Re: [alfa] re: fuel pump pressure

[bearbvd@xxxxxxxxxxxxxx (Greg Hermann)]

At 3:31 PM 10/26/03, Robert Wilkinson wrote:
>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.

Or explode either itself or some part of the system downstream of it if the
pump driver develops enough torque to develop the pressure to do this
without stalling or breaking. It is EXTREMELY bad practice to install a
positive displacement pump without a downstream pressure relief  and/or
safety valve !!

  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.

True for a positive displacement pump.

 If you over-pressurize the input, the pump will resist that too,
>drawing more current.

No, actually the pump (if electrically driven) will act like a rather
inefficient generator, and feed current back into the circuit which would
normally drive it.

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

Don't really feel like giving a graduate level seminar on pump curves,
performance, etc.

To be brief, let's just say that 'impedance' is usually a term used in
describing a characteristic of an electric circuit, not a fluid
circuit--this despite the fact that there are many parallels between the
flow of electricity and the flow of fluids !

Greg
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