Detailed explanation of VGT variable cross-section turbocharger

VGT variable cross-section turbocharging technology is common in diesel engines, and what is the role of some?

With the development of technology, people's requirements for automobile engines are becoming more and more demanding, not only to have strong power, but also must have very high efficiency and clean enough emissions. This requires the engine in a variety of working conditions can be to achieve its most efficient working condition, so it must meet the engine under various operating conditions for the demand for air intake. This requires the engine components to be “variable” to meet the conditions under different operating conditions. For example, we are familiar with the variable valve timing / lift technology, variable intake manifold technology are so. Then in the diesel engine common VGT variable cross-section turbocharging technology, and what is the role of some? Let's find out below.

"The exhaust gas drives the turbine, and the turbine drives the impeller to pressurize the air, thereby effectively increasing the intake volume"

Turbocharging technology is one of the common technologies on the engine, and its principle is actually very simple: the turbocharger is equivalent to an air pump driven by the exhaust gas emitted by the engine. In the entire combustion process of the engine, about 1/3 of the energy goes into the cooling system, 1/3 of the energy is used to push the crankshaft, and the last 1/3 is discharged with the exhaust gas. Take a 200 kW engine, for example, according to the above mentioned ratio, it will consume about 70 kW of power in the exhaust. A large part of this power is consumed in the form of heat energy with the high-temperature exhaust gas, and the kinetic energy of the exhaust gas itself may be only ten kilowatts. But do not underestimate this ten kilowatts, to know that the household floor fan power is only about 60 watts! In other words, even ten kilowatts is enough to drive more than two hundred electric fans! It can be imagined that the supercharging effect brought by the exhaust gas turbine to drive the air is very impressive.

"BMW's Parallel Twin Turbine Technology"

Although the exhaust energy of the engine under full load is very impressive, when the engine speed is low, the exhaust energy is very small. At this time, the turbocharger cannot reach the working speed due to insufficient driving force. As a result, the turbocharger cannot play a role at low speed. At this time, the power performance of the turbocharged engine is even less than that of a naturally aspirated engine with the same displacement, which is what we often call the "Turbo lag" phenomenon.

"The small size turbine of the Volkswagen 1.4TSI engine has a low starting inertia"

For traditional turbocharged engines, one way to solve the phenomenon of turbine hysteresis is to use small-sized lightweight turbines. First of all, small turbines have small moment of inertia, so the turbine can achieve the best working speed at low engine speed, thus effectively improving the phenomenon of turbine hysteresis. However, the use of small turbines also has its disadvantages: when the engine is at high speed, the small turbine will increase the exhaust resistance (exhaust back pressure) due to the small exhaust cross section, so the maximum power and maximum torque of the engine will be affected to a certain extent. For a large turbine with small back pressure, although it can have excellent supercharging effect at high speed and the engine will have stronger power performance, the turbine is more difficult to drive at low speed, so the turbine hysteresis will be more obvious.

"Porsche has applied variable section turbine technology to petrol engines"

In order to solve this contradiction and ensure good supercharging effect of turbocharged engine at both high and low speeds, Variable Geometry Turbocharger (VGT) or VNT variable cross-section turbocharging technology came into being. In the field of diesel engines, VGT variable section turbocharging technology has been widely used. Since the exhaust temperature of the gasoline engine is much higher than that of the diesel engine, reaching about 1000°C (about 400°C for the diesel engine), and the hardware material used by the VGT is difficult to withstand such a high temperature environment, so the technology has been delayed in the application of the gasoline engine. In recent years, BorgWarner has joined forces with Porsche to overcome this problem, using heat-resistant aviation material technology to successfully develop the first gasoline engine with a Variable cross-section turbocharger, which Porsche calls VTG (Variable Turbine Geometry) variable turbine blade technology.

Using two VTG variable section turbochargers, the Porsche 911 Turbo squeezes out a maximum power of 368kw/6000rpm and a maximum torque of 650Nm/1950-5000rpm using only a 3.8L displacement. In supercharged mode, the power is increased to 390kw and the maximum torque is increased to a staggering 700Nm, while the liter power is also reached a staggering 102.6kw/L. Best of all, this engine, with the help of VTG technology, can maintain a maximum torque output of 650Nm from 1950-5000rpm, and the turbine lag is almost imperceptible at low RPM.

From the principle point of view, there is no essential difference between the VGT technology of diesel engine and the VTG of Porsche, and the basic principle and structure are similar. Below, we will take a look at the working principle of variable section turbochargers through Porsche VTG technology.

"The red blades on the outside of the turbine in the picture are the guiding blades"

The core part of VGT technology is the guide blade with adjustable eddy current section. As can be seen from the figure, a guide blade with Angle controlled by the electronic system is added to the outside of the turbine. The relative position of the guide blade is fixed, but the Angle of the blade can be adjusted. Control the flow and velocity of gas flowing through the turbine blades, thereby controlling the speed of the turbine. When the exhaust pressure of the engine is low at low speed, the opening Angle of the guide blade is small.

"A typical turbine doesn't have a vane structure"

According to the principle of fluid mechanics, at this time, the air flow rate at the inlet of the turbine will be accelerated, and the pressure at the turbine will be increased, so that the turbine can be more easily driven to rotate, so as to effectively reduce the phenomenon of turbine hysteresis, and improve the response time and acceleration ability of the engine at low speed. With the increase of the speed and exhaust pressure, the opening Angle of the blades gradually increases. Under full load, the blades remain fully open, reducing the exhaust back pressure, and thus achieving the supercharging effect of a general large turbine. In addition, since changing the blade Angle can effectively control the turbine speed, which also realizes the overload protection of the turbine, so the turbocharger using VGT technology does not need to set the exhaust pressure relief valve.

It should be pointed out that VGT variable section turbochargers can only change the characteristics of the turbine by changing the cross-cutting area of the exhaust inlet, but the size of the turbine will not change. If understood from the turbine A/R value, the principle of variable section turbines will be more intuitive.

"There are also manufacturers who call this technology VNT, such as Volvo and Audi, which are essentially the same"

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

The opening of the guide blade can affect the air velocity of the guide turbine blade. The opening of the guide blade is small at low speed (as shown in the upper left picture), which increases the air velocity; the opening of the guide blade is large at high speed (as shown in the upper right picture), and the negative exhaust pressure is reduced.

When the A/R value is smaller, it means that the flow rate of exhaust gas through the turbine is higher, which can effectively reduce the turbine hysteresis, and the turbine can obtain higher supercharging in the lower speed region, while the engine will produce larger exhaust back pressure at high speed, so that the power is limited at high speed. On the contrary, when the A/R value is larger, the response speed of the turbine is slower, and the turbine hysteresis is obvious at low speed. However, at high speed, the turbine has smaller exhaust back pressure and can make better use of exhaust energy, thus obtaining stronger power performance.

The variable cross-section achieved by VGT technology means changing the A value. When the blade Angle is small, the cross-cutting area of the exhaust inlet will be reduced accordingly, so the A value will change accordingly, and the small turbine has the characteristics of fast response. When the blade Angle increases, the A value increases, and the A/R value increases, thus obtaining stronger power output at high speed. In summary, by changing the Angle of the blades, the VTG system can change the A/R value of the exhaust turbine at any time, thus taking into account the advantages of the large and small turbines.

Although the structure and principle are very simple, but VGT variable section turbine technology for the boost effect is very significant, in the current mainstream turbocharged diesel engine, this technology has been widely used. However, due to the limitations of hardware materials, this technology has only just started on gasoline engines with higher exhaust temperatures, and the cooperation between Porsche and BorgWarner can be said to have created a precedent. However, with the advancement of material technology, this technology will be more widely used in future gasoline engines.