# Modelling of pneumatic engine mount

Amélie Renault
Göteborg : Chalmers tekniska högskola, 2005. 60 s. Examensarbete - Institutionen för bygg- och miljöteknik, Chalmers tekniska högskola; 2005:48, 2005.

The goal of this Master thesis has been to build a mathematical model of a pneumatic damped engine mount, which could be a cost effective alternative to hydraulically damped engine mount. This model will help the pneumatic mount and its damping mechanism. An engine mount must satisfy at least two essential but conflicting criteria. First, the mount has to carry the static gravity load of the engine. The second requirement is that the compartment should be isolated from vibrations and acoustically comfortable: the mount has to be compliant and lightly damped for small excitation amplitudes and over the higher frequency range. The third requirement is to isolate the car from the motions due to bumpy roads, abrupt vehicle acceleration or deceleration, and braking and cornering. That means that the mount has to be stiff and highly damped. Conventional rubber mount cannot satisfy both requirements simultanelously and a compromise between resonance control and isolation is inevitably needed, which is to get non-linear stiffenss and damping dependant on frequency. There are many disadvantages to the hydro mounts: it is big, heavy, complicated and expensive. Then, an air damped engine mount could be an issue to drecrease the size of the damping device (it has only one chamber) and lower the price (it is quite simple and is based on air compression). But a pneumatic mount is less damped than the hydro mount and the damping has to be increased in order to damp heavy engines. In this report will be presented an overview about hydro-mechanical mount, pneumatic mounts and some tools that will be used to model the rubber dynamic properties, the air volume and the orifice behavior. After that, the model will be explained. Finally, some proposals how to increase the damping will be explained. After having understood how an hydralic mount is working and explored some simple pneumatic spring, the model of the pneumatic mount has been implemented in MATLAB and finally, the calculated curved are compared to the measured one. The mount has physically been divided in three parts in order to model it more easily and the conclusion of the model-experiment study shows that: - The rubber part is well modelled with a spring in parallel with a dashpot, - The orifice impedances seems to be quite correctly modelled since the global model predicts really well the resonance frequency, - The air compliance is not modelled accurately enough. It can also be remarked that the simplified orifice impedance is working well for the chosen frequency range and for this application.

Nyckelord: engine mount, pneumatic mount, damping modelling, oriface damping, engine suspension.

CPL ID: 6668

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