Editing Turbocharger (section)

====Variable-geometry====
[[File:VariableGeometryTurbo 1.JPG|thumb|Cutaway view of a [[Porsche]] variable-geometry turbocharger]]{{Main|Variable-geometry turbocharger}}
Variable-geometry turbochargers (also known as ''variable-nozzle turbochargers'') are used to alter the effective [[aspect ratio]] of the turbocharger as operating conditions change. This is done with the use of adjustable vanes located inside the turbine housing between the inlet and turbine, which affect flow of gases towards the turbine. Some variable-geometry turbochargers use a rotary [[Actuator#Electric|electric actuator]] to open and close the vanes,<ref>{{cite book|last=Hartman|first=Jeff|title=Turbocharging Performance Handbook|publisher=MotorBooks International|url=https://books.google.com/books?id=SvG0gq4DxecC&pg=PA95|year=2007|isbn=978-1-61059-231-4|page=95}}</ref> while others use a [[pneumatic actuator]].

If the turbine's aspect ratio is too large, the turbo will fail to create boost at low speeds; if the aspect ratio is too small, the turbo will choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo's aspect ratio can be maintained at its optimum. Because of this, variable-geometry turbochargers often have reduced lag, a lower boost threshold, and greater efficiency at higher engine speeds.<ref name="eight"/><ref name="one"/> The benefit of variable-geometry turbochargers is that the optimum aspect ratio at low engine speeds is very different from that at high engine speeds.