Inhaling Clean Air, Exhaling Pure Sounds

3D acoustic simulation helps MANN + HUMMEL GmbH engineer the perfect acoustics of air intake systems and reduce its manufacturing costs

The light turns from red to green. A sports car starts off with a roar. You can feel the envious glares from nearby pedestrians as they cannot resist the temptation to look. That powerful sound of a sports engine is a carefully engineered mix of tunes and tones, bass and booms. But even the familiar engine sound of a luxury limousine or a businessman’s sedan is thoughtfully designed by acoustics engineers. That distinctive engine sound mirrors the brand identity of a vehicle or manufacturer.

Creating the right sound has long been the task of engine acoustic specialists and exhaust acoustic engineers. But not anymore. The air flow tubes and ducts, the air inlet of a car, are sound media that cannot be neglected. Engine noise originating from the combustion process is radiated into the ducts up to the inlet, producing undesirable sounds. Not only should a car’s engine sound right, it should also be fairly quiet. Noise emission regulations are becoming more and more stringent. In order to reach demanding target levels, car manufacturers and parts suppliers (from air intake to exhaust suppliers) need to work hand in hand to get the sound right.

Performance, durability, acoustics - the multiple roles of an air intake system

MANN+HUMMEL is one of the original equipment suppliers that have taken the acoustics challenge seriously. The company’s product portfolio includes air filter systems, intake manifold systems, liquid filter systems, cabin filters, and cylinder head covers. MANN+HUMMEL designs and manufactures complete air intake systems, from air inlets to air manifolds.

“More engine power and less fuel consumption”; this is the shared objective amongst all car manufacturers. The air intake system functions as the lungs of a car by supplying and filtering ambient air to the engine. It delivers the right amount of clean air to it to ensure the most efficient combustion process. It filters and separates oxygen from dust or humidity particles. What remains is pure air that enters the combustion chamber.

It is not only engine performance, but also engine reliability, which requires flawless air intake. Clean air is essential for smooth, long-term engine operation. Pollutants, like dust or soot, could enter the combustion chamber and cause premature wear on the engine. Air intakes are designed to ensure maximum performance and durability of the engine under any operating conditions. Next, the acoustic needs to be considered.

MANN+HUMMEL, a global team, is dedicated to relentlessly improve the acoustics of air intake systems, which have critical acoustical behaviors by nature. Air cleaning systems are typically deemed as troublemakers. The acoustics engineering team performs full acoustical analyses on those air cleaners, from concept to validation and troubleshooting. The task is always difficult because the engineers have to adjust a design that has already been optimized for maximum engine performance and reliability. They can hardly modify the design for the purpose of acoustics optimization. While acoustic optimization comes last in the process, it is not the least of engineering challenges. In a competitive world, perfect air cleaner acoustic is a strong asset that enhances the value of a brand.

Recently, MANN+HUMMEL has taken acoustic engineering to the next level by introducing the Symposer, a sound quality optimizer. The Symposer is an additional component, placed at the heart of the engine block, with a membrane that resonates at a fixed frequency. It transmits a desired sound into the car’s interior. With the Symposer, the intake system is now used as an instrument to emphasize the acoustic character of the car.

Sand, dust and water - local constraints influence the design

Due to local constraints, regional teams in France, Korea, US, Japan and China have also adapted sound characteristics. In some Asian countries, air intakes need to be fitted with an extra valve to prevent water from entering the air duct. This influences the overall acoustic behavior and necessitates additional analyses to ensure that the target sound pressure level is met. Temperature variations, from extremely hot in equatorial regions to extremely cold around the polar circle, or humid air conditions under tropical latitudes also influence the air intake acoustical design. When taking up the challenge of optimizing sound design around the world, MANN+HUMMEL acoustics engineers need easy-to-use, reliable tools that will let them understand, compare and share the acoustical behavior of numerous designs within the core German team as well as with remote engineers worldwide. 

3D acoustic assessment is indispensable

In the engineering world, prototypes are scarce valuables. Engineers at MANN+HUMMEL seldom have access to full vehicle or parts prototypes. Under these circumstances, they need to rely on methods that will allow them to optimize the product without the need for a prototype. Simulation is essential in the product development process to guarantee that the final product will meet customer’s requirements.

In low frequencies, simulation engineers consider that the system’s acoustical behavior is uniform on any section of the air duct, with unidirectional sound pressure waves. However, when frequencies jump over 500 Hz, transversal acoustic effects distort the analysis. Neglecting the rising transversal effects at higher frequencies leads to huge assessment errors. The trend towards gasoline turbochargers that boost engine performance has made high frequency peaks, with frequencies up to 7 kHz, a common phenomenon.

With this turbocharging trend, what acoustics engineers at MANN+HUMMEL need is a fast, efficient and reliable acoustics simulation tool that understands multi-directional sound effects. This tool should help fine-tune parameters with maximum precision at any stage of the development cycle. It should also be easy to use, so that experts and beginners around the world can work consistently with it. To efficiently solve its acoustic issues, MANN+HUMMEL has relied on LMS Virtual.Lab Acoustics since 1995.

Julia Schempp, NVH & Acoustic Engineer at MANN+HUMMEL, is an expert user and advocate of the software: “LMS Virtual.Lab Acoustics is very reliable and when we compare test to simulation results, we see a good match.” Matthias Alex, Head of the Center of Competence, and her manager, acknowledges that the software efficiently supports his global team when fulfilling the difficult task of accurate acoustics prediction:  “Thanks to an easy-to-use and dependable tool like LMS Virtual.Lab Acoustics, our local acoustic experts are able to respond quickly to any customer’s request.”

“Any size, any shape” - the subtle task of air intake acoustic optimization

There is no unique process for the acoustics optimization of an air intake system. It first

starts with the customer requirements. Some automotive OEMs may ask for a relative sound transmission loss (delta dB) on parts of the system, while others will deliver a target curve, showing sound pressure level against rpm, which the developed part has to comply with. The constraints, however, remain the same in all cases. Acoustics engineers perform improvements on a draft design that has been computed and optimized by the CFD department. Concrete data such as shapes, lengths, diameters of ducts and other elements can only be modified in exceptional situations. The air intake package size is also fixed. The final design needs to fit in a prearranged box, approximately the size of a shoe box, for the air cleaner. Following the motto “any size, any shape”, engineers adjust all parameters to create a design that will fill the often tiny space under the hood. Finally, there are the absolute laws of physics that engineers have to deal with. A long neck (total length of tubes connecting the intake to the engine) will induce low frequency resonances. A flat air cleaner box amplifies sounds like a loudspeaker. Therefore, there seems to be little freedom left for the engineers to tune the parameters.

In order to trap unwanted noise, the engineers add a rectangular piece, called a resonator, to the design. The engineers will place a resonator to target each annoying frequency peak, voiding the disturbing sound waves. The resonator achieves the desired acoustical effect without obstructing the air flow. It is only when troubling noise happens on a large frequency range that a re-design of the system, such as an increase of duct diameter, is considered to optimize both air flow and acoustics.

Because design optimization should happen as early as possible in the development process, the integration of turbochargers in the engine block brings a new challenge. Turbochargers require calibration, which takes place towards the end of the development cycle and often modifies existing frequency peaks. It can also create a shift of the resonance in the frequency range. The size, shape or position of the resonator needs to be adapted. Acoustic engineers have to solve this issue at a development stage where they have even less freedom to adjust parameters.

A fit-for-purpose solution

LMS Virtual.Lab Acoustics is the integrated solution to minimize noise and optimize sound quality in air intake designs. It simulates both internal and external acoustic radiation. Koen De Langhe, the Product Line Manager of Noise and Vibration at LMS, a Siemens business, explains, “LMS Virtual.Lab Acoustics for air intake and exhaust systems addresses the multiple noise issues that acoustic engineers typically face. Standard transmission loss analyses based on Finite Element Acoustics technology provide fast and clear insights into potential noise problems. Engineers also rapidly obtain dimensions of countermeasures such as resonators or quarter wavelength tubes, using a parameterized CAD for geometry-based optimization. In addition, LMS Virtual.Lab users can customize the software for their specific applications. The software perfectly fits the internal development processes and fully integrates into the CAE environment.” Beyond standard applications, LMS Virtual.Lab Acoustics tackles the most complex noise problems. Flow-induced noise, resulting for example from acoustic leaks in the air intake system, can be analyzed with LMS Virtual.Lab Aero-Acoustics, based on the input from CFD analyses. Internal pressure fluctuations or engine vibration can also excite the shell of the intake manifold. To address this issue, engineers need a vibro-acoustics solver, such as the integrated LMS Virtual.Lab structural FEM solver, that resolves the fully-coupled problem instantly and accurately. The effects of temperature and air flow on the acoustic performance are captured with high fidelity. Coupling these sophisticated acoustic technologies with LMS’s unique FEM Automatically Matched Layer (AML) technology results in accurate, fast and efficient noise simulations.

Automation helps keep up with competitive pressure

Time is always a constraint. Engineers sometimes have less than three weeks to optimize a design. A complete system model takes up to 100,000 degrees of freedom into account. “Fast solvers are essential to deliver results on schedule,” says Schempp. “The speed of the LMS Virtual.Lab Acoustics solver is a great advantage in that regard. But we need to go some steps further and find new ways to optimize our simulation processes. Automated tools are required. To predict sound pressure levels on the air cleaner, we have developed an automated process and have implemented this in LMS Virtual.Lab Acoustics. It lets us compute immediately a transmission loss curve, with all the correct boundary conditions. This saves significant development time.” Another automation tool provides orifice noise analyses, based on the absolute sound power level emitted by the engine. On top of saving time, automation standardizes processes. MANN+HUMMEL engineers will introduce more automated tools in the future.

Schempp explains: “We are currently in the deployment phase of a new library of standard components in LMS Virtual.Lab Acoustics that will especially help us to get through the quotation phase faster”.

An optimized design at optimized costs

Designing right the first time can have a big impact on profitability.  Size, position, but also total number of resonators may vary from one design to the other. Adding or deleting a resonator respectively increases or decreases the complete part’s price. Have a few cents multiplied by the total number of produced parts and price estimations might become totally erroneous. A quick and accurate price estimation based on fast and accurate simulation is a must. The engineering team actually has to give a price estimation on a design overnight.

There are other cost constraints that even the manager of an acoustics department has to consider. On the manufacturing side, huge scale economies can be realized when reducing the number of design variants. The number of engine variants has exploded in the past few years. Manufacturers do not focus anymore on the traditional 1.2 or 2.0 liter engines but introduce many variants such as 1.8, 2.2 or 2.4 liter engines. Meeting the customers’ requirements becomes more difficult from a technical point of view, but also raises the issues of manufacturing costs. In some cases, it might be beneficial to over-engineer a system that will be then be used together with both downsized and high-performance engines. A single design will fulfill the performance, durability and noise requirements of multiple engine variants, and can bring manufacturing costs down.

Engineers at MANN+HUMMEL tackle daily challenges when optimizing the acoustics of air intake systems. Extreme precision is required to fine-tune parameters within strict boundary conditions. LMS Virtual.Lab Acoustics is the trusted tool to perform accurate prediction on tough deadlines.

For more information, visit http://www.plm.automation.siemens.com/en_us/