Optimizing commercial vehicle muffler size, volume via CFD

  • 13-Oct-2014 11:20 EDT
Fig7-combined.jpg

The airflow fields of the existing (top) and downsized (bottom) designs are shown. The top figure showing that chambers C2 and C3 contribute least toward the muffler functioning. The bottom figure shows that exhaust flow is denser in C2 and C3 in the downsized version.

Vehicle lightweighting can be achieved by either using lightweight materials or by reducing the size of the existing components. Researchers at VE Commercial Vehicles Ltd. in India used the latter approach to design a commercial vehicle exhaust system using the ANSYS CFD tool Fluent, resulting in a 14.1% reductions in both size and volume and a mass savings of 2%.

Muffler optimization is one of the critical challenges to the automotive engineer. Conventionally, muffler design includes the exhaustive physical trial for various combinations of muffler by varying the diameter of pipe/perforated hole size/baffle position/damping volume, etc. This process can be streamlined with the use of simulation tools. Usage of simulation tools has improved the process by balancing the conflicting requirements such as backpressure and noise.

VE Commercial Vehicles researchers modeled the geometry of the muffler in CATIA V5, where the complete wireframe and face data are generated. The data was then translated to Initial Graphics Exchange Specification (IGES) format and read into Altair’s HyperMesh software, where the fluid surface model was extracted followed by tri surface mesh generation. The shell mesh size was kept to 1 mm (0.04 in) at the surface where holes are created and gradually increased up to 3 mm (0.12 in) at flat surfaces like baffles and pipe.

The options available for downsizing are along the length or width of the muffler, or both simultaneously. Downsizing along the minor axis of the muffler is difficult due to pipe construction and position. Further it will lead to increased tooling cost and new development of baffles. The decision of downsizing direction is further reinforced when layout of the vehicle is studied and it is found that the majority of issues came due to the length of the muffler rather than its width.

Based on this information, researchers decided to downsize along the length of the muffler. Chambers C1 and C4 remained untouched during downsizing. Reduction/expansion of C1 and C4 would lead to an increase/decrease in backpressure. C2 and C3 contribute least to the muffler functioning, with minimum contribution toward noise and pressure drop.

CFD analysis was then performed with downsized chambers C2 and C3. Exhaust flow became dense in chambers C2 and C3, leading to an increase in back pressure from 4.5 to 4.7 kPa. At the later stage of analysis, the number of holes in the inlet pipe was increased from 80 to 120. Pressure then dropped to 4.3 kPa. The obtained pressure drop is comparable and better with respect to the existing muffler. The result shows that there is improvement in exhaust flow restriction by 4.5%.

Pressure and velocity plot will determine the quality of noise. Exhaust gases in pulses will end up with discontinuous gas flow and create different noise. While high velocity escaping gases might create the whistling noise. The flow velocity and pressure remained the same for the downsized muffler, which shows that the quality of sound was not deteriorated with the existing design. Pressure is the main parameter for evaluating the noise level in a system. Therefore by comparing the pressure contours of existing and downsized muffler, inference can be derived for no change in noise level.

Verification of backpressure was performed on the downsized muffler at the vehicle level. Backpressure is measured by inserting the probe to the center of the pipe at its straight length. The barometer is connected to measure the backpressure. Similarly, pass-by noise testing is conducted on vehicle with the existing and downsized muffler. Results are encouraging and correlating with the simulated result for pressure drop.

In conclusion, the change in the number of holes in the inlet pipe and baffle position led to a change in backpressure value. To balance the same at par with existing, CFD iterations were done that showed the number of holes to be increased from 80 to 120 and distance between baffles to be kept as 220 mm in place of 275 mm.

The downsized muffler is at par with the existing muffler. However, it is better in terms of backpressure. There is 6.4% less pressure drop in downsized muffler with respect to existing design. There is no deterioration in the quality of noise and vehicle level noise (justified by pass-by noise test). The overall volume reduction in muffler is 14.1% leading to reduction in muffler weight by 2% and yielding cost benefits.

This article is based on SAE International technical paper 2014-01-2440 by Ashok Patidar, Shivdayal Prasad, Umashanker Gupta, and Mohan Subbarao of VE Commercial Vehicles Ltd.

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