On February 26, 2009, Our company accepts a Weifang customer to sell pistons of Binzhou Bohai piston Co., Ltd (model: WD615 e2-3a). A serious cylinder tamping accident occurred shortly after the replacement of the cylinder block, crankshaft and four supporting parts of the engine. Since the customer did not provide all the damaged parts, we can only organize relevant technicians to analyze the cause of the accident according to the damaged piston, piston ring, cylinder liner and connecting rod provided by the customer of Steyr heavy truck.
It is understood that the user has changed the cylinder block, crankshaft and four supporting parts; During load running in, the driver continued to drive for 2-3km on the premise of abnormal noise of the engine until the engine stopped due to cylinder tamping. The local maintenance station generally says that this phenomenon is caused by the product quality problem of the piston. Let’s make a detailed analysis below:Â
A. The piston, piston ring and cylinder liner of this cylinder pair are seriously damaged into broken blocks, and the small end of the connecting rod is bent along the axial direction.
B. The relatively large fragments that can be found in the piston are shown in the figure above, in which the damage shows many formal characteristics.
C. The damage of cylinder liner is also in many forms.
D. The piston ring is basically broken, and the piston pin circlip is seriously worn.
E. The cylinder liner is broken from the support shoulder, and the broken stubble is extremely irregular.
The piston is called the heart of the engine. It is one of the most important parts in the engine. Because the piston is constantly doing high-speed reciprocating motion under the action of high-temperature and high-pressure gas, it bears the thermal load and mechanical load of high-temperature and high-pressure, and its working conditions are very bad. The piston is the only aluminum part in the main moving parts of the engine. The failure of any related part in the engine or the working state of the engine is likely to cause the piston to be damaged or broken. Therefore, the engine fault is often exposed to the piston, but the part that really causes the engine fault is not necessarily the piston, Specific analysis needs to be carried out according to specific conditions. Many friends don’t know how to analyze a pile of damaged parts. In fact, they can see the essence through the phenomenon. After mastering enough basic knowledge, they can still analyze it as long as they deliberate carefully!
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1. Piston fragment analysis:
A fragment in the combustion chamber (as shown in the figure below) may be because a part of the piston broken due to external force is inserted and separated from the piston head after collision with the cylinder head.
For this guess, we will send the two fragments in the figure below to Shandong Binzhou Bohai piston Co., Ltd. for metallographic test.
2. Piston ring block analysis:
Generally, it is rare for the piston ring to break into several sections (usually, it is not because the supporting shoulder of the cylinder liner is broken; most of the problems of the piston ring are caused by the resonance of the piston ring after the natural vibration frequency is close to the high-speed and high burst impact frequency due to the reduction of the ring size of the piston ring after wear under the condition of high speed and large load), but the first piston ring in this accident is worn The marks of material wear and melting wear also have deformation areas caused by external force.
The severe abrasive wear in the figure above usually occurs in the early slight cylinder pulling phenomenon. When the lubrication effect of the piston ring becomes poor and the gas ring and the nickel iron insert ring of the piston have semi-dry friction, the magazine in the engine oil is clamped in it to form this wear.
For Euro II engine equipped with e2-3a piston, the second gas ring is generally anti twisted rectangular ring. This structure can not only effectively reduce oil consumption, but also better improve air tightness. However, through observation, the second gas ring in this accident found that the second gas ring was installed reversely during assembly!
Since the oil ring of the engine has been completely damaged, we do not make graphical analysis here.
3. Piston pin circlip
The function of piston pin circlip is to limit the axial jump of piston pin. Under normal conditions, the piston is in a semi floating state in the cylinder liner due to the support of the piston ring and the role of gas compression; Therefore, the piston pin rarely has axial impact force on the piston pin circlip. When the fit tolerance between the piston and cylinder liner exceeds the limit or the piston ring is seriously worn and loses its support function, part of the component force will lead to axial jump of the piston pin and wear of the piston pin circlip. If the cylinder liner is damaged and the piston directly loses its guidance, the increase of this axial component will lead to serious wear of the snap ring!
4. Cylinder liner and cylinder liner support shoulder:
5. Connecting rod:
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The red mark on the right is the cylinder head; The yellow area is the part where the piston is partially torn from the position of the first gas ring because the first piston ring is stuck on the stubble of the cylinder liner after the cylinder liner is broken. When the cylinder liner is pulled to a certain position by the piston ring, the crank arm of the crankshaft collides with the cylinder liner during rotation, resulting in serious damage to the cylinder liner.
When the piston goes up, due to the loss of support, the piston will continue to go up under the obstruction of the Yellow piston residue, but the direction will change, which is very likely to squeeze with the cylinder liner bearing hole of the cylinder block, Because the organizational structure between the cylinder liner bearing hole and the cylinder block water channel of the engine is the thinnest (axial) between the two adjacent cylinders (low strength), most of the positions where extrusion damage occurs are in the axial direction. When the residual piston connecting rod group is driven by the crankshaft and continues to rise, the connecting rod will bend axially.
The cylinder tamping accident of this engine is not caused by piston fracture. The piston has been tested by Binzhou Bohai piston Co., Ltd. and the material is qualified.
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