Automotive NVH

Vibration control of engine mount system

Vibration control of engine mount system

Engine is the main source of vibration that generates unwanted rattle noise and shakes in automobiles and industrial machines.
Vibration excited by the engine transfers to the engine mounts then disperses through the attached chassis.
Many studies have been done to reduce this unwanted vibration in vehicles, and introduced theoretical and practical solutions for optimizing the engine mount system. In our study of vibration control within vehicle engine mount system, Matlab is used to simulate the mount to reflect the exact same property condition of the actual mount used in the vehicle.
We first estimate the stiffness of the mount which can be represented with the complex stiffness including the loss factor (damping).
Then, the engine excitation model is mimicked according to the actual engine specifications which consist of dimensions of crank, piston, pressure and etcetera.
These specifications need to be carried out thoroughly when mimicking the engine because it could severely affect the engine dynamic forces (bounce, roll, pitch, and yaw).
Also, for our simulation we varied the resilient values of the mount since it deviates with normal distribution force according to the ambient conditions and parameter in real life.igh confidence.

Frequency-dependent mount rubber stiffness

The stiffness of engine mount is normally expressed in a complex term, a combination of resistive stiffness and dissipated stiffness which represents the damping or loss factor, respectively.
It is important to set the test condition exactly same as the operation condition since the dynamic stiffness and the loss factor has different frequency, preload, pulsing, and displacement values.

Engine excitation force

· Longitudinal : x-direction translation
· Lateral : y-direction translation
· Bounce : z-direction translation
· Roll : x-direction rotation
· Pitch : y-direction rotation
· Yaw : z-direction rotation

The engine excitation forces consist of the inertia force on the z-direction, the moment along the y-direction and the torque on x-direction. Internals such as crank shaft, pistons and connecting rods are very heavy and hence generates enormous excitation forces when the moment of forces are exerted on them throughout the combustion cycles.
The excitation order and level depends on the number of engine cylinders and specifications and therefore, the precise engine modeling is crucial.

Reliability-based design optimization
· Objective function
: Minimize [α*Low idle + β*High idle]

· Constraint
- Mount position
- Roll mode purity
- Reliability target

· Design parameter
- Mount position
- Mount stiffness

The resilient property of the engine mount varies with circumstances such as ambient temperature and etcetera.
To achieve a very reliable design for the engine mount, it is essential to set the ambient temperature and humidity to real life conditions when simulating.
For example, the engine mount will get softer during the summer and relatively stiff during the winter hence, we iterate the simulation with varying parameters and constraints to optimize the design.