Knock (engine)

Knock is abnormal combustion in a spark-ignited or Otto-cycle gas engine, where part of the unburned end-gas autoignites ahead of the flame front and burns almost instantly, producing a sharp pressure spike and a metallic rap. In marine low-pressure dual-fuel two-strokes such as the WinGD X-DF and the MAN ME-GA, which burn premixed lean gas on the Otto cycle, knock sets the upper limit on compression ratio, gas admission, and load in gas mode. Engine controls watch cylinder pressure and retard timing, trim the air-fuel ratio, or trip to fuel oil to avoid it. High-pressure diesel-cycle gas engines such as the ME-GI inject gas late and burn by diffusion, so they do not knock.