Innovative Passive Exhaust Valve Improves Sound Quality and Reduces Muffler Volume without Backpressure Penalty 2020-01-0410

Exhaust systems traditionally require a specific amount of muffler volume to reduce sound levels appropriately. However, as hybridization evolves, the packaging area becomes smaller, reducing available muffler space and requiring alternative solutions to attenuate exhaust sound with less volume. Passive exhaust valves are a key solution, leveraging the physics of the exhaust (flow, temperature, and pressure) to cycle the valve. Passive exhaust valves typically operate in a closed position under low-flow conditions (low engine speeds and loads), which helps to reduce low-frequency boom, moderately increasing backpressure when it is not detrimental to engine efficiency. Conversely, under higher engine speed and load operating conditions, when exhaust flow increases and backpressure is critical to achieve desired power output, the passive valve opens to reduce its impact. Such valves are often positioned within the muffler, fully immersed and exposed to exhaust heat, humidity, and corrosion, as well as vibrations from road and engine loads. A next-generation passive valve is detailed in this paper, highlighting its operating advantages, including compact size (fits inside mufflers), low mass (all sheet metal forming, no castings), and reduction of backpressure (valve opens without applying force against the flow because of its innovative spring-lever design). These valves scale across various diameters to support applications across a range of engine sizes, and each can be further tuned by varying spring 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. This paper highlights results, conclusions and recommendations in support of future development enhancements.