Variable area turbine nozzle:
The VATN is a whole new deal. The vane of the VATN pivots to present
varying areas to the discharge stream, changing the exhaust gas velocity
as it enters the turbine, permitting the speed of the turbine to vary.
The
merit of the VATN lies in several areas: it acts like a small A/R when
asked to do so, a large A/R when required, and it produces a smooth transition through all points
between the two extremes.
The VATN can create such a huge A/R that
turbine speed over the entire range of operation can be controlled by
varying the A/R alone. Thus the VATN becomes its own boost control, and
no wastegate is required. When no wastegate is present, all exhaust gas
energy is available to power the compressor, and "waste" becomes
a thing of the past.
Turbine performance can take on whole new
dimensions. Since turbine speed is always controlled by the VATNs, the
A/R ratio is always the largest possible for the boost pressure at that
instant. If the A/R ratio is always the largest possible for the boost pressure
at that instant. If the A/R ratio were smaller, turbine speed would
rise, creating more boost, which would raise turbine speed, which would
raise boost again. This situation will always keep exhaust gas back
pressure at its lowest for any given boost pressure. This creates the
wonderful condition of the exhaust back pressure being less than the
boost pressure.
When this "crossover" occurs, power production
takes on new dimensions. This condition is not generally feasible with
conventional turbos without the turbine's being so large that it becomes
unresponsive at low speeds.
by Corky Bell from his book,
Maximum Boost |
