Clean Snowmobile Challenge Muffler

The goal of this project is to design a muffler that significantly reduces emissions and noise output compared to the stock muffler without noticeably reducing engine performance or increasing weight. We must have a fully assembled muffler before the Clean Snowmobile Competition, held in March. The muffler must be tested pre-competition with relative data that can be presented in the team’s technical paper and presentation.

Background
For the past 15 years, the University of Idaho has had a team compete in the SAE Clean Snowmobile Challenge. This challenge is based on making modifications to stock snowmobiles in order to reduce noise and emissions while increasing fuel economy, so that the snowmobile industry has less effects on the environment. This year, the UI CSC senior design team will be re-designing the muffler on the snowmobile in order to further address two aspects of the competition - Noise and Emissions.

Deliverables

 * Background research on various sound reduction technologies
 * CAD models and flow simulations of different sound reducing technologies and their effects on backpressure
 * Assembled muffler

Design Development
Although the UICSC team has made noise a focus for development in the past, it has remained a problematic area. To further address this issue, the team developed new testing apparatuses and procedures to better understand snowmobile sound production and mitigation. This includes determining sources of noise as well as controlled testing of various noise attenuation devices. All final sound reductions on chassis were found using the J1161 sound test.

Anechoic Sound Box
To improve understanding of sound attenuation, the UICSC team manufactured an anechoic sound box testing apparatus, referred to as the UI sound box (UISB). The initial design of the UISB was based on an existing design, which was used to test the acoustic effectiveness of quarter-wave and Helmholtz resonators [10]. The anechoic sound box was designed to emit pure frequencies through a waveguide (pipe) without interference. The box contains amplified speakers and tweeters acting as a sound source directed into the UISB. The housing was built using 1.91 cm (0.75 in) high density fiber board internally lined with the studio-foam. Attached to the sound box is the waveguide, which extends to the environment with a removable center section. This section in the center of the pipe is removable for the purpose of testing individual acoustic components. The component length was held constant so the waveguide spanned the same distance for each test. Four microphones were placed along the pipe at the specified locations from the outlet of the UISB. The microphone’s signals were analyzed using a Digilent electronics explorer board.

Muffler Design
The following strategies contributed to the largest reduction in sound: expansion chambers with larger cross-sectional areas, ordering expansion volumes from small to large in series, and “V”-shaped geometric interference plates of the same size. Solidworks was used to simulate fluid flow and back pressure of the muffler components. A flow bench with variable flow rate was used to compare back pressure values to simulations. The average difference between the Solidworks simulations and flow bench measurements was 58%. This was due to the simulations being calculated based on steady flow while the flow bench utilizes vacuum motors that create pulses. The UICSC muffler design is called Red Dawn (AM)2. Using this information, the UICSC team designed four mufflers that were simulated through Solidworks for fluid flow and back pressure, while Sidlab was used for transmission loss.

Flow Simulations
The arrows represent the pressure in each chamber of the muffler. Through the design process round edges were placed to ensure the smoothest flow characteristics and reduce the chance of creating high frequency noise

Before Competition Results
The Red Dawn (AM)2 muffler performed better than stock at higher frequencies, which are weighted higher in the A-weighted scale. All testing assumed 56.3 kph (35 mph) at 30% throttle. This gives an accurate representation of typical cruising speeds. The results for the Sidlab simulations are shown in the Figure. The muffler was tested using the J1161 with a result of a 1 dBA loss compared to the 2016 configuration. The UISB also was used to validate the (AM)2 showing that at the lower frequencies there was improved performance over stock, shown in the Figure

Additional Information
A muffler’s acoustic performance is typically a function of its increased back pressure. Some back pressure is needed for optimum engine performance, but too much results in power loss. The muffler was designed to be a standalone component and have similar back pressure to the stock component. The measured back pressure of the 2017 muffler with catalyst is 75% higher than stock at the max flowrate of the engine. This resulted in very little power loss from the engine after recalibration. The Table represents the various configurations tested.



Final Muffler Results
The custom muffler was able to reduce noise emissions by 5 dBa compared to stock. It maintained the performance of the stock system as no significant power loss could be directly attributed to the muffler. The muffler was able to reduce the weight and cost of the muffler and increase the operational temperature compared to stock.



Future Work
After extensive use on the dynamometer and at the UICSC competition, an analysis of the muffler was need to view the durability of the muffler. The outer heat shield was removed and viewing ports were cut into the sides. The picture shows an example view of this process. Both chamber previously contained sound material. However due to temperature and flow going through the sound material the sound material was missing.



Along with missing sound material cracks and holes were in locations were the stresses and heat were high. A hole was located at the end of the piping where the flow met the upper wall. This hole was due to vibrations, impacts, and brittleness due to high temperatures. Cracks were located around the outlet of the muffler. This was due to stressed caused by the high temperature and the muffler supporting its weight at this location. The metal was also repeatedly elastically deformed at high temperatures due to external forces and the weight of the emission pipe.

After the analysis, the design of this muffler has the potential to be improved. Higher grade steel and better welding would have better resistance to heat and vibrations. A higher temperature ceramic fiber should be used for the catalyst temperatures. A mounting technique would allow for better support for the muffler reducing weight bearing stresses. For the design itself, adjusting the outlet so the pipe doesn’t block the flow would decrease the back pressure of the muffler. Larger volume and size of the muffler would allow for better attenuation especially and lower frequencies, but chambers designed for peaks of exhaust noise emissions is highly recommended.