In the last article, we talked about the working principle of turbochargers, but the development of turbochargers has not been smooth sailing, and there are various shortcomings during this period.
From the invention of turbocharging technology in 1905, more than 100 years have passed now, during this period, turbocharger technology has also continued to make progress, let's take a look at the technical development of turbochargers.
A two-pronged twin-scroll turbine
In the early days, the turbine used a single scroll tube, which can be simply understood as an air intake channel, which led to some turbo hysteresis, and the intuitive feeling was that the acceleration was not linear.
Lightly step on the accelerator, the car loves to move, and then continue to step deeply, the car seems to be kicked out, and the driving feeling is very abrupt. For the same engine, the larger the size of the supercharger (impeller), the more severe the turbo lag.
The reason why the power output is not linear and why there is turbo hysteresis is that at idle and low engine speeds, the exhaust gas is low enough to push the turbine to rotate at high speed. When the engine speed is increased, the turbocharger value generated by the turbine can be satisfied when the exhaust gas flow is large.
In order to solve the turbo hysteresis, changing the exhaust mode of the engine and modifying the exhaust gas channel can effectively reduce the turbo hysteresis.
By splitting the exhaust manifold of the engine into two streams, the exhaust flow velocity is increased by reducing the interference of the exhaust air flow between the two cylinders that are adjacent to each other, so that the turbocharger can enter the working state earlier.
To put it simply, in the past, four-cylinder engines were equipped with ordinary turbos, and only one pipe led to the exhaust turbine after the four exhaust manifolds came out, so that the exhausts of the four cylinders would interfere with each other.
If a twin-scroll turbo is used, the four-cylinder engine allows two cylinders to use one scroll tube, which reduces the exhaust interference between the four cylinders, effectively alleviates the hysteresis at low speeds, makes the peak torque of the engine explode earlier, and the fuel economy is better.
Variable geometry turbocharging technology
The variable geometry turbine, to put it simply, is arranged around the exhaust chamber impeller with a circle of louver-style deflectors. By controlling the angle of the guide vanes, the flow rate and flow rate of the gas are controlled, and thus the rotational speed of the turbine is controlled. This technology was also created to reduce turbo lag.
When the engine is low, the exhaust pressure is low, and the guide vanes open at a small angle, which increases the velocity of the air flowing through it, increasing the pressure at the impeller, which makes it easier to push the turbine to rotate, thereby reducing the phenomenon of turbo lag.
As the engine speed increases, the exhaust pressure increases, and the guide vanes gradually increase the opening angle and reduce the exhaust back pressure, so as to achieve the supercharging effect of the general large turbine.
In addition, turbochargers using VGT technology generally do not need to be equipped with an exhaust relief valve, since the turbine speed can be effectively controlled by changing the blade angle, which also enables the turbine to be overloaded.
In addition, the adjustable vane turbocharging technology is called differently in different manufacturers. Volvo calls it VNT (Variable Nozzle Turbine) and Porsche calls it Variable Turbine Geometry, also known as VGT (Variable Geometry Turbocharger).
Electric turbocharger
Due to the limitations of the principle characteristics of conventional exhaust gas turbines, the engine must reach a certain speed in order to push the blades to turn. In order to solve the turbo hysteresis, the aforementioned twin-scroll turbine and more complex variable section turbines were born, which effectively solved the problems of turbo hysteresis and insufficient linearity of acceleration.
However, in the new energy era, things have taken a further turn. This means that what was not possible before can now be done, and that is to use electric motors to drive turbines.
In the past, car batteries were all 12V voltage, and if you want to drive an electric turbine with tens of thousands of revolutions, you can quickly squeeze out the battery capacity. However, with a higher system voltage and a larger battery, this is not a problem.
Volvo's previous classic exhaust gas turbo + supercharged T6 engine, after upgrading the 48 hybrid system, the supercharger was replaced with an electric turbo.
Taking a certain brand of electric turbine as an example, relying on the 48V power supply, the rated power of the electric turbine reaches 5kW, the maximum speed is as high as 72,000 rpm, and the instantaneous peak power is 6.2kW, which can achieve 90% of the maximum speed within 230 milliseconds.
Since it is electrically driven, there is no turbo hysteresis, the electric turbine can build up pressure at any time if necessary, and the electric turbine is simpler and more compact.
Teach students according to their aptitude and adapt measures to local conditions
The solution to turbo hysteresis, in addition to the previously mentioned improvement of turbine technology, is also a solution to optimize the arrangement of turbines.
In view of the turbo hysteresis phenomenon, two turbines, one large and one small, are connected in series or parallel, and when the engine is low, less exhaust can drive the turbine to rotate at high speed to generate sufficient intake pressure, which can also reduce the turbo hysteresis effect.
The Audi 3.0 diesel engine is powered by two turbo wheels that operate in series-parallel mode. When the engine is at a low load, the turbine switching valve opens and the exhaust gases directly drive the low-pressure turbopump gas. When the load gradually increases, the turbine switching valve closes, and the exhaust gas flows through the high-pressure turbine first, then through the low-pressure turbine, and the two turbines work together to pump the gas.
Tri-turbocharged
Or Audi, at the 2016 Paris Motor Show, Audi showed off the tri-turbo 4.0TDI diesel engine in the Audi SQ7. The engine is equipped with both an electric turbine and two exhaust gas turbines.
The two exhaust valves of the engine cylinders are independent of each other and are connected to two exhaust gas turbines, which are available in series and parallel.
At idle or low-to-medium revs, only one exhaust gas turbine and electric turbocharger operate.
At medium to high speeds, the two exhaust gas turbines work in tandem.
At high speeds, the two exhaust gas turbines are in parallel mode.
The evolution of the turbine has led to the development of automobiles, and many interesting technologies have been born. To this day, we can still see the turbine continue to exert its residual heat in new energy plug-in hybrid vehicles. Stay tuned for a series of articles on those stories of Turbo.