What are the hazards of engine knocking?
1 Answers
When knocking occurs, it is accompanied by shock waves, causing vibrations and producing a metallic knocking sound. In mild cases, it affects normal comfort, while in severe cases, it can have adverse effects on the engine. The hazards are as follows in three aspects: 1. Decline in engine economy and power performance: When slight knocking occurs in a gasoline engine, it approaches constant-volume combustion, slightly improving power and thermal efficiency. However, during moderate or severe knocking, the sudden increase in pressure and temperature at the knocking site generates multiple pressure shock waves. These shock waves reflect multiple times between the cylinder walls, piston top, and combustion chamber ceiling, creating high-frequency oscillations. The shock waves disrupt the laminar boundary layer on the cylinder walls, significantly increasing heat transfer to the outside, raising heat loss, and reducing output power, thereby decreasing the engine's economy and power performance. 2. Reduced reliability of engine components: During knocking in a gasoline engine, the rate of pressure rise and maximum explosive force increase, with the rate of pressure rise being 4 to 8 times that of normal combustion. This imposes tremendous stress on engine components, accelerating wear on piston connecting rod bearings and main bearings. 3. Worsened engine emissions: During knocking combustion, the increased temperature in the combustion chamber accelerates the decomposition of combustion products into CO, HC, and NOx. In severe cases, the liberated free carbon does not have enough time to form CO2 and is expelled with the exhaust, increasing the PM content in the exhaust and aggravating air pollution.
