Variable-geometry turbocharger

See also turbocharger with VTG

A variable - turbine geometry loader ( VTG charger ) or English Variable Nozzle Turbocharger ( VNT ) is an exhaust gas turbocharger, which works on the same principle as a wastegate turbocharger. It also has a turbine and a compressor which are connected with each other on a shaft. The difference with the conventional turbocharger is that the VGT-unit has, on the input side of variable guide vanes.

The vanes are mounted with their shafts on a support ring. On the back of the carrier ring, the shafts of the guide vanes have a guide pin, which engages in an adjusting ring. All vanes are rotated simultaneously via the adjusting ring. The adjusting is moved either by a servo motor or by a vacuum actuator.

In the largest negative pressure on the - attached to the outside of the turbo - vacuum cell vanes reach their thinnest position. At low vacuum at the pressure cell, the vanes are made steeper by a spring in the pressure cell. They reach their steepest position. The adjustment causes the turbine wheel will always be flowing perfectly through the exhaust, even if different amounts of exhaust gas becomes available. With a small amount of exhaust gas, the guide vanes are provided on flat, it is obtained for the gases, a lesser cross-section in the vanes. Thus, while the same amount of exhaust gas can flow through the vanes in the same time, the few exhaust gases must flow faster. This leads to more rapid flow and thus a greater rotation speed of the turbine wheel, thereby conveyed on the other hand, more fresh air and a higher charging pressure is reached. So this happens for example at low speeds and full load when a fast ' charge pressure build-up ' (= increase the boost pressure ) is required.

With increasing amount of exhaust gas, the vanes are made steeper. The cross section increases, the flow to the turbine wheel remains optimal and the boost pressure and the turbine power remain approximately constant.

Since most VTG charger with vacuum or be electrically controlled, he must - in contrast to wastegate turbochargers (which are controlled with its own gauge) - are constantly regulated. The steepest position of the guide vanes and thus the maximum inlet cross section is also the emergency operation of the VGT- on failure of regulation. This position usually makes a negligible loading pressure, thus preventing an uncontrolled overcharging of the motor.

Benefits of VGT-:

  • Reduce or even avoid the turbo lag
  • Adjustable over the entire speed range (about engine braking on the overrun )
  • Large motor torque in the lower and upper speed range
  • Reduction of poisonous substances in the exhaust gas over the entire speed range

Disadvantages of the VTG turbocharger:

  • Incorrect engine timing contamination of VTG adjustment with flue gas residues is possible, often at short distances or " comfortable " ride.
  • The moving parts in the hot exhaust gas stream may be susceptible to interference
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