The phenomenon of piston knocking can usually be divided into two types: knocking with excessive clearance between the cylinder liner and the piston, and longitudinal knocking of the piston. The knocking sound with excessive clearance between the cylinder liner and piston is a rhythmic questioning sound, which occurs at the upper part of the cylinder. Some sounds are like the "click" sound when lightly tapping the cement floor with a small hammer; Some sounds are slightly like the "when" sound made when a glass ball falls on a cement floor; Some make a "clanging" sound when lightly tapping a steel pipe with a hammer. When the noise is severe, it is often accompanied by a decrease in diesel engine power, a decrease in compression force, and intermittent noise or smoke at the oil inlet. This sound is obvious at diesel engine temperature or low speed, and weakens or disappears as the temperature increases; When the diesel engine speed suddenly changes, the sound is obvious and clear; If the speed steadily increases above the medium speed, the sound usually weakens or disappears.
1. Main reasons
The longitudinal knocking sound of the piston cylinder is generally a rhythmic continuous sound, which often appears during high-speed or high-temperature operation of the diesel engine, sounding a bit muffled and continuous "creaking" sound. The sound is enhanced during the fuel cut-off test.
Knock the cylinder due to excessive clearance between the piston and cylinder liner. Normal wear and tear during use, too tight fit between the piston pin and the pin hole causing deformation of the piston, excessive cylindrical skirt of the piston, bending and twisting deformation of the connecting rod, etc., result in the piston head hitting the inner wall of the cylinder liner; Failure to clean thoroughly during assembly, insufficient or unclean engine oil, clogged or removed air filters, and large amounts of dust entering the cylinder causing early abrasive wear,and long term overload or low high temperature operation of diesel engines accelerates the wear of cylinder liners and pistons.
(2) The piston strikes the cylinder longitudinally. The axis of the connecting rod journal is not parallel to the main journal of the crankshaft, or it is not aligned during cylinder boring resulting in longitudinal deviation of the cylinder barrel after boring and bending of connecting rod or longitudinal deviation of small end bushing.
2. Check and diagnose
(1) On the opposite side of the valve train, inspect along the entire length of the cylinder barrel from the cylinder wall downwards. Gradually change from low speed to standard speed, while auscultating the suspected cylinder by cutting off the fuel supply for 3-5 seconds (i.e. cylinder cut-off method). When the knocking sound changes or disappears after the cylinder is disconnected, it indicates that it is the knocking sound of the piston. If this knocking sound is only noticeable when the car is cold and gradually disappears after the diesel engine is warmed up, it can be considered normal. When the cylinder piston is severely worn, it can be observed that the compression force and power of the diesel engine decrease, and the ventilation hole (or oil filling port) of the crankcase blows out (this phenomenon also occurs when the piston ring is severely worn, but there is no such sound).
(2) Diagnosis of excessive cylinder plug clearance noise. Place the stethoscope opposite the valve mechanism and perform auscultation along the entire length of the cylinder liner from top to bottom. The sound near the cylinder is strong and there is a slight vibration sensation. In conjunction with the fuel cut-off test, when a cylinder that produces a noise is disconnected, if the noise becomes weaker or disappears, it can be concluded that the cylinder is making a noise. Pistons with excessively large skirt cylinders often experience a reduction in noise during oil break tests, and the noise changes from intermittent to continuous. When the diesel engine is running at low to medium speeds, check the shaking throttle, and the sound is usually more noticeable when the throttle is retracted. The sound characteristics are the dull knocking sound of "whistle" or the friction sound of "dada". It is particularly noticeable when the speed suddenly changes or when the speed is low. If the speed steadily increases, the knocking sound actually decreases, which is difficult to hear when the diesel engine is working. When the repaired locomotive feels heavy when turning the crankshaft by hand, this kind of sound can also be heard.
To further confirm the knocking of a certain cylinder, the diesel engine can be turned off, the fuel injector assembly can be removed, a small amount of engine oil can be poured into the cylinder, and then reassembled and started for inspection. If the sound disappears or weakens, and then reappears after a while.If there are a few sounds within seconds of the fire, then the sound disappears and reappears after a while, it can be diagnosed as knocking the cylinder. During use, if the noise only occurs when the car is cold and gradually disappears after the temperature returns to normal, it can be considered normal and does not need to be ruled out. When it is diagnosed that exclusion is necessary, the cylinder wall of the sounding cylinder can be checked for any signs of strain and whether the lubrication conditions are good. Extract the piston and connecting rod assembly again, further inspect the piston for any strains, burns, or knock marks, and measure the cylinder clearance. If the cylinder clearance is too large or the piston is deformed or severely damaged, the piston should be replaced; If the connecting rod is severely twisted and deformed, it should be corrected or replaced; If there is poor splash lubrication, the cause should be carefully investigated and eliminated.