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Name: Cormac
Status: student
Grade: 9-12
Country: Ireland
Date: Summer 2009

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?

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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