Paul,

A fellow SeaWind builder and I both plan on using 300Hp rotary engines
and we are at the fuel system design stage.  In trying to understand how
the fuel pressure regulator work, it appears that there exists the
possibility of alarmingly large amounts of return flow under some flight
conditions.

Picture high  altitude cruise and a 45psi rail pressure.  The main  pump
is capable of supplying 30 gal./hour with the engine perhaps only
requiring 15gal/hour.  As I understand it, the regulator trades volume
for pressure and reject (returns) unneeded flow to keep the fuel rail at
the user set pressure value. A fuel pressure sensor monitors this value.

In this case it appears that 15 gallons per hour  (30 minus 15) ends up
being returned from the regulator. ... And that becomes a major pain in
the butt in the design and as you have shown in earlier illustrations,
the flow must be returned from whence it came. System efficiency is not
so elegant either because we are expending energy to pump fluid in a
circle. With the booster pump on the return flow amounts can be greater,
even if the engine is under full power.

Yes I know the solution is to  PWM control the pump so it only pumps a
wee bit more than is needed at any given moment and perhaps the return
line then can then go to the three gallon sump/header tank.

To illustrate the problem to my friend I am looking a worst case scenario
amount of by pass flow in gallons/per hour.  Have any ideas?


Thanks

Doug in Japan

Correct. I think you will need the fuel sump tank in the cowl
to return the flow too fed by low pressure pumps from each wing tank.

There are at least three ways of doing that.
A. The sump tank is pressurized to a low pressure. Built strong!
B. The sump tank has a cut off float valve like a carb.
C. A float switch that shuts off the low pressure pump
  when the sump tank is full.