What is the principle of turbocharging?
If more air and fuel mixture can be forced to enter the cylinder (combustion chamber) for compression and explosion in the same unit time (a small displacement engine can "inhale" large displacement air to improve volumetric efficiency), it can produce greater power output than a naturally aspirated engine with the same speed. It's like blowing an electric fan into the cylinder, forcing the wind to fill the cylinder, thus increasing the amount of air inside and getting more horsepower, but the fan is driven by the exhaust gas from the engine, not the motor.
Generally speaking, with such a "forced air intake" action, the engine can increase the extra power by at least 30%-40%. Such amazing effect is the reason why the turbocharger is so attractive. Moreover, achieving perfect combustion efficiency and greatly improving power is the greatest value that the turbocharged system can provide to vehicles.
The system comprise a turbocharger, an intercool, an intake bypass valve, an exhaust bypass valve and supporting intake and exhaust pipelines.
The connection mode of automobile turbocharger is shown in the following figure:
Process piping
We hope that you can understand the working sequence of turbocharging through the following simple steps, so that you can clearly understand the working principle of turbocharging system. The schematic diagram is as follows:
First, the exhaust gas from the engine pushes the turbine wheel) ② At the exhaust end of the turbine, making it rotate. Therefore, it can simultaneously drive the compressor wheel (③) on the other side connected with it to rotate.
Second, the compressor impeller forcibly sucks air from the air inlet, and after being compressed by the rotation of the blades, it flows out into the diffuser channel with larger and larger diameter, and these compressed air is injected into the cylinder for combustion.
Thirdly, some engines are equipped with intercoolers to reduce the temperature of compressed air, improve the density and prevent engine knocking.
Fourthly, the compressed (and cooled) air enters the cylinder through the intake pipe to participate in combustion and work.
5. The burnt exhaust gas is discharged from the exhaust pipe and enters the turbine, and then the above-mentioned (1) action is repeated.
turbocharger
The turbocharger body is the most important part of the turbocharger system, which is what we generally call "worm" or "screw". The turbine is named because it is shaped like a shell on the back of a snail or a conch on a seafood stall.
Turbocharger body is the core component to improve the charging efficiency, and its basic structure is divided into: intake end, exhaust end and intermediate connection part.
The air inlet includes a compressor housing including a compressor inlet, a compressor outlet and a compressor impeller.
The exhaust end includes a turbine housing including a turbine inlet, a turbine outlet and a turbine wheel.
Between the two shells, there is also a central shell, which is equipped with a shaft connecting and supporting the compressor impeller and the turbine impeller, and dealing with tens of thousands of revolutions, as well as the corresponding oil inlet and oil outlet (even including water inlet and water outlet).
"High temperature" is the biggest test that turbocharger faces when working. When the turbine is running, the first contact is the high-temperature exhaust gas (the first heat source) discharged by the engine, which pushes the turbine impeller and drives the compressor impeller on the other side to run synchronously. The rotating speed of the whole blade shaft is 120000- 160000 rpm. Therefore, the heat generated by the high-speed rotation of the turbine shaft is amazing (the second heat source), and the temperature of air after being compressed by the compressor impeller rises (the third heat source), which becomes the most severe high temperature burden of the turbocharger. Turbocharger becomes an independent working system integrating high-temperature components. So "heat dissipation" is very important for turbocharger. There are special oil passages (heat dissipation and lubrication) inside the turbine body, and many of them are designed with oil passages and water passages at the same time, which can reduce the temperature of the supercharger through oil cooling and water cooling.
turbine shaft
The bearing looks like a simple metal tube, but it is actually a precision part with 120000- 160000rpm rotation and ultra-high temperature. Its fine machining error and profound material application and treatment are the core technologies of all turbine plants. The traditional turbine shaft uses a bushing bearing structure. It is really just a metal tube, which completely depends on the high-pressure engine oil entering the bearing chamber to realize heat dissipation, so it can rotate at high speed.
The new ball bearing has gradually become the development trend of turbine shaft. As the name implies, ball bearing is to install balls on turbine shaft instead of oil as bearing. Ball bearing has many advantages: the friction is smaller, so it will have better turbine response (which can reduce turbine lag), which is more conducive to the ultimate extraction of power; The dynamic control of turbine shaft rotation is more stable (traditionally, bearings are made of engine oil and the stroke fluctuates); The requirements for oil pressure and quality can be relatively reduced, which indirectly improves the service life of the turbine. However, its disadvantage is that its durability is not as good as that of the traditional Bos bearing, and it will reach its life limit in about 70-80 thousand kilometers, and it is not easy to maintain and expensive to maintain. Therefore, turbine manufacturers who value durability (such as KKK) will not introduce this turbine.
Turbine impeller and compressor impeller
Blade types of turbine impeller can be divided into "waterwheel" blade (straight blade design, which makes the exhaust gas collide to generate cyclotron force, which is directly combined with cyclotron motion) and "windmill" blade (curved blade design, which not only uses collision force, but also effectively uses airflow to enter between blades to obtain the expansion energy of exhaust gas). The impeller diameter and blade number of turbine impeller will affect the linearity of horsepower. Theoretically, the fewer blades, the worse the response at low speed, but the explosiveness and persistence at high speed are not comparable to that of multi-blades.
Most of the blades of turbine impeller are made of high heat-resistant steel (some of them are made of ceramic technology), but because of the large mass of iron itself, the blades of titanium alloy are light and strong. Only in the production cars, only Mitsubishi Lan Se EVO ⅸ RS model has a turbine equipped with titanium alloy blades (the titanium alloy turbine model of EVO is TD05-HRA, generally TD05-HR, for readers' reference). Among the modified products, only Garrett's racing turbine is made of titanium alloy, which has not been heard of for the time being.
Compressor impeller
Blades are the power source of turbines. However, the role of compressor impeller and turbine impeller is different, so the blade shape is of course different. Basically, how to squeeze air into the compression channel efficiently is regarded as the primary task of the compressor impeller, and then its shape is determined.
Generally, the compressor impeller of the original turbine adopts the design of full blades, that is, the blades are designed from top to end. In order to increase the channel area of air intake and improve the efficiency of high-speed rotation, there have been many impellers with half blades inserted beside the whole blades (this design is mostly found in modified products).
Another purpose of compressor impeller design is to balance the speed of compressed air. The traditional impeller is a "radial compression wheel", and the gas velocity between its two blades changes rapidly: the air in front of the impeller is squeezed by the blades, so the speed is very fast. However, due to factors such as suction resistance and back pressure, the air velocity behind the blade is slow. When the throttle valve is half open, the speed of the compressor impeller decreases, and the speed of air entering the compressor wheel decreases. However, if the amount of air compressed before is relatively too much, there will be a "vacuum" state, air can not be transported (the rotation speed of compressor impeller can not produce pressure greater than the air pressure in the intake pipe), and relative pressure (pressure feedback) can not be produced, which is the so-called "compressor surge" phenomenon.
The so-called surge effect is like we stir the water in a bucket with our hands. The faster the hand is stirred, the more the water in the bucket will spread to the side of the bucket, and then the water level in the bucket will become lower and lower. In the end, the water in the bucket will only rotate around the bucket and will not fall. This phenomenon can also happen in aerodynamics. You can imagine that the air inlet of the compressor is like a bucket, and the surrounding air is like water. As for turbine blades, they are like stirring hands. Once the rotating speed of turbine blades increases, the airflow in the air inlet will gradually spread around. The higher the rotating speed, the closer the airflow will be to the surroundings, which will lead to the central position of the turbine blade becoming less and less able to absorb air, and even finally it will be in a vacuum state, and the air can only enter from around the blade, and the air intake efficiency will of course decrease. The blade with large windward angle has better intake efficiency, but it is easy to produce surge effect at high speed, while the blade with small windward angle is the opposite.
In order to prevent the phenomenon of "air stripping", the "reverse" compressor impeller designed to reduce the blade angle to the running direction (closer to the turbine axis direction) has gradually become the mainstream of modified products, that is, the so-called "diagonal flow" blades in the field of modification. The "diagonal flow" blade is usually under the original main blade, plus one and a half blades (generally, its angle is closer to the turbine shaft, that is, more vertical). If you look directly at the compressor impeller from the air inlet, you can see that the two blades overlap, indicating that this is a "diagonal flow" impeller. The compressor impeller of hybrid turbine usually adopts "oblique flow" blades (with the back plane flattened) with funnel-shaped enlarged suction port to increase air output. In addition, there is a new design of adding a circulating exhaust hole at the air inlet of the compressor to circulate the lost compressed air twice to reduce the surge effect (not detailed here, HKS T04Z has this design).
Built-in bypass valve
Built-in exhaust bypass valve (commonly known as actuator) is the most common pressure relief device in steam turbine system at present, also known as linkage exhaust pressure relief valve. The "actuator" is directly arranged on the turbine, and a connecting rod is used to control the valve in the exhaust of the turbine. Once the pressurization value at the compressed air end of the turbine reaches the limit level, the intake pressure will push the connecting rod of the "actuator" to open the bypass valve at the exhaust side of the turbine, and part of the exhaust gas will be directly discharged to the exhaust pipe without passing through the turbine wheel. In this way, the flow rate of exhaust gas blowing the turbine impeller is reduced, and the rotation speed of the turbine impeller is reduced, and at the same time, the rotation speed of the compressor impeller is reduced. Therefore, the "actuator" is not only a device to limit the maximum speed of the turbine, but also a device to keep the boost pressure at the inlet of the turbine at a stable value (not too high for a long time).
External bypass valve
External wastegate (commonly known as wastegate), also known as exhaust pressure relief valve, has roughly the same function as "actuator", but its structure and installation position are different. Structurally, the wastegate eliminates the connecting rod and exhaust valve in the turbine. On the other hand, the wastegate is installed between the turbine and the exhaust pipe head in an independent way, instead of being attached to the turbocharger body like an actuator. Once the turbocharging value reaches the set upper limit, the "wastegate" discharges excess exhaust gas (which can be directly discharged into the atmosphere or led back to the exhaust pipe), reducing the exhaust gas flow that "blows" the turbine impeller, thus keeping the turbocharging value stable. "Waste gate" has greater supercharging capacity (large spring) and sensitive response than "actuator", so it is more suitable for high horsepower or high supercharged turbine engines, especially for hybrid turbines with great differences. It is a must-have item!
air cooler
The intercooler is located in the "cooling row" between the compressor outlet and the throttle valve. Its structure is a bit like a water tank, that is, many small flat aluminum tubes are used to divide the air, and then the cold air outside is used to blow the fins connected with the thin tubes to cool the compressed air, so that the intake temperature is close to normal temperature.
The engine hates hot air because it will reduce horsepower. Especially in subtropical areas where the seasons are hot. However, because the turbocharger will forcibly compress the gas inhaled into the engine, the air density will increase, but at the same time, the air temperature will also rise sharply. The increase of temperature will lead to the decrease of oxygen content in compressed air. In addition, this hot gas will enter the high-temperature cylinder without cooling, which will lead to irregular pre-combustion (knocking) of fuel, further aggravate the temperature rise of the engine and increase the possibility of piston melting.
In order to improve the air density and give consideration to the oxygen content in the air, we need to reduce the temperature of the intake air (to a greater extent) after compressing the air. The intercooler was thus produced. The larger the area and thickness of intercooler, the stronger its heat dissipation capacity. Due to the large area and thickness, the number, length and cooling blades of small flat tubes in the intercooler increase, so that the high-temperature compressed air in the intercooler has more contact area and contact time with the atmosphere outside the intercooler, and the heat exchange (cooling) area and time are more sufficient, and the cooling effect is better. Although the cooling efficiency of large-capacity intercooler is better, it prolongs the heat dissipation path and increases the air intake, which will bring relative pressure loss and the turbine lag will easily become larger.
Intake bypass valve
The air inlet pressure reducing valve is also commonly called "air inlet pressure reducing valve". Installed on the intake pipe near the throttle, it is the original pressure relief device for most turbocharged engines when they leave the factory.
Because the turbine is driven by the power of exhaust gas, the throttle is closed when it is closed during driving (such as shifting gears and braking suddenly). Turbine blades (compressor wheels) still keep rotating under the action of inertia. At this time, due to the truncation of the throttle and the continuous pressurization of the blades, the air pressure in the intake pipe (between the throttle and the turbine) will rise rapidly. In order to protect the supercharging system, when the pressure reaches a certain limit value, the intake bypass valve is opened to guide excess air (pressure) back between the filter and the turbine to realize the function of depressurization protection.
Bleed valve (BOV), commonly known as "deflated guy", also belongs to the intake bypass valve. But it is generally used as a modification to replace the safety valve. Its function is basically the same as that of the safety valve, the only difference is that the valve of the drain valve will not be affected by the inlet pressure as easily as the safety valve (causing the inlet pressure to drop). In addition, after the throttle valve is closed, the relief valve directly releases the residual pressure to the atmosphere, instead of re-pressurizing between the turbine and the filter. Therefore, in addition to protecting the turbine system, the relief valve is also superior to the original relief valve in terms of pressure relief response. However, for small displacement or small turbocharged turbine engines, the dynamic response of the bleed valve to refueling will become worse. In addition, when the air release valve releases pressure, it will produce more air release sound, which will make people more excited and become the most special sound effect of turbocharged vehicles.