Working principle of fuel injection pump

- Jul 06, 2020-

Working principle of fuel injection pump

Oil absorption process: the plunger is driven by the cam of the camshaft. When the convex part of the cam leaves the plunger, the plunger moves down under the action of the plunger spring, the volume of the oil chamber increases and the pressure decreases; when the plunger sleeve When the radial inlet of the radial is exposed, the fuel in the low-pressure oil chamber flows into the pump chamber along the inlet.

Pumping process: When the convex part of the cam lifts the plunger, the volume in the pump cavity decreases, the pressure increases, and the fuel flows back to the low-pressure oil cavity along the radial oil hole on the plunger sleeve; when the plunger When the radial oil hole on the plunger sleeve is completely blocked, the pressure on the pump cavity increases rapidly; when this pressure overcomes the preload of the oil valve spring, the oil valve moves upward; when the oil valve is on When the pressure relief ring leaves the valve seat, the high-pressure diesel is pumped into the high-pressure fuel pipe and injected into the cylinder through the fuel injector.

Oil return process: As the plunger continues to move up, when the chute on the plunger communicates with the radial oil hole on the plunger sleeve, the fuel in the pump cavity will pass through the axial oil passage on the plunger. The oil holes on the oil passage and plunger sleeve flow back to the low-pressure oil chamber, and the pump oil stops.

 

Take the plunger injection pump as an example to see how the "pump" works:

The fuel injection pump must have a power source to run. The lower camshaft is driven by the engine crankshaft gear.

The key part of the fuel injection pump is the plunger. If the common syringe in the hospital is used as an analogy, then the movable plug is called the plunger, and the syringe is called the plunger sleeve. Assume that a spring is installed inside the needle to press the post At one end of the plug, the other end of the plunger contacts the camshaft. When the camshaft makes one revolution, the plunger will move up and down once in the plunger sleeve. This is the basic movement of the plunger of the fuel injection pump.

Plunger and plunger sleeve are very precise matching parts. There is an inclined groove on the plunger body. There is a small hole in the plunger sleeve called the suction port. This suction port is filled with diesel. When the plunger inclined groove is facing the suction port, the diesel enters the plunger sleeve and the plunger is camshaft. When it reaches a certain height, the inclined groove of the plunger is staggered from the suction port, and the suction port is closed, so that the diesel can neither be sucked nor pressed out. When the plunger continues to rise, it presses the diesel, and the diesel pressure will open one way. The valve swarmed into the fuel injection nozzle, and then entered the cylinder combustion chamber from the fuel injection nozzle.

It is particularly important to point out here that diesel engines have an oil inlet and return pipe. The oil inlet pipe is easy to understand, so what is the purpose of the oil return pipe? It turns out that each time the plunger discharges a certain amount of diesel, only part of it is injected into the cylinder, and the rest is discharged through the oil return hole, and the amount of fuel returned is increased or decreased to adjust the injection amount.

When the plunger rises to "top dead center" and moves down, the plunger inclined groove will meet the suction port again, and the diesel will be sucked into the plunger sleeve again. Repeat the above action again. Each in-line plunger system of the in-line fuel injection pump corresponds to one cylinder. There are 4 groups of plunger systems in 4 cylinders, so the volume is relatively large, and it is mostly used in medium-sized cars and above. For example, diesel engines on buses and large trucks generally use in-line fuel injection pumps.

Fuel injection pumps on diesel engines of cars and light vehicles are generally distributive type, which has the advantages of small size, light weight, and few parts, and simple structure. It is pressurized with two sets of plunger systems (or a set of plunger systems), and the diesel fuel is fed into each injector separately.

Its basic working principle is that there are two sets of opposed plungers in the pump installed on the impeller. When the impeller is driven by the engine to rotate, the plunger also rotates. Because the convex part of the cam ring presses the plunger, the plunger It acts like a pump to send diesel oil to the oil feed hole in the center of the impeller. At this time, the diesel fuel sent out fills the inlet of the distributor, and then the injection is arranged in order of each cylinder.

Since the rotation speed of the two groups of plunger systems (or a group of plunger systems) increases in proportion to the number of cylinders, this type of fuel injection pump is limited by the number of cylinders and the maximum speed.

With the development of diesel engine technology, a new type of single fuel injection pump (called a single pump or pump nozzle) has emerged, which actually "turns the above two types of fuel injection pumps to zero", each engine Cylinder fuel injection is accomplished by its own independent injection unit (single pump or pump nozzle).

For the single pump, the fuel injection pump and the fuel injection nozzle are connected by a short high-pressure fuel pipe, while for the pump nozzle, the fuel injection pump and the fuel injection nozzle are integrated into one body, which is directly installed on the cylinder head of the diesel engine. Driven by overhead camshaft. Their biggest advantage is that they can reduce or eliminate the influence of pressure waves formed in the high-pressure fuel pipes during the flow and injection of diesel fuel. Because this pressure wave will hinder the good matching of the fuel injection system to the load and speed, and will increase as the length of the high-pressure pipe increases.