The variable geometry turbine allows significant flexibility over the pressure ratio across the turbine. In diesel engines, this flexibility can be used for improving low speed torque characteristics,reducing turbocharger lag and driving EGR flow.
Variable Turbine Geometry
The most common designs of variable geometry turbochargers include the pivoting vane design and the moving wall design.
VNT—Variable Nozzle Turbine (Honeywell/Garrett),
VTG—Variable Turbine Geometry (BorgWarner and ABB)
VG—Variable Geometry turbocharger (MHI)
VGS—Variable Geometry System turbocharger (IHI)
VTA—Variable Turbine Area (MAN Diesel & Turbo)astegate.
Variable-geometry turbochargers (VGTs), (also known as variable nozzle turbines/VNTs), are a family of turbochargers, usually designed to allow the effective aspect ratio (A:R) of the turbo to be altered as conditions change. This is done because optimum aspect ratio at low engine speeds is very different from that at high engine speeds. If the 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 türbine (variable geometry türbine) housing to help the engine accelerates, the turbo's aspect ratio can be maintained at its optimum. Because of this, VGTs (Variable Geometry Turbochargers) have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds. VGTs (Variable Geometry Turbochargers) do not require a wVGT—Variable Geometry Turbocharger (Cummins/Holset),