As cars go deep into people's daily lives, more and more car problems are exposed. Engine knock is also a relatively common problem. Many friends who don't know much about this are very confused after listening to this word. They think it is esoteric and not easy to understand. In fact, engine knocking is not complicated. As long as the principle of knocking and the factors that cause knocking on gasoline engines are clarified, the occurrence of such hazards can be reduced more fundamentally.

Formation of engine knock

After the mixture (air and fuel are fully mixed) enters the combustion chamber during the intake stroke, the piston compresses it during the compression stroke. After the spark plug ignites the high-pressure mixture, the pressure generated by its combustion is converted into engine operation. power. Although engine combustion can be described in a few words, the combustion research of internal combustion engines alone is complicated, so it takes a lot of work to really understand the engine. It is precisely because the combustion of the engine is very complicated that it requires quite precise design and control. A slight control error or abnormality will cause abnormal combustion, and "knocking" is a kind of abnormal combustion. Simply put, knocking is a pressure aberration in the combustion chamber caused by abnormal combustion.

"Knock" not only aggravates engine vibration, but also produces knocking sound, reduces engine output and damages engine structure. Knock can be said to be the natural enemy of engine designers. Many designs that increase horsepower, reduce fuel consumption, and reduce pollution, such as increasing compression ratio, supercharging devices, and increasing cylinder wall working temperature, are all limited by the generation of knock.

Characteristics of Knock

The characteristic of knocking is that the ignition and combustion wave propagation are normal at the beginning, but a part of the oil and gas that should be burned is called "tail gas", because it is compressed by the expansion of the gas after combustion, which reduces its volume and temperature. And the pressure rises. Before the combustion wave reaches there, the temperature of a part of the oil and gas has reached the spontaneous ignition point, after reaching the spontaneous ignition point, after a period of delay in spontaneous ignition, it will ignite itself and spread outward rapidly. When the combustion pressure waves in the opposite directions of normal combustion and detonation meet, violent gas vibration will be generated and a unique metal impact sound will be emitted, so it is called "knock".

Classification of knock

A slight knock cannot be detected by human senses. Here we call it non-sensory knock. Therefore, when you can feel the noise and vibration generated by the engine knock, the knock situation at this time is already serious. Exceeded your imagination, we call it sensory knock. After the felt knock lasts for a period of time, it will cause serious damage to the piston, cylinder head, valve, piston ring, etc.