applies a force to it and causes the coil to move within the magnetic field. The diaphragm is deflected in accord-ance with this movement. It applies pres-sure to the fluid in the actuating cham-ber, so that the rubber support spring is caused to move, . This movement does not take place until the excitation fre-quencies are above 20Hz and the damp-ing channel is closed hydraulically.

COMPLETE SYSTEM ARCHITECTURE

Apart from the two active engine mounts, the complete engine mount sys-tem includes two acceleration sensors and a new control unit especially devel-oped for the active mounts, . As well as being connected to the engine mounts and the relevant sensors, the control unit is incorporated into the complete vehi-cles CAN network. There is also a direct line to the engine management control unit, for transmission of the speed signal from the crankshaft.

SYSTEM FUNCTION

Various state-of-the-art methods for influencing mount characteristics or for active control are known. If the adaptive engine mount system uses magneto-rhe-ological fluid, the mounts dynamic properties can be modified, but with this type of mount an active counter-force cannot be generated. The mount can only be adapted to suit various require-ments by altering the viscosity of the fluid. Active systems using an open-loop control principle are in use. Actuating signals calculated from a range of data are supplied to the engine mount, and influence the transmission path. There is

no feedback concerning the quality of the regulating action.

The Audi S8 uses a closed-loop system with a signal feedback. The increase in vibration resulting from less smooth run-ning in the four-cylinder mode is detected by the acceleration sensors and transmit-ted to the active engine mount control unit. At the same time this receives the direct crankshaft speed signal from the engine management control unit which needs to be without shift in time or phase. From these signals the algorithm in the control unit calculates a counter-signal in real time and uses it to energise the actuators. The actuators then generate a counter-force in the mount that acts on the rubber supporting body, and is superimposed on the vibration from the

engine. If the vibration from the mount has the same amplitude as the engine vibration but a phase displacement of 180, the two vibrations cancel each other out according to the destructive interference principle.

The acceleration sensor on the mount measures the resulting actual body accel-eration and feeds it to the algorithm in the closed loop. This enables the actual amount of compensation to be determin- ed and, in the next calculation step, the new parameters to be used to modify the following actuator adjusting signal. In this way the system can adapt itself continu-ously to changed levels of disturbance.

The algorithm is designed to deter-mine the actual sine and cosine propor-tions in the frequency range from the

Layout of mount and actuator with lines of magnetic flux

Complete system architecture

01I2013 Volume 74 37

engine-excited sine-wave vibrations over a period. The vector of the resulting vibration is then reduced as much as possible by a sequence of iterative adap-tations of the calculation parameters (phase position, frequency and ampli-tude) with reference to the crankshaft signal, which supplies information on the frequency and phase position of the engine vibration orders. In ideal circum-stances the vector of the resulting vibra-tions is reduced until close to zero. To energise the actuator the vector of the actively generated vibration is converted back into a sine-wave signal and sup-plied to the mount at a suitable power level. In this way the orders of engine vibration can be reduced to the basic noise level, where they are indistinguish-able for the cars occupants. For the sec-ond and fourth engine orders, this takes place between 25 and 250Hz in the four-cylinder operating mode. If necessary, the mount can develop a force of up to 120N. It is thus also capable of compen-sating fully for the higher-amplitude vibrations that occur when the vehicle is driven at low engine speeds with heavy loads, (schematic view). The active engine mounts remain operational when the engine is idling. Since no switching unit is available in the active engine mount for amplitude decoupling as pre-viously known, the task of lowering dynamic stiffness when idling is per-formed by the active mounts. The system

then compensates fully for the dominant 4th engine order.

Various challenges have to be accepted when applying the algorithm in real terms to a vehicle. Compared with a conven-tional hydraulic engine mount, two transmission paths have to be taken into account for every active engine mount: one of them, already known, is the pas-sive element from the base of the engine mount through the rubber to the body, taking the characteristics of the fluid into account; but there is also an active element leading from the actuator through the fluid to the body. Both affect the transmission of vibration, with dif-fering influences on phase position and amplitude, which must be taken into consideration when determining the algorithm values.

This additional transmission path from the active element must be recorded for the algorithm by a technical measure-ment process, so that the calculation can be carried out with the appropriate parameters. It should be noted, however, that the two engine mount transmission paths change on account of vehicle com-ponent ageing (in particular rubber age-ing) and under the influence of tempera-ture changes. The algorithm must either be suitably amended or must adapt itself in such a way as to ensure stable opera-tion for the vehicles entire working life.

A further challenge is that the vibra-tion transmission path passes through

both engine mounts. For this reason, both of them are of the active type. Since the mounts are linked together through various structural noise trans-mission paths in the body and the engine, they can influence each other, depending on performance and operat-ing situation. This cross-coupling has to be taken suitably into consideration in order to avoid undesirable control effects.

OUTLOOK

The active engine mounts are the first mechatronic system to be applied to a motor vehicles drivetrain mountings. Counter-vibrations generated by the mounts are essential if full value is to be obtained from cylinder on demand. The V8 TFSI engine can then be run in the four-cylinder mode in load areas that promise the greatest fuel saving poten-tial, without the customer being aware of any shortfall in refinement. Media com-ments confirm success in the fuel econ-omy, reduced vibration and engine acoustic areas. There is no basic reason why active engine mounts should not be adopted for other drivetrains, especially for vehicle and driveline concepts in which conventional mounts come up against their limits. In such cases active engine mount systems will be used more frequently in the future to compensate for unwanted vibration.

Schematic view of measured results in driving mode

DEVELOPMENT AcOUSTIcS | NVH

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01I2013 Volume 74 39

The Audi plant in Neckarsulm re

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Head of Thermodynamics and Ap

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