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Accident Analysis Of Engine Cylinder Tamping Caused By Improper Maintenance And Detection

On June 28, 2008, our company accepted an accident of cylinder tamping (piston damage, cylinder liner crushing and cylinder liner bearing hole cracking) after overhaul of WD615 engine in Shangqiu, Henan Province. Because the owner and maintenance plant only brought some damaged parts, we mainly analyzed the damaged piston:

Accident Phenomenon:

1. (dictated by the owner) after the engine was operated for more than 40 days after the overhaul, the cylinder liner bearing hole of the second cylinder block cracked (a phenomenon of tamping the cylinder). After the overhaul, the engine has been subject to abnormal smoke exhaust and rough operation.

2. Damaged piston and connecting rod assembly:

3. Undamaged piston

4. The fuel injection pressure of the six fuel injectors of this engine (including the damaged fuel injector caused by the tamping cylinder) has been calibrated and checked to be 23-25mpa.

Accident Analysis:

1. The shape of piston combustion chamber of WD615 Series non common rail diesel injection engine is ω Type, friends who have a relative understanding of diesel engine structure should know that the characteristics of this type of combustion chamber are:

1) mainly rely on porous spray (4-7 holes), the use of oil bunches and combustion chamber to form a mixture of space.

2) The orifice diameter is small (hole type), and the orifice is easy to be blocked. The injection pressure is high, about 22-30mpa.

3) Compact structure, small heat dissipation area, small heat loss, high thermal efficiency, good economy and easy to start.

4) During the ignition delay period, there is more mixture, rough operation and high combustion noise.

For the porous fuel injector of WD615 engine, when the fuel injection pump starts to supply oil, the high-pressure diesel enters the fuel injector body from the oil inlet, enters the annular groove of the fuel injection nozzle valve body along the oil passage, and then enters the high-pressure oil chamber below the needle valve body through the inclined oil passage. The high-pressure diesel acts on the conical surface of the needle valve and generates a force to lift the needle valve upward, When this force overcomes the pre tightening force of the pressure regulating spring, the needle valve rises upward, opens the fuel injection hole, and diesel is injected into the combustion chamber through the fuel injection hole.

However, if the preload of the pressure regulating spring is lower than the calibrated value, the actual injection time of the fuel injector will be relatively advanced and the shutdown will be relatively delayed, resulting in too long ignition delay period of the engine and incomplete diesel combustion. The negative compression work increases, the power decreases, the heat load on the top of the piston increases, the side pressure increases, and accompanied by a loud knocking sound.

Therefore, for this type of combustion chamber engine, if you want to achieve the best combustion efficiency, you must adjust the injection pressure and injection advance angle of the engine’s fuel injector.

2. When the engine is under normal conditions, the fuel injector is working and the piston is closed

Near TDC, the fuel is mixed and burned in the combustion chamber.

The top of undamaged piston of this engine has traces of early fuel injection by fuel injector (formed by high-pressure fuel scouring carbon deposit)

3. Due to the high head temperature of the damaged piston, the inner cavity of the piston is still overheated and discolored on the premise of forced cooling of the oil nozzle. After inspection by the maintenance personnel, the damaged cylinder liner did not stick or pull the cylinder in a large area.

Accident Cause Analysis:

The root cause of the cylinder tamping accident of the engine is that the piston side pressure is too large after abnormal combustion of the engine; Due to slight deformation of the cylinder liner bearing hole of the cylinder block, the outer wall of the cylinder liner cannot fully contact the cylinder liner bearing hole of the cylinder block, and excessive side pressure relatively forms a high-frequency impact load (the piston strikes the inner wall of the cylinder liner, and the outer wall of the cylinder liner strikes the inner wall of the cylinder liner bearing hole).

Because the support shoulder of the cylinder liner is fixed, while the cylinder liner bears the tensile stress during the reciprocating movement of the piston up and down, the additional impact load forms a shear stress at the tool withdrawal groove of the support shoulder of the cylinder liner. When the fatigue limit of the cylinder liner is exceeded, the cylinder liner cracks in the tool withdrawal groove of the support shoulder, and then the support shoulder breaks. The cylinder liner descends under the action of tensile stress. When the piston descends, the first gas ring is stuck in the stubble of the cylinder liner, and the accelerating downward cylinder liner collides with the crank of the crankshaft. Driven by the rotation of the crankshaft, the piston is torn and falls off at the upper part of the first gas ring. When the cylinder liner is damaged to a certain extent, the upward direction of the piston loses its guidance, and after collision with the damaged piston residue, it squeezes with the cylinder liner bearing hole of the cylinder block, resulting in extrusion damage to the cylinder block.

This engine is WD615 produced by Weifang Diesel Engine Factory 46371ps engine, The model selected for the piston is e2-3a (cold in the oil chamber). According to the maintenance personnel of the repair shop, while overhauling the mechanical part of the engine, the maintenance personnel asked the owner to calibrate the oil pump and injector at the location of the high-pressure oil pump. However, in just over 40 days, how can the injection pressure of the injector be reduced to 23-25mpa (the injection pressure of the fuel injector of this engine is calibrated as 30MPa)!

We suspect that there is a high possibility of deviation during the calibration of fuel injectors!

For the corresponding test report, please contact the manufacturer or relevant technical appraisal organization directly.

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