Twin Scroll Turbo sometimes called a twin scroll turbocharger or twin scroll turbine is an improvement to the normal or
single scroll turbocharger.
The twin scroll turbocharger is connected to the exhaust manifold via two input lines leading to the turbine and two scrolls
where separate wastegates controls the gases from each input.
The two input lines is used to separate cylinders whose exhaust gases would interfere with each other because of their firing
sequence in the four stroke cycle therefore allowing a more effective flow of exhaust gases
to the turbo.
The pulse of exhaust gases from the cylinders interfere with each other because the exhaust valves of one cylinder will be
open when it finishes the exhaust stroke and begins the intake stroke and at the same time the exhaust valves of another cylinder
will be opening as it begins the exhaust stroke.
This overlap could cause some of the exhaust gases from one cylinder to mix with the fresh intake of air/fuel of another
cylinder thereby reducing the amount of gasses going to the turbo and it would reduce the power from the combustion of the fresh
air/fuel mixture in the new cylinder.
Attention must now be paid to the firing order of the cylinders so no cylinder would be paired with one that directly behind
it in the firing sequence. The design of the intake to the turbo ensures that the pairs don't mix.
When the gases reach into the turbo it rotates its own small scroll to spin the turbine. Although the entire turbo operates
as one unit the separate scrolls act as small turbos separately but together form a large turbo equivalent to a single scroll turbo
of the same size.
A twin scroll turbo offers the following benefits over normal turbocharging;
Twin scroll turbocharging is more complicated and more expensive to set up than a regular turbo but the benefits are usually
worth the extra expense and complications especially on V engines or engines with more
than four cylinders.
- More pressure is sent to spin the turbo which adds more boost.
- Reduction in turbo lag as the turbo can function as small turbos and a large turbo simultaneously.
- A larger quantity and better quality of fresh air or air/fuel mixture can be forced into the cylinders on each stroke.
Regular turbocharging already has its benefits and with all the issues listed above it has still justified its existence but
twin scroll turbo has pushed the benefits further.