Air Bleed Lesson by John Kyle
In my mind, the bigger the bleed, the more air is introduced and the more and sooner fuel is introduced. Like a 20 year old gal givin a BJ VS a 35 year old gal. The 35 YO would suck your wiener inside out.
We have to understand that there is a pressure differential that causes fuel to move up the main well and out to the booster.
I’ll see if I can explain like I understand it.
The booster is X distance above the fuel level in the bowl. And we have a fuel that weighs Y. We need to encourage (or discourage depending on what we want) the fuel to go up the main well and to the booster. I’ll use somewhat easy numbers to make this make sense. The pressure differential between atmospheric, manifold and booster pressure (negative or vacuum) is what makes the fuel start to move. So let’s say the float level (X) is .400 above the first air bleed, and we are using a fuel with the specific gravity (Y) of .750, which means that the fuel weighs 75% of an equal amount of water. And we use inches of water for testing booster draw.
When the engine is off, there is atmospheric pressure (whatever it may be at your locality) on top of the MAB, in the manifold and the booster. And the fuel in the main well is in the emulsion well. All it equilibrium.
Once the engine cracks off there is a signal at the booster. However low it may be, the booster is pulling on the main well. There is still atmospheric pressure on the bleeds and whatever manifold vacuum you have. If at idle manifold vacuum was lower than whatever the booster is seeing fuel
would flow from the booster. We never see that but that is what would happen.
To initiate fuel at the nozzle the manifold vacuum needs to drop far enough so the pressure differential across the booster can pull the fuel that .400 up from the main well to the booster. As I said, there is always a booster signal when the engine is running. Different booster designs on the same engine can have far different pressure drops across them. Which means you’d need a different MAB for each booster type. A straight leg booster won’t provide the same signal as a down leg booster which won’t have as much signal as a good annular booster.
We want to lift the fuel that .400 with a fuel that weighs 75% of an equal amount of water. It takes .4 inches of water pressure drop across the booster to start moving water up the main well. Since we don’t use water but fuel that is 75% the weight of water, we only need .3 inches of water to get the fuel moving.
The MAB has atmospheric pressure on it at all times. The air pushing down on the fuel in the main well comes through the MAB. As you close the size the MAB there is less air pushing down on the fuel in the main well, so it discourages the fuel from heading up the main well until the pressure drop is higher, meaning less manifold vacuum verses atmospheric pressure.
At higher pressure drops across the booster, there is less air pressure on the main well and the fuel curve goes rich(er) because it’s less air introduced into the main well.
A bigger MAB has more air in the main well at lower booster pressure drop, so it encourages fuel to flow to the nozzle at the same pressure drop.
Im not sure that makes sense as I typed it. Taylor in his book says that at low air flow (lower pressure drops across the across the booster) the MAB acts like an emulsion jet. So the smaller the MAB the there is less air to get the fuel moving up the main well.
As you can see, float level affects when the nozzle starts pulling fuel. The booster itself affects when fuel flow starts. The specific gravity of the fuel (weight) affects when fuel starts moving to the nozzle. And the size of the MAB affects when fuel stars to the nozzle. The height of the top emulsion jet relative to float level affects when the fuel starts moving to the booster. And they all affect each other.
I’ll say it like this. The more air in the main well, the lighter the fuel is and less air in the main well and the fuel is heavier.
A smaller air bleed has less air in the main well, so the fuel is heavier at lower pressure drop across the booster and that discourages fuel flow up the main well.
A larger air bleed has more air in the main well, so the fuel is heavier at lower pressure drop across the booster and that discourages fuel flow up the main well.
A smaller MAB delays the start of the booster. A bigger main air bleed hastens the start of the booster.
John Kyle got it wrong.