Abstract: An HVAC system comprises an air flow path, a heat exchanger in the air flow path, a blower in the air flow path to move air through the heat exchanger, and at least one Helmholtz resonator in the air flow path. The Helmholtz resonator may be placed in numerous places throughout the air flow path. The Helmholtz resonator may be adjustable. The Helmholtz resonator further may be automatically adjustable. The automatic adjustment may be based on a characteristic of at least one of the blower or the air in the air flow path. Characteristics of air moving through the air flow path may include airspeed, absolute pressure, a pressure change through a portion of the HVAC system, or an audio characteristic. The automatic adjustment may be based on a feedback pressure or an electrical signal from a sensor. The system may have multiple Helmholtz resonators.

Abstract: An engine exhaust gas system includes a conduit for transferring exhaust gas, and a butterfly valve within the conduit for controlling communication of exhaust gas through the conduit. The butterfly valve is mounted on a shaft for pivoting relative to the conduit. The shaft is supported on a bushing in a transverse bore. Leakage through the clearance between the bushing and the shaft acts upon the transverse face of a washer which is freely slidable on the shaft, urging the latter into engagement with a retaining ring carried on the housing to effect sealing of the clearance. As the pressure of exhaust gases increases, the washer is urged more forcefully against the retaining ring, thereby improving the effectiveness of the seal.