Engine Mount: Description and Operation

Dynamic Engine Mounting







To further improve the driving dynamics as well as the driving and vibration comfort, the optional Sport Chrono Package Turbo also includes dynamic engine mountings.

The dynamic engine mountings automatically adjust rigidity and damping depending on the driving situation. This variability eliminates apparent contradictions. The dynamic engine mountings improve both driving dynamics and driving comfort. They significantly minimize the transmission of vibrations in the entire drive unit and in particular of the engine to the body. The system does this using a magnetizable (MRF) damper fluid and an electrically generated magnetic field. Using a defined electric current, a magnetic field is generated and the particles in the fluid are magnetized to a greater or lesser extent. This changes the viscosity of the fluid and the engine mountings are made harder or softer.







NOTE: Magnetorheological fluids (MRF) are flowable suspensions (Latin: suspendere to suspend) whose consistency can be changed quickly and steplessly between liquid and solid by a magnetic field. Such materials can be used for a large number of applications (e.g. adjustable shock absorbers and vibration dampers). An MRF is a suspension of small, magnetically polarizable particles that are finely distributed in a carrier fluid. These particles are normally iron particles in an oil, such as mineral or silicone oil. The particles are polarized in a magnetic field and align themselves in chains along the lines of magnetic flow. The suspension becomes more viscous as a result of such a structure formation, whereby the viscosity increases as a function of the field strength. This effect can take place within a few thousandths of a second. After the magnetic field is switched off, the solidified material returns to its initial fluid state.

Operating Principle

In racing vehicles, the drive units (engine and transmission) are bolted to the body without engine and transmission mountings (unit mountings) for high driving dynamics. In road vehicles, unit mountings with harder or softer tuning are used depending on the required level of comfort. Dynamic driving situations, e.g. braking and steering into a corner or a series of alternating bends, produce a high level of relative motion between the drive unit and the body with conventional unit mountings. The engine mountings play a major role here, particularly in vehicles with a rear engine where the mountings are located far to the rear.

When braking before a corner, the vehicle is subject to delayed lifting with conventional engine mountings on the rear axle due to the mass moment of inertia of the engine and the flexibility of the engine mountings. The result is a reduced wheel load and lower braking potential on the rear axle. This effect is intensified on uneven road surfaces in particular due to the instability of the entire system. The dynamic engine mountings minimize this effect and produce more stable braking with increased braking potential on the rear axle.

When steering into a corner, the vehicle follows the steering movement directly. However, the engine tries to continue following a straight line in accordance with the principle of mass moment of inertia. The engine will only follow the steering direction once the elasticity in the engine mountings has been overcome. In the case of vehicles with a rear engine, the result is a delayed momentum on the rear axle, which in turn produces an oversteer tendency when driving very dynamically. In this driving situation, the dynamic engine mountings are automatically set to hard. There is no delayed momentum and the vehicle can move more precisely and without disruptive side effects in extreme driving situations, just like a racing car.

To reduce the effects of the mass moment of inertia of the engine, damping of the dynamic engine mountings is also increased when driving through a series of alternating bends and in the event of load changes. This reduces "pushing" from the rear and allows more stable and precise handling.

The dynamic engine mountings also offer significant advantages during acceleration from a standing start and at full throttle. Vertical engine vibrations are largely reduced. The result is more uniform and higher drive power at the rear axle with higher traction and better acceleration. At low speeds and for comfort-orientated driving, the dynamic engine mountings are made softer according to the condition of the road surface. This reduces the amount of natural vibrations from the engine to the body and therefore to the passenger compartment, particularly when driving on poor road surfaces. The result is enhanced driving comfort with reduced vibration.

Function of Unit Mountings

The dynamic engine mountings contain an electric coil and two chambers filled with magnetorheological fluid. The upper chamber is coupled to the body, while the lower chamber is connected to the drive unit. The chambers are connected to each other by means of an annular gap. The electric coil is located directly next to this annular gap. Relative motion between the engine and the body presses the fluid through the annular gap. An electromagnetic field is generated in this annular gap by a defined electric current through the coil. The viscosity of the magnetorheological fluid then changes and the engine mountings are made softer or harder.

When there is no current flowing through the coil, the fluid has a relatively low viscosity and the flow resistance is low. The mountings have low rigidity and damping and are thus soft. When a defined current is applied to the coil, the iron particles in powder form in the fluid are magnetized and join together to create chains. The fluid becomes viscous and the flow resistance high. The mountings are then characterized by high rigidity and damping and are therefore hard.

The system is controlled by a separate control unit with a highly dynamic control loop and response times of just a few milliseconds when switching between soft and hard engine mountings. A large amount of information is processed, including the steering angle, lateral, longitudinal and vertical acceleration as well as the fluid pressure in the respective engine mounting. To ensure excellent driving dynamics and functionality, vehicles with dynamic unit mountings feature separate and independent control of the right and left engine mountings. The control strategy of the dynamic engine mountings is also influenced by the PSM vehicle stability system pre-setting.

System Components:







- 2 magnetorheological engine mountings with integrated sensors and control
- 1 control unit with corresponding power electronics

Engine mounting consists of:
Connection dimensions (interface between body and engine carrier)

The control unit includes:
- Controller functionalities (situation detection, safety logic)
- Sensor system (body acceleration signal from the PASM control unit)
- Actuator (one coil per engine mounting)
- CAN Drive
- Power electronics

"Dynamic engine mounting" allows the stiffness and damping to be varied continuously on both engine mountings (left and right) in accordance with the detected driving situation. This reduces or minimizes the unit movements and modes caused by road and drivetrain excitations. These system adaptations lead to improvements in the driving dynamics (traction, handling) and driving comfort (initial spring response, jerking, jitter).







The control algorithms for driving situation detection and control of the unit mountings are processed in the control unit. The control unit functions include the following:
- Reading, filtering and quantifying the input signals
- Supplying the algorithm for driving situation detection
- Supplying the algorithm for the unit mountings
- Executing the algorithms
- Executing the diagnostic functions
- Fault detection and fault management
- Actuation of the mountings
- Output of the fault status to the instrument cluster
- Communication in the CAN network

Driving Situation Detection

The prevailing driving situation is detected by an algorithm in order to optimally adapt mounting control to the respective situation. The following situations can be detected:
- Level road surface (Normal)
- Level road surface (Sport)
- Poor undulating road surface
- Rapid increase in engine torque (pedal potentiometer)
- Rapid reduction in engine torque (pedal potentiometer)
- Racetrack driving
- Full braking
- Rapid acceleration (racing start)

NOTE: Depending on the function setting - PSM ON / Sport Chrono (SC) OFF / SC+TC (Traction Control) OFF - the pre-set damping of the engine mountings is decreased or increased as required for sporty driving. In the sportiest setting SC+TC OFF, for example, a control strategy suitable for race circuits with very high damping of the engine mountings is selected.

The control parameters of the algorithm are adapted on the basis of the detected driving situation. This ensures that the required stiffness and damping can be adjusted continuously at all times.

NOTE: Explanation of algorithms:
Algorithms are exactly defined rules or procedures for solving a task in a finite number of steps.