**********************************************************
>> 2. The pump relay is activated by a signal from the ignition
>> pulse indicating that the engine is either cranking or running. A
>> switch from the oil pressure sender can also be incorporated as a
>> secondary source of power to the fuel pump relay. Either one will
>> independently allow the engine to start and run. An engine
>> stoppage will cause both to turn off the pump.
>>*************************************************************
Joel,
This might be a useful safety measure in some circumstances, but if I
built in any "safety" device that might inadvertently shut down the fuel
pump in flight, I'd insure that I at least provide a manual bypass
switch
that would become part of my "engine restart" checklist. I'm still not
sure this safety measure enhances safety in the aircraft application.
Mark Waldron
Mark,
If you reread the information in (2) you will see that two
independent circuits are actuating the pump and only one of the
circuits is needed to keep the pump engaged. Starting with only
the oil pressure switch will take several revolutions of the engine
to bring the pressure up (~4psi) and activate the switch. Power
from the oil pressure switch goes directly to the pump and the pump
will work even with a relay failure.
Pump failure is rare, but I would personally have a parallel fuel
delivery system which would include a pump, relay, filter, and a
second power source. The pumps have a built in check valve so you
might not have to have a manual line shut off valve to prevent
return of the fuel thru the inoperative pump.
I'm thinking of using a primary header tank to contain both fuel
pumps. It would be kept full at all times and would have a warning
sensor to alert me to a fuel shortage. Depending on capacity, this
would give from 10 minutes to an hour to make an emergency landing
with power before running out of fuel. It might also be better for
those of you who don't watch the artificial horizon and fly up side
down..
------------------------------------------------------------------------
George
George (down under) asked about a leak down regulator by-pass. He is
concerned about maintaining pressure in the fuel system after
shutdown. I an not familiar with it so if you can give me an
application reference, maybe I can comment on it.
Pressure is maintained the fuel system after shutdown to allow a
quick start during the next start up. If pressure is relieved at
shutdown, the fuel in the top loading injectors will begin to
vaporize as the engine temperature increases and could actually
displace all of the liquid fuel in the injector. It would take a
considerable amount of engine cranking to clear the vapor from the
injectors.
There are injectors called "bottom loading" where the fuel enters at
the bottom of the injector instead of the top. Bottom loaders can
clear the vapor much faster than top loading injectors.
Joel
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