A Variable Geometry Turbocharger (VGT) uses a mechanism to alter the flow area of the exhaust gas entering the turbine, so the turbocharger's performance can be matched to the engine over a wide load and speed range, instead of being fixed at one design point as in a Fixed Geometry Turbocharger (FGT).

Working principle: In a FGT the turbine nozzle area is fixed, so the turbocharger is matched to the engine at one speed/load point (usually around full load). At low load the small exhaust energy makes the turbocharger spin slowly and deliver inadequate boost, giving poor air/fuel ratio, black smoke and high pumping work; at high load a fixed nozzle can cause the turbocharger to over-speed or the turbine to choke. In a VGT the turbine entry/nozzle area is varied by rotating vanes (or a moving ring/sleeve) that surround the turbine wheel. By adjusting the nozzle area the exhaust gas velocity (and hence the energy extracted) is controlled:

• At low load/speed, the nozzle vanes are closed (area reduced), increasing the exhaust gas velocity onto the turbine blades so the turbocharger speeds up earlier and delivers higher boost, improving the air/fuel ratio and reducing smoke.
• At high load/speed, the vanes are opened (area increased), limiting the gas velocity to prevent turbocharger over-speed and provide more flow.

This is controlled electronically/hydraulically based on engine speed and load to hold a target boost (charge air pressure).

How it differs from fixed geometry: the FGT has a fixed nozzle so its match is a compromise; it gives poor part-load performance, poor transient response and smoke. The VGT can maintain good boost and air/fuel ratio across the load range, giving better part-load fuel economy, lower smoke, faster response, and protection against turbocharger surging/over-speeding.

Importance for marine engines and maintaining the right air/fuel ratio: Marine engines, especially for auxiliary power and for variable-speed propulsion (and for complying with emission limits), must operate over a wide load range. The VGT keeps the scavenge/charge air pressure and the air/fuel ratio correct at all power levels. At low engine power a FGT would deliver too little air, causing excess fuel-to-air ratio, incomplete combustion, soot/smoke, high exhaust temperature and poor combustion - the VGT closes to deliver more air. At high power the FGT might over-speed; the VGT opens to limit boost and protect the turbocharger, and to limit peak cylinder pressures. By maintaining the correct air/fuel ratio the VGT ensures complete combustion, low smoke and particulate emissions, lower fuel consumption, and controlled NOx - which is important for Tier II/Tier III compliance. It also gives better load acceptance and manoeuvring response which a fixed-turbocharger engine cannot achieve.