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Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.
In megayachts, the engines and alternators let out unwanted noise and vibrations. To solve this, the solution is a double elastic suspension where the engine and alternator are mounted with vibration dampers on a common frame. Then, this is mounted elastically between the common frame and the hull.
The torsional deflection of a simple cylinder cannot radiate efficiently acoustic noise, but with particular boundary conditions the stator can radiate acoustic noise under torque ripple excitation. [8] Structure-borne noise can also be generated by torque ripple when rotor shaft line vibrations propagate to the frame [9] and shaft line.
The smaller, enclosed fans produce less noise, particularly at higher machine speeds. [8] Alternators can also be water-cooled in cars. Larger vehicles may have field coil alternators similar to larger machines. [9] The windings of a 3 phase alternator may be connected using either the delta or star (wye) connection regime set-up. [10]
To detect knock, a piezoelectric knock sensor (basically a microphone) bolted to the engine block responds to unique frequencies caused by engine knock. The sensor generates a small voltage that is sent to the electronic control unit , which processes the signal to determine if, in fact, knock is occurring.
In space, no one can hear you scream -- but you may hear a knock. When he was alone in a spacecraft in 2003, astronaut Yang Liwei reportedly heard a "knock" despite being alone.