[Diy_efi] RE: Throttling intake air

[Greg Hermann]

At 3:42 PM 1/16/03, Adam Wade wrote:
>--- Perry Harrington <pedward at apsoft.com> wrote:
>
>> Please state your reasoning.  If you plug up the
>> intake of a vacuum cleaner or a water pump both will
>> simply cavitate and draw less power.  I would expect
>> a compressor to cavitate and draw less power from
>> the turbine.
>
>You caught me.  Neither fluid flow not turbos are my
>field of expertise.
>
>To what degree does cavitation have an impact?
>Realistically, I would imagine there being SOME
>decrease in turbine speed when throttling the intake.
>Or does cavitation take over completely and allow
>freewheeling?

CAVITATION happens with a non-compressible fluid (such as water).

SURGE happens with a compressible fluid (such as air).

Throttling the intake to a compressor lowers the density, and moves you to
the RIGHT on a compressor map which is plotted against mass flow. MUCH
easier to see what goes on if you use a map plotted against CFM !! It's
pretty hard to get into surge by throttling the intake to a compressor--but
overspeed CAN be an issue.

You have GOT to figure out what the compressors power usage is-- it goes up
with pressure ratio, and goes up much more dramatically with mass flow (and
vice versa)--- before you can determine what the damn turbine is gonna do
!!

Turbine output depends on pressure ratio, inlet temp, and mass flow.
Turbine SIZE (nozzle, etc.) defines how much pressure ratio the turbine
will have at a given mass flow/fluid density, and then inlet temp and
turbine efficiency determine how much power the turbine can make at those
conditions.

Greg



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