Quote:
Originally Posted by Shiny
All makes sense but still curious about the pressure at carb inlet if both pumps are operating. I guess I'm asking what the pump curves are by themselves and in series. To me, if an electric pump puts out only 7 psi, it will still add to the pressure built by the mechanical pump and end up maybe too high for a carb needle and seat.
"both pumps doing some of the work" makes sense to me from a flow perspective but without flow, wouldn't the pressure be roughly additive from tank (0 or atmosphere) to carb inlet?
Mike
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If the fuel demand is low, there is a resistant to flow, the system pressure increases, and the bypass is operating on the electric pump, the electric pump is carrying the load. The first bypass from the pump is what sets the system pressure until the second pump says too do otherwise. The mechanical pump can raise the pressure if the bypass on it is set higher than the electric pump, it can lower the pressure also but it really needs a fuel return on the mechanical pump to lower the pressure or it really will struggle to control the pressure. It will do funny stuff with a dead head type pressure bypass.
At idle the electric pump already has the pressure metered for demand at the bypass pressure, the mechanical pump’s bypass really doesn’t need to do much. The mechanical pump can gain a couple lbs of pressure though with the suction pressurized. But usually not much, the relief valves are independent of each other. I think the pressure increase is becuase the mechanical pump is operating a higher efficiency with the suction charged.
Once demand goes up, and the fuel system is operating below the bypass pressure, then the two pumps split the load. Then the pumps are operating at lower pressures than they would be by themselves, and operate at a different places on their performance curves where they are more efficient, enabling them to flow more working together.
Edit. I just read Sam’s comment, and that probably explain it better than what I posted here.