A bilayler metamaterial silencer allows substantial fluid through the apparatus, while mitigating the propagation of sound through the apparatus, and while providing a form factor that is significantly more compact than previously-known devices. Moreover, illustrative embodiments allow a designer to specify one or both of the frequency or frequencies at which the apparatus mitigates sound propagation, and/or the bandwidth around the frequency or frequencies at which the apparatus mitigates sound propagation.

An SCR system includes a box body, a urea nozzle and a mixer. The mixer is arranged inside the box body, and is cylindrical, and the first end of the mixer faces the urea nozzle. The mixer comprises a check ring, a swirl tube and a perforated pipe which are connected in sequence. The check ring is arranged close to the urea nozzle, and is provided with a plurality of first through holes. Multiple second through holes are formed in the swirl tube, and multiple third through holes are formed in the perforated pipe. The mixer further comprises a retaining ring, an inner ring of the retaining ring is connected to an outer wall of the swirl tube and/or the perforated pipe, and an outer ring of the retaining ring is connected to an inner wall of the box body. Also provided is an engine.