A few days ago we were chatting about turbos and compresso surge. I surfed
around and found this really good explanation from Borg Warner Turbo
Systems:
"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". "
Ok, this means that if you have a centrifugal compressor and you are at some
level of prformance and you release the throttle to the closed position, the
air in the duct works backs up and with no place to go, the compressor blade
cannot "grip" the air and move it forward and can in some instances reverse
the flow (harmful if carried on to long). When the compressor either slows
down or the flow is reestablished bye either the opening of the throttle
blades or opening a blow off valve then the surge condition disappears. If
the compressor is sized to big for the system and it over pressurizes the
system then surge can also occur. However the waste gate, in this case acts
to limit the performance of the compressor by keeping it at a value that
does allow over boost. It does this by releasing exhaust gas before it can
enter the turbine section of the turbo, thus keeping it at some lower level
of operating performance.
There was another interesting part of the definitions and that was the
"choke" line. This is opposite the surge in that it is on the right hand
side of the compressor map. It is the point where the shaft speeds bein to
go vertical and is cause by sonic flow at the inlet of the compressor. This
occurs when the pressure differential accross the inles is such that the
conditions for mach 1 flow are established. At that point no more flow can
cross into the inlet and the flow becomes choked.
So biggest turbo that can be driven to the performance level you want/need
is best (IMHO) because choked flow is not expected and surge flow can be
corrected via BOV and waste gate.
mayf, the red necked ignorant desert rat in PAH rump
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