Types of engine supercharging. Types of supercharging of internal combustion engines Modern methods of supercharging in internal combustion engines

Since the advent of the internal combustion engine, designers have been faced with the task of increasing its power. And this is only possible in one way - by increasing the amount of fuel burned.

Ways to increase engine power

To solve this problem, two methods were used, one of which was to increase the volume of the combustion chambers. But in the context of ever-tightening environmental requirements for car power units, this method of increasing power is now practically not used, although previously it was a priority.

The second method of increasing power comes down to a forced increase in the amount of combustible mixture. As a result, even small-volume power plants can significantly improve performance.

If there are no problems with increasing the amount of fuel supplied to the cylinders (the fuel supply system is easily adjusted to the required conditions), then with air it is not so simple. The power plant pumps it in independently due to the vacuum in the cylinders and it is impossible to influence the volume of injection. And since for maximum efficient combustion in the cylinders, a fuel-air mixture with a certain ratio must be created, increasing the amount of fuel alone does not give any increase in power, but on the contrary, consumption increases and power decreases.

The way out of the situation is to force air into the cylinders, the so-called engine supercharging. Note that the first devices that pump air into combustion chambers appeared almost from the moment the engine itself appeared, but for a long time they were not used in vehicles. But superchargers were widely used in aviation and on ships.

Types by method of creating pressure

Engine supercharging is a theoretically simple idea. Its essence boils down to the fact that forced injection allows you to significantly increase the amount of air in the cylinders compared to the volume that the engine itself sucks in, and accordingly, more fuel can be supplied. As a result, it is possible to increase the power of the power plant without changing the volume of the combustion chambers

But this is all simple in theory, but in practice many difficulties arise. The main problem comes down to determining which supercharging design is the most efficient and reliable.

In general, three types of superchargers have been developed, differing in the method of pumping air:

Roots
Lysholm (mechanical supercharger)
Centrifugal (turbine)

Each of them has its own design features, advantages and disadvantages.

Roots

The Roots style supercharger was originally introduced as a conventional gear pump (something similar to an oil pump), but over time the design of this supercharger has changed greatly. The modern Roots supercharger replaces the gears with two counter-rotating rotors mounted in a housing. Instead of teeth, the rotors have blade cams that engage the rotors with each other.

The main feature of the Roots supercharging is the way it is charged. Air pressure is created not in the housing, but at the exit from it. Essentially, the rotor blades simply capture air and push it into an outlet passage leading to the intake manifold.

Design and operation of the Roots supercharger

But such a supercharger has several significant drawbacks - the pressure it creates is limited, and air pulsation is still present. But if the designers were able to overcome the second drawback (by giving the rotors and output channels a special shape), then the problem of limiting the created pressure is more serious - either it is necessary to increase the speed of rotation of the rotors, which negatively affects the service life of the supercharger, or to create several stages of discharge, which is why the device becomes very complex in design.

Lysholm

The Lysholm-type supercharging engine is structurally similar to the Roots, but instead of rotors it uses spiral-shaped augers (like a meat grinder). In this design, pressure is created in the supercharger itself, and not at the outlet. The idea is simple - air is captured by the augers, compressed during transportation by the augers from the inlet to the outlet, and then pushed out. Due to the spiral shape, the air supply process is continuous, so there is no pulsation. This supercharger creates more pressure than the Roots design, operates silently and in all engine modes.

Lysholm type supercharger, another name is screw.

The main disadvantage of this supercharging is the high manufacturing cost.

Centrifugal type

Centrifugal blowers are the most common type of device today. It is structurally simpler than the first two types, since it has one working element - a compression wheel (ordinary impeller). Installed in the housing, this impeller captures air from the inlet channel and pushes it out into the outlet channel.

Centrifugal blower with gas turbine drive

The peculiarity of the operation of this supercharger is that in order to create the required pressure it is necessary for the turbine wheel to rotate at a very high speed. And this in turn affects the resource.

Drive types, their advantages and disadvantages

The second problem is the supercharger drive, and it can be:

Mechanical
Gas turbine
Electric

In a mechanical drive, the supercharger is driven from the crankshaft through a belt, or less often a chain, transmission. This type of drive is good because the boost starts working immediately after the power plant starts.

But it has a significant drawback - this type of drive “takes away” part of the motor power. The result is a vicious circle - the supercharger increases power, but immediately takes it away. A mechanical drive can be used with all types of supercharging.

The gas turbine drive is currently the most optimal. In it, the supercharger is driven by the energy of burnt gases. This type of drive is used only with centrifugal charging. A supercharger with this type of drive is called turbocharging.

To harness the energy of exhaust gases, the designers essentially simply took two centrifugal superchargers and connected their impellers to a single axis. Next, one supercharger was connected to the exhaust manifold. Exhaust gases leaving the cylinders move at high speed, enter the supercharger and spin the impeller (it is called a turbine wheel). And since it is connected to the impeller (compressor wheel) of the second supercharger, it begins to perform the required task - pumping air.

Turbocharging is good because it does not affect engine power. But it has a drawback, and a significant one - at low engine speeds, due to the small amount of exhaust gases, it is not able to effectively pump air; it is effective only at high speeds. In addition, in turbocharging there is such an effect as “turbo lag”.

The essence of this effect comes down to the fact that turbocharging does not provide an instant response to the driver’s actions. If there is a sudden change in the operating mode of the engine, for example, during acceleration, at the first stage the energy of the exhaust gases is not enough for the boost to pump in the required amount of air; it takes time for processes to take place in the cylinders and the amount of exhaust gases to increase. As a result, when you sharply press the pedal, the car “stumbles” and does not accelerate, but as soon as the boost picks up speed, the car begins to actively accelerate – it “shoots”.

There is another not very pleasant effect - “turbo lag”. Its essence is approximately the same as that of “turbo lag”, but its nature is somewhat different. It boils down to the fact that supercharging has a delayed reaction to the driver’s actions. This is due to the fact that the supercharger takes time to capture, pump air and deliver it to the cylinders.

Indicative graphs of the effects of “turbojam” and “turbolag” depending on power

“Turbojam” appears only in superchargers powered by exhaust gas energy, but in devices with mechanical drives it does not exist, since the charging performance is proportional to engine speed. But “turbo lag” is present in all types of superchargers.

Electric supercharging drives are beginning to be introduced in modern cars, but they are just in their infancy. For now they are used as an additional mechanism to eliminate “turbo lag” in the operation of turbocharging. It is possible that soon a development will appear that will replace the superchargers we are used to.

Electric supercharger from Valeo

For them to work effectively, they need more high voltage, so a second network with its own 48-volt battery is used. The Audi concern generally plans to transfer all equipment to a higher voltage - 48 volts, as the number of electronic systems and, accordingly, the load on the vehicle network increases. Perhaps in the future all automakers will switch to higher voltage on-board networks.

Other problems

In addition to the injection method and type of drive, there are many more issues that have been successfully resolved or are being resolved by designers.

These include:

heating air during compression;
"turbojam";
efficient operation of the supercharger in all modes.

During injection, the air heats up greatly, which leads to a decrease in its density, and this in turn affects the detonation threshold of the air-fuel mixture. This problem was resolved by installing an intercooler - an air cooling radiator. Moreover, this unit can carry out cooling different ways– by counter air flow or due to a liquid cooling system.

Options for supercharging systems

But installing an intercooler gave rise to another problem - an increase in “turbo lag”. Because of the radiator, the overall length of the air duct from the supercharger to the intake manifold has increased significantly, and this has affected the pumping time.

The problem with “turbo lag” is solved by automakers in different ways. Some reduce the mass of their components, others use variable turbo drive geometry technology. In the first solution to the problem, reducing the mass of the impellers leads to the fact that less energy is required to spin up the boost. This allows the supercharger to start working earlier and provide air pressure even at low engine speeds.

As for the geometry, due to the use of special impellers driven by an actuator installed in the turbine wheel housing, it is possible to redirect the flow of exhaust gases depending on the operating mode of the engine.

Some manufacturers decide to increase the efficiency of the supercharger in all operating modes by installing two or even three superchargers. And here every car company acts differently. Some install two turbochargers, but of different sizes. The “small” supercharger operates at low engine speeds, reducing the effect of “turbo lag”, and when the speed increases, the “large” boost comes into operation. Other automakers use a combined scheme in which a mechanically driven supercharger is responsible for low speeds, which completely eliminates turbo lag, and at high speeds turbocharging is activated.

Finally, we note that the above are only some of the main problems associated with forced air supply to the cylinders; in reality, there are more of them. These include overblowing and surge.

The increase in power by a supercharger, in fact, is limited by only one factor - the strength of the components of the power plant. That is, power characteristics can only be increased to a certain level, exceeding which will lead to the destruction of motor components. This excess is called overblowing. To prevent this from happening, the forced air injection system is equipped with valves and channels that prevent the impeller from spinning above the set speed; it turns out that the charging performance has a limit. Additionally, when certain conditions are reached, the power supply ECU adjusts the amount of fuel supplied to the cylinders.

Surge can be described as “reverse movement of air.” The effect occurs when there is a sharp transition from high to low speeds. As a result, the supercharger has already pumped in a large amount of air, but due to a decrease in speed it becomes unclaimed, so it begins to return to boost, which can cause its breakdown.

blow-off valve

The problem of surge is solved by using bypass channels (bypass), through which compressed unconsumed air is pumped to the input channel in front of the supercharger, thereby softening, but not eliminating, the load during surge. The second system that completely solves the problem of surging is the installation of a bypass valve or blow-off, which, if necessary, releases air into the atmosphere.

Installing air blowers on power plants is still the most optimal way to increase power.

Autoleek

Since the need for supercharging engines became apparent, many variants of supercharging have appeared. The main types of supercharging are the following:

Figure 1 - Types of supercharging

Supercharging systems can be qualified by:

1) the method of supplying air without a blower due to the inertia of the column of air or gas itself;

2) supercharger design;

3) type of supercharger drive;

4) the type of connection between the supercharging unit and the engine.

Inertial charging (without a supercharger, also called “resonant”, “wave”, “acoustic”) is carried out due to pressure fluctuations in the intake pipe of a piston engine. The wave of pressure decrease in the intake pipe at the entrance to the cylinder during the intake stroke moves at the speed of sound to the opposite open end of the pipe, is reflected from it and, in the form of a pressure wave, moves again at the speed of sound to the intake valve. By choosing the length of the pipeline so that the pressure wave approaches the final period of intake, it is possible to ensure that charge is supplied to the cylinder under excess pressure, thereby supercharging the engine (Figure 2).

Figure 2 - Intake tract diagram 1 - air cleaner housing or special resonator

The pipeline length l required for this can be calculated from the time f of the wave passing from the valve to the open end of the pipeline and back.

The energy to “accelerate” the air column in the intake manifold is taken from the additional work of the piston, i.e. due to increased pumping and mechanical losses of the engine.

Inertial charging as an independent charging system is used in passenger car engines. The length of the intake pipe can vary depending on the engine speed, thereby ensuring high filling of the engine cylinders in a wide range of modes.

In combination with gas turbine supercharging, inertial supercharging was used in diesel engines of trucks - Scher's combined supercharging system (Figure 3).

The level of increase in boost pressure during inertial charging is relatively small, so such systems are usually used not to increase the maximum engine power, but to improve the flow of the torque characteristic.

Figure 3 - Combined supercharging system proposed by G. Sher

Another well-known method of supplying air to engine cylinders under increased pressure is the use of exhaust gas pressure waves in the gas-dynamic machine “Comprex” (the name “Comprex” comes from the English words compression and expanding) (Figure 4).

The principle of operation of this system is based on the fact that the pressure wave passing through the pipeline channel is reflected negatively at the free end, i.e. as a rarefaction wave, and at the closed end as a pressure wave, and, conversely, a suction wave at the open end is reflected as a pressure wave, and at the closed end as a suction wave.

The Kompreks system consists of a rotor with axial channels - cells of a trapezoidal cross-section, open at the ends. The rotor, mounted in bearings and surrounded by a casing, is driven through a belt drive from the engine crankshaft. The power required to rotate the rotor is small, because it is spent only to overcome friction in bearings and ventilation losses.

Figure 4 - Diagram of the Kompreks supercharging system 1 - exhaust pipeline; 2 -- inlet pipeline; VND -- air low pressure; VVD - high pressure air; HPG - high pressure gas; GND - low pressure gas; R - rotor.

Air and gas channels converge at the end sides of the housing. The axial channels - the rotor cells - alternately coincide with the end walls of the supercharger housing, then with the inlet or outlet pipelines leading either to the engine or to the atmosphere through an air cleaner or muffler.

The supercharging units can be driven:

1) from the crankshaft of the internal combustion engine directly or through a switchable device (“drive superchargers”);
2) from an external energy source, for example, the so-called “e-drive” - from an electric motor (“electrically supported supercharging”);

3) from a turbine that uses the energy of the exhaust gases of the internal combustion engine (turbocompressors).

As drive superchargers, either positive displacement superchargers (piston, rotary-gear (Roots type), rotary-screw, rotary-plate (vane)) or blade (usually centrifugal) are used. The Roots drive supercharger (Figure 5) has two specially shaped rotors, the axes of which are interconnected, connected through gears to the supercharger drive gear, which in turn is connected to a pulley driven by the crankshaft via a belt drive. Rotors rotating in opposite directions literally “suck” air through the inlet, pushing air currents into the so-called. distribution compartment.

Figure 5 - Roots drive supercharger

Another representative of mechanical superchargers is a screw (Linholm supercharger) in its shape and structure very similar to the Roots supercharger (Figure 6), but in reality it differs radically from it.

Figure 6 - Linholm Driven Supercharger

The shapes of the screw supercharger rotors are more pointed, and they themselves resemble self-tapping screws or meat grinder screws. When the rotors rotate, the air entering the supercharger is forced through this conveyor of spirals and is already in a compressed state when it exits the housing. In addition, the air is already compressed inside the device, which means that there will be nowhere to counteract the forces that push the air back in the Roots supercharger.

Drive centrifugal blowers (Figure 7) are made in the shape of a snail and have approximately the same properties as turbines.

Figure 7 - Driven centrifugal blower

Air entering the supercharger housing is picked up by the blades of the impeller and, unwinding, is pressed against the outer walls of the housing by centrifugal forces. At this stage, the air flow reaches enormous speed, but its pressure is still too low. Then, using a diffuser, the opposite effect is achieved: when leaving the supercharger, the air flow speed decreases, and the pressure, on the contrary, increases due to the air “pressing” from behind. The efficiency of centrifugal superchargers is proportional to engine speed. At low speeds the increase in power is practically not felt (although it is greater than that of the same turbine), but at medium and high speeds the power soars.

Gas turbine supercharged engines are often referred to as "turbo-piston engines" or "combination engines".

With a turbocharger (Figure 8), the compressor wheel and turbine wheel sit on the same shaft. The energy of the exhaust gas flow, which is not used in conventional engines, is converted here into torque - the exhaust gases leaving the engine cylinders are supplied to the turbine wheel, where their kinetic energy is converted into mechanical rotational energy (torque). The compressor wheel draws in fresh air through the air filter, compresses it and supplies it to the engine cylinders. The amount of fuel that can be mixed with air can be increased, allowing the engine to develop more power. There are also many other turbocharger designs.

Figure 8 - Turbocharger

The task of increasing the power characteristics of the power unit has always been relevant. There are quite a few methods for improving engine power; for example, it is possible to increase the overall dimensions of the cylinders, the number and number of engine revolutions. However, all of the above methods lead to a significant increase in the overall dimensions and weight of the power unit, as well as an increase in the load on its structural elements.

There is a much more effective method for improving the power characteristics of the motor. The idea itself is quite simple: the more air you can “push” into the cylinder of the power unit, the more fuel you can burn and, as a result, increase engine power. This method is called engine supercharging. Its main advantage is the fact that the overall dimensions and weight of the motor remain the same, but its power characteristics will be higher.

In a conventional power unit, the combustible mixture is supplied to the cylinders at a pressure that is significantly less than atmospheric. In this case, it is necessary to take into account the presence of “obstacles” to the passage of the combustible mixture in the form of a throttle valve, an air filter element, turns and the rough surface of the channel walls. By supercharging the engine, the pressure under which the fuel is supplied increases significantly, which allows you to obtain high engine power.

Application of mechanical circuit

Mechanical air blowers were used on vehicles in order to increase the power of the power unit back in the 30s. At that time, such devices were called compressors. Currently, they are mainly called turbochargers, which, in fact, will be discussed further. It is worth noting that there are quite a lot of mechanical structures of this type, but despite this, the development of new modifications is still relevant today.

The picture above shows air blowers with a standard mechanical type design. Such turbochargers have a simple design and are not difficult to operate.

However, there are also unusual air blowers developed by various companies. One of them is the “Comprex” wave air blower developed by Asea-Brown-Boweri. The rotor of this turbocharger has axially arranged cells. During the rotational movements of the rotor, air enters the chambers, after which it approaches the hole in the housing and through it hot exhaust gases from the power unit enter the cell. Interacting with cold air, a pressure wave is formed, which moves at high speed, due to which the air is forced into the opening of the exhaust pipe, to which the camera manages to approach during this period of time. Since the rotor is spinning all the time, the exhaust gases do not enter this hole, but exit as the rotor moves into the next one. Such superchargers have been used by many vehicle manufacturers; for example, Mazda has been using them on some car models since 1987.

Another interesting development is the spiral supercharger - G40. It was first used by the German car manufacturer Volkswagen in 1985.

In 1988 it appeared new modification spiral air blower G-60, which had higher power and was used on Corrado and Passat cars.

Structurally, such superchargers consist of two spirals, the first of which is stationary and acts as part of the housing. The second spiral plays the role of a displacer and is placed between two turns of the first. This spiral is attached to the shaft. The shaft is driven by a belt drive of the power unit with a ratio of one to two.

The operating principle of this design is quite simple and is as follows: during rotation of the shaft, the spiral located inside the housing carries out oscillatory movements and crescent-shaped cavities are formed between them, moving towards the center and thereby moving air from the periphery to the motor under low pressure. In this case, the amount of compressed air supplied directly depends on the speed of the motor shaft.

This supercharger design has two important advantages: fairly high efficiency and wear resistance (due to the absence of rubbing structural elements).

Application of turbochargers

Currently, in order to improve the power characteristics of the power unit, not mechanical air blowers are used, but turbochargers. Such devices are much easier to manufacture, which compensates for a number of disadvantages that are inherent in them.

Modern turbochargers differ from the above schemes primarily in their design features and the operating principle of the drive. IN in this case a rotor with blades is used, that is, a turbine rotated due to the influence of the exhaust gas flow of the power unit. The turbine rotates a compressor mounted on the same shaft, presented in the form of a wheel equipped with blades.

This operating principle of the drive determines the main disadvantages of a gas compressor. It should be noted that in this case the engine speed is quite low, which means the amount of exhaust gases is also small, which negatively affects the performance of the turbocharger.

In addition to an engine with an installed turbocharger, it most often has a so-called, that is, a slow response of the engine to an increase in the amount of hot water supplied. In this case, the driver needs to sharply press the gas pedal all the way, and the power unit reacts only after a certain time. The explanation for this phenomenon is quite simple - it takes a certain amount of time to spin up the turbine, which is responsible for rotating the compressor.

The developers tried to minimize the above-mentioned disadvantages of turbochargers using various methods. And first of all, the mass of the structural rotating elements of the compressor and the turbine itself was reduced. The compressor rotor used today has become so small that it fits in the palm of your hand. In addition, the lightweight rotor significantly increases the efficiency of the compressor even at low speeds of the power unit.

However, reducing the size of structural parts is not the only method of improving the efficiency of a gas compressor. Today, new materials are used for their manufacture, which help reduce the weight of rotor elements, which improves its performance. For example, quite often match silicon carbide is used for these purposes, which is resistant to high temperatures and at the same time it is light in weight.

That is, we can say with confidence that modern turbochargers do not have many of the disadvantages of previous models of similar devices. Due to this, such installations are successfully used on vehicles from different manufacturers. The choice of turbochargers should be based on the initial power of the car, as well as the financial capabilities of the car owner. The installation of such units must strictly be carried out at service stations or auto repair shops.

Which is better to choose a mechanical air blower or a turbocharger?

Increasing the speed performance of your car is quite actual question for many vehicle owners. Today, this problem can be solved in many ways, but the greatest demand is to install a mechanical air blower or turbo compressor. So which of these two options is better? We will try to answer this question in this article.

For this purpose, you first need to understand the operating principle of a mechanical and gas compressor.

Principle and features of the mechanical circuit

There are several types of such devices:

Volumetric air blowers. Such installations supply air to the power unit in equal portions, regardless of the speed limit, which is an advantage when driving at low engine speeds;
Mechanical diagrams of external air compression. Such compressors are perfect where there is a need for a large amount of supplied air at low engine speeds. The disadvantage of this approach is the possibility of creating a reverse air outflow, since the compressor itself does not provide the required pressure. In addition, such installations have low efficiency;
Internal compression units. Their use is relevant at high speeds of the power unit; moreover, the effect of reverse air outflow is much less. The disadvantages of such schemes are: quite high cost (due to high requirements regarding the material) and the possibility of jamming, especially in case of overheating;
Dynamic air blowers. Such installations operate only after reaching a certain number of revolutions, but at the same time their efficiency is much higher in comparison with higher-level installations.

Since mechanical air blowers operate at the expense of the engine crankshaft through an additional drive, the compressor speed directly depends on the speed of the power unit.

Features of the turbocharger

These air blowers operate using energy obtained from exhaust gas emissions. At its core, a turbocharger is a combination of a centrifugal compressor and the turbine itself (a wheel equipped with blades).

The principle of its operation is as follows: exhaust gases rotate a turbine mounted on a shaft at high speed. A centrifugal pump is mounted at the other end of the shaft, the main task of which is to pump a large amount of air into the cylinders.

In modern compressors, an intercooler is used to cool the air supplied to the turbine.

Disadvantages and advantages of mechanical and gas compressor

The turbocharger is ideal for use to enrich fuel with oxygen. However, such schemes also have their drawbacks:

the turbine is presented in the form of a stationary device and, accordingly, there is a need for connection to the power unit of the vehicle;
at low engine speeds, such a compressor is not capable of providing high speed, but only at high speeds is its operation effective;
when switching from low to high speeds, a so-called “turbo lag” often forms, and the higher the power of the turbocharger, the more significant this effect will be.

It is worth noting that nowadays you can buy a turbocharger that will perfectly cope with its main task both at low and high speeds of the power unit. However, their price is quite high, both for the equipment itself and for maintenance. But despite this, many owners prefer turbochargers.

Mechanical air blowers, in turn, are easier to install and maintain. Such devices operate at both low and high speeds. In addition, they require too much time and financial costs for restoration and repair. This is because, unlike a turbocharger, a mechanical supercharger is an independent device.

The turbine, in addition to being expensive and difficult to install, is also quite demanding in terms of quality and technical specifications the fuel mixture used.

Mechanical air blowers have a significant problem - a fairly high fuel consumption, with a relatively low efficiency. But at the same time they are simpler in design and maintenance.

In this case, the choice of one or another installation depends only on the driver and his wishes, as well as the initial characteristics of the car.

Video

Engine power can also be increased by supercharging. For supercharging, special compressors are used, driven by the crankshaft, using part of the power, or gas turbines, in which air or a combustible mixture is compressed before entering the cylinders. Supercharging systems The most typical supercharging schemes are: mechanical supercharging; turbocharger supercharging gas turbine centrifugal compressor; combined mechanical gas turbine supercharging; The compressor is located before the carburetor, sealing is required;...

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LECTURE 15

CHARGING SYSTEM

1 General information about engine supercharging

The main trends in improving internal combustion engines are increasing power while reducing fuel consumption and toxic exhaust gas components. An analysis of the development of land transport shows that piston engines will retain their leading position for a long time. Engine design is usually assessed by liter power

The increase in power is mainly due to an increase in engine crankshaft speed. Increasing the speed is effective if the filling coefficient is large. To this end, it is necessary to reduce losses in the intake and exhaust systems, use inertial phenomena in them and improve gas distribution systems. To increase effective power with increasing speed, it is necessary to reduce mechanical losses (use of appropriate materials, oils, temperature stability, oil purification from mechanical impurities and its cooling, precision of parts manufacturing and quality of mechanical surface treatment).

Engine power can also be increased by supercharging. For supercharging, special compressors are used, driven by the crankshaft (using part of the power) or gas turbines, in which air or a combustible mixture is compressed before entering the cylinders. Charge compression occurs faster than the increase in charge temperature, so the charge density after compression is greater than the charge density before the compressor. The mass amount of charge entering the engine cylinder per cycle will be greater than when entering from the atmosphere.

2 Supercharging systems

The most typical boost patterns are:

mechanical supercharging;
turbocharger supercharging (gas turbine + centrifugal compressor);
combined supercharging (mechanical + gas turbine);
the compressor is located before the carburetor (sealing is required);
the compressor is located after the carburetor (mixture formation improves, operating conditions of the compressor blades worsen due to fuel);
pulse boost system;
non-pulse pressurization system.

Blade-centrifugal compressors are most widely used for pressurization. The main parameters of a compressor are the pressure ratio, compressor performance and adiabatic efficiency. The work spent on compressing 1 kg of air in a compressor from pressure Po to Pk (adiabatic compression) is determined by:

In reality, the compression process occurs in the presence of heat exchange with environment and internal losses, which increases the work required. This is taken into account by the adiabatic efficiency (0.65 ─ with a pressure increase ratio of 1.3. As the degree increases, the efficiency decreases to 0.5). To achieve high boost pressures, rotary gear compressors are used.

In high-speed engines, high-speed centrifugal or axial compressors are used (). With a decrease in efficiency and an increase in the degree of supercharging, the work spent on compressing the charge in the compressor and the temperature increase significantly, while the efficiency of the supercharging decreases. When the boost ratio increases beyond a certain value, the effective power does not increase due to a decrease in mechanical Engine efficiency due to an increase in power spent on compressor drive.

Supercharging slightly changes the nature of the combustion process due to an increase in pressure and temperature at the end of compression. With supercharging, the amount of fuel participating in combustion increases, therefore, the maximum values of pressure and temperature at the end of combustion increase, and the thermal stress of parts increases. When the valves close during boost, better cooling of the valves occurs. These circumstances should be taken into account when applying supercharging.

In carburetor engines, the use of supercharging is limited to the conditions under which detonation combustion occurs and is most often used when operating vehicles in mountainous conditions. If supercharging is used in a carburetor engine, correction of the compression ratio is required. Application comparatively high pressures boost ( more than 0.2 MPa ) requires a change in valve timing and the use of a refrigerator to reduce the charge temperature after compression. The use of boost is most effective in diesel engines, where the increase in boost pressure is limited only by the thermal and mechanical strength of the engine structure. In this case, the power of the blower increases by 20-30% and the average effective pressure increases to 0.9-0.95 MPa.

3 Gas turbine supercharging

With a turbocharger, part of the energy of the exhaust gases is used to compress air and force it. This makes it possible to partially utilize the difference between the pressure at the end of the expansion process in the engine cylinder and atmospheric air pressure. Engine power with gas turbine engines can increase up to 50%, and the toxicity of exhaust gases is reduced. The design of the engine includes the use of appropriate materials, which increases the cost of manufacturing the engine, but the cost of the engine per unit of power is less than without supercharging. Air enters the compressor through an inlet in the center of the housing. The impeller and guide vane provide an increase in potential and kinetic energy, then the air enters the diffuser and air collector, from where it is distributed among the cylinders when the valve opens. The absolute speed of air movement in the wheel reaches 300-350 m/s.

The turbocharger consists of a single-stage centrifugal compressor and a radial centripetal turbine. The main components of a turbocharger are: the compressor stage, the turbine stage and the sealed bearing assembly. The compressor and turbine wheels are located at opposite ends of the rotor shaft, cantilevered relative to the bearings. The compressor impeller is cast from AL4 type alloy into plaster molds obtained using elastic models. The wheel is put on the shaft with tension, so when installed on the shaft it heats up to 1100-1300 degrees Celsius. The turbine impeller of a semi-open type with radial blades is made by investment casting from a heat-resistant nickel alloy such as INKO-713S, ANV-300 and the like. It is connected to the shaft by friction welding. The body is made of heat-resistant cast iron. The turbocharger uses a “floating” plain bearing with a non-rotating mono-bushing. The rotor is held against axial movements on both sides by ring holder bushings pressed onto the turbocharger rotor shaft. Bearing lubrication is carried out from the engine lubrication system, under pressure, into the bearing housing. For stable operation of the engine at all speeds, reducing the “turbo lag” effect, a pressure control system is used, using a regulator, by bypassing gas past the turbine.

The exhaust gases enter the blades of the nozzle apparatus in the housing. As gas passes through the nozzle apparatus, its speed increases. At this speed, the gas enters the blade channels of the turbine impeller. The tangential action of the gas jet on the blades causes the appearance of torque. A rotating output straightener is installed at the turbine outlet. The peripheral speed of the turbocharger impellers is determined by the pressure developed by the turbocompressor. V= 280-350 m/s. At average temperatures of about 700 degrees Celsius or more, turbine wheels are made of nickel-based alloys. To ensure high acceleration of the turbocharger, they try to reduce the outer diameter and moment of inertia of the impeller. Based on the peripheral speed and diameter of the impeller, the rotor rotation frequency is calculated, which can reach 50,000-80,000 rpm.

4 Characteristics of supercharged automobile engines

The design characteristics of the turbocharger should provide a torque development pattern similar to that of a naturally aspirated engine. In this case, the greatest air supply should occur at a speed mode at which the torque is maximum. With an increase in the cyclic supply, the excess air coefficient decreases, but its reduction should be such that there is no increase in the smoke of the exhaust gases. Some turbocharger designs have adjustable nozzle channels, which, when the crankshaft speed is reduced, with the help of a special device, rotate the blades of the nozzle apparatus in the direction of reducing the flow area. As a result of this, the gas pressure at the inlet increases and the exhaust velocity increases, which increases the rotation speed of the TC shaft and the pressure of the fresh charge. Specific fuel consumption remains virtually the same as engine power increases.

In the pipelines of high-speed automobile engines, fluctuations in the gas flow occur during the intake and exhaust processes. This phenomenon in the intake and exhaust pipes can be used for dynamic charging. If you configure the exhaust system so that by the end of the exhaust process, at the moment the valves close, a vacuum forms near the exhaust valve, then the amount of residual gases will decrease and the filling of the cylinder will improve. With a similar organization of the intake process at the end of the intake, the pressure of the fresh charge increases, which causes an improvement in the filling of the cylinder. The dynamic exhaust system is adjusted by changing the length of the exhaust pipe for each group of cylinders. A properly tuned exhaust and intake system provides an increase in effective engine power of up to 10%.

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In order to burn more fuel in the volume of the cylinder and, as a result, obtain greater useful power, it is necessary to proportionally increase the amount of air from the condition α ≈ const. This problem found its solution in supercharging. Supercharging is an increase in the charge of air supplied to the cylinder due to an increase in its density as a result of pre-compression to a pressure P k > P o , and accordingly an increase in the amount of fuel burned. The degree of boosting of diesel engines by supercharging is estimated by the “degree of supercharging” λ n:

λ n= R en / R e, (№1)

Where R e And R en — average effective pressure of the engine without supercharging and with supercharging.

In principle, the amount of air in the cylinder can be increased not only by precompressing it, but also by lowering the temperature (the specific gravity of air is proportional to Pk and inversely proportional to Tk: γk / γo = Pk To / Po Tk), as well as by increasing the cylinder filling factor η n by better cleaning the cylinder. These factors are used in combination when supercharging. So, after preliminary compression, the air is cooled to a temperature of 30÷45 o C, after which it is supplied to the cylinder. Better cleaning cylinder is ensured by careful development of the gas exchange system, using combustion chamber purging in 4-stroke internal combustion engines.

Gas turbocharger of a marine diesel engine

The use of supercharging made it possible to increase the cylinder power of diesel engines by 4–5 times compared to naturally aspirated engines, but required the solution of a number of serious technical problems associated with increased mechanical and, deterioration of lubrication conditions, increased wear of the cylinder-piston group, coordination of the characteristics of supercharging and diesel units etc. These problems constantly confront diesel manufacturers with further boosting of engines.

The following methods of supercharging are distinguished:

Inertial;
Mechanical;
Gas turbine and combined.

Attempts to use inertial supercharging took place in the initial period of boosting 4-stroke internal combustion engines. In this case, each cylinder was supplied with a specially selected long intake pipe. An increase in air pressure at the end of the intake was achieved due to the kinetic energy of the air column in the intake pipe and the corresponding organization of resonant oscillations in it. Inertial supercharging made it possible to increase power by 15÷25%.

Inertial charging of a marine engine

With mechanical charging, the air blower is driven by the engine crankshaft. Piston, rotary or centrifugal compressors are used as superchargers, driven from the crankshaft directly or through a gear ( gear, chain, electric).

The most widely used methods in internal combustion engines are gas turbine and combined charging methods. With gas turbine charging, the energy of the exhaust gases is used to drive the supercharger. A gas turbine and a centrifugal compressor sitting on the same shaft constitute a single unit - a gas turbocharger (GT). Gases from the working cylinders, giving part of the energy to the gas turbine, are sent further to the recovery boiler and into the atmosphere. Air sucked from the atmosphere is compressed in a compressor to pressure Pk, supplied to the air cooler and then to the purge receiver and working cylinders. With slight compression in the compressor, when the temperature does not rise above 45÷50 o C, the refrigerator may be absent.

Combined supercharging means a system that simultaneously uses gas turbine and mechanical supercharging. It is resorted to in cases where the power of gas turbines is insufficient to drive the supercharger. A special case of a mechanical supercharger is the use of working cylinders of crosshead engines in conjunction with a gas turbocharger.

Mechanical supercharging of a marine diesel engine

An assessment of the degree of perfection of a particular supercharging system can be given on the basis of a qualitative analysis of the mechanical efficiency of the engine. For a naturally aspirated engine, a relationship can be written;

ηfur= Ne/ Ni = (Ni - Nmex) / Ni = 1 - Nmex / Ni:

η fur= 1 - N fur / Ni

With inertial charging, all other things being equal, the power of mechanical losses of the engine N mech will not change, but will increase without any additional energy costs for driving the air supercharger. Consequently, the mechanical efficiency of the engine will increase. However, inertial charging has not found application in marine diesel engines due to the bulkiness of the intake system and the relatively low level of boost.

In an engine with mechanical supercharging, the power of mechanical losses increases by the amount NB of the cost of driving the air supercharger; mechanical efficiency is:

η mn fur= 1 - ((N fur + Nв) / (Ni + ΔNi)), (№2)

Where Ni + ΔNi= N in— indicated power of the supercharged engine.

Obviously, any increase in diesel power requires an increase in boost pressure Pk. At the same time, the power Nв for the air supercharger drive also increases. If the indicator power increases more rapidly than the power of mechanical losses, then the mechanical efficiency increases. In this case, as Pk increases, the average effective pressure Ren also increases: Re n = Pi n η mn fur.

When a certain boost level is reached, the cost of driving a mechanical air supercharger begins to grow more rapidly than the increase in indicated power; mechanical efficiency decreases. Despite the increase in Pk, the average effective pressure may even decrease (if the degree of decrease in η mech exceeds the degree of increase in Pi). In the limiting case of mechanical supercharging, it is possible to create an engine in which all the indicator work will be absorbed by the compressor, the mechanical and effective efficiency will be equal to zero.

According to experimental data, the limit of a reasonable increase in An with purely mechanical supercharging is within the limits:

λн= 1.2÷1.3.

Wherein RK= 1.3÷1.5 or η mn fur= 0.70÷0.85.

With further boosting of the engines, too much power is required to drive the supercharger, which reduces η mech and η e. For this reason, purely mechanical supercharging is not used in modern engines. It can be found in engines of older design ( ZD-100, 37D, DR 30/50, DR 43/61 and etc.).