The fuel oil circuit of a diesel engine can be divided into a negative pressure oil circuit, a low pressure oil circuit and a high pressure oil circuit.
The negative pressure oil circuit refers to the oil circuit from the fuel tank to the fuel pump. When the engine is working, the pressure in this section of the oil circuit is lower than the atmospheric pressure, so the fault may only be an air leak, not an oil leak. Air leakage will reduce the vacuum in this section of the oil circuit, reducing the amount of fuel (diesel) entering the fuel transfer pump, resulting in insufficient fuel supply of the fuel transfer pump or even fuel failure, and the engine cannot be started.
When the air leakage is small, the fuel delivery pump can still continue to supply fuel, but the engine is difficult to start or the engine will self-extinguish soon after starting. The reason for the above phenomenon is that the air is compressible, and a small amount of air that enters the oil circuit is sucked into the pump cavity from the oil hole of the fuel injection pump plunger and becomes air bubbles, and its volume changes with the reciprocating movement of the fuel injection pump plunger.
When the plunger is in the ascending (pressure oil) stroke, the bubble is compressed, so that the oil pressure cannot rise to the required injection pressure; when the plunger is in the descending (return) stroke, the bubble expands and returns to its original volume , So that the pump cavity can not produce a vacuum that sucks oil, as if it is blocked. Therefore, it is absolutely not allowed to have even the smallest leak point in the negative pressure oil circuit.
The fuel of the diesel engine is sucked out of the fuel tank by the fuel transfer pump and delivered to the fuel injection pump. If the fuel line leaks, the fuel transfer pump cannot deliver the fuel. Air is elastic and compressible.
When there is air in the oil circuit of the diesel engine fuel supply system, with the reciprocating movement of the fuel injection pump plunger, the volume of the bubble changes. When the plunger is in the compression stroke, the bubble is compressed, so that the oil pressure cannot rise to the specified level. Injection pressure. When the plunger is in the oil suction stroke, the bubble expands and returns to its original volume, so that the suction force of the oil cannot be generated in the oil circuit, so if there is air, the oil supply is insufficient or even interrupted. The engine is gradually stalled during driving; it cannot be restarted after stalling; turn on the bleed screw of the high-pressure oil pump, pump oil with the hand oil pump to remove the air in the oil circuit, and then start the engine to catch fire, but the flameout phenomenon occurs again after a while, it can be judged Air enters the oil circuit.
When the diesel engine is working, the pressure of the oil path from the fuel tank to the fuel transfer pump section is lower than the atmospheric pressure, so as long as there is a small leak point, the external air will be sucked in, which will cause the engine to work unstable or even stall. The method to confirm whether this section of the oil path is leaking is to loosen the bleed screw of the fuel injection pump after the engine is turned off and pump oil with a hand oil pump. If a large amount of air bubbles begin to be discharged from the bleed screw, and repeatedly pump oil with the hand oil pump , When the oil flow is still not completely free of bubbles, it can be determined that there is an air seepage point in this section of the oil path. Except that the oil level in the fuel tank is so low that it is close to the suction port that air enters the tubing, the air infiltration points in this section of the oil path mostly appear at the oil pipe joints of the oil pump inlet. Generally, the copper gasket is damaged or improperly installed, and the tubing is cracked. Damage, etc. are easy to seep here.
When the diesel engine is working, the pressure of the oil path from the fuel delivery pump to the fuel injection pump is higher than the atmospheric pressure. Even if the oil path has leakage points, it can only leak oil, not gas, so it will not affect the normal operation of the engine, but when the engine is turned off , After the oil leaks from the leak point, the oil pressure higher than the atmospheric pressure cannot be maintained, and even causes air backflow, making it difficult to start again. In order to confirm whether there is air leakage in this section, a fuel pipe can be used to skip the diesel filter from the fuel transfer pump and directly connect to the fuel injection pump, and then start the engine to observe after exhausting. If the fault disappears, it can be determined that there is a seepage point in this section of the oil circuit. If the fault still exists, it may be caused by oil leakage at the plunger sleeve positioning screw and bleed screw of the fuel injection pump.
The troubleshooting method of the low-pressure oil circuit leakage is as follows.
When encountering air leakage in the low pressure oil circuit, first check whether the bleed screws at the diesel filter, fuel injection pump, etc. are tightened; then check the joints of the oil circuits, especially the hose parts that are prone to air leakage, and should be carefully checked , Find out the cause, and eliminate it if necessary. Open the bleed screw and remove the air in the oil circuit with a hand oil pump; tighten all pipe joints to remove the loose seal.
When replacing the filter element or cleaning the oil-water separator, a large amount of air will also enter the low-pressure oil circuit, so it must be exhausted. When the oil pump is full of diesel without bubbles, you can tighten the bleed bolt. Then start the engine, check that there are no leaks in all parts, and repair if necessary until there is no leakage.