1. The working principle of the oil supply system for excavator maintenance The Liebherr R944C excavator is equipped with an oil supply system (abbreviated as PLD oil supply system) composed of a single pump, high-pressure pipes and fuel injectors. While the camshaft of the diesel engine drives the intake and exhaust valves, it drives the single pump tappet. When the cam rotates, the single pump tappet is pushed upward, and the single pump plunger is pushed up by the tappet. The controller (ECU) controls the energization and de-energization of the solenoid coil of the oil return sealing valve on the top of the unit pump according to the electrical signal sent by the diesel engine speed sensor, and then controls the opening and closing of the oil return sealing valve. The fuel is pressurized during the closed period of the return oil shut-off valve. When the fuel pressure reaches the injection pressure, the fuel is injected into the combustion chamber through the fuel injector, thereby realizing the operation of the diesel engine. This type of excavator diesel engine oil supply system is shown in Figure 1. The oil return closing valve is installed on the upper part of the unit pump, and the duration of the electromagnetic coil energization determines the oil supply of the unit pump. When the solenoid is de-energized, the oil return channel of the unit pump is opened, and even if the plunger in the unit pump has started to pump oil, high pressure cannot be established. Only when the solenoid valve is energized and the return oil passage is closed, the oil pressure can be quickly increased, and the high-pressure fuel enters the injector through the high-pressure fuel pipe to inject fuel. When the electromagnetic coil is de-energized, the oil return oil passage is opened, the unit pump quickly overflows to relieve the pressure, and the oil supply stops.
The four working strokes of monomer 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 oil return sealing valve. Pre-compression 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 a low pressure. Output stroke: The cam continues to drive the plunger upward. At this time, the electromagnetic coil is energized, so that the oil return closing valve is closed, and the fuel pressure in the plunger cavity increases. When the fuel reaches the injection pressure, the injector injects fuel. Pressure relief stroke: The cam continues to rotate, driving the plunger upward. At this time, the solenoid coil has been de-energized, the fuel injection is completed, the oil return sealing valve is opened, and the plunger cavity is drained.
2. Troubleshooting for excavator repair First, check that the diesel engine air filter is not blocked, the supercharger is operating normally, and the intercooling is normal. Secondly, using Liebherr's special SCULI software to check the fuel supply of the diesel engine, it was found that the fuel supply was constantly changing during the low idling operation of the diesel engine, thereby determining that the diesel fuel supply system had a fault. Finally, check the diesel engine speed sensor, diesel engine controller (ECU), diesel filter, fuel pump, fuel injector, and fuel supply line and found no problems. However, after replacing the new unit pump, it was found that the diesel engine operated stably at low idling speed, and the abnormal noise of "buzz, buzz" disappeared. It is thus determined that the fault is caused by the wear of the single pump.
3. Analysis and analysis of the maintenance excavator believes that the unstable speed of the diesel engine at low idle speeds is caused by unstable fuel supply (injection). The source of the unstable fuel supply is the failure of the unit pump. There may be two places where the failure occurs: one is the leakage of fuel in the plunger cavity, and the other is the leakage of fuel in the closed oil return valve controlled by the electromagnetic coil. The single pump is installed on the cylinder of the diesel engine, and the lower side of the plunger cavity is above the cam. If the plunger is severely worn, it will cause diesel to enter the oil in the cylinder. Since no oil deterioration or increased oil level of the diesel engine was found during the on-site inspection, it can be determined that there is no leakage failure in the plunger cavity. The oil return closing valve of the single pump is controlled by the electromagnetic coil. When the diesel engine is running, the oil return closing valve needs to be closed and opened at high speed. The reciprocating motion of the high-speed opening and closing of the return valve will produce frequent friction. If the quality of the diesel is not good, it will cause wear. Serious wear of the return valve will inevitably lead to internal leakage of fuel. After disassembling and inspecting the four oil-return closed valves of the single pump, it was found that the inlet and outlet ports have been severely worn and cannot play a sealing role. The leakage position of the oil return sealing valve of the unit pump is shown in Figure 3. The internal leakage of the oil-return closed valve of the single pump causes the diesel engine to have insufficient fuel supply at low idle speed, that is, the speed is unstable. The reason why the diesel engine is stable at high idle or working speed is that after the diesel engine speed increases, the fuel supply also increases accordingly. At this time, the internal leakage of the single pump return oil sealing valve has less effect on the operation of the diesel engine .
