Variable Turbo Chargers Geometry (VTG)

VTGs, in short, are a group of turbochargers, designed to allow the effective aspect ratio (A/R Ratio) of the turbo to be changed with changes in conditions. This is owing to the difference in optimum aspect ratio at low engine speeds and that at high engine speeds.

If the aspect ratio is too large, the turbo will fail to develop boost at low speeds; if the A/R ratio is too small, the turbo will choke the engine at high speeds, leading to high pumping losses, high exhaust manifold pressures, and ultimately lower power output. By changing the turbine housing geometry, as the engine accelerates, the turbo’s aspect ratio can be maintained at its optimum.

What are the most common designs?

The two most common include a ring of aerodynamically-shaped vanes in the turbine housing at the turbine inlet. Generally, for light duty engines, the vanes rotate in unison to vary the gas swirl angle and the cross-sectional area. Generally for heavy-duty engines, the vanes do not rotate, but instead, the axial width of the inlet is selectively blocked by an axially sliding wall. Either way, the area between the tips of the vanes changes, leading to a different aspect ratio.

Actuation

Often the vanes are controlled by a membrane actuator identical to that of a waste gate, but increasingly electric servo actuation is used. Hydraulic actuators have also been used many times.

Common uses

In trucks, they are used to control the ratio of exhaust re-circulated back to the engine inlet. Although excessive engine back pressure is not good for overall fuel economy, ensuring a sufficient EGR rate even during transient events can be enough to reduce nitrogen oxide emissions down to that required by emissions legislation.

Another application of the sliding vane type of turbocharger is as downstream engine exhaust brake, such that an extra exhaust throttle valve isn’t required. Moreover, the mechanism can be modified to reduce the turbine efficiency in a predefined position. This mode could be selected to sustain a raised exhaust temperature to promote "light-off" and "regeneration" of a diesel particulate filter. Actuation of a VGT for EGR flow control or to implement brakes or regeneration modes that generally needs hydraulic or electric servo actuation.

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