Variable turbine geometry
The first turbochargers used on Audi’s TDI engines had rigid blades, but in 1995 the 81 kW (110 hp) 1.9 TDI was given adjustable blades on the exhaust side. Known as variable turbine geometry (VTG), this principle allowed torque to build up smoothly and without delay at much lower engine speeds than before. If the driver presses the gas pedal down firmly at a low engine speed, the turbine blades move to a shallower angle. This reduces the cross-section of the inlet to the turbine housing and forces the exhaust gas flow to speed up and strike the outer face of the blades. The turbine wheel rotates faster, the volume of fresh air delivered by the turbocharger increases and boost pressure builds up instantly. As the volume of exhaust gas increases, or if less boost pressure is needed, the turbine blades return to a steeper angle. This increases the inlet cross-section, so that the exhaust gas flows less rapidly and the speed of the turbine rotor is reduced. Boost pressure and turbine output remain more or less constant in this operating situation.