In auto design, fuel economy saves money but silence is golden

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MSC Actran cavity_mode_neon_p7

Guest post by By Ze Zhou and Philippe Hebert.

Long before Rolls Royce boasted that the only thing passengers could hear inside their cars was the dashboard clock ticking, silence was literally golden in automotive design. Nothing has changed. As Elevating Sound reported in the blog post The Price of Quiet Driving, the quietest cars on the road are the priciest – Lexus, Mercedes, BMW, Volvo – while the loudest are economy models.

A quiet ride, however, is increasingly conflicting with an equally important design imperative: fuel economy.

Government mandates, fuel prices and the public’s growing concern for the environment have made mileage a key factor in vehicle design. Oak Ridge National Laboratory has determined that reducing a car’s mass by 10 percent increases mileage by 7 percent. The EPA says that for every 100 pounds taken out of the vehicle, the fuel economy is increased by 1-2 percent. There are also cost benefits to mass reduction. Using 10 to 20 percent fewer materials in a vehicle can reduce its costs by 5-15 percent.

As a result, auto manufacturers are experimenting with new designs, drivetrains and materials to decrease vehicle weight and improve gas mileage.

The problem is that lighter vehicles are also noisier vehicles. New lightweight materials like fiber-reinforced plastics (FRPs) and lightweight metals vibrate more than conventional materials – primarily steel – and can create more noise in passenger compartments. Though lightweight materials can also be used as noise insulation, engineers need a comprehensive view of their designs’ noise profile to identify noise sources and the best ways to mitigate them.

Engineers have known for years how steel deflects, vibrates and blocks noise in different applications. That knowledge had enabled them to approximate their designs’ acoustic properties and make vehicles consistently quieter. Their approximations were based on performance benchmarks accumulated through decades of working with steel.

Those benchmarks held up for all of those years largely because steel is an isotropic material, meaning that it has a uniform composition and behaves consistently under loads. Once engineers had benchmarks for steel’s performance in different applications, the benchmarks didn’t change because steel didn’t change.

Benchmarks derived from steel do not apply to new metals, and benchmarking doesn’t work at all with composite materials such as fiber reinforced plastics (FRPs). While steel is isotropic, composites are said to be “anisotropic”, which means that their structure and behavior under loads can vary widely.

The behavior of a composite material is different depending on the direction considered in the material. It is very stiff if the material is solicited in the fiber direction, while it will be very soft if it is loaded transversely to the fibers. Another challenge set by composite material is the fact that in addition to their anisotropy, their microstructure – fiber orientation or fiber content – can vary over the entire part.

In addition to composites, automotive engineers are often dealing with other new materials with unfamiliar properties. Companies are experimenting with laminate metals in auto bodies and using visco-elastic materials for interior sound dampening.

Automotive design engineers are not used to dealing with so much variability in their materials. So how can they incorporate lightweight materials into vehicle designs to increase fuel efficiency while making them quiet enough to please customers?

The answer is new vehicle design processes built around simulation technology that accurately models a vehicle’s sound profile from the beginning of the design process all the way through production.

Finite element acoustic simulation technology has been around for almost 20 years, but has been too complicated and expensive to use throughout the design process. Acoustic analysis has been confined to teams of highly trained Ph.D. analysts working in research departments, as opposed to design engineering. There were few solutions on the market because acoustic simulation required so much computational power that few companies could afford it.

Acoustic simulation’s price and complexity have dropped over the last several years. Automotive companies have steadily adopted acoustic simulation solutions such as MSC Actran, but the best example of integrating acoustic simulation into design processes might be at the aerospace company Airbus. It can serve as a model for automotive companies, which are only now starting to deal with noise issues at the level that Airbus and its predecessor have for almost 40 years.

Airbus is the successor to the consortium founded by French aerospace company Aérospatiale and the British Aircraft Corporation to develop the supersonic Concorde airliner in the 1970s. The Concorde was a technological success, but it was so loud that few airports would grant it landing rights.

Partly because of its predecessor’s experience, Airbus has worked steadily over the last 15 years to “democratize” access to acoustic simulation technology among its design teams. Today, acoustic simulation is fully integrated into Airbus’ in-house engine noise optimization processes.

Design engineers at Airbus’ Tolouse, France headquarters use Actran at the beginning of the process to get a broad idea of which designs and material use will yield the best balance of silence and fuel economy. As they get closer to a final design, they can adjust the parameters for optimal performance. This system eliminates guesswork and needless iteration. It avoids costly late-stage errors through constant simulation that reveals when an idea is going awry.

Airbus design engineers can initiate acoustic simulation calculations to determine, for example, engine noise levels at various speeds, temperatures and altitudes, and how changing the design of the nacelle liner design would affect them. Working in a simulation model on the network, they change parameters then submit them for calculation. MSC Software’s Actran runs the simulation and reports the results back to the engineers.

Airbus also uses acoustic simulation to manage interior noise by making material choices for fuselage design. Engineers use the acoustic simulation solution to virtually design the acoustic behavior in the cockpit or cabin, and the noise transmission from exterior to interior. They can compare noise levels between designs using traditional aluminum or newer composite materials on the fuselage. The resulting insight into their designs’ sound profiles gives Airbus engineers the ability to balance noise with fuel efficiency considerations through the process.

It’s easy to see how the Airbus model would apply in the automotive industry. The heart of the design process would be a simulation model created by various design groups working in a common data format. Part and assembly models would include the properties of their constituting materials. Every design engineer would be able to run acoustic simulations against the design and material property data in the model through desktop or server-based applications.

This on-demand access to acoustic simulation technology gives engineers previews of their designs’ sound profiles well before they commit to expensive prototypes. Armed with advanced knowledge, they can experiment with different shapes and masses to reach a balance between a quiet ride and fuel economy.

Ze Zhou is a senior application engineer and product marketing manager at Free Field Technologies, an MSC Software company; Philippe Hebert is an application engineer at e-Xstream Engineers, also an MSC company.

To learn more about cars and sound/noise, get your copy of the Quiet Cars eBook – Your Guide to a Quieter Ride – plus join the Quiet Cars online community.

Quiet Cars - An Introductory Guide to a Quieter Ride (eBook and online community)