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Innovative Passive Exhaust Valve Improves Sound Quality and Reduces Muffler Volume without a Backpressure Penalty
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
Exhaust systems traditionally require a given amount of muffler volume to reduce sound levels appropriately. However, as hybridization continues, package space is limited, reducing available muffler space, requiring alternative solutions to attenuate exhaust sound with less volume. Passive exhaust valvesare a key solution, leveraging the physics of the exhaust (flow, temperature, and pressure) to cycle the valve. Passive exhaust valves are typically closed under low flow conditions (low engine speeds and loads), which helps reduce low-frequency boom, moderately increasing backpressure when it is not detrimental to engine efficiency. Conversely, under higher engine speed and load operating conditions, as when power is on demand, exhaust flow increases, and backpressure is critical to achieve desired power output, thus the passive valve opens to reduce its impact. And, such valves are often positioned within the muffler, fully immersed and exposed to exhaust heat, humidity, and corrosion, as well as the vibrations from road and engine loads. A next-generation passive valve is detailed in this paper, describing the engineering development to evolve the design, as well as the operating advantages, including compact size (fits inside mufflers), low mass (all sheet metal forming, no castings), reduction of backpressure (valve opens without applying force against the flow because of its innovative spring-lever design) and muffler volume. These valves are sized in various diameters to support applications across a range of engine sizes, and each can be further tuned by varying spring and mass damper characteristics. Flow and thermal conditions are modeled with both 1D and 3D tools, verified with appropriate bench testing, and validated across a range of conditions, including durability and corrosion. The paper will highlight results, conclusions and recommendations in support of future development enhancements, such as adopting in-pipe configurations upstream of the muffler.