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Expert Guide: Principles and Functions of Turbochargers

Time: 2018-11-17

A turbocharged engine is an engine that relies on a turbocharger to increase the intake of the engine. The turbocharger (Turbo) is actually an air compressor. It uses the exhaust gas from the engine as the power to push the turbine in the turbine chamber (located in the exhaust passage). The turbine drives the coaxial impeller in the intake passage. The impeller compresses the fresh air sent by the air filter pipeline and sends it. Into the cylinder. When the engine speed is increased, the exhaust gas discharge speed and the turbine speed are also accelerated, the degree of air compression is increased, and the intake air amount of the engine is correspondingly increased, so that the output power of the engine can be increased.

The biggest advantage of a turbocharged engine is that it can greatly increase the power and torque of the engine without increasing the engine displacement. When a turbocharger is installed in an engine, the maximum output power can be increased by approximately 40% or more compared to the uncharged supercharger.

Twin turbo

Twin turbo is generally called Twin

Turbo or Biturbo, twin turbo is one of the ways to turbocharge. For turbine turbocharging of exhaust gas turbocharging, one or two small turbines or two parallel turbines are connected in series. At low engine speed, less exhaust gas can drive the turbine to rotate at high speed to generate sufficient intake air. Pressure to reduce turbo lag effect.

In a twin-turbocharged car, you will see two sets of turbines connected in series or in parallel. Parallel means that each set of turbines is responsible for the operation of half of the cylinders of the engine. Each set of turbines is of the same specification. Its advantage is that the supercharging reaction is fast and the complexity of the pipeline is reduced.

The tandem turbine is usually a large and small set of two turbines connected in series. It drives a small turbine with a fast response at low turn, which makes the low-torque torque rich. When the turbine rotates at high speed, it provides sufficient air intake and power output. improve.

Single turbine twin scroll

The single-turbo twin scroll can be said to be BMW's unique technology. The single-turbo twin-vortex is to divide the flow of a turbocharger into two streams when passing through the scroll, and the airflow is responsible for three cylinders. In contrast, the design of a single turbine also reduces the interference of exhaust pulses. The single-turbo twin-scroll engine has gradually become popular in various BMW cars.

Compared to the N54B30's 3.0 twin-turbo engine, it uses only a single turbocharger optimized by TwinPower. TwinPower simply says dual intake, single-turbo twin scroll is driven by every three cylinders of the twin-turbo A turbine evolved to drive a turbine through each of the three cylinders through a turbine intake line to reduce engine weight and fuel consumption.

Variable section turbine

To better understand the advantages of variable-section turbines, let us analyze the shortcomings of conventional turbocharged engines.

The exhaust energy of a conventional turbocharged engine is very impressive at full load, but when the engine speed is low, the exhaust energy is small, and the turbocharger cannot reach the working speed due to insufficient driving force. As a result, the turbocharger does not work at low speeds. At this time, the turbocharged engine's power performance is even smaller than a naturally aspirated engine with the same displacement. This is what we often say. "Turbo lag" phenomenon.

For conventional turbocharged engines, one way to solve the turbo lag is to use a small-sized, lightweight turbine. First, the small turbine will have a smaller moment of inertia, so at lower engine speeds, at lower engine speeds. The lower turbine can achieve the best working speed, which can effectively improve the turbo lag. However, the use of a small turbine also has its disadvantages: when the engine is running at a high speed, the small turbine will increase the exhaust resistance (generating exhaust back pressure) due to the small exhaust cross section, so the maximum power and maximum torque of the engine will be limited. influences. For a large turbine with a small back pressure, although it can have excellent supercharging performance at high speed, the engine will have stronger power performance, but the turbine is more difficult to drive at low speed, so the turbo lag will be more obvious. .

Principle of variable section turbocharging

In order to solve the above contradictions, the turbocharged engine can guarantee a good supercharging effect at high and low speeds, VGT (Variable)

GeometryTurbocharger) or VNT variable-section turbocharging technology came into being. In the field of diesel engines, VGT variable-section turbocharging technology has long been widely used. Because the exhaust temperature of the gasoline engine is much higher than that of the diesel engine, it reaches about 1000 °C (the diesel engine is about 400 °C), and the hardware material used by the VGT is difficult to withstand such a high temperature environment, so this technology is also Delayed application on gasoline engines. In recent years, BorgWarner and Porsche have teamed up to overcome this problem by using high-temperature aerospace material technology to successfully develop the first gasoline engine with a variable-section turbocharger. Porsche calls this technology VTG (Variable Turbine Geometry) variable turbine blade technology.

The core part of the VGT technology is the guide vane with adjustable vortex cross section. From the figure, we can see that the outer side of the turbine adds a guide vane that can be controlled by the electronic system. The relative position of the guide vane is fixed. However, the blade angle can be adjusted. When the system is working, the exhaust gas is sent to the turbine blade along the guide vane. By adjusting the blade angle, the flow rate and flow rate of the gas flowing through the turbine blade are controlled to control the turbine speed. When the engine's low-speed exhaust pressure is low, the angle at which the guide vanes open is small. According to the principle of fluid mechanics, the flow rate of air introduced into the turbine at this time is increased, and the pressure at the turbine is increased, so that the turbine can be more easily pushed, thereby effectively reducing the hysteresis of the turbine and improving the response time at low engine speed. And acceleration capabilities. With the increase of the rotational speed and the increase of the exhaust pressure, the blade also gradually increases the opening angle. Under the full load state, the blade maintains the fully open state, reducing the exhaust back pressure, thereby achieving a general large turbine. The boost effect. In addition, since changing the blade angle can effectively control the rotational speed of the turbine, which also achieves overload protection for the turbine, the turbocharger using the VGT technology does not need to provide an exhaust pressure relief valve.

It should be noted that the VGT variable-section turbocharger can only change the characteristics of the turbine by changing the cross-sectional area of the exhaust inlet, but the size of the turbine does not change. The principle of a variable-section turbine is more intuitive if understood from the turbine A/R values.

The A/R value is an important indicator of the turbocharger to express the characteristics of the turbine and is often indicated in the turbocharger sales brochure for the modified market. A denotes the Aera area, which refers to the narrowest cross-sectional area at the turbine exhaust side inlet (that is, the "section" in the variable-section turbine technology), and R (Radius) means the radius, which means the entrance. The distance from the center point of the narrowest cross-sectional area to the center point of the turbine body, and the ratio of the two is the A/R value. Relatively speaking, the impeller at the compressor end is not affected by the A/R value, but the A/R value is of great significance to the exhaust turbine.

The opening degree of the guide vane can affect the airflow speed of the guide turbine blade. The opening degree is small at low speed (left), the air flow rate is increased, and the opening degree is high at high speed (right), and the exhaust negative pressure is reduced as A/ The smaller the R value is, the higher the flow rate of the exhaust gas passing through the turbine. This characteristic can effectively reduce the turbo lag, and the turbine can also achieve higher supercharging in the lower speed region, and the engine will have a larger high speed. The exhaust back pressure limits the power at high speeds. Conversely, when the A/R value is larger, the response speed of the turbine is slower, and the turbo lag is obvious at low speeds, but at high speeds, it has a smaller exhaust back pressure and can better utilize the exhaust energy. In order to obtain stronger dynamic performance.

The variable cross-section achieved by VGT technology refers to changing the value of A. When the blade angle is small, the cross-sectional area of the exhaust inlet will be correspondingly reduced, so the value of A will change accordingly, so that the small turbine has a fast response. When the blade angle increases, the A value increases, and the A/R value increases, thereby obtaining a stronger power output at a high rotation speed. All in all, by changing the angle of the blade, the VTG system can change the A/R value of the exhaust turbine at any time, taking into account the advantages of the large/small turbine.

Although the structure and principle are simple, the VGT variable-section turbine technology has a significant boost to the boosting effect. This technology has been widely used in current mainstream turbo diesel engines. However, due to the limitations of hardware materials, this technology has just started on the gasoline engine with higher exhaust temperature. The cooperation between Porsche and BorgWarner can be said to be the first. However, with the advancement of materials technology, this technology will certainly be more widely used in future gasoline engines.

Pressure relief valve

When a modified car flies past, we often hear the sound of an engine acceleration of "嗡嗡...", and then a "呲..." sound - this gives people a drive Feeling full. Where did the voice come from? Why are there no such sounds on ordinary civilian cars or on some high-performance sports cars (and some ordinary modified cars)?

In fact, this unique "呲..." sound is the sound of the turbocharger's pressure relief valve when it is relieved. It can be said that all turbocharged engines produce this sound, but for everyday civilian vehicles. In terms of design, the manufacturer will treat this sound as noise and reduce it as much as possible. The practice is to vent pressure into the intake manifold, so the noise is small – this type of pressure relief is called internal venting.

For ordinary civilian vehicles equipped with turbocharged engines, it is generally unnoticeable to hear the "click" when unloading pressure. This is like a normal civilian car, engine noise and rumble exhaust sound are negative parameters (and high-performance cars can sometimes emphasize this sound), the original design to eliminate this sound as much as possible. For those who like to drive fun, this sound has become a catalyst to stimulate their driving passion (we often see that many cars' exhaust pipes have been changed like gun barrels, and the sound is rumbling.)

In addition to stimulating driving passion, “Hyun” is also one of the appeals of the conversion. Many people want to change their car like a super-performance car, even if the car is not particularly strong. In general, the size of the "squeaky" sound of a turbocharged engine is related to the boost strength - a large turbocharger is more likely to make such a sound. Therefore, if the sound is relatively large, it can be seen that the supercharger of this car is large, and people have a very cool feeling.

Many modified enthusiasts also used this purpose to change the internal venting pressure of their own civilian car engines (such as Bora 1.8T and Passat 1.8T) into a leak-proof type, and further increase the "click" sound, the reason and the gun The barrel exhaust pipe is very similar. So why does a turbocharged engine make such a sound?

When we step on the accelerator pedal to accelerate, the throttle opens, and the engine exhausts high-temperature and high-pressure exhaust energy to drive the exhaust turbine to rotate. When the turbocharger is running (that is, the turbine rotates at 100,000 rpm or more) The turbocharger compresses the surrounding air to increase the engine intake and increase the engine's power.

『HKS external pressure relief valve』

When we close the oil, the throttle opening is rapidly reduced until it is in an idle state, that is, the engine does not need to be inflated, or the airflow in the intake pipe is blocked at the throttle. But at this moment the turbocharger did not stop working! Due to inertia, the turbocharger continues to rotate at a speed of more than 100,000 revolutions per minute. It is now conceivable that the air at this time will continue to be continuously compressed into the intake pipe. If this part of the high-pressure air in the intake pipe cannot be removed in time, the pressure in the intake pipe will rise rapidly, possibly causing the festival. Valve damage or bursting of the intake pipe.

『Relief valve installed in the intake pipe』

At this time, it is necessary to install a pressure relief device in the intake pipe to remove excess high pressure air from the intake turbine after the pipe is compressed. In fact, the pressure relief valve is a valve installed on the intake pipe to control the boost pressure. The opening and closing of the pressure relief valve is controlled by an electromagnetic coil operated by an ECU (Electronic Control Unit). The ECU will make a judgment based on the pressure of the turbo outlet boost. Once the pressure exceeds the critical value, the solenoid will be energized or de-energized to open or close the pressure relief valve.

When the pressure relief valve is closed, it is based on ensuring sufficient intake pressure in the intake pipe. When the valve is opened, excess gas can be released into the atmosphere, reducing the pressure in the intake port and protecting the intake pipe of the engine. Therefore, we heard the sound of “呲, 呲” on the modified car is the sound of the pressure relief valve when it is released from pressure.

In fact, the sound of "squeaking and squeaking" on the modified car has no meaning for improving the performance of the engine, but it can render an illusion of a large boost value. Relatively speaking, the larger the boost value, the more air is discharged from the pressure relief valve, and the theoretical noise is generated. When the car is modified, the sound when the pressure relief valve is released is amplified, but there is no practical effect.