Variable Turbine Geometry (VTG) OR Variable Nozzle Turbine (VNT)

The simple turbocharger though increasing the power output of IC engine, also has some disadvantages at low and high RPM of engine. Turbo-lag (producing no turbo boost at low engine RPM) and Engine choke ( Due to high RPM of compressor, highly compressed air enters cylinders which is difficult to be compressed more in compression stroke). The waste gate technology was used to avoid the engine choke at high RPM by bypassing the exhaust gases directly to catalytic converter thus removing one disadvantage. The VTG is a technology that removes both the disadvantages of a turbocharger to some extent. The VTG has a movable guide vanes, which can change the angle and speed of exhaust gases striking on the turbine.

LOW RPM

At low RPM of engine the exhaust pressure is very low to rotate the turbine or compress the inlet air in order to produce turbo boost. The guide vanes adjust in a closed position to provide nozzle like shape (increasing velocity of exhaust gases) and exhaust gases hit tangentially on blades of turbine, thus rotating the compressor with high speed at even low rpm.

HIGH RPM

The turbocharger highly compresses the air at high engine RPM. The maximum RPM of turbocharger can go up to 150,000 RPM. The highly compressed air is very difficult to be compressed by engine in compression stroke, thus can choke the engine. At high RPM the guide vanes are fully open, changing the direction of exhaust gas in radially inward direction. The exhaust loses only a little pressure on turbine thus turbine slows down.

The TATA SAFARI storme 2015 in india has VTG thus, turbo boost is obtained at 1700RPM rather than waiting for 2000RPM