[alfa] re: fuel pump pressure

[Robert Wilkinson <wilk@xxxxxxxxxxxxxxxxx>]

Previous post:
Most fuel pumps are piston, or rotary gear/vane type.  If these have 2 psi
at the intake, and are designed to deliver 2psi at the out let, they
deliver 2 psi at the outlet. 
Kevin, Orem Utah

Previous response:
Absolutely incorrect, Kevin. Any pump, regardless of whether centrifugal or
positive displacement, will ADD its pressure to the inlet pressure. 
Greg Hermann

So-called positive displacement pumps (as on my Spica car) are of a class
known as "high impedence;" centrifugal fans, etc are "low impedence."  An
ideal high impedence pump generates whatever pressure is needed (up to what
it's capable of) to achieve a certain design rate of flow (constant
displacement).  Thus it often must have bypass (oil pump) or return (fuel
pump) circuitry or it will fry itself trying to pump into a dead end.  If
you pressurize the input, the pump still pumps its rated flow (volume per
unit time), and will still generate exactly the pressure needed to do that.
 Pressure at the input causes the pump to be "loaded" less--if you apply
precisely the pressure needed to achieve the rated flow the pump will see
no load at all, but will still spin at about the same rate, drawing less
current.  If you over-pressurize the input, the pump will resist that too,
drawing more current.  Real world pumps have finite impedence (in between
ideal low and ideal high).  But pumps like we're talking about are
sufficiently ideal to not significantly increase pressure if there is
another pump in series upstream.  The pressure is determined by the
circuit--resistance to flow in the path from pump to the engine's FI--which
I believe is calibrated in Spica engines by a restriction near the Spica
pump.  Some pumps have built in bypass, which I'm not considering.

Bob Wilkinson
72 Spider
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