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.
Operating characteristics
The compressor operating behavior 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.
Surge line
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".
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Compressor map of a turbocharger for
passenger car applications |
Choke Line
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 recognized by the steeply descending speed lines
at the right on the compressor map.
