1. Working principle of excavator maintenance oil supply system Liebherr r944c excavator is equipped with an oil supply system composed of unit pump, high-pressure pipe and fuel injector (PLD oil supply system for short). While the camshaft of diesel engine drives the intake and exhaust valves to move, it also drives the tappet of single pump to act. When the cam rotates, push the tappet of the single pump upward, and the tappet pushes the plunger of the single pump upward. The controller (ECU) controls the energization and de energization of the solenoid coil of the oil return closing valve on the upper part of the single pump according to the electrical signal transmitted by the diesel engine speed sensor, and then controls the opening and closing of the oil return closing valve. When the fuel pressure reaches the injection pressure, the fuel is injected into the combustion chamber through the fuel injector, so as to realize the operation of the diesel engine. The oil supply system of diesel engine of this excavator is shown in Figure 1. The oil return closing valve is installed on the upper part of the unit pump, and the energization duration of its solenoid determines the oil supply of the unit pump. When the solenoid is powered off, the return oil passage of the unit pump is opened, and the plunger in the unit pump cannot establish high pressure even if it has started pumping oil. Only when the solenoid valve is energized and the return oil passage is closed, the oil pressure can rise rapidly, and the high-pressure fuel enters the fuel injector through the high-pressure oil pipe to inject fuel. When the solenoid is powered off, the return oil passage is opened, the monomer pump overflows and relieves pressure rapidly, and the oil supply stops.
The four working strokes of the single pump are as follows. Suction stroke: when the cam rotates, it drives the plunger downward, so that the oil from the oil transfer pump enters the plunger cavity through the return oil sealing valve. Preloading stroke: the cam drives the plunger upward to pressurize the fuel in the plunger cavity. At this time, the oil return closing valve is not closed, and the fuel in the plunger cavity remains at low pressure. Output stroke: the cam continues to drive the plunger upward. At this time, the solenoid coil is powered on to close the oil return closing valve and increase the fuel pressure in the plunger chamber. When the fuel reaches the injection pressure, the fuel injector injects fuel. Pressure relief stroke: the cam continues to rotate to drive the plunger upward. At this time, the solenoid coil has been powered off, the oil injection is completed, the oil return closing valve is opened, and the oil in the plunger cavity is drained.
2. Troubleshooting of excavator repair first, check that the air filter of diesel engine is not blocked, the supercharger operates normally and the intercooling is normal. Secondly, the special sculi software of Liebherr was used to check the fuel supply of the diesel engine. It was found that the fuel supply was constantly changing during the low idle operation of the diesel engine, so it was determined that there was a fault in the fuel supply system of the diesel engine. Finally, check the diesel engine speed sensor, diesel engine controller (ECU), diesel filter, fuel transfer pump, fuel injector and fuel supply pipeline, and no problems are found. However, after replacing the single pump with a new one, it was found that the low idle operation of the diesel engine was stable, and the "buzzing, buzzing" abnormal sound disappeared. Therefore, it is determined that the fault is caused by the wear of the monomer pump.
3. Cause analysis of excavator maintenance shows that the unstable speed of diesel engine at low idle speed is due to the unstable fuel supply (fuel injection). The source of unstable oil supply is the failure of the single pump, which may occur in two places: one is the fuel leakage in the plunger cavity, and the other is the fuel leakage in the return closed valve controlled by the solenoid coil. The unit pump is installed on the diesel engine cylinder block, and the lower side of its plunger cavity is located above the cam. If the plunger is seriously worn, it will cause diesel oil to enter the oil in the cylinder. Since no deterioration of diesel engine oil and high oil level were found during on-site inspection, it can be determined that there is no leakage fault in the plunger cavity. The oil return sealing valve of the single pump is controlled by the solenoid coil. When the diesel engine is running, the oil return sealing valve needs to be closed and opened at high speed. The reciprocating motion of the high-speed opening and closing of the oil return closed valve will produce frequent friction. If the quality of diesel is poor, it will cause its wear. After the oil return sealing valve is seriously worn, it will inevitably lead to internal leakage of fuel. After dismantling and inspecting the return oil sealing valves of four monomer pumps, it is found that their oil inlet and outlet ports have been seriously worn and can not play a sealing role. The leakage position of the return oil sealing valve of the unit pump is shown in Figure 3. The internal leakage of the return oil sealing valve of the monomer pump causes insufficient fuel supply of the diesel engine under low idle operation, that is, the speed is unstable. The reason why the diesel engine operates stably at high idle or working speed is that after the speed of the diesel engine increases, the oil supply also increases accordingly. At this time, the influence of the internal leakage of the return oil sealing valve of the monomer pump on the operation of the diesel engine is reduced.
