1) The fit clearance between piston and cylinder is too small
The fitting clearance between piston and cylinder shall be in strict accordance with the provisions of the diesel engine operation manual. If the clearance is too large, it will make it difficult for the diesel engine to start cold. When the engine is cold, it will make a cylinder knocking sound, and the power will drop. If the clearance is too small, it will cause cylinder pulling, cylinder expansion and other failures.
Especially when the linear expansion coefficient of the aluminum alloy used for the piston is relatively large, the above situation is more likely to occur. When repairing a diesel engine, if there is no operation manual for the engine, you can refer to the matching clearance of similar models. Generally, the matching clearance of air-cooled diesel engine is slightly larger than that of water-cooled diesel engine. Generally, the matching clearance of a four stroke water-cooled diesel engine with a cylinder diameter of 100mm is 0.120 ~ 0.150mm when using an aluminum piston.
2) Piston ring opening clearance is too small
The opening clearance and back clearance of the piston ring are too small, the friction between the ring and the cylinder wall is too large, the carbon in the cylinder is serious, and the dirt or oil is not clean during assembly, resulting in strain.
The opening clearance of the piston ring is too small. After the old cylinder is replaced with a new ring, the first gas ring impacts the cylinder liner shoulder or the opening of the piston ring expands too large during assembly, which is easy to break.
The side clearance is too small, and the special tools are not used during disassembly and assembly, resulting in the spiral shape of the ring, elastic effect or serious carbon deposition in the cylinder, which will lead to the seizure of the piston ring.
Excessive opening clearance and side clearance of piston ring, reverse installation of twisted ring and conical ring in the up and down directions, and dirty lubricating oil can easily lead to oil channeling.
The opening clearance of piston ring is too small, and the opening is impacted; The side clearance is too large, and the annulus collides with the annulus; The old cylinder is replaced with a new ring, and the first gas ring collides with the cylinder liner shoulder, which will cause noise.
3) Broken piston ring
The piston is deflected too much in the cylinder, resulting in ellipse and taper, and the upper part is stepped. The working surface of the inner wall of the cylinder liner often forms uneven wear in the moving area of the piston ring, exceeding the limit. Generally, it forms a cone in the reciprocating direction and is ground into an irregular ellipse in the circumferential direction.
At the position where the cylinder liner has the greatest wear, the ovality is often the largest, and the non-contact parts between the cylinder liner and the piston ring are not worn, so there are obvious wear steps. During the power stroke, the gas pressure forces the piston to move to the downward dead center, changing from reciprocating to rotating. The piston ring has the greatest side pressure on the cylinder liner in the direction of the crank motion plane, resulting in eccentric wear of the cylinder liner, that is, the cylinder liner perpendicular to the crankshaft has the greatest wear, the crankshaft is bent, and the connecting rod is bent, The center line of the cylinder liner is not perpendicular to the axis line of the crankshaft. If the axial clearance of the crankshaft is too large, the piston ring and the cylinder liner can be broken due to eccentric wear and uneven stress.
If the bearing bush clearance is too large and the piston pin and connecting rod copper sleeve clearance is too large, the first gas ring is most likely to break when obvious wear steps appear on the upper part of the cylinder liner. It is stuck on the piston, or the piston pin circlip is broken or falls off.
4) The piston and piston ring shall be tilted to one side and pressed tightly on the cylinder wall;
As the piston is deformed, the piston hole is offset, the cylinder is honed and skewed, the connecting rod is bent or twisted, and the crank pin and the main journal are unbalanced, the pressure of the piston on the cylinder wall is relatively concentrated, and the oil film between the cylinder wall and the piston ring becomes thin or even ruptured under great pressure, thus losing lubrication and forming dry friction, resulting in cylinder pulling.
5) Poor atomization of fuel injector (carburetor) for a long time
The fuel injection nozzle is the most precise component, and the engine mainly burns gasoline during operation. During the combustion process, the high temperature will cause carbon deposition on the surface or pores of the fuel injection nozzle, and at the same time, more colloid will also be deposited on the inner wall of the fuel injection nozzle and the surface of the needle valve in the long-term use, affecting the fuel injection effect, blocking and adhesion of the fuel injection nozzle, resulting in fuel leakage, poor atomization, or even no fuel injection, resulting in increased fuel consumption, decreased engine power, unstable idle speed, poor acceleration and difficult cold start. The test data show that when 10% of the fuel injection quantity is blocked, the engine combustion will be incomplete, the performance will be degraded, the fuel consumption will increase and the exhaust temperature will rise. At this time, the fuel injection nozzle should be cleaned in time to improve the combustion efficiency of the engine.
6) Piston cooling nozzle port failure.
Individual engines rely on the connecting rod bearing to the piston pin piston injector to inject oil. Due to various reasons, the normal oil injection cannot be carried out, resulting in overheating of the piston top, resulting in melting of the piston top and cylinder pulling. (functions of engine oil: cooling, lubrication, cleaning, sealing, (rust prevention)
7) Poor engine cooling and supercooling operation
Running the engine when the water temperature is below 65 ° C is called cold running. When the engine is not fully operated and the water temperature reaches a certain level, it will start to work, or when the opening temperature of the thermostat is too low, the cooling water enters the large circulation too early, which will cause supercooling. When the cylinder wall temperature drops from 800 ° C to 500 ° C, the wear of the cylinder liner increases by about 5 times. However, when the cylinder wall temperature reaches 80 ° ~850 ° C, the wear amount decreases significantly. The water temperature is too low, the temperature rise of diesel in the combustion chamber is slow, the ignition delay period is long, the combustion process is deteriorated, and the operation performance is poor.
