To address the issue of sluggish acceleration at low speeds in a diesel vehicle with a fixed-geometry turbocharger, a variable geometry turbocharger (VGT) can be implemented. Here's how it works and why it effectively resolves the problem:

Variable Geometry Turbocharger (VGT)

A VGT modifies the internal geometry of the turbocharger's turbine, allowing for dynamic adjustments to optimize performance across different engine speeds.

Explanation

1. Purpose of Turbocharger: A turbocharger is used to increase the power output of an engine by forcing extra air (and oxygen) into the combustion chamber to support more fuel burning.
2. Fixed-Geometry Turbochargers: These turbochargers have a fixed geometry and are typically tuned to provide maximum boost at higher engine speeds, which means they may not perform as efficiently at lower speeds, often leading to 'turbo lag'.
3. Functionality of VGT: The VGT can change the angle and area of the turbine blades based on engine operating conditions. At low engine speeds, the VGT reduces the outlet area of the turbine, increasing exhaust gas velocity and effectively creating boost pressure earlier.
4. Resolving the Sluggishness: By altering turbine geometry, the VGT provides much-needed boost pressure even at lower engine speeds, which results in better acceleration and response.

Conclusion

The correct answer is: By altering turbine geometry to create boost at low speed. This explanation highlights why a VGT is superior in providing quicker, more efficient boost, eliminating the sluggishness experienced with fixed-geometry turbochargers at low engine speeds.

Other Options Explained:

• By reducing exhaust manifold pressure at idle: While helpful in some contexts, reducing exhaust manifold pressure at idle does not address the issue of increasing boost at low speeds.
• By raising exhaust gas temperature automatically: Raising exhaust gas temperature alone does not effectively solve the problem of low-speed boost as efficiently as altering turbine geometry.
• By increasing fuel supply at low engine speeds: Increasing fuel supply could increase power output but would lead to inefficiencies and is not how VGT addresses the problem of turbo lag at low speeds.