We often talk about turbo petrol and the two types of turbines. Let's see together the characteristics of each, highlighting their strengths and weaknesses.

Very often we hear the word " turbo ", in relation to engines and their performance. This word is the abbreviation for “ turbocharger ”, a mechanical component created and designed to supercharge the car's heat engine.

How many types of turbochargers are there on the market?
Different types of turbo for different types of fuel?
Is there an absolute better turbo than another?
And finally reliability. Does supercharging the engine create reliability problems?
Let's see all these aspects together.

Let's begin to understand how the turbocharger is made and how it works, more specifically. The latter is made up of two components, the compressor and the turbine : the first compresses the air, the second discharges the gases produced by combustion in the cylinders. To work, each component has an impeller inside, also called an impeller (think of a fan), and they are connected to each other by a metal shaft and inserted in housings called "snails". The turbine is the first to come into action, since, receiving the exhaust gases generated by the combustion in the cylinders, it mechanically moves, thanks to the common shaft, the compressor, which as the word suggests compresses the air at the intake and introduces it in the combustion chamber (to be clear, the cylinders). In this way the turbo, thanks also to the work of the system control valves ( wastegate and pop-off ), is able to improve the filling coefficient and therefore the efficiency of the engine, increasing the power and torque values developed.

However, not all engine speeds have benefits: below 2,000-3,000 engine revolutions, due to the inertia of the turbine impeller, the exhaust gases come out more slowly, thus creating a turbo delay (in English turbo lag ).

What we have just described is the classic version, called a fixed geometry turbine. Over the years, engineering has introduced new proposals for turbo configurations and probably the best known variant is the variable geometry turbocharger . The innovative aspect resides in the turbine impeller: there is in fact a ring of stator vanes with variable incidence, that is, the angle of incidence with respect to the rotating vanes of the impeller, can vary thanks to the intervention of the electronic control unit (called ECU). In simple terms, depending on the number of revolutions of the engine, the control unit modifies the angle of these stator vanes to favor the speed or gas flow rate. A fundamental role is obviously that of the electronic control unit, whose correct mapping allows an optimization of the operation of the turbo system, and therefore of the entire engine.

Over the years, engines with two turbos have also been presented, the so-called biturbo , the structure of which can be in series or in parallel. The former are normally mounted on in-line engines and have two turbos of different sizes: the small one to work at low revs, while the larger one takes over when needed at high engine revolutions. The biturbo in parallel is used instead in engines with V configurations, where the two turbos are of equal size and each manages one of the two engine banks. Example of a twin-turbo car? BMW 535d.

Let's now pass to the question which is the natural consequence of our discussion: is the turbine with fixed or variable geometry better?
Surely the variable geometry allows to optimize the gas output, according to the number of engine revolutions: at low revolutions the stator vanes remain "closed" in order to limit the gas flow, while at high revolutions they "open", so as not to obstruct the exit of the gases themselves. In this way, below 2,000 RPM, the variable geometry solves the problem of inertia of the fixed geometry, with benefits also linked to driving comfort, since the progression remains smooth and without recoils.

Reading the article one would almost ask oneself: why then does everyone not adopt the variable geometry version?
The main reason is the delicacy of the system and therefore reliability problems, due to the presence of moving parts of the stator. For this reason, variable geometry is almost exclusively present in diesel engines, due to the lower exhaust gas temperatures compared to petrol engines. Returning therefore to the initial question "different turbos for different types of fuel?" we can say yes: the variable geometry is used almost exclusively in diesel , while in petrol engines it is instead the norm to find turbochargers with fixed geometry. An exception? Porsche 911.

In conclusion, we can say that, from a general technical point of view, the fixed geometry is no better than the variable one and vice versa. In the details of use, however, the fixed geometry turbine is mainly used in petrol engines. The car manufacturers, on the other hand, are now choosing, almost for each production model, whether to mount one or more turbos and what type. An almost personalized choice, in order to maximize the performance of the engine and to make the driving experience of us motorists as pleasant as possible.

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