Influence of a Hydra-mount on overall Vehicle-NVH

Since 3 decades, special hydraulic engine-mounts are in market for luxurious passenger cars as a part of passive vibration control. They are considered to give superior secondary ride-comfort (in comparison with normal rubber mounts) to Drivers while passing over a coarse road surface.

On such roads, tyre-road impacts excite a rigid body bounce mode of the engine-mounting system of the vehicle close to 10 Hz. Tuned damping of a hydra-mount at this frequency is supposed to kill such a resonance keeping the engine vertical movement under control without affecting dynamic performnace at higher frequencies.

This is especially useful on highways of developed countries which are otherwise smooth (enabling car-drivers to keep a high speed of driving over them > 50 km/hr) but can suffer from unexpected regularities at junction of 2 road patches. The hydra-mount is supposed to assure then a carpet ride to the passengers over a long drive.

But is this benefit realized on pur Indian roads ? Majority of our hatchbacks and sedans are without such hydra-mounts and there have been no complaints from drivers on different roads. Some of our high-end SUVs use them and with their recent trend of using mono-coque bodies (meaning, avoiding rubber mounts between chassis and cab), hydra-engine mounts promise to raise their NVH refinement to a higher level.

It should be however noted that design of the hydra-mount needs special attention.

In one of a recent diesel engine-powered SUV models, first there were found issues of fluid cavitation giving field complaints of rattle heard from the mount on rough roads at low speeds like 20 km/hr. Then, it was surprisingly observed that high frequency noise had increased (reducing % Articulation Index) inside the cabin on smooth roads at all high speeds above 50 km/hr as compared with a normal rubber (non-hydra) mount.

The NVH deterioration was due to a fact that dynamic stiffness of the hydra-mount was higher (as compared with the rubber mount of the same static stiffness) at all frequencies away from the tuned one viz., 10 Hz (which was close to the engine-mounting bounce natural frequency) of this particular SUV. This increased mid/high frequency vibrations of the body and led to higher structure-borne noise.

The same experience was observed in another small proto-car in the past, where a hydra-mount (HM) was deployed to reduce vibrations at the Driver’s seat at idling for 1st engine-order (1EO) near 14 Hz. It was tuned at this freq. & gave a good benefit but at the sacrifice of comfort at higher firing orders.

Thus, design of the hydra-mount--- related to decoupling of the upper and lower fluid- chamber thru’ a separator plate & transfer holes (inertia track) as shown above----- is critical for assuring the best NVH of the vehicle for both low and mid/high frequency road & power-train excitations.