I. The Relationship Between Abnormal Noises and Engine Speed
The presence of the most common abnormal noises in the engine depends on the engine’s speed state.
1. Abnormal noises only exist at idle speed or low-speed operation. The causes of the noise include excessive clearance between the piston and the cylinder wall; excessively tight assembly of the piston pin or connecting rod bearing; excessive clearance between the tappet and its guide hole; wear of the cam profile; sometimes, the loosening of the starter clutch causes the pulley to make noise (obvious when the speed changes).
2. When maintaining a certain speed, the sound is chaotic, and short sounds are emitted successively when decelerating abruptly. The causes of the noise include the camshaft timing gear being broken or its fixing nut being loose; the crankshaft being damaged; the piston pin bushing being loose; the axial clearance of the camshaft being too large or its bushing being loose.
3. Abnormal noises appear when the engine is accelerated suddenly, and the sound still exists when maintaining high-speed operation. The noise causes include the connecting rod bearing being loose, the bearing shell being burnt or the size not matching and rotating; the crankshaft bearing being loose or the bearing shell being burnt; the piston pin being broken; and the crankshaft being broken.
II. The Relationship Between Abnormal Noises and Load
Many abnormal noises in the engine have a clear relationship with the load. During diagnosis, the method of relieving the load on a cylinder-by-cylinder basis can be used for testing, usually by using the single-cylinder or double-cylinder misfire method to alleviate the load of one or two cylinders to identify the relationship between the abnormal noise and the load.
1. When a certain cylinder is misfired, the abnormal noise disappears or is reduced. The causes of the noise include the piston knocking on the cylinder; the connecting rod bearing being loose; the piston ring leaking; and the piston pin being broken.
2. When a certain cylinder is misfired, the sound is intensified, or the original soundless becomes noisy. The causes of the noise include the piston pin bushing being loose; the piston skirt taper being too large; the piston pin being out of place; the connecting rod bearing cap fixing bolt being excessively loose or the connecting rod bearing shell alloy being burnt and completely detached; the flywheel fixing bolt is excessively loose.
3. When two adjacent cylinders are misfired, the abnormal noise is reduced or disappears. The cause of the noise is: that the crankshaft bearing is loose.
III. The Relationship Between Abnormal Noises and Temperature
1. Abnormal noise occurs at low temperatures, and the sound is reduced or even disappears as the temperature rises. The causes of the noise include excessive clearance between the piston and the cylinder wall; the piston due to the main bearing oil groove depth and width being inaccurate; low oil pressure and poor lubrication.
2. After the temperature rises, there is a sound, and the sound is reduced or disappears when the temperature is lowered. The causes of the noise include pre-ignition caused by overheating; the long and short axes of the piston skirt ellipse are opposite in direction; the piston ellipse is small, and the clearance between the piston and the cylinder wall is too small; the piston is deformed; the gaps between the piston rings are too small.
IV. The Relationship Between Abnormal Noises and the Engine Working Cycle
Abnormal noise faults in the engine are often clearly related to the engine’s working cycle, especially the noises caused by the crank connecting rod mechanism and the valve mechanism are related to the working cycle. For a four-stroke engine, any sound caused by the crank connecting rod mechanism will sound twice for each working cycle of the engine; any sound caused by the valve mechanism will sound once for each working cycle of the engine.
1. The causes of abnormal noises caused by the crank connecting rod mechanism include the piston knocking against the cylinder wall; the knocking sound emitted by the piston pin; the piston hitting the cylinder head; the connecting rod bearing being excessively loose; the piston ring is leaking.
2. The causes of abnormal noises caused by the valve mechanism include excessive valve clearance; excessive clearance between the tappet and its guide hole; wear of the cam profile; excessive clearance between the valve stem and its guide; valve spring breakage; radial fracture of the camshaft timing gear; valve seat circle loosening; valve sticking and unable to close.
3. If the abnormal noise is unrelated to the working cycle, attention should be paid to the area where the sound is produced. Generally, the sound caused by the gap unrelated to the working cycle is mostly due to faults in the engine accessories; if it is a continuous metallic friction sound emitted by a machine part unrelated to the working cycle, it is possible to consider that some rotating parts have faults.
V. The Relationship Between Abnormal Noises and Engine Parts
When the engine makes abnormal noises, it will inevitably produce a certain degree of vibration. The characteristics and parts of the vibration can assist in diagnosing the causes of the abnormal noises.
If you are looking for a suitable CNC machine tool or want to learn more about related technical applications, please feel free to contact our professional team. We will provide you with customized solutions to help your company achieve the transformation and upgrading of intelligent manufacturing.