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.