Beware the NVH pitfalls of weight reduction

  • 14-May-2013 11:41 EDT
DSD 05-13 Findlay.jpg

DSD's Mark Findlay noted that the use of aluminum means that any noise generated in the driveline is more easily transmitted and may call for mass to be added back into a component or system.

“The weight savings possible from the use of alternative materials are very tempting—as much as 25% in some areas of transmission and driveline design. But there are pitfalls waiting to trap the unwary,” warns Mark Findlay, Managing Director of Drive System Design (DSD), a U.K.-based engineering consultancy.

Cutting CO2 emissions by reducing vehicle weight has what he terms “inevitable consequences” for the strength and stiffness of the systems and structures affected.

“For example, the use of aluminum means that any noise generated by the driveline is more easily transmitted. This can result in the need to add weight back into the driveline in the form of heavy mass damper solutions to reduce the levels of excitation and response,” he said.

And the larger-scale integration of carbon fiber into structures could create even worse issues, he believes. He said that if the use of carbon fiber construction becomes more mainstream rather than predominantly for supercars (such as in BMW's 2014 i3 hybrid and EV), noise transmission will become an even greater challenge due to the high stiffness of the material and its lack of damping.

Harmonic issues arise

Findlay noted that fundamental conflict exists between weight reduction and refinement because mass is a key parameter in the dynamics of any system. “Using lighter materials with higher strength-to-weight ratios to produce designs with higher power density often changes the inherent damping, the local loading conditions, and the resonant frequencies of the assembly,” he explained.

Although these challenges are generally understood by an auto industry well versed in the need to reduce mass, Findlay believes that if not implemented carefully, lightweighting strategies can still lead to NVH issues that are potentially difficult to eradicate because they arise from complex system interactions.

Thinner-viscosity oils and some types of low-friction bearings cut parasitic losses, but they also reduce system damping. Replacing ferrous casings with lighter materials introduces differential expansion when temperatures rise, affecting preloads, backlash, and running clearances, and the change in heat transfer properties between older, familiar materials and newer alternatives can affect heat rejection rates and local temperature gradients.

DSD specializes in the design and development of mechanical transmission systems and design of vehicle chassis systems within the motorsport, renewable energy, and hybrid/electric vehicle sectors. The company's engineers are currently investigating the design of driveline components specifically to meet the particular requirements that new lightweight vehicles have created.

Findlay regards the latest computer modeling techniques as allowing greater insight into system interactions, enabling critical modes and frequencies to be identified and appropriate action taken. He is also aware of a growing realization in the auto industry that the need to save weight must be linked with the resultant effect on the outputs of established vehicle systems. This is generating a new way of thinking about the way in which geared drives are designed, placing the focus more on system integration and the dynamic interrelationship with the surrounding vehicle structure.

DSD’s technical director, Alex Tylee-Birdsall, also underlines the NVH challenges of weight saving for EV drivelines because of the vehicles’ low cabin noise levels. 

“The range of an EV is affected by its efficiency, and the common measures to improve that efficiency, such as cutting friction and making components lighter, reduce the damping in the system," he said. "Achieving satisfactory refinement levels requires a thorough understanding of the interactions between different frequencies and the various damping factors involved.”

Tylee-Birdsall cited recent experience of how a new EV multi-speed transmission generated an audible whine in two ratios. “We established that the issue was not primarily related to the gear sets but to casing resonant mode alignment, triggered at certain meshing contact conditions," he recalled. "The addition of a rib to the casing walls to separate the modes was the only modification needed. This merely required metal removal from the die—a realistic solution in both cost and time.”

Supplier-OEM challenges

Findlay warned of an added element in the light weight/NVH discussion. He said the challenges are not only technical, but commercial boundaries between organizations could also create barriers to achieving an optimum solution because knowledge remained segmented across the supply chain.

“The OEM has the vehicle-level understanding but may lack the system-level knowledge to exploit it," he asserted. "The system supplier is expert within that domain but delivers against targets set by the OEM. Commercial boundaries often restrict the exchange of crucial information because of intellectual property concerns.

Much of DSD's work involves "breaking down these barriers, turning the OEMs into ‘expert’ customers, able to direct their suppliers towards an optimum solution,” Findlay noted.

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