What is a plunger and what are the differences between it and a piston?
As far as the automotive industry is concerned, the plunger is often referred to as a piston and the piston as a plunger.
This is not quite the right interpretation, since there is a definite difference between them.
It is necessary to understand the design features and nuances of plungers, as well as to understand how plungers differ from pistons, whether there is a significant difference between them or whether the words of many car enthusiasts about their identity are fair.
What is a Plunger
Plungers are called special displacers, which are characterized by their cylindrical shape.
An important aspect is that a plunger has a length far greater than the diameter of the part. Some even compare it to a jump rope on a handle.
It is important to understand the difference between an ordinary piston and the plunger in question. It really is. In fact, it is a kind of piston. Only it can be used in mechanisms and devices where you need to create higher pressure parameters in comparison with classic piston-type pumps.
The plunger has another peculiarity. Here, the seal is located directly on the cylinder and moves on the plunger surface at the moment of reciprocating motion. This is commonly referred to as a plunger pair.
Plungers have gained widespread popularity in the field of radial and axial-plunger hydraulic machines, as well as in the plunger-type pumps themselves.
The most striking example of the use of this element is the fuel supply system in diesel engines, implemented on the basis of the fuel injection pump.
It is in the high-pressure fuel pumps that plunger pairs are actively used.
If compared with pistons, the elements in question allow the creation of pumps that provide higher pressure values. Specialists explain this by the fact that plungers require increased cleanliness of machining exactly on the side of the outer surface of the cylinder. With piston counterparts, the main emphasis is placed on the accuracy of machining the inner cylindrical surface, which is technically much more difficult to implement.
This gives much insight into the difference between such elements as a plunger and a piston, and why it is wrong to consider them synonymous.
If we talk in simple terms about what a plunger is, it can be described as follows. It is a type of piston that can create high pressure in mechanisms.
Significant characteristics
How much fluid volume the mechanism will displace depends on the stroke length parameters of the piston used. By varying this value, it is possible to adjust the fluid supply per time interval or interval.
If you emphasize the operation of the plunger or rotary-plunger mechanism, these hydraulic machines have a very high accuracy of detail. As a result, the clearance available between the outer and inner surface of the cylinder in the plunger pairs used may not exceed 2-3 µm.
The plunger pairs are characterized by their ability to withstand very high pressures.
Again, as an example, it is worth considering the fuel injection process on diesel engines. Here, the pressure can reach values of about 200 MPa.
Plungers in the fuel injection pump
The fuel pump, more precisely the high pressure pump, is installed on diesel engines and is designed to deliver fuel at high pressure. It is the increased pressure parameters that make it clear what the plunger is for. It uses a plunger pair, which is rightly referred to as one of the key components of the pump.
This pair consists of the sleeve and the plunger itself. The plunger is in the form of a long cylindrical piston. It moves inside the sleeve when the pump is running. The plunger, installed in the pump assembly in the vehicle, performs reciprocating movements, thereby pressurizing and sucking fuel.
Special holes are provided on the sleeve of this pair. With their help, the fuel ends up in the discharge unit. In parallel, the plunger (displacer) acts as a regulator, dosing the amount of fuel.
All this means that the pair is able to precisely measure the volume of fuel supplied to the internal combustion engine cylinders, creating the required pressure parameters at a particular moment of liquid supply. In order for the steam to fulfill its function, the elements are subject to special requirements.
Here we are talking about the following requirements
- High-strength materials are used for the surface of the sleeve;
- High-strength materials with additional hardening must also be used for the plungers;
- the pair must be thermally hardened in the factory;
- Depending on the technology the hardness is between 58 and 75;
- the pair must be able to move freely while avoiding even minor fluid leaks;
- the clearance between the bushing and the plug must not exceed 1-3 microns;
- Each part should be selected individually;
- Once the bushes are matched to the displacers, they are further matched.
That is how the desired operating effect is achieved. And here there is a tangible difference between the plunger, because the latter is not designed to work under conditions of increased pressure. The plunger copes with such tasks perfectly.
Operational peculiarities
Continuing the subject of the fuel injection pump, which is most relevant to the discussion of the plunger as part of automotive assemblies, it is worth noting some operational features.
The constructive feature provides that in the plunger pair there is a minimum gap between the surfaces of the two elements. Therefore, when operating the pump, there are higher requirements for maintaining the optimal condition of the internal combustion engine power system.
For the plunger-hub pair to work effectively, it is recommended to fill the tank only with high quality fuel. The presence of water, as well as other foreign impurities, may negatively affect the operation and condition of the plunger.
When water penetrates into the microscopic gap, the fuel film, which also acts as a lubricant for this part, is torn. Dry operation contributes to severe friction and overheating.
As a result, the plunger can jam, and the entire fuel pump will fail. Even a small amount of water in the fuel can cause corrosion on the surfaces. In the case of mechanical particles, the plunger can jam.
Failure Diagnosis
There are several characteristic features and signs, as far as car engines are concerned, that indicate a failed or worn displacer.
This can manifest itself in the form of symptoms such as:
- Difficulty in starting the engine;
- After starting the engine, floating revolutions are observed;
- the engine is unstable;
- The engine may start to stall;
- the noise level during operation increases, which has not been observed before;
- in some cases it is possible to notice the knocking of the plunger when the fuel pump is running;
- working under load, the power unit decreases its power; accompanied by perceptible jerks;
- In some situations, the engine goes to pieces.
To check the plunger in the high pressure pump, you need to use special equipment. With its help you can find out how worn out the displacer and bushing.
There are two ways to restore the operation of the high-pressure pump:
- perform a complete replacement of worn and failed components;
- to carry out repair of parts.
The essence of the repair is to restore the exact fit, to provide the necessary clearance to the required factory parameters. But to do it in garage conditions will be extremely problematic. It will be necessary to address in an auto service center.
To sum up
Displacers, which have a cylindrical shape and a length significantly greater than the diameter of the part, are really different from pistons. Therefore, separately distinguish between plunger-type and piston-type pumps. The latter cannot cope with the tasks that their counterparts based on a plunger-sleeve pair perform without problems. These are mainly high-pressure operating conditions.
The most striking example of the plunger in relation to the car is the fuel injection pump. This device ensures the stable operation of the diesel engine. But at the same time there are high requirements to the plunger pair production, their fitting to each other, as well as to the provision of operating conditions.
Piston and plunger pump
The piston fluid pump is one of the first representatives of pumps. Mechanical displacement of fluid is one of the first principles of fluid pumping.
What distinguishes piston pumps from other positive displacement hydraulic machines is their irreversible effect on water. In other words, such pumps cannot operate as hydraulic drives.
Contents of the article
- Principle of operation
- Design
- Characteristics
- Piston air pump
- Advantages, disadvantages and area of application
Nowadays, the design of the piston pump has undergone many improvements and the modern version has a sturdy housing and has a wide range of possibilities for interaction.
Principle of operation of a reciprocating pump
The operation of the reciprocating liquid pump is based on the principle of displacement. The main working bodies of such equipment are: a cylinder and a piston. The piston moves in the cylinder in reciprocating motion.
The operation of a piston pump in general is as follows
The piston (item 
moves in the cylinder (item 7) rigidly connected to the rod (item 9), which is the executive part of the drive crank. As the piston moves to the right, the effective volume of the cylinder, i.e. the volume to be filled with fluid, increases, causing the pressure in the cylinder to decrease.
The suction valve (item 4) is lifted and the fluid, under the action of external pressure on its surface, usually atmospheric pressure, enters the cylinder through the vessel (item 1), the open check valve (item 2) and the suction pipe (item 3).
When the piston moves “to the left”, the fluid that previously entered the cylinder is pushed out by the moving piston. This increases the pressure in the pump cylinder. The suction valve (position 4) closes and the discharge valve (position 5) rises and the liquid flows from the cylinder to the discharge line (position 6). The supply of liquid to the discharge line is caused by the moving piston displacing pre-sucked liquid from the cylinder.
High pressure plunger pumps
A plunger pump is a type of displacement pump. The plunger pump is distinguished by its operating element, the plunger. Its task is to move along the axis of the cylinder. The plungers are moved by means of an electric drive, which spins the crankshaft.
The fluid is drawn into the pump cylinder as the plunger moves upward. This raises the suction valve K1 and the liquid enters the pump cylinder under external pressure. On the downward return movement of the plunger, valve K1 is pressed against its seat, closing it, and discharge valve K2 opens, allowing the fluid displaced from the cylinder into the discharge line.
Pump plug 1 in operation is in contact only with packing elements 2, sealing the plug in the cylinder. The thoroughness of the sealing is achieved by the packing compressed by the packing cup, which reduces friction and wear on the contacting surfaces. As a result, the plunger pump cylinder is not subject to wear and serves only as a reservoir which is filled and emptied depending on the direction of the plunger movement.
Classification of piston pumps
Now that you know the peculiarities of these two types of equipment, we propose to highlight their basic classification:
By type of action: simple-acting piston pump – one side of the piston is working; double-acting piston pump – both sides of the piston are working;
By type of arrangement of cylinders: horizontal; vertical.
By type of drive: driven – runs from the engine, connected to the pump through a connecting rod; direct action – mounted on a common rod with a steam engine.
Construction of a piston pump
At the heart of the structure of piston pumps is a hollow metal cylinder, in which all working processes take place.
A reciprocating water pump generally consists of: 1. valves 2. piston moving in the cylinder 3. connecting rod 4. crank
The purpose of the valves is to let the water in, while preventing it from moving back out. The valves can be a ball or a diaphragm, depending on the design.
Hydraulic piston pumps use a ball made of glass, plastic or metal as the sealing element in the check valve.
A diaphragm piston pump has a rubber plate (diaphragm) attached to one side as a valve.
The movement of the piston in the cylinder is achieved by rotation of the crank fixed on one shaft with an electric motor.
The piston pump of the modern type has several valves whose rods are attached to a single crank. By rotating in bearings such variable displacement pumps are able to provide a stable flow rate.
High-pressure plunger pumps can handle aqueous media and any liquids like water, which are of low viscosity and cannot react with the metal parts of the equipment. The device works like a dispenser. The plunger pump can be manual or automatic. In this case, the dosing pump pumps the liquid by means of high pressure.
In contrast to the piston pump, a special feature of the plunger pump is the absence of an internal piston seal. This leads to their extensive use in the high-pressure range.
the plunger pump high pressure has a number of advantages: the pump is quite easy to mount, the high pressure plunger pump is easy to control, the lubrication system is provided, allowing easy access to it there is a possibility to adjust the high pressure plunger pump to the output of the necessary working pressure
Besides, axial and radial piston pump types are structurally distinguished.
The difference between piston and plunger
Piston pumps can be either a circular piston or a plunger type according to the construction of the operating element that forces the fluid out of the cylinder.
The pump piston (shown left) is disk-shaped, sealed in the cylinder by special spring-loaded split metal (and most often cast iron) rings. A disc piston can also be carefully sealed in the cylinder with rubber or leather sleeves.
In contrast to the piston, the plunger (shown right) is a hollow cylinder that is much longer in length than the diameter. It moves in the packing gland without making contact with the walls of the operating cylinder. Plungers are made as a rod (piston rod).
Working Characteristics
Displacement of a piston pump
The delivery rate of a pump is the volumetric quantity of liquid the pump puts into the discharge line per unit of time. This definition applies to all pumps, regardless of pump design.
The delivery of a piston pump Q is expressed by the product of the volume V displaced per stroke by the number of strokes per time unit.
Volume V=f*S, where f is the area of the piston and S is its stroke.
Q = f*(S*i/60), where i is the number of strokes per minute.
S*i/60 = Vcf is the average velocity of the piston, taking into account the movement only at the working stroke.
Thus Q = f*Vcf.
If to consider the pump characteristic, the piston pump delivery varies cyclically in time, the graph of liquid delivery into the pressure line for the pump of single-acting has discontinuous character.
Double-acting piston pumps are used to equalize the delivery schedule.
Plunger Pump Flow
The delivery of a triple-acting plunger pump is equal to triple the delivery of a single-acting pump.
A triplex plunger pump creates a more uniform delivery of fluid into the discharge system than a piston pump and generally does not require special devices to equalize the delivery schedule.
This property is an essential advantage of this type of pumps.
Power and efficiency
The power and efficiency of piston and plunger pumps are the main characteristics that speak for the quality of the equipment performance. Efficiency, the coefficient of performance, speaks to the losses in the pump and is made up of two values.
Hydraulic efficiency is the power loss due to hydraulic resistance:
Mechanical efficiency – shows the mechanical losses in the equipment, such as friction, etc.
Useful power of a reciprocating pump:
N = Q – ρ – g – H, where Q – pump flow rate; ρ – water density; H – total height of liquid lift.
Piston air pump
A piston pump, which draws in gas or air at a pressure below atmospheric pressure and vents it to the atmosphere, is called a vacuum pump.
In the food industry they are mainly used for drawing out non-condensable vapours and gases in evaporation stations, in breweries and factories with vacuum pumps and for creating vacuum in the vacuum sections of vacuum filters. Low-vacuum pumps, which create a vacuum of up to 92-95% of atmospheric pressure at their suction nozzle, are most commonly used.
In principle, a reciprocating air pump is a compressor that draws in gas at a reduced pressure, compresses it, and then pressurizes the gas. Although in practice the discharge pressure is not much higher than atmospheric pressure, the degree of compression in a reciprocating air pump is much higher than in a conventional compressor.
With this degree of compression, the volumetric efficiency is low, about 35%. In order to increase the volumetric efficiency, technical methods are used to equalize the pressure at the suction and discharge of the pump, so that a high volumetric efficiency is achieved.
Advantages and disadvantages of the piston and plunger pump
A great advantage of the pump is its reliability and high maintainability. These two parameters result not only from the operating principle, but also from the construction of the pump – the pump is made of highly durable materials. The pump is able to work with media with high requirements for starting conditions. A huge advantage of this type of pump, in contrast to the circulation pumps, is the possibility of “dry” suction, which can boast not every pump.
Of the disadvantages, it should be noted the low productivity. Currently, there are models at the market, where this figure is at an acceptable level, but these pumps have higher requirements for the parameters of operation, which translates into a high cost of the pump.
Application area
In displacement pumps, the head value is not limited in principle. An increase in the delivery rate can only be achieved by increasing the design dimensions and the number of working strokes (number of revolutions).
In piston and plunger pumps, due to the cyclic motion of the displacement body, the liquid flow is unsteady, and the increase of the flow rate and, therefore, the flow rate by increasing the number of revolutions is limited by the inertia phenomena.
Plunger pumps are useful when high pressures are required at relatively low flow rates.
Pumps with pressures of 1000 atmospheres and more are built in the presses and the chemical industry. Specialized piston pumps can be used for aggressive media, explosive mixtures and some types of fuel. But the scope of application of this type of pumps is not limited to the industrial field only. These pumps are also used to provide clean water for domestic purposes.
Although the piston liquid pump is not designed for large circulation volumes, it is highly reliable and with timely maintenance can work for a very long time.
The piston pump is a type of displacement pump. For an opinion on this type of pump, read the article on screw pumps.
Piston pumps occupy a separate niche in the market, they meet the requirements of both private users and large industries. The demand for this type of pumps in domestic needs is due to both the simplicity of their design and undemanding maintenance, as well as the high operating life of this type of equipment.
