With all due respect let me offer these thoughts..
Compressor surge.
Premise.
All well designed turbo systems for racing vehicles have a turbo large
enough to supply the required air flow at the desired pressure.
They all have a waste gate to limit pressure.
They all have an exhaust housing and turbine trim to provide boost over a wide
rpm range.
They all have a way to reduce charge air temps to regain density loss from
compressors that are not 100% efficient.
Application
Look at a compressor map like the one at DrMayf's web site it is
typical. When your required flow and pressure put you to the left side
of the line on the left side of the map the compressor will surge. It
is called the surge line for that reason. Surge is not an over boost
condition. It is an aerodynamic one internal to the compressor. It is
not over boost as this can happen well below your target boost
number. A turbo should be sized compressor wise and turbine wise to
make it very hard to get in this area. A properly designed waste gate
is to both limit overall boost and control the curve as well as most
are electronically controlled now. I program them from the start to
stay away from this area by opening the waste gate when the rpm and
throttle position are in an area that would surge. I also program them
to open if if the boost is beyond the desired number. The real issues
can also arise when you take a known good working system and change
altitude and demand the same boost higher up. Now the pressure ratio
goes up for the same flow in lbs/min this can easily put you into
surge which is quite destructive to the turbo and possibly the engine
if the compressor wheel fails..
A BOV or 'Compressor bypass' is also fitted at times, it is to unload
the compressor and help it stay spooled up as well when the throttles
are closed. It is made to save the compressor wheel and shaft and
thrust bearings. It operates by dumping boost from the system to the
atmosphere when the throttles are closed. It is mounted before the
throttle blades and after the compressor with a reference line
attached to the engine side of the throttle and the diaphragm in the
valve. When the throttles are closed and the pressure below the blades
is less than atmospheric the BOV opens. In the surge issue the
throttle may be open so the BOV will do nothing to remedy the
situation so it will not save you from a poorly sized turbo. A turbo
has to be selected for the engine size and operating range as well as
the amount of boost. Many time multiple compressor maps will do what
you need to get done and have to be carefully looked at to see where
the surge line is relative to where you are going to use the engine. A
well designed system is one that you spend as much time as possible in
the center island of the map and as boost goes up you are working to
the up and right of it. Over speed will obviously be the other issue
if you demand too much airflow from a given compressor as well.
I added the subject in the subject line so they might be searched
later and be found if any one chooses to go and look at them later..
Time for me to go back to work..
Dave
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