What is Mechanical supercharger?
The operation of the internal combustion engine (ICE) is based on the fact that the fuel must be mixed with the required amount of oxidizer, i.e. oxygen. This will ensure complete and efficient combustion of the combustible mixture and will allow to achieve the highest possible power. More burned - more power. Oxygen in air is only 21% by volume and 23% by weight (this is at sea level, at certain pressures and temperatures). For normal engine operation, the proportions of the fuel-air mixture are taken at approximately 1: 14.7. If we add to the standard pressure one atmosphere, for example, one more, we get 2 times more air, and hence the oxygen entering the cylinders.
So, we must get 2 times more power from the motor. The engine capacity of 1.5 liters at a pressure of slightly aspirated pressure of the atmosphere is almost equivalent to a three-liter "aspirated". This, of course, is rude arithmetic, but the idea is exactly that. And, by the way, such an increase is by no means the limit. You can go the way of increasing the volume of motors. The greater the working volume of the cylinder - more air-fuel mixture with all the ensuing consequences. So did the American manufacturers. Huge, high volume engines with incredible fuel consumption, but impressive torque. In Europe, and especially in Japan, they made small, compact and efficient engines. But power, however, was also in demand by car buyers. Perhaps this was one of the reasons why the first developments of superchargers appeared on the old continent.
As a pioneer who developed a supercharged car engines, we can mention such companies as Mercedes-Daimler, Fiat, Sunbeam, Alfa Romeo. The very idea of forced air injection into the cylinders was proposed shortly after the invention of the internal combustion engine. Already in 1885, Gottlieb Daimler received a German patent for a supercharger. The idea was that some kind of external fan, pump or compressor pumps an increased charge of air into the engine. In 1902 in France, Louis Renault patented the project of a centrifugal supercharger. A number of cars were produced, but then all the work in this direction was turned off. The principle of the turbocharger, working on exhaust energy, was first described and patented by the Swiss inventor Alfred Büchi in 1905, but here too the technology of that time slowed down the introduction of such devices.
The Roots brothers developed a volumetric supercharger as early as 1859. These rotary gear compressors are now called “roots” compressors. On cars, devices of this type appeared in the 20s of the last century thanks to the company Mercedes. The screw compressor was developed in 1936. The patent was received by Alf Lishholm (Alf Lysholm) - Chief Engineer of SRM (Svenska Rotor Maskiner AB). The then level of technology development did not contribute to the proliferation of such devices, but now they are quite popular. There were other types of superchargers. Over time, they are naturally divided into mechanical (driven by the crankshaft or other means) and turbo (driven by the exhaust system). The latter, although they have common roots and purpose, are still a rather isolated branch of the development of superchargers.
The rest of this article will cover several basic types of mechanical superchargers.
Such blowers in tuning are currently the most common. By design, they are closest to turbocharging, because they have the same principle of air injection. Differ only ways to drive. The work is as follows. The main part of the centrifugal supercharger is the impeller or impeller. It has a rather complex conical shape. Impeller blades play the most important role. The resultant efficiency of the entire supercharger depends on how correctly they are designed and manufactured
So, the air, passing through a narrowing air channel into the supercharger, falls on the radial blades of the impeller. The blades are twisted and discarded by centrifugal force to the periphery of the casing, where there is a diffuser. Often the diffuser has blades (sometimes with adjustable angle of attack), designed to reduce pressure loss. Next, the air is pushed into the circumferential air tunnel (air collector), which most often has a cochlear shape (the air collector, describing a circle, gradually expands in diameter). This design creates the necessary pressure of the air flow at the outlet of the supercharger. The fact is that inside the ring the air initially moves quickly, and its pressure is low. However, at the end of the cochlea, the channel expands, the air flow rate decreases, and the pressure increases. This creates the necessary support for pumping cylinders "compressed atmosphere." By virtue of the principle of operation, a centrifugal supercharger has one major drawback. To work effectively, the impeller must not only rotate quickly, but very quickly. The pressure actually generated by the centrifugal compressor is proportional to the square of the speed of the impeller. The speeds can be 40 thousand rpm and more, and for high-pressure diesel compressors they approach the figure of 200 thousand r / min.
And since the drive is carried out from the crankshaft by means of a belt transmission to the turbine pulley, the noise from such a device is quite strong. Although for many this particular whistle warms the soul. There were even deceptions mimicking the sound of a working turbine. The problem of noise and resource elements of the drive is partially removed by the introduction of an additional multiplier. Here it is worth mentioning an interesting solution from Powerdyne. Inside the single case of the supercharger there is an additional boost belt drive. It does not require maintenance, lubrication and is designed for mileage of more than 80 thousand km. This allows to reduce the gear ratio of the external, main belt transmission, than to reduce its workloads. High working turns impose special requirements on the quality of the materials used and the accuracy of manufacturing (taking into account the enormous loads from centrifugal forces). The disadvantages of the principle of injection can also be attributed to a certain delay in actuation, although it should be noted that this delay is not as noticeable as that of a turbocharger.
And one more note. As a rule, the centrifugal supercharger gives an increase at fairly high engine speeds. At first, the pressure increases slowly, but then, with increasing speed, it increases quite sharply. This feature makes centrifugal superchargers most suitable for those cases when it is more important to maintain high speeds, and not acceleration intensity. As noted above, centrifugal superchargers are very popular. The relatively low price and, most importantly, ease of installation contributed to the fact that compressors of this type almost supplanted other, more expensive and complex types.
Especially in the field of tuning. Currently, centrifugal superchargers are manufactured by a number of companies. Here are the most famous ones: Paxton Automotive, Powerdyne Automotive, ATI ProCharger, RK Sport, Vortech.
Roots compressors belong to the class of volumetric superchargers. Their design is quite simple and most of all resembles an engine oil gear pump. In the oval-shaped body, two rotors with a special profile rotate in opposite directions. The rotors are mounted on axles connected by identical gears. A small gap is maintained between the rotors themselves and the housing. The main difference of this method of injection is that the air is not compressed inside, but as it were, outside the compressor, directly in the discharge pipe. That is why they are sometimes called external compression compressors. The air is as if scooped out by cams (entering the space between the rotors and the housing) and squeezed into the discharge pipeline. The main disadvantage of this method of injection is that, since the process of air compression is carried out outside the compressor, its effective operation is possible only up to certain values of boost. No matter how accurately the parts of the compressor are made, as the pressure in the injection pipeline increases, air infiltration back increases, and its efficiency significantly decreases. By increasing the speed of rotation of the rotors, it is possible to somewhat reduce air leakage, but this is possible only up to certain limits. Further, the power expended on the rotation of the supercharger itself may exceed the additional engine power.
To increase the boost pressure, designs with two or more stages were used. They allowed to raise the total pressure values by 2, 3 times and more. But due to the fact that these compressors lost one of their main advantages - compactness, such multi-tiered structures did not take root. Another major drawback. In compressors of this type, when extruding uncompressed air into compressed in the injection pipe, turbulence is created, which contributes to an increase in the temperature of the air charge. That is, along with the usual increase in temperature from a direct increase in pressure, additional heating occurs in the rut-compressors.
In this regard, such blowers are necessarily equipped with intercoolers (a special device for air cooling). The noise from the work of volumetric compressors is not as strong as that of centrifugal ones, and has a slightly different tonality. But, unlike the latter, the work of rotary gear superchargers is accompanied by pressure pulsations. This happens because of the uneven air supply. To reduce the noise and the amplitude of the pulsations, recently, the most widely received are three-tooth rotors of spiral shape. In addition, for the same purpose, the inlet and outlet windows of the compressor are triangular. These constructive tricks allow to achieve the fact that such compressors work fairly quietly and evenly. At present, modern technological capabilities have brought such compressors to a very high level of performance. Automobile giants such as DaimlerChrysler, Ford and General Motors install mechanical-type superchargers on some of their cars. There are several reasons for this. First of all, volumetric superchargers, as opposed to centrifugal, are effective already at low and medium engine speeds. This feature of the ruts-compressor made them the most suitable for drag racing, where the acceleration dynamics is valued first of all. Another important advantage is the relative simplicity of the design.A small number of moving parts and low rotational speeds make these blowers one of the most reliable and durable.
However, the complexity in the manufacture and installation, and hence the high price (relatively centrifugal) somewhat reduced their market popularity. Apart from the manufacturers listed above, for the secondary market, several companies produce such blowers. Here are some of them: Jackson Racing, Kenne Bell Superchargers, Magna Charger. Separately, it is worth noting the company Eaton Automotive. That it is, as they say, the locomotive of the promotion of ruts-type superchargers. By the way, these are its compressors and are installed on Ford and GM engines.
On behalf of the founding father, these compressors are sometimes referred to as Lysholm bulk blowers. They somewhat resemble ruts-compressors with spiral-shaped rotors, but most of all this design resembles a meat grinder. With only one difference: the auger is not one, there are two of them, and they engage in a special way, having complementary profiles. Two rotors ("father" and "mother"), capturing the incoming air, begin a mutual counter-rotation. A portion of the air is pushed forward, like meat along a meat grinder auger. The rotors have extremely small gaps between them. It provides high efficiency and rather small losses. The main difference between a screw compressor and volumetric rotary gear superchargers is the presence of internal compression.
This provides them with high discharge efficiency on almost the entire engine speed scale. In order to achieve high pressure values, it may be necessary to cool the compressor casing. But at the standard, not extremely high pressures of boost, the air is not heated as much as in the rutches-compressors. Another advantages: high efficiency, reliability and compact design. In addition, screw compressors are fairly quiet. They work almost in a “whisper” (of course, with proper, accurate design and manufacture). Here lies the, perhaps, their only minus. The fact is that such compressors are rather difficult to manufacture and, as a result, expensive. For this reason, they practically do not occur in mass automotive production. For the same reason, there are not so many companies producing these progressive blowers. I managed to find only two brands from serious manufacturers: Comptech Sport and Whipple Superchargers. Such devices also produce some Western tuning studio - for example, Kleemann, AMG. The most interesting thing is that such completely expensive superchargers can be found here as well.
In the 1980s, Volkswagen experimented with rather unusual spiral superchargers. In automotive applications, they are better known as G-Lader. Now this direction by VW is minimized. However, you can still find cars Golf, Passat and Corrado with such injection devices, and, in addition, a number of companies (mainly German) continue to produce such compressors. Piston superchargers, the most common scheme of conventional air compressors at the moment, have not taken root in cars at all. But on marine engines, they were used quite widely. An interesting method of injection podporsnevym pump. Here, the piston itself is used as a supercharger, which pushes the air under it when moving to BDC (lower dead center). An interesting fact is that the originally famous Wankel rotary engine was designed as a supercharger. And, by the way, for some time it was successfully used in this capacity. There are also so-called axial compressors. The movement of air in them is carried out in the axial direction. Now you can find electric "blowers", built on this principle. One or a pair of series or parallel fans with a motor, being installed in the air path, push the air along itself back into the filter or after it into the intake manifold. Some manufacturers of such products claim 20 liters. with. and more power increase. I will not argue the opposite, but if these devices overcome at least the resistance of the filter elements, the effect is already quite good. Another interesting solution that is actually not an artificial method of forcing air is the resonant pressurization system. The idea is based on the fact that in order to better fill the cylinders, it is necessary to provide an overpressure in front of the intake valve immediately at the moment of its opening. And therefore, you just need to “ride” the compression wave, and this is exactly how the air behaves in the intake manifold when the engine is running: alternating tides. With change of turns the amplitude of these fluctuations changes. And in order to "catch" the wave, you need to change the length of the intake manifold. At first, we went through a rather primitive in meaning, but rather difficult in terms of translating the path: several air ducts of different lengths and a valve opening one or another channel. Currently, this idea has found its logical embodiment in the devices of the intake manifold of variable length.
For example, BMW uses a device that provides a change in the length of the intake tract. Of course, this is not a complete replacement of the boost, but there is some benefit from this. And the energy of the motor on such a "supercharger" is practically not spent.
Many believe that the use of superchargers can adversely affect the engine resource. This is so and not so. Everything needs measure. Start with the fact that, as a rule, increased engine speed causes engine damage. Consequently, the use of a supercharger that increases the torque at low and medium speeds, on the contrary, may have a positive effect on the engine's resource. On the other hand, if you achieve really big growth in power, many of the regular parts will have to be replaced with more durable ones. For example, forged pistons and connecting rods will be completely superfluous. In particular, given the more serious thermal loads in the combustion chamber, manifested in supercharged engines. When using blowers, the temperature has a completely fundamental effect. Physics can not be fooled. So it turns out that the compression of air is always associated with an increase in its temperature. In some compressors this increase is not so significant, but in any case, the air must be cooled to increase the air charge and reduce the power loss to the blower drive (by reducing the back pressure). But even more important is another problem that few people think about - detonation. The fact is that the high temperature and pressure of the air supplied to the cylinders can lead to the fact that at the end of the compression stroke, when the piston compresses the already compressed fuel-air mixture in the cylinder, its temperature and pressure can be so high that it will cause its premature detonation, i.e. explosion. In order to avoid such problems (and detonation can “kill” the engine rather quickly), you can switch to higher octane grades of fuel, but more often than not, this is not enough. At sufficiently large values of pressure, it is necessary to produce decompression, i.e., to reduce the degree of compression. In addition, you should carefully approach the adjustment of the ignition timing. When using superchargers, it is recommended to change the ignition setting. Proper selection of spark plugs is also important. In fact, when installing pressurization issues arise much more. Installing a compressor on a serial engine may produce different results.
And even ready-made kits from well-known companies can not provide all the nuances of your car. In any case, the installation of supercharging requires high professionalism of installers who can choose the right compressor and competently adjust the engine. Then there is the certainty that the result will not lead to undesirable consequences