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Butterfly Valve Carburetor
Name: Cormac
Status: student
Grade: 9-12
Country: Ireland
Date: Summer 2009
Question:
I have an airflow/Bernoulli carburetor question please. I understand that,
as the throttle is gradually closed on a running engine and the available area (around
the butterfly valve) becomes smaller, that the fuel/air mixture gradually accelerates
faster and faster through this ever decreasing gap. My question then is ...Would this
not result in effectively the same amount of fuel/air mixture entering the engine per
unit of time such that the engine would RPM would not actually drop? In other words in
the first case with a wide open throttle there is a certain amount of mixture passing
past the throttle but as the throttle is closed slightly I thought that this mixture
would increasingly accelerate through the restriction now being formed by the closing
valve such that the same amount of air would effectively be entering the engine as in
the wide open throttle case. Carrying on from this assumption the revs then would only
die when the valve was fully closed? I know that in reality that this does not happen
and the relationship between throttle opening and engine RPM is or appears to be,
linear (assuming engine not under undue load) but I am wondering why, what i have
described does not actually happen in reality. Is it because that whilst there is a
pressure drop on the downstream side of the throttle valve and subsequent acceleration
of the fuel/air mixture, it is only relatively small and does not impact on maintaining
mass flow. I must be discounting some aspect of fluid flow?
Replies:
The air through the closing butterfly valve may accelerate in that immediate region due
to the restriction, but *above* the valve (where fuel comes in) air velocity is less
because less can make it through the valve below it. Due to the Bernoulli principle,
the lower air velocity lowers the vacuum at that location, which pulls less fuel in,
which lowers the RPM. The so-called high and low speed circuits operate on this principle,
but the Idle circuit is separate. The Idle circuit is *below* the BF valve, so when the
valve is closed off, the idle jet continues to supply a small amount of fuel for idling.
Paul Bridges
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Update: June 2012
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