Autoblog

Pouya @ Dec 31st 2007 7:38PM

Actually twin-scroll turbos and variable geometry turbos (e.g., variable area turbine nozzle) turbos work completely differently and do not exactly accomplish the same thing. For one, twin-scroll turbos do not accomplish much in comparison to non-twin scroll turbos top end, whereas VGTs accomplish a lot with its variable turbine a/r. The only thing VGT and twin-scroll turbos have in common is that they tend to perform better than conventional turbos low end, but even then there are some conventional turbos, such as the Garrett GT Ballistic Series, that outperform twin-scroll turbos.

Variable geometry turbos are designed to allow the effective aspect ratio (A/R Ratio) of the turbo to be altered as conditions change for better performance. For example, at lower rpms the a/r is less for a lower boost threshold, less lag, better response. At higher rpms the a/r ratio changes to a greater ratio so that the turbo doesn't choke at higher engine speeds, exhaust manifold pressures are decreased, and ultimately power output is increased as a result.

Twin scroll turbos work differently by requiring a divided exhaust manifold to separate the rotors or cylinders whose cycles interfere with one another to best utilize the engine's exhaust pulse energy. For example, on a four-cylinder engine with firing order 1-3-4-2, cylinder #1 is ending its expansion stroke and opening its exhaust valve while cylinder #2 still has its exhaust valve open (cylinder #2 is in its overlap period). In an undivided exhaust manifold, this pressure pulse from cylinder #1's exhaust blowdown event is much more likely to contaminate cylinder #2 with high pressure exhaust gas. Not only does this hurt cylinder #2's ability to breathe properly, but this pulse energy would have been better utilized in the turbine. The proper grouping for this engine is to keep complementary cylinders grouped together-- #1 and #4 are complementary; as are cylinders #2 and #3. Because of the better utilization of the exhaust pulse energy, the turbine's performance is improved and boost increases more quickly.