Single turbochargers alone have limitless variability.
Differing the compressor wheel size and turbine will lead to completely different torque characteristics.
Large turbos will bring on high top-end power, but smaller turbos will provide better low-end grunt as they spool faster.
There are also ball bearing and journal bearing single turbos. Ball bearings provide less friction for the compressor and turbine to spin on, thus are faster to spool (while adding cost).
Just like single turbochargers, there are plenty of options when using two turbochargers.
You could have a single turbocharger for each cylinder bank (V6, V8, etc).
Alternatively, a single turbocharger could be used for low RPM and bypass to a larger turbocharger for high RPM (I4, I6, etc).
You could even have two similarly sized turbos where one is used at low RPM and both are used at higher RPM.
On the BMW X5 M and X6 M, twin-scroll turbos are used, one on each side of the V8.
A turbo is powered by exhaust gases that are redirected to spin turbine blades and force air into the engine.
Now, an engine’s cylinders fire in sequence, meaning that exhaust gases enter the turbo in pulses.
As you can probably imagine, these pulses can easily overlap and interfere with one another when powering the turbo, and a twin-scroll turbocharger solves this issue by using a divided-inlet turbine housing and a specific exhaust manifold that pairs the right cylinders to each scroll.
In a four-cylinder vehicle, you can then have the first and fourth cylinders powering one scroll, and two and three powering another.
This means that there are less pulse overlap and less lag.
A variable geometry turbo (VGT) is an expensive and complex power solution that’s especially prevalent in diesel engines.
A VGT has a ring of aerodynamically-shaped vanes in the turbine housing that can alter their area-to-radius ratio to match the revolutions of the engine.
At low revs, area-to-radius ratio creates more pressure and velocity to spool up the turbo more effectively.
At higher revolutions, the ratio increases to let in more air. The result is a wider boost range and less lag.
A variable twin-scroll turbo combines a VGT with a twin-scroll setup, so at low revolutions, one of the scrolls is closed completely, forcing all the air into the other.
As you speed up, a valve opens to allow air into the other scroll (this is a completely variable process, meaning the valve opens in small increments), you get good high-end performance.
You get the sort of performance from a single turbo that you’d normally only be able to get from a twin-turbo set-up.
A very recent development is the introduction of turbos with electric compressors.
An example is BorgWarner’s booster, which is an electrically powered compressor.
The compressor provides an instant boost to the engine until the turbocharger has spooled up enough.
A similar version of this can be found in Audi’s SQ7. With the instant boost, lag becomes a thing of the past, but again, the system is expensive and complex.
A compressor needs a motor, which in turn needs to be powered, so this is not a simple system to implement.
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