The ABCs of Carburetion

UM-900 (1959)


Page 10 of 19

To form a combustible mixture of air and gasoline, certain proportions of each are needed. But the pressure differences needed for the right flow of each are vastly different.
As an example, suppose that a 2-pound pressure difference will supply the right air flow for a given condition.
move, the less time they have to sit around and exert pressure.
Now we have just what we need: the right air flow and a reduced pressure in one area to allow gasoline to flow. To refer back to the example we used before, we would still have a 2-pound difference for the air flow, and the venturi restriction supplies the 3-pound vacu­um needed for gasoline flow.
But the gasoline is many times heavier and needs a pressure difference of 3 pounds before it will flow.
In order to get both fuel and air to flow in the right proportions, there must be some way to increase the pressure difference for the gasoline without making any more air flow.
This is accomplished with a "venturi," which may be pictured as a pipe line contain­ing a restriction with rounded corners for smooth flow.
Air flowing in the pipe will be forced to speed up through the small section to keep up with the flow in the large section (consider water going slowly through a hose and very fast through the small hole in the nozzle). The smaller the restriction, the faster the flow.
Air going through the restriction can't change in total energy, but more energy is used for speed and less for pressure. One way we might think of it is that the faster the air particles
If we wish to lower the pressure even more, we can even put a second venturi within the first; using various sizes and combinations of venturi, we can regulate the flow of gasoline and air to any desired ratio. (Exception: for idle conditions, there is very little air flow and fuel is supplied by another method to be described later).
Venturi size is determined according to en­gine requirements. Air flow is controlled by the driver as he opens and closes the throttle; the venturi is designed to lower the pressure enough to maintain the required pressure difference for fuel flow.



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