What's the difference between single, twin, twin-scroll, variable geometry, or even electric turbochargers? What are the advantages of each setup?

The world of turbocharging has about as much variety as engine layouts. Let’s take a look at the different styles:

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.

Twin-scroll turbochargers are better in nearly every way than single-scroll turbos. By using two scrolls, the exhaust pulses are divided. For example, on four cylinder engines (firing order 1-3-4-2), cylinders 1 and 4 might feed to one scroll of the turbo, while cylinders 2 and 3 feed to a separate scroll. Why is this beneficial? Let’s say cylinder 1 is ending its power stroke as the piston approaches bottom dead centre, and the exhaust valve starts to open. While this is happening, cylinder 2 is ending the exhaust stroke, closing the exhaust valve and opening the intake valve, but there is some overlap. In a traditional single-scroll turbo manifold, the exhaust pressure from cylinder 1 will interfere with cylinder 2 pulling in fresh air since both exhaust valves are temporarily open, reducing how much pressure reaches the turbo and interfering with how much air cylinder 2 pulls in. By dividing the scrolls, this problem is eliminated.

Perhaps one of the most exceptional forms of turbocharging, VGTs are limited in production (though fairly common in diesel engines) as a result of cost and exotic material requirements. Internal vanes within the turbocharger alter the area-to-radius (A/R) ratio to match the RPM. At low RPM, a low A/R ratio is used to increase exhaust gas velocity and quickly spool up the turbocharger. As the revs climb, the A/R ratio increases to allow for increased airflow. The result is low turbo lag, a low boost threshold, and a wide and smooth torque band.

Could this be the solution we’ve been waiting for? While attending SEMA 2015 I stopped by the BorgWarner booth to look into the latest in turbocharging, among the concepts is the variable twin-scroll turbo as described in the video above.

Throwing a powerful electric motor in the mix eliminates nearly all of the drawbacks of a turbocharger. Turbo lag? Gone. Not enough exhaust gases? No problem. Turbo can’t produce low-end torque? Now it can! Perhaps the next phase of modern turbocharging, there are undoubtably drawbacks of the electric path as well.

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