Pugh's Decision Matrix for Vehicle NVH Sign-off

Making scientific efforts for Vehicle NVH Refinement under different customer driving operating conditions is on one side and implementing necessary the solutions in a mass production is on the other side.

There is always resistance by the Management to take ahead various NVH recommendations mainly due to cost addition, long development time and durability concerns.

In addition, the design changes made for improving small amplitude but higher frequency vibrations on smooth roads and structure borne noise are not necessarily suitable for transient large-displacement load cases on pot-holed roads or ones with sudden torque reversals.

For example, soft rubber-mounts supporting a Power-train (PT) on a stiff chassis are good for increasing the vibration isolation and hence for assuring the best possible In-cab NVH. However, they allow large PT- motion under sudden torque changes (tip-in-tip-out or key-on-key-off or 1st gear launch) or while running over bumpy roads. Durability of the rubber-mounts under rough road impacts will be another concern.

Active Vibration control or advanced switchable hydra-mount can be then a good solution but their cost addition will be serious from business profitability point of view. In addition, their robustness is yet to be proved on Indian roads for a warrantee life more than 1 lakh km of running. .

Another option is to provide secondary isolation to the vehicle-suspension sub-frame or cradle over which PT is mounted with the rubber-mounts.

They have the potential to maximize the vibration isolation efficiency against the combustion induced or electromagnetic forces. Here, the chassis or body should have 5 to 10 times dynamics stiffness higher than that of the cradle-bushes over a frequency range at least till 600 Hz. Hence, here, too, the Cost to Benefit Ratio might be on higher side.

Hence, every time, a famous Pugh's decision matrix becomes a part of Engineering Sign-off for the NVH solutions; a sample example below elaborates the challenge before the Management which is eager to launch a new vehicle in the market:

The above Pugh's Matrix shows that the chassis modifications enabling ideal position of PT-mounting is a winner under all conflicting performance demands! An elastic axis coinciding with the Power-train Roll axis will decouple all of its rigid body modes balancing both requirements of steady state and transient load cases.

Next to this solution is idle engine-RPM tuning to have modal separation between the firing order and the natural frequencies of PT- rigid body modes! Engine Calibration Unit [ECU] tuning to limit the combustion forces as well as flywheel angular fluctuations will play a critical positive role to improve NVH of the vehicle.

However, often both these counter-measures (either chassis designed in line with the PT-Roll-axis or ECU tuning) are not taken ahead during tool-kick off stage due to some constraints faced by the Management.

Then the solutions like the softer rubber mounts or the active control will be necessary where their fatigue life or Cost & time of development will be a concern!

Even providing a Counter-balancer to cancel inertia forces of reciprocating masses of the engine will be considered which will give good benefit to NVH at all speeds but increase significantly the PT- cost & its development time.

In conclusion, a Pugh's decision matrix will be always needed for all NVH-critical subsystems or components of the vehicle influencing its conflicting performance parameters!