How a turbo and a supercharger work: types of forced induction

Forced induction systems allow a significant increase in engine output. Basically, when we force-induct an engine, the aim is to pump more air into the cylinders than the engine could draw in on its own at atmospheric pressure.

Why do we want more air in the combustion chamber? Put very simply, engine power depends on the amount of oxygen (and therefore air) we can make react with the fuel to produce a powerful explosion – or combustion, depending on whether we are talking about petrol or diesel – that pushes the piston.

To do this, the intake air must be compressed using a forced induction device, which can be of two types: turbo (also called turbocharger) or supercharger.

Let’s look at the operating principle of a turbo and a supercharger. That way, you’ll better understand the advantages and drawbacks of each.

Before that, it’s worth remembering that neither the supercharger nor the turbo are recent technologies.

Forced induction has been used in motorsport since the first decade of the 20th century. In the case of superchargers, a mechanism already used in other industrial applications was adapted. As for the turbocharger, it was first developed for aircraft engines, to compensate for the lower air density as altitude increases.

How a turbo works

A turbo basically consists of two turbines connected by a shaft that rotate inside a housing. The operating principle is to harness the kinetic energy of the exhaust gases.

The exhaust gases are passed through the first turbine, making it spin. Since the second turbine is on the same shaft, it also rotates. In this case, the movement of the second turbine is used to build pressure by forcing air into the intake manifold.

There are several important things to know about turbocharged engines. Naturally, the larger the turbo, the greater its capacity to build pressure. However, it also takes more exhaust gas to spin the turbine up, which can cause a delay between pressing the accelerator and the delivery of power. This is known as turbo lag.

More points. Turbines can spin at very high speeds, over 150,000 rpm. That’s why proper lubrication of the turbo (specifically of its shaft) is so important to avoid failures. For this, it’s essential to understand the importance of engine oil.

Any excess pressure generated by the turbo is controlled by a wastegate valve. Adjusting this valve, which lets air “escape”, is what allows the boost pressure to be regulated.

In addition, to improve performance, turbocharged engines usually have a radiator called an intercooler. This radiator cools the intake air after it passes through the turbo, since it has heated up during compression, losing oxygen density.

To finish with turbochargers, it’s worth mentioning that this system is particularly well suited to diesel engines, which work by compression ignition rather than by igniting the mixture with a spark plug.

Petrol engines require a mixture with a specific proportion of oxygen and fuel (for the curious, this is called the stoichiometric ratio) for combustion to be efficient, whereas a diesel is able to burn all the oxygen you manage to get into the combustion chamber.

In fact, naturally aspirated diesel engines have not been available on the market for some time. In petrol engines, the turbo began to gain importance with the arrival of electronic fuel injection, which allowed the mixture to be controlled more precisely than in turbo engines with carburettors. But that would be another story. Today, some brands offer excellent non-turbo petrol engines. One example is the 2.0 Skyactive Mazda that we tested in the CX-30.

Types of turbos

There are different types of turbochargers, depending on the application they are intended for. According to their “geometry”, they are divided into fixed and variable turbos.

• Fixed-geometry turbocharger. In this type of turbocharger, the airflow hitting the exhaust-driven turbine always does so in the same way. Manufacturing is simpler, as there are fewer moving parts. The problem is that when the engine runs at low revs, the exhaust gas flow does not carry enough energy to spin the turbine up quickly, producing the aforementioned turbo lag or response delay.
• Variable-geometry turbocharger. These are also known as VGT (Variable Geometry Turbocharger). They have vanes around the turbine that are actuated pneumatically or electrically. They allow the way exhaust gases strike the turbine to be varied, mainly to speed up its rotation when the gas flow is weak. They largely eliminate turbo lag.
• Twin-scroll turbo: Also called double-entry turbos. They work by splitting the exhaust gas flow from the cylinders so that backpressure is not created in the exhaust manifold

In this video you can see how a turbocharger works, specifically a variable-geometry one.

https://youtu.be/AuQor-YnbZ8

How a supercharger works

Now we move on to the supercharger. The goal is still the same as with the turbo: to introduce compressed air into the combustion chamber. But here the energy of the exhaust gases is not used; instead, the compressor mechanism is driven by the engine itself, usually via a belt connected to the crankshaft.

Its use also dates back to the early 20th century, when it was used in competition models as a means of achieving high power. As it does not depend on exhaust gases, it offers no response delay. But at high revs it entails a significant loss of engine power, which reduces efficiency.

Types of superchargers

There are different types of superchargers, depending on their architecture.

• Roots-type supercharger. Also called a “lobe” supercharger, after the figure-of-eight rotors that turn in opposite directions inside a housing, pushing the air along.
• Eaton supercharger. Also known as “screw” superchargers. They consist of two helical elements that turn meshed together, so that the air entering between the two spinning parts is trapped in a volume that decreases, increasing pressure.

Another type of supercharger, which Volkswagen used unsuccessfully in the 1990s, is the G-compressor, known as the scroll compressor. It consists of a fixed spiral and an internal moving spiral which compress the air as they move. They were discontinued due to the complexity of their manufacture and their low reliability.

Roots supercharger video

https://youtu.be/nsA9YsZhREA

Eaton supercharger video

https://www.youtube.com/watch?v=stjvbAO_6JQ

Which is better, a turbo or a supercharger?

If you’ve made it this far, you’ll be able to form your own opinion on whether a turbo or a supercharger is better. Because there is no single answer to this question. It depends on the type of engine and what you want to achieve. In fact, as you’ll see, some brands use both systems together.

To sum up, the supercharger provides an almost instantaneous throttle response from very low revs. But they are bulky, and driving them takes power away from the engine.

The turbocharger does not “steal” energy from the engine; it uses that of the exhaust gases, which would otherwise be wasted. But it has the problem of response delay when there are not enough exhaust gases to spin the turbine, and this worsens the larger the turbo is. This has led to solutions such as using more than one turbocharger (small for low revs, large for high power at high rpm); Mercedes-Benz or BMW have even used up to three turbos on some of their engines.

Some brands have also turned to double forced induction to combine the advantages of both systems. And not only in sports models. To name just one example, many models from the Volkswagen Group used a supercharger and a turbo on the popular 1.4 four-cylinder engine fitted to, among others, the Volkswagen Golf.

The new era of electrification has opened the door to electrically assisted turbochargers, such as the EPC from Audi, with an electric compressor that spins up the turbine to reduce turbo lag at low revs. Another solution, also seen in recent Audi performance models with TDI engines, is combining an electric compressor with a turbo.

One of the leading specialists in forced induction, BorgWarner, has already developed its eBooster technology, which uses an electric compressor to assist the turbo driven by the exhaust gases. You can see how it works in this video.

https://youtu.be/iEGisoOnewg