What is turbocharging
Undoubtedly, each of us at least once in our life noticed the “turbo” label on a normal-looking car. The manufacturers, as if on purpose, make these nameplates of small size and place them in inconspicuous places so that the uninitiated bystanders will not notice and pass by. And an understanding person will certainly stop and take an interest in the car. The following is a story about the reasons for this behavior.
Automotive designers (since the appearance in the world of this profession) are constantly concerned about the problem of increasing the power of engines. The laws of physics state that engine power is directly dependent on the amount of fuel burned per cycle. The more fuel we burn, the more power. And, say, we wanted to increase the "livestock of horses" under the hood - how to do it? This is where problems await us.
The turbocharger consists of two “snails” - the exhaust gases pass through one, and the second “pumps” air into the cylinders.
The fact is that for the burning of fuel requires oxygen. So it is not the fuel that burns in the cylinders, but the fuel-air mixture. It is not necessary to interfere with the fuel with air by eye, but in a certain ratio. For example, for gasoline engines, 14–15 parts of air are relied on for one part of the fuel, depending on the mode of operation, the composition of the fuel and other factors.
As we can see, quite a lot of air is required. If we increase the fuel supply (this is not a problem), we will also have to significantly increase the air supply. Conventional engines suck it in independently due to the pressure difference in the cylinder and in the atmosphere. The dependence is a direct one - the larger the cylinder volume, the more oxygen it will get on each cycle. So did the Americans, producing huge engines with breathtaking fuel consumption. Is there a way to drive more air into the same volume?
Exhaust gases from the engine rotate the rotor of the turbine, which, in turn, drives the compressor, which forces the compressed air into the cylinders. Before this happens, the air passes through the intercooler and cools down - so you can increase its density.
There is, and for the first time invented by Mr. Gottlieb Wilhelm Daimler (Gottlieb Wilhelm Daimler). Familiar last name? Still, it is used in the name DaimlerChrysler. So, this German thought very well in the engines and in 1885 figured out how to drive more air into them. He guessed to pump air into the cylinders with the help of a supercharger, which was a fan (compressor), which received rotation directly from the motor shaft and forced compressed air into the cylinders.
Swiss engineer-inventor Alfred Büchi (Alfred J. Büchi) went even further. He was in charge of the development of diesel engines at Sulzer Brothers, and he categorically disliked that the engines were large and heavy, and the power developed little. He also did not want to take energy from the “engine” in order to rotate the drive compressor. Therefore, in 1905, Mr. Buchi patented the first injection device in the world, which used exhaust energy as a propeller. Simply put, he came up with a turbocharger.
The idea of a smart Swiss is simple, as all ingenious. As the winds rotate the wings of the mill, the exhaust gases also rotate the wheel with blades. The only difference is that the wheel is very small, and there are a lot of blades. A wheel with blades is called a turbine rotor and is mounted on one shaft with a compressor wheel. So conventionally, the turbocharger can be divided into two parts - the rotor and compressor. The rotor receives rotation from the exhaust gases, and the compressor connected to it, working as a “fan”, forces additional air into the cylinders. All this tricky design and called the turbocharger (from the Latin words turbo - a whirlwind and compressio - compression) or a turbocharger.
In a turbo engine, the air that enters the cylinders often has to be further cooled — then its pressure can be made higher by driving more oxygen into the cylinder. After all, compressing cold air (already in the cylinder of the engine) is easier than hot.
The air passing through the turbine, is heated by compression, as well as from parts of the turbocharged, heated by exhaust gases. The air supplied to the engine is cooled using a so-called intercooler (intercooler). This is a radiator installed in the air path from the compressor to the engine cylinders. Passing through it, he gives his heat to the atmosphere. And the cold air is more dense - it means that it can be pushed into the cylinder even more.
The more exhaust gas enters the turbine, the faster it rotates and the more additional air enters the cylinders, the higher the power. The effectiveness of this solution compared to, for example, with a driven supercharger in that quite a bit of engine energy is spent on “self-service” of supercharging — only 1.5%. The fact is that the turbine rotor receives energy from the exhaust gases not due to their slowing down, but due to their cooling - after the turbine, the exhaust gases are still fast, but colder. In addition, the compressed air expended free energy increases the efficiency of the engine. And the ability to remove more power from a smaller working volume means less friction losses, less engine weight (and the machine as a whole). All this makes turbo cars more economical compared to their atmospheric counterparts of equal power. It would seem, here it is, happiness. But no, not so simple. The problems have just begun.
Firstly, the rotational speed of the turbine can reach 200 thousand revolutions per minute, secondly, the temperature of hot gases reaches, just try to imagine 1000 ° C! What does all this mean? The fact that to make a turbocharger, which can withstand such not weak loads for a long time, is very expensive and not easy.
For these reasons, turbocharging became widespread only during the Second World War, and even then only in aviation. In the 50s, the American company Caterpillar managed to adapt it to their tractors, and the craftsmen from Cummins designed the first turbo diesel engines for their trucks. On serial passenger cars turbo appeared later. It happened in 1962, when the Oldsmobile Jetfire and Chevrolet Corvair Monza saw the light almost simultaneously.
But the complexity and high cost of construction are not the only drawbacks. The fact is that the efficiency of the turbine depends heavily on the engine speed. At low revs the exhaust gas is slightly, the rotor spins slightly, and the compressor almost does not blow additional air into the cylinders. Therefore, it happens that up to three thousand revolutions per minute the motor does not pull at all, and only then, thousands after four or five, “shoots”. This spoon of tar is called turbo lump. And the larger the turbine, the longer it will unwind. Therefore, motors with a very high power density and high-pressure turbines, as a rule, suffer from turbojam first. But for low-pressure turbines, there are almost no failures, but they do not raise the power too much.
There are more sophisticated designs. For example, engineers have come up with installing two turbines on the motor, not one. One works at low engine speeds, creating cravings on the "bottoms", and the second is turned on later. This solution was called twin-turbo and allowed to kill two birds with one stone - both the turbo lagoon, and the problem of lack of power. At the end of the last century, cars with a sequential turbine connection scheme had some popularity; they were produced by Nissan, Toyota, Mazda and even Porsche. However, due to the complexity of the design, the age of such devices was short-lived, and other ideas became popular.
For example, parallel turbocharging, or biturbo. That is, instead of one turbine, they put two small identical turbines, which operate independently of each other. The idea is as follows: the smaller the turbine, the faster it spins, the more “responsive” the engine turns out. As a rule, two small turbines were installed on V-shaped engines, one for each "half".
Another option - the turbine with two "snails", or twin-scroll. One of them (slightly larger) receives exhaust gases from one half of the engine cylinders, the second (slightly smaller) - from the second half of the cylinders. Both supply gases to one turbine, effectively spinning it both at low and at high speeds.
But on this the designers did not calm down. Naturally, than to fence two turbines, it is much easier to manage with one. It is only necessary to make the turbine work equally effectively throughout the entire rev range. So appeared turbines with variable geometry. This is where the fun begins. Depending on the speed, special vanes are rotated and the shape of the nozzle varies. The result is a “super turbine” that works well throughout the rev range. These ideas have been in the air for more than a dozen years, but they have been implemented relatively recently. And first, turbines with variable geometry appeared on diesel engines, the benefit is, the temperature of the gases there is much less. And out of gasoline cars, the first one tried on such a Porsche 911 Turbo turbine.
The design of the turbo engine brought to mind for a long time, and recently their popularity has increased dramatically. Moreover, the turbochargers turned out to be promising not only in terms of speeding up the engines, but also in terms of improving the efficiency and cleanliness of the exhaust. This is especially true for diesel engines. Rare diesel today does not carry the prefix "turbo". But the installation of the turbine on gasoline engines allows you to turn an ordinary-looking car into a real “lighter”. The one with a small, barely noticeable tag "turbo".