Variable turbine geometry (VTG)

One goal of a regulated turbine is to expand the usable flow rate range in practical applications while maintaining a high level of efficiency. To accomplish this, the turbine output is regulated by changing the inflow angle and inflow speed at the turbine wheel inlet. In the case of the VTG turbocharger from BorgWarner Turbo Systems this is achieved using guide vanes located in front of the turbine wheel.

When the guide vanes are in the closed position, the high circumferential components of the flow velocity and a steep enthalpy gradient lead to a high turbine output and therefore to a high charging pressure. When the guide vanes are in the fully open position, the turbine reaches its maximum flow rate and the velocity vector of the flow has a large centripetal component. The advantage of this type of output control over bypass control is that the entire exhaust mass flow is always directed through the turbine and can be converted to output. The guide vanes adjustments can be controlled by a series of different pneumatic or electrical regulators.

BorgWarner Turbo Systems currently offers various sizes of turbochargers with variable turbine geometries for diesel engines in automobiles and light commercial vehicles. The sizes range from 1.2 liters to 3.2 liters of displacement per turbocharger. This corresponds to an engine output range of 50 kW to 180 kW per turbocharger.

The demands placed on turbochargers with variable turbine geometries have steadily increased in the past several years. Just a while ago a per liter output of 35 kW was sufficient, but now the current state of the art demands 50-58 kW/l. A per liter output of up to 65 kW can be reached with improved turbocharger technology.

As a result of these higher demands, the exhaust temperatures and pressure conditions in the exhaust system increases. BorgWarner Turbo Systems currently offers turbochargers with variable turbine geometries for exhaust temperatures up to 850°C. In the future there will be turbochargers with VTGs for diesel engines with exhaust temperatures up to 900°C. Refinement of the VTG technology for use at even higher exhaust temperatures will expand the possible range of applications to include gasoline engines.

The mechanical demands placed on a VTG in a commercial vehicle are significantly higher than those placed on one in a passenger car since the rotary vanes also need to function as a highly efficient motor brake. This is necessary since future commercial vehicle engines will always have a lower displacement and the exhaust flaps used today at the end of the exhaust pipe will just not be enough anymore. Furthermore, the variable turbine geometry will be used to control exhaust gas recirculation, especially in modern commercial vehicle engines. When this is done, the pressure in front of the turbine is regulated by the VTG so that there is a sufficiently large pressure difference between the exhaust gas side and the fresh gas side after the compressor. Only then will the exhaust be drawn into the inlet duct through an exhaust gas recirculation valve.

BorgWarner Turbo Systems also offers a wide spectrum of VTG turbochargers that will meet all demands for use in commercial vehicle engines.