- What is a plunger?
- Plunger pair of injection pump
- Discharge valves
- Features of the operation of the plunger pairs of the fuel pump
- How to determine the malfunction of the plunger pairs yourself
Often, mechanics, when talking about repairing the fuel system of a diesel engine, mention a term such as a plunger pair. Let us consider in more detail what kind of mechanism it is, the peculiarity of its operation, why it is needed, and how the plunger malfunction is determined.
What is a plunger?
Plunger pairs, as the mechanism name suggests, are two small parts that go into a high pressure fuel pump (injection pump) device. The first is called a plunger and is represented as a thick, recessed finger. The second is the plunger sleeve and it looks like a thick-walled sleeve into which the first part is inserted.
The plunger or piston itself acts as a fuel displacer from the bushing cavity. This element is used to create high pressure in the fuel supply system line.
It should be noted that this mechanism is not exclusively used in diesel engine pumps. For example, a similar element is used in the gas distribution mechanism as hydraulic compensators. However, the principle of operation of the devices is identical - reciprocating actions move the piston in the sleeve, and through the coincidence of the cuts and holes in these two parts, liquid enters the cavity and is pumped into the main line.
Plunger pair of injection pump
We will discuss in more detail the functions of the plunger pair using the example of a diesel injection pump. As the name suggests, the pump pumps diesel fuel from the main tank to the high-pressure line (for example, to the fuel rail), from where it is sprayed into the engine cylinders under strong pressure.
The main element that creates such a pressure is just the plunger pair. The fuel is then distributed to the cylinders in accordance with the design of the fuel system. The types of pumps are described in another article.
During pump operation, push rods and actuator return springs move the piston up / down inside the plunger bushing, thereby reciprocating. So the design sucks in the diesel fuel through the hoses from the fuel tank and pumps it into a closed tank, which creates pressure in it. To prevent this parameter from increasing excessively, the pump device has a number of valves designed to hold or dump excess pressure in the system.
The piston itself has an uneven slot, which allows it to dose the fuel entering the tank with a slight axial displacement. This process depends on the position of the gas pedal in the car - the minimum clearance is when the accelerator is released, and the maximum is when the pedal is fully depressed.
Since the steam creates a high pressure so that it does not fail, it is made of strong steel, and the walls are thick enough to withstand a pressure of several hundred atmospheres. This makes the mechanism reliable even under high loads.
Another feature of the plunger pair is that both parts are created only for each other. That is, it is impossible to take a bushing from one mechanism and a piston from another and combine them. To prevent steam from letting in diesel fuel, the gap in it is created as minimal as possible. For this reason, one part is never replaced - the pair always changes (their parameters are adjusted on high-precision factory equipment).
Here is a short video on how the plunger pair is restored:
The sequence of the plunger pair
The amount of fuel pumped in one piston cycle depends on the height of its working stroke. This regulates the operation of the pump to ensure idle speed. But as soon as the driver presses on the gas pedal, the plunger turns a little. The notch in that part is increased, therefore, the amount of fuel will be supplied in a larger volume.
This is how the most common plunger modification works. However, today there are many models that provide dosage in slightly different ways (often regulated by the electronics of the machine). The plunger pushers themselves are driven by the rotation of the crankshaft.
When the piston is lowered, through the inlet of the bushing, the fuel moves into the vacant cavity of the above-piston space due to the vacuum that has formed in it. As soon as the piston rises, the liner bore is closed off by the plunger body, and the fuel presses on the valve, opening it. Next, the fuel enters the high pressure tank. When the downward movement starts, the valve closes, and a vacuum (or vacuum) is formed in the cavity of the plunger pair. The cycle is repeated.
Each high-pressure fuel pump is equipped with pressure valves, the purpose of which is to shut off the part of the line in which the fuel is at rest from the one where the diesel is already under pressure. Also, valves are needed to maintain static pressure in the system (while the engine is running, the pump continues to pump diesel fuel into the tank) - they dump the excess back into the fuel tank.
There are several types of discharge valves that are used in plunger pumps. Here are their distinctive features.
Constant volume valve without return flow restriction
The design of this valve includes a retraction piston (part of the valve design). When the plunger is lifted, the helical slot is closed by the bushing body, the discharge valve is closed. The piston moves into the rod sleeve guide.
At this moment, that part of the line is cut off, in which high pressure is formed from the supra-plunger cavity. Due to this, the volume of fuel in the container with high pressure increases insignificantly - only by the amount that has entered through the retraction piston into the cavity of the working stroke of the plunger.
Fixed volume valve with return flow restriction
When fuel is atomized through the nozzle, after closing the needle, backflow pressure is created in the line. This effect can lead to wear on some of the valve itself. For this reason, some pump models use a backflow limiting valve. It acts as a damper to prevent back pressure from acting on the valves.
The device of such a discharge valve includes the following elements:
- The valve body is threaded (it is screwed into the pump structure);
- A spring that serves as a damping element that prevents a negative effect on the damper blade itself;
- Plate - cuts off the high pressure cavity from the cavity above the plunger space;
- The holder against which the valve plate abuts (has a hollow structure - the delivery piston enters the cavity).
Often, these valves are used as an additional mechanism to facilitate the operation of the check valve.
Constant pressure valve
In addition to the main elements of the valve, the device of this mechanism also includes a ball and a restrictive channel. These valves are capable of delivering fuel pressures in excess of 800 bar.
Its design includes two mini valves - pressure and stabilization. The first element supplies fuel, and the second maintains the generated pressure. This function allows it to maintain a static pressure between injection phases.
The valve modification depends on the parameters of the engine in the car. Some valves are not triggered by mechanics, but by a signal that comes from an electronic control unit.
Features of the operation of the plunger pairs of the fuel pump
There are no special steps for servicing the plunger pair of the fuel pump. However, the car owner can do something to make the mechanism work as long as possible.
First, it is worth considering that a diesel engine runs on a special fuel, which can contain a large number of microscopic particles. If you use low-quality diesel fuel, then the gap between the plunger and the bushing may increase due to the content of abrasive particles, water and other impurities in the diesel fuel.
For this reason, the only service that a car owner can perform is to monitor the quality of the fuel, prevent condensation in the line and change the filter in time.
At first glance, the presence of water droplets in diesel fuel does not seem so critical, but because of this, the fuel film in the gap of the plunger pair will collapse, and the mechanism will not be able to create an appropriate pressure. Also, diesel oil lubricates the surfaces of parts, preventing friction when dry, and protecting the device from overheating.
If the fuel filter is not changed in time, its element may burst. Because of this, dirty fuel will be pumped through the pump, in which small particles may be present. In this case, there is a high probability of pump failure, since the plunger pair will simply jam.
How to determine the malfunction of the plunger pairs yourself
The stability of the machine's power unit depends on the correct operation of the plunger pair. Since this mechanism is the main element of the injection pump, its malfunction will lead either to unstable operation of the pump, or even to its failure.
To check the efficiency of the pump, you will need to diagnose it. Most repair shops have special equipment for this. It allows you to determine what exactly the malfunction is - even determine the condition of the plunger pair. According to the results of diagnostics, specialists will offer appropriate repairs. If the plunger fails, the entire kit must be replaced.
The fact that there are problems with the plunger pair is evidenced by the "symptoms" characteristic of a breakdown of the fuel pump. Among them:
- Difficult start-up of the unit;
- Unstable engine idle speed;
- Engine triplet;
- A sharp loss of internal combustion engine power;
- During engine operation, extraneous noises are clearly audible;
- Increased "gluttony" of the motor.
A supplementary review examines the difference between a cold and hot plunger test:
To make sure that the plunger is faulty, one should not skimp on diagnostics. Only specialists with the help of specialized equipment are able to accurately determine the malfunction. Thanks to this, the cost of repairs will be justified - you will not have to change parts that will work for a long time.