Troubleshooting of 3 faults in construction machinery engines

The NT855 diesel engine used in the D85A-18 bulldozer works weakly and emits black smoke

Upon shutdown inspection, it was found that there was no oil leakage from the engine oil radiator and no faults in the oil circuit. When disassembling and inspecting the engine, it was found that there were small rusted holes in the cylinder liner, which proved that the engine oil entered the radiator through the rusted holes. The repair personnel believe that the malfunction was caused by a large cylinder wall clearance, too small a compression ratio, and incomplete fuel combustion. After replacing the cylinder liner, piston, piston pin and piston ring of the engine and re-testing it, black smoke still came out from time to time and the earthmoving operation was weak. The calibration of the fuel injector indicates that the fuel injection pressure is within the standard range and the atomization is good. No problems were found during the calibration of the PT pump either. The engine was disassembled and inspected again. It was found that there were no problems with the timing and piston rings. However, when measuring the height of the camshaft CAM, it was discovered that its height was only two-thirds of the standard height. Analysis suggests that the CAM height has decreased due to wear, causing the piston to fail to burn when moving to the top dead center position, resulting in delayed ignition and causing the engine to emit black smoke and operate weakly. After replacing the camshaft, the fault was eliminated.

The 6135AZKa diesel engine used in the Zhengshan brand T200 bulldozer makes abnormal noises during operation

When disassembling and inspecting the engine, it was found that the connecting rod bolt of cylinder IV had been stretched significantly. It was initially determined that the cause of the fault might be the loosening of the connecting rod bolt, which led to increased force and deformation under repeated impacts. It could also be that there is a quality issue with the connecting rod and screw. For this purpose, other connecting rod bolts were inspected, and the results showed no issues. The tightening torque also met the requirements. After replacing this connecting rod bolt, the installation operation indicated that everything was normal and the fault was eliminated. Later, it was learned that during the major overhaul of the machine, the connecting rod of Cylinder IV was slightly deformed. After the repair personnel corrected it by themselves, they reassembled it.

Therefore, the real cause of this malfunction is that it was not expected that the deformation of the connecting rod would cause plastic deformation of the connecting rod bolt, resulting in a decrease in its tensile strength. It was precisely because new connecting rod bolts were not replaced as required but old parts were still used for assembly that the bolts elongated due to the reduction in tensile strength.

The TATRA3-929-11 diesel engine used in the TATRA815S1 dump truck has experienced bearing seizure

After the major overhaul, the machine only operated for 1,000 hours before experiencing the phenomenon of bearing seizure. After disassembling the engine, the first thing to check was the engine oil. This machine uses Great Wall 15W/40CD engine oil. Although the viscosity of the oil has decreased at high temperatures, it is still within the normal range. Moreover, the contamination degree, suspension property, pH value, etc. are all qualified. When the oil circuit was checked again, it was also unobstructed and there was no oil leakage. When disassembling and inspecting the oil pump, it was found that the gap at the mating surface between the gear end face and the front and rear covers was too large, thus causing internal leakage. Since the aluminum pump casing cannot be repaired by a grinding machine, a surface milling machine was used for processing to make the gap between the gear end face and the front and rear covers reach the standard value, and then it was polished smooth with metallographic sandpaper. After reinstallation, there was no internal leakage during the test, and the operation has been normal throughout the subsequent transportation and installation, indicating that the fault has been eliminated.

Analysis suggests that the cause of this malfunction is as follows: During the major overhaul of the engine, it was in the spring when the temperature was relatively low. At that time, the viscosity of the engine oil was high, so the leakage from the oil pump was small, and the oil pressure could reach the normal value. Therefore, the repair personnel neglected to inspect the oil pump. During the summer construction, the machine operated continuously at high temperatures, which reduced the viscosity of the engine oil and increased the internal leakage of the oil. As a result, an effective oil film failed to form, leading to the occurrence of a bearing seizure accident.