Design and Function of a Turbocharger: Compressor
The turbocharger's basic functions have not fundamentally changed since the times
of Alfred Büchi. A turbocharger consists of a compressor and a turbine connected
by a common shaft. The exhaust-gas-driven turbine supplies the drive energy for
Design and function
Turbocharger compressors are generally centrifugal compressors consisting of three
essential components: compressor wheel, diffuser, and housing. With the rotational
speed of the wheel, air is drawn in axially, accelerated to high velocity and then
expelled in a radial direction.
The diffuser slows down the high-velocity air, largely without losses, so that both
pressure and temperature rise. The diffuser is formed by the compressor backplate
and a part of the volute housing, which in its turn collects the air and slows it
down further before it reaches the compressor exit.
The compressor operating behaviour is generally defined by maps showing the relationship
between pressure ratio and volume or mass flow rate. The useable section of the
map relating to centrifugal compressors is limited by the surge and choke lines
and the maximum permissible compressor speed.
The map width is limited on the left by the surge line. This is basically "stalling"
of the air flow at the compressor inlet. With too small a volume flow and too high
a pressure ratio, the flow can no longer adhere to the suction side of the blades,
with the result that the discharge process is interrupted. The air flow through
the compressor is reversed until a stable pressure ratio with positive volume flow
rate is reached, the pressure builds up again and the cycle repeats. This flow instability
continues at a fixed frequency and the resultant noise is known as "surging".
The maximum centrifugal compressor volume flow rate is normally limited by the cross-section
at the compressor inlet. When the flow at the wheel inlet reaches sonic velocity,
no further flow rate increase is possible. The choke line can be recognised by the
steeply descending speed lines at the right on the compressor map.