TURBO - What it is and how it works:

The turbocharger, known as turbo-group or turbo, is a mechanical system with the purpose of supercharging the internal combustion engine which uses exhaust gases. It consists of the coupling between a centripetal turbine, called the "hot" side or exhaust side, where the high temperature exhaust gases are received, and a centrifugal compressor, called the "cold" side or suction side, in the which the air to be compressed is sucked in. The turbo is installed on the engine by mounting the hot side on the exhaust manifolds which, for example, can be of the 4 in 1 or 4 2 1 type to concentrate the flow. This wording indicates the way in which the manifolds were made and in particular , the evolution of the curves. The turbo is used in engines with injection systems and in most carburetor systems, and the advantage is to limit fuel dispersion along the walls of the fuel system. Inside the compressor and the turbine there are two impellers that perform two separate functions: they are joined by a joint shaft and rotate at the same speed inside their snail-shaped housing. The turbine wheel, located inside the cast iron exhaust scroll, is set in rotation both by the action of the exhaust gases pushed out of the cylinders thanks to the pistons, and by the combustion of the air/petrol mixture, which create a pressure positive between the exhaust valve and the turbine. The compressor impeller, located inside the "cold side" is made of a light alloy which allows you to reduce resistance to acceleration by trying to have the opposite effect of the flywheel, for example. Its function is to compress the air and introduce it into the intake manifold, providing the cylinders with a volume of air greater than what a naturally aspirated engine could suck in the same unit of time. As mentioned before, the two impellers are connected by a shaft that passes through the "core assy", the real heart of the turbo. In summary, the turbine collects the energy of the exhaust gases and transforms it into mechanical energy useful for rotating the compressor and therefore the rotation of the exhaust impeller simultaneously activates the intake impeller which compresses the incoming air towards the intercooler and the throttle body. This system is a highly efficient complex that uses the residual, otherwise wasted, energy of the exhaust gases to provide more power to our engine.

Why don't turbo engines suffer from thin air?

Thanks to this system, turbo engines do not suffer from changes in atmospheric pressure as they are calibrated through pressure sensors known as "wastegates" which constantly work on pre-set values. On road cars we are talking about values ​​varying between 0.5 bar and 1 bar of positive pressure on average, while on racing cars decidedly higher pressures can be reached. Suffice it to say that the racing cars of the 80s reached pressures close to 3 bars, all obviously to the detriment of reliability, but rightly, if you are racing, the result of conservation is more important. The wastegate maintains the indicated pressure by only varying the revolutions per minute of the two impellers, for example: at sea level a turbo must make 1000 rpm to maintain the pressure at 1 bar, in the mountains, to maintain the same pressure, the turbo will turn at around 15000 rpm.

But what is the wastegate?

The wastegate valve is a fundamental component for a turbocharger. The latter, receiving a lot of kinetic energy from the exhaust gases, must stop when the operating pressure set by the manufacturer is reached so as not to exceed this limit in order to avoid unwanted breakages. This valve is controlled by the positive pressure taken from the cold side which pushes a spring diaphragm and, through a mechanical shaft, opens the valve, removing pressure from the exhaust manifolds, reducing the revolutions of the impellers and the pressure of the compressor.

What is the difference between fixed and variable geometry turbines?

There are five types of turbines on the market that we will analyze in this video: single scroll, twin scroll, fixed geometry turbines, variable geometry turbines and electric turbines. Fixed geometry turbines are the ones we described before and ensure that the turbine and compressor keep the air volume fixed. Those with variable geometry, on the other hand, have "movable blades" placed around the turbine wheel with the function of varying the section of the channels delimited by the blades themselves and the angle of incidence of the exhaust gases. Simply put, the position of the vanes determines the speed of the turbocharger rotor. The main advantage of the variable geometry compared to the fixed one is the reduction of turbo lag which improved the linearity of operation of the supercharged engine compared, for example, to what could be found in the first turbocharged diesel engines. The turbo lag consists in the delay in the response of the turbine and derives from the fact that the engine at low speeds offers a limited flow of exhaust gases which does not allow the fixed turbine to transfer sufficient rotation to the rotor, preventing the compressor from offering the engine the pressures useful for a concrete increase in torque and power values. With the variable geometry, however, a high angular rotation speed of the compressor is guaranteed starting from low rpm since the vanes implement a restriction of the section in which the gases affect, guaranteeing a prompt response even at idle. As the engine rpm increases, the exhaust gases acquire speed, significantly increasing the rotor rpm which guarantees greater compression of the air entering the engine. This in turn will allow more power to be delivered from low revs as the quantity of fuel that the fuel system will be able to supply to the engine will be greater thanks to the increase in oxygen introduced into the cylinders. At this point a vicious circle is triggered: there will be more speed of the exhaust gases which will produce more rotation of the rotor and the compressor will send more compressed air to the engine. With the old fixed geometry turbochargers the opposite situation was obtained: a large mass of exhaust gases entering the turbine produced high back pressures in the exhaust with the effect of complicating the expulsion of the latter, thus increasing the temperature of the same gases with the actual reduction in engine efficiency. This negative effect, in the case of variable geometry turbochargers, is very limited: in fact, as the engine rpm increases (and therefore the flow of exhaust gases), the incidence of the moving vanes varies. The exhaust gas passage section increases with the effect of both "modulating" the rotation speed of the turbine (avoiding over-revving) and allowing a more effective flow of exhaust gases which guarantees low back pressure values ​​in the exhaust manifold . The advantages thus obtained are numerous: reduced fuel consumption and increased engine efficiency throughout the entire operating speed range. To obtain the advantages listed, it is important that the variable geometry adjustment is very precise: otherwise the engine operation would not be smooth and could cause premature failure of the turbocharger which would operate in over-speed conditions. It is therefore crucial that the engine management electronics effectively monitor the position assumed by the variable geometry. The variable geometry is controlled by an actuator which is essentially divided into three main types: pneumatic actuators; electro-pneumatic actuators; electronic actuators.

Single scroll or twin scroll?

Given the difference between fixed geometry turbines and variable geometry turbines, let's move on to another solution adopted to reduce the response lag of turbo-compressors, the twin scroll turbines. Before listing the advantages, let's clarify what the term means, imagine the traditional turbo with a single incoming exhaust manifold coming from the 4 in 1 manifolds, now imagine having the 4 2 2 manifolds, i.e. manifolds coupled two by two which do not they join together up to the exhaust impeller, in this case we would have the collectors and the twin scroll impeller. But what does this mean? It means cleaning the flow of exhaust gases coming out of the cylinders, reducing the back pressure in the manifolds since the explosion phases of a 4 cylinder in some moments with the 4 in 1 manifolds could hinder the speed of the flows by increasing back pressure and temperatures. In this way the exhaust gases reach the hot side impeller much faster without disturbances but that's not all, there are further advantages. In this way the flows coming from each individual cylinder will be clearly distinct from each other, preventing their pulsations from obstructing each other. To give you an example, in a 4-cylinder engine with 1-3-4-2 combustion order, the manifolds 1 with 4 and 2 with 3 are grouped together, trying to use manifolds of equal length so as not to generate different pressure losses due to the different lengths of the same collectors. In essence, therefore, twin scrolls work with multiple flows, exploiting the impulse theory which is already very useful in managing the charge in the cylinder. On the contrary, a single scroll turbocharger works with a single flow at constant pressure. The result is an extremely ready turbo capable of eliminating the efficiency problems that afflict single scroll turbos at low rpm and high loads. A further advantage of twin scrolls is the adoption of different intake manifolds, so in addition to having cleaner flows we can have flows directed in a different way trying to create the effect of variable geometry turbos. With the two different intake manifolds in the turbo we will have a smaller manifold directed in such a way as to help the rotation of the impeller at low rpm and another slightly larger manifold angled in such a way as not to create drag on the exhaust gases at high rpm. regimes.