Function and Fault Analysis of Three Precision Couplings of Diesel Engine
Plunger pair, outlet valve pair and injection valve pair are three precision components of diesel engine fuel supply system. Most of these parts are made of high-quality alloy steel, and have undergone fine processing such as grinding and grinding, with extremely high manufacturing accuracy. However, after long-term use, they may also fail due to wear and stagnation, resulting in abnormal operation of the fuel system of the diesel engine.
1.Plunger coupling
Plunger and plunger sleeve are collectively referred to as plunger couplers, which are the most important precision couplers of fuel injection pumps. The plunger and plunger sleeve should not only pressurize the fuel, but also accurately control the fuel supply and injection time, so there should be a strict fit clearance (about 0.002~0 003 mm) to ensure fuel pressurization and certain lubrication clearance. In fact, it is difficult to ensure interchangeability under such high precision, so it is necessary to select and match pairs of parts, which cannot be interchanged. In addition, due to the high relative movement speed between the plunger and plunger sleeve, the working surface is required to be very smooth, which needs to be achieved by grinding. The normal service life of plunger coupling is 2000 h. Plunger wear is mainly abrasive wear, so fuel cleaning is extremely important. Generally, when the plunger diameter is worn 3 μ After m, the plunger coupling will fail, which will result in reduced fuel supply, delayed fuel supply time, uneven fuel supply, difficult starting, smoke, insufficient power and other faults. When the plunger is stuck, it will affect the normal operation of the governor and cause the engine to run fast. If it is not handled in time, it will cause serious accidents. The main faults of plunger coupling are wear and seizure.
Causes of plunger wear:
(1) The diesel filter has not been maintained for a long time and is in poor working condition, resulting in too many diesel engine impurities entering the plunger coupling parts and wearing the plunger mating surface.
(2) The newly replaced plunger coupling did not clean up the anti-rust oil, and scratched the mating surface of the plunger during operation.
(3) Improper adjustment of the injection pressure, high pressure or inflexibility of the needle valve body of the injection nozzle make the injection pressure too high, resulting in early wear of the plunger mating surface.
Causes of plunger sticking:
(1) If the plunger accessory is not carefully transported, stored and assembled, the plunger will be slightly bent and jammed during operation, which should be replaced in time.
(2) Because the plunger is not cleaned during assembly, or impurities enter between the plunger pairs, and the plunger is not careful during assembly, the plunger will be bruised and pulled, resulting in plunger jamming.
(3) If the new plunger pair is stored for a long time, it is easy to cause oil loss and oxidation reaction, which makes the plunger rusty, and the plunger is assembled without cleaning, and the plunger is jammed during operation.
2.Outlet valve coupling
The upper end face of the plunger sleeve is equipped with a spring tight oil outlet valve coupling, which is a one-way valve.
The function of the oil outlet valve is:
(1) When the fuel supply is stopped, the conical surface of the oil outlet valve and the valve seat are closely matched under the action of the spring to prevent the fuel in the high-pressure oil pipe from flowing back to the injection pump.
(2) When the outlet valve falls, the pressure reducing ring enters the pilot hole of the valve seat, which immediately stops the fuel from entering the high-pressure fuel pipe. The outlet valve continues to drop, and the pressure reducing ring completely falls into the valve seat, which increases the volume of the high-pressure oil pipe and plays a pressure relief role. The pressure in the high-pressure oil pipe drops sharply, avoiding the phenomenon of oil dripping in the later stage of fuel injection.
The delivery valve coupling is also a pair of precision couplings of the fuel injection pump, and the plunger coupling and the needle valve coupling of the fuel injector are commonly known as the three pairs of precision couplings of the fuel supply system. Like other two pairs, the conical seating surfaces of the oil outlet valve and the valve seat have been ground and cannot be interchanged after matching. In order to ensure the tightness of the oil outlet valve coupling, the width of its sealing belt shall not be greater than 0.4~0 5 mm。 The wear at the sealing cone will increase the return oil volume of the high-pressure oil pipe, reduce the residual pressure of the high-pressure oil pipe, and delay the injection time. When the pressure reducing part is seriously worn, the pressure reducing belt will fail, and the fuel injection cannot be cut off quickly, resulting in dripping.
The faults of the oil outlet valve include:
(1) The outlet valve plate is strained. There are two situations: one is that the upper plane of the oil outlet valve pad is strained, resulting in oil leakage; The second is that the lower surface of the oil outlet valve pad is seriously strained, and some high-pressure oil ejected from the oil outlet valve flows back to the low-pressure oil chamber from the lower part of the valve pad, thus reducing the amount of high-pressure oil, making the oil pressure insufficient, causing insufficient fuel injection, power reduction, and slow acceleration.
(2) The oil outlet valve gasket is deformed. Generally, due to long-term use, the gasket of the oil outlet valve has plastic deformation, which makes the inner hole of the gasket form a tight fit with the oil outlet valve body. When the tight seat bolt of the oil outlet valve is tightened, the copper gasket is forced to squeeze and deform, which causes the oil outlet valve to be stuck, unable to form high-pressure oil, and the engine stops working.
(3) The thickness of oil outlet valve gasket does not meet the requirements. The gasket is too thick, which causes the spring pressure of the outlet valve to be insufficient and the valve to close slowly, resulting in back drops and engine smoke; The gasket thickness is too thin, the spring force is too large, and the valve opening is delayed correspondingly. Because the injection duration is shorter, the engine power can not be fully utilized. In addition, if the gasket is too thin, it will increase the fuel supply pressure of the plunger, promote the wear of the plunger and shorten the service life of the fuel injection pump.
3.Needle valve coupling
The needle valve coupling is composed of the needle valve and the needle valve body. It is a high-precision coupling. It cannot be interchanged after being ground in pairs. The upper part of the outer circle of the needle valve is provided with a support shoulder. During assembly, the injector cap tightly presses the upper end face of the needle valve body and the lower end face of the injector body through the support shoulder. The needle valve head has two conical surfaces and an inverted cone shaped pin. The pin of the inverted cone is inserted into the spray hole of the needle valve body to form a circular gap of the inverted cone. When the needle valve is raised to inject oil, a positive cone atomized oil spray can be formed. The main reasons for the clogging of the nozzle hole of the fuel injector are that the nozzle hole is clogged due to the dirty diesel fuel, the fuel injector coupling is not tightly sealed and the oil drips to form carbon deposits, and the diesel engine combustion chamber overheats to form carbon deposits. The clogging of the orifice is manifested by the thinning of the orifice diameter and the change of the shape and direction of the jet column. When it is suspected that a cylinder of the diesel engine is not working normally or not, touch the high-pressure oil pipe of the cylinder with your hand. If you feel that the pulsating pressure is obviously high, remove the fuel injector of the cylinder for comprehensive inspection.
The technical status of needle valve coupling parts can also be judged from the quality of fuel atomization. After the injection pressure is adjusted, when uneven fog spots or oil lines appear in the spray, the injector coupling parts are generally severely worn; When the oil mist distribution is uneven or the center line of the oil line is skewed, it indicates that the spray hole is not clean; When there is oil dripping after spray, the needle valve sealing cone is not tightly sealed. Grind the sealing ring of the needle valve coupling with abrasive paste, and clean it after grinding. If it still fails to meet the requirements after grinding, replace the needle valve with a new one.
