Turbocharger design & working principle

Variable Geometry (VGT) VS Sequential Turbocharger

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What is Variable Geometry Turbocharger or VGT?

Variable Geometry Turbocharger or VGT is a type of turbocharger. It is a mechanical device that increases the pressure of air by compressing it. Another name for the Variable-Geometry Turbocharger (VGT) is Variable Nozzle Turbines (VNT). VGT allows an effective aspect ratio (A:R) for the turbocharger according to varying conditions. VGTs are usually designed to deliver refined performance.

Variable Geometry Turbocharger (Courtesy: Cummins)
Variable Geometry Turbocharger (Courtesy: Cummins)

Manufacturers employ VGT because the optimum aspect ratio at low engine speeds is very different from that at high engine speeds. If the aspect ratio is too large, then the turbocharger will not be able to create the required boost at low speeds. If the aspect ratio is too small, the turbocharger will fill the engine with more air at high speeds. This results in high exhaust manifold pressures, high pumping losses. And, ultimately, it lowers the effective power output.

How Variable Geometry Turbocharger or VGT works?

By varying the turbine housing geometry as the engine accelerates, the turbocharger can maintain the aspect ratio at its optimum levels. Because of this, VGT offers minimum lag. It has a low boost threshold and is very efficient at higher engine speeds. VGT does not need a waste-gate.

Applications of Variable Geometry Turbocharger:

The two most common applications include a ring of aerodynamically-shaped vanes in the turbine housing at the inlet. Manufacturers use the first type in the passenger cars, race cars, and light commercial vehicles, which use light-duty engines. In this design, the vanes rotate in unison. They vary the swirl angle and the cross-sectional area of the turbine.

Manufacturers use the second type for heavy-duty engines, such as in commercial vehicles. In this design, the vanes do not rotate. However, instead, an axially sliding wall mechanism selectively blocks the width of the turbo inlet. In this design, either this mechanism partially covers the vanes of a moving slotted shroud, or they partially move against a stationary slotted shroud. It varies the area between the tips of the vanes. Thus, it results in a variable aspect ratio.

Parallel Turbochargers:

Manufacturers use the term ‘Parallel Turbochargers’ when they employ two turbochargers that are identical in size and function. Generally, V engines employ the Parallel turbochargers as one turbo serves to each bank of cylinders. A 'V' engine has two separate exhaust manifolds with plumbing on both sides of the cylinder banks. Some manufacturers refer to this design on the V engine as bi-turbo.

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