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dwightlooi

Twin Scroll Turbos - explained

2 posts in this topic

dwightlooi    259

There has been much confusion and misunderstanding as to what twin scroll turbos do and why they are used.

First of all let's get a few things out of the way. Having two scrolls in the turbine housing:-

  • Does not make the turbocharger itself more efficient or more responsive
  • Having two scrolls have nothing to do with optmizing one for low speed and one for high speed flow
  • In fact, having two scrolls introduce additional passage restrictions to the turbocharger and reduces its turbine efficiency slightly.
  • Twin scroll turbos do not benefit all engine configurations

So why are twin scroll turbos used? Well, they are used to solve an exhaust problem that may exist on some engines upstream of the turbo itself. Let me try to explain this...

(1) Most engines have some amount of exhaust and intake valve overlap. That is the intake valves open before the exhaust valves are closed. This is necessary for good breathing and more complete aspiration. The ingress of intake air help push the last remaining exhaust gases out.

(2) In some engine types, a piston end up at the bottom of the stroke when another is at the top of the stroke. If another piston happens to be at the bottom of its power stroke and opens its exhaust valves at the very moment when the aforementioned overlap period occurring, what do you think happens? Well, as the high pressure exhaust gases from the just opened exhaust valves enters the exhaust manifold and encounters resistance from the turbocharger's turbine, some of it makes its way back into the cylinder in overlap which -- being at the end of its exhaust stroke and the beginning of its intake stroke -- is at a much lower pressure than the new exhaust pulse. Instead of the intake charge purging the remnants of the exhaust charge, you end up having exhaust going back through the exhaust valves into the cylinder and sometimes into the intake tract.

(3) There are two solutions to this. The traditional solution is that turbocharged engines experiencing this problem simply use very little overlap -- either opening the intake valves late or closing the exhaust valves early or both. This minimizes the back flow problem. However, it also renders off boost aspiration incomplete and inefficient. Result? Higher emissions, inferior cruise economy and slower turbo compounding response. The other solution is to keep the exhaust between the cylinders 180 degrees apart separate until it reaches the turbo and use a dual scroll turbo which continues to keep the flows separate right until it hits the turbine. This eliminates the overwhelming majority of the feedback pulse and allows proper overlap to be used. The downside is that segregated manifolds and turbos are more complex, more expensive and actually more restrictive.

twinscroll.jpg

  • I3 does not have this problem
  • I4 has this problem
  • H4 has this problem
  • I5 does not have this problem
  • V6 (60 deg) has this problem
  • V6 (90 deg) does not have this problem
  • I6 does not have this problem
  • H6 does not have this problem
  • V8 has this problem
  • V10 does not have this problem
  • V12 does not have this problem

To answer the question of whether an engine benefits from a dual scroll turbo, all you have to ask yourself is whether the engine has pistons at the top and at the bottom of their travel at the same time. Because this is when the problem of exhaust back feeding manifests itself.

Edited by dwightlooi
  • Upvote 1

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