A few days ago, I wrote an article, which simply said that the underlying causes of engine pulling cylinders, burst cylinders, top cylinders and other failures occurred, the response was still relatively good, and the netizens commented and interacted more warmly. Today, some netizens asked me, saying that the engine knocks on the cylinder, why does the engine occasionally have a "bell" sound when it starts cold? This problem is related to the above several faults, which is a knowledge point omission, and this pit is filled in today. The so-called engine knocking cylinder refers to the phenomenon that the piston in the engine hits the cylinder wall and makes a "bell" sound. The biggest difference between it and the above several faults is that the pull cylinder, the burst cylinder, and the top cylinder are all serious engine mechanical failures, and the knocking cylinder is not necessarily a failure! For all reciprocating piston engines, there will be a knockout phenomenon, but the severity is different; and in some special operating conditions, the engine will inevitably have a knocking phenomenon. Of course, if it is a serious knocking sound, it is a mechanical failure. As mentioned in the previous article, there is a gap of 0.05 to 0.10mm between the piston and the cylinder wall of the engine, the size of the gap is different for different engines, and the gap for the engine at different temperatures is also different, the gap is larger when the general cold engine is cooled, and the gap is smaller when the heat machine is hot. Due to the existence of this gap, when the piston is reversed at the upper and lower stop points, it will hit the opposing cylinder wall, making a knocking sound.
Let's elaborate on that. For example, the piston moves from the lower stop point to the upper stop point, under the action of the connecting rod thrust, the piston is up along the left side of the cylinder wall; when the piston runs to the upper stop point, it changes direction and runs downward, and the piston will go down along the right side of the cylinder wall under the pull of the connecting rod. In this way, when the piston is at the upper stop point, there is a transition from the left side to the right side. In this process, the piston will have an impact on the cylinder wall under the action of side pressure, which is the basic principle of knocking the cylinder.
It can be seen that all the reciprocating piston engines will send a life plug at the upper and lower stop points, so there will be a knocking cylinder phenomenon. The engine has four strokes in one work process, and the piston is reversed four times, so theoretically there will be four knocks. Only at the lower stop point, the cylinder pressure is small, the lubrication is better, the cylinder wear is also small, the knocking sound is not obvious, and it is generally ignored; and at the upper stop point, it may be the end of the exhaust stroke, or it may be the end of the compression stroke, the start of the work stroke, in these two cases, the pressure at the top of the piston is relatively large, so the piston reversing speed is relatively fast, and the knocking sound is more obvious. Especially at the end of the compression stroke and the beginning of the work stroke, the piston is almost "tapped" from one side to the other under the action of huge pressure, and the sound of knocking cylinder is more obvious.
In order to alleviate the phenomenon of engine knocking, the engine has made structural improvements, the most common way is piston pin seat bias. That is, the piston pin seat is offset by 1 to 2mm in the direction of the pressure on the working side. In this way, when the piston is close to the upper stop point, the gas pressure at the top of the piston makes the piston tilt, and the skirt at the lower end is first reversed; then the piston crosses the upper stop point, the side pressure reverses, and the skirt of the lower end of the piston is the support point bias. In this way, the piston commutation is divided into two steps to complete, and the commutation time is extended, the impact force is reduced, and the knocking phenomenon is greatly reduced.
The engine is in the cold engine state, the gap between the piston and the cylinder wall is larger, the lubrication between the two is not good, and the impact force when the piston is reversed is also larger, so many cars will have a cylinder knocking sound when the cold engine is in the state. The "bell" sound that some engines make during cold start is this kind of knocking sound. Especially diesel engines, this phenomenon is particularly obvious. When the engine temperature rises, the gap between the piston and the cylinder wall becomes smaller, the lubrication is improved, and the knocking sound is less obvious. This is a normal phenomenon.
During normal operation of the engine, the highest pressure point in the combustion chamber occurs after the piston crosses the upper stop point and begins to descend. If the engine ignition time is too early (or the injection time is too early), the piston has reached the upper stop point, the highest pressure point has appeared in the combustion chamber, at this time the piston is reversed very quickly, and the piston violently slaps against the cylinder wall on the opposite side, making a loud noise. This phenomenon generally occurs in the case of premature ignition time (or premature injection time), high-load operation of the engine (such as heavy-duty climbing, sharp acceleration, etc.), commonly known as "calling rod". In this case, the general engine will rise slightly to meet the needs of the car's short-term overload operation. If you are in this condition for a long time, the engine will wear out badly and the life will be shortened.
There is also a situation, that is, when the engine deflagration occurs, the combustible mixture at the top of the piston is multi-point combustion, the pressure inside the combustion chamber is abnormally elevated, the air flow is disordered, the piston is under the impact of high temperature and high pressure gas, high-frequency oscillation occurs, and the left and right swings repeatedly impact the cylinder wall, making a sharp sound, which is also a kind of knocking cylinder. This belongs to the knocking cylinder failure caused by abnormal engine combustion, as long as the deflagration is eliminated, the failure disappears. The most common is the use of gasoline that does not meet the label, such as the use of No. 95 gasoline, and the owner of the car uses No. 92 gasoline, when the engine is heavily loaded, the gasoline deflagration in the engine, resulting in a knocking cylinder failure.
In addition, the phenomenon of knocking the cylinder of the engine is a mechanical failure. For example, the gap between the piston and the cylinder wall due to excessive wear leads to increased knocking cylinder; or poor lubrication between the piston and the cylinder wall, the metal part of the two is in direct contact, there is no oil film isolation; or the connecting rod deformation, cylinder block deformation leads to cylinder centerline offset, piston partial cylinder, or piston size deviation is too large, etc., will also lead to knocking cylinder failure.
When the engine knocks on the cylinder, a continuous rhythmic "bell" metal knocking sound is emitted in the middle and upper parts of the engine, and when the engine speed and temperature change, the knocking sound will disappear or weaken. In daily work, it is usually used to judge the specific cylinder position by cutting off the fire or oil: if a cylinder is broken or the cylinder is cut off, the knocking sound disappears, and it can be judged that the cylinder has a knocking cylinder failure. Generally speaking, if the engine only knocks the cylinder when the cold car is cold, and disappears after the hot car, it can be dispensed with; if the cold car is normal, the hot car knocks the cylinder, or the cold and hot car knocks the cylinder, it belongs to the engine mechanical failure and must be dismantled and repaired.