Improved Control Source Concept for Active Cancellation of Exhaust Noise

This work is based on a current project related to auxiliary power unit (APU) exhaust silencing technology development, specifically an active exhaust noise cancellation technology. The overriding technical challenge is identified as development of a frequency and amplitude controllable noise source that can survive in a hot and corrosive exhaust environment. Installation of an APU enables vehicle platforms to turn off the main engine and still utilize auxiliary equipment that requires electrical power. An APU with a small engine potentially consumes much less fuel than a main engine at low power levels. However, past APU integrations have resulted in unacceptable noise levels, often times louder than the main engine at the equivalent operating point. Exhaust tones created by combustion pose a unique challenge to external acoustic reduction because they generally occur at low frequency, often less than 100 Hz. Low frequency noise propagates efficiently due to its long wavelength, and requires significant space when treated with a conventional passive silencer or tuner. The performance of a passive silencer is highly dependent on volume and/or length, and effective silencing requires expensive packaging and weight burdens. The APU application is a very good fit for an active noise control solution due to its predominantly steady state operational characteristics. Typical automotive applications deal with rapid changes in engine speed, which introduce tracking and response time challenges for active control. APU's typically operate at relatively constant speed, eliminating the speed tracking challenge. A review of literature [ 1,2,3,4,5] indicates expanding success in integrating active noise cancellation in less severe environments such as headphones, electronics cooling systems, industrial stacks, and aerospace applications, yet limited success has been realized in developing a robust and durable system for use as an engine exhaust silencer. Several promising concepts have been proposed and patented in exhaust applications, but without commercial success primarily due to the lack of a suitable control source. Commercially available sensor technology is more mature and offers durable performance in the exhaust environment. The active exhaust silencing concept presented in this paper offers the potential for a high payoff solution to the challenges and compromises associated with combustion noise and passive silencing devices. Analysis also indicates that it is very effective in a small, lightweight package. The proposed control source includes an electrodynamic shaker that forces a flexible bellow mimicking a piston in a cylinder. The bellow acts as a barrier, effectively separating the control source from exposure to exhaust gas. The concept eliminates any need to seal a moving joint while providing tunable broadband performance. The system offers flexibility to perform under changing conditions and can be adapted to other engine applications without major modification to system architecture. The space claim of the selected concept is also smaller than an effective passive silencer targeting the same frequencies. Analysis indicates cancellation performance of roughly 30 dB of broadband attenuation, with 40-60 dB of attenuation in frequencies below 300 Hz.