REFLECTOR: Fuel system issues

[Al Gietzen]

I've heard several people talking about the remote shutoff valves being 

difficult to set to the open position with a cable attachment.  Is this 

because the torque is too high causing the cable core to bend?

 

Yes; but the purpose of the valve is emergency shutoff only, or possibly
downstream disassembly.  

 

One thing this discussion has revealed is the variety of opinions and
misconceptions about the fuel system. Probably not too surprising since we
have a number of different  combinations involved: Certified aircraft
engines and alternative engines, fixed gear sumps and RG sumps, Fuel
injection systems and carburetor systems, Avgas and Mogas. That leads to a
lot permutations.

 

One issue I'd like to clear up is the 'vapor lock' and 'bubbles' or 'foam'
in the fuel is not about "air".  It is about the fuel, or fractionates in
the fuel, flashing to vapor.  Gasoline is very volatile, it evaporates
easily, otherwise it wouldn't work. Even at room temperature, it takes
relative little negative pressure for some components to flash to vapor;
more so for auto fuel than avgas.  

 

Bubbles of vapor can form in a fuel line if it is warm, and/or under
negative pressure.  That's how vapor lock occurs on the suction side of a
pump.

 

Foam or bubbles in the line are not caused by the pump, and don't exist
downstream of the pump because of the positive pressure.  In fuel injection
systems there has been evidence that small bubbles could downstream from the
pressure regulator, where the pressure of 45-60 psig in the fuel rail
suddenly goes to zero, and the fuel is warm because of some heating by the
pump, and picking up heat in the engine environment.  These bubbles will
collapse again when cooled, as by mixing with cooler fuel, but apparently it
takes a little time for that to happen. So maybe there could be some 'foam'
returning the sump as a result, if that is how you plumbed the system, that
will attempt to make an exit out the vent line.

 

 Keep in mind that at if you're at 10,000 ft the ambient pressure is 30%
less than at sea level, and that will increase to chances of vapor forming.
Again, these things are more of a concern with mogas than avgas because
avgas is formulated to have lower vapor pressure.

 

Another issue - water in the fuel is certainly a concern. But on my RG sump,
made in accordance with the plans, the sump drain is at the lowest point,
and the fuel exit fitting is maybe an inch, inch and a half, above the
bottom. On mine that means there could be nearly a quart of water in the
sump before it would go out to the pump. I would assume that the fixed gear
sump is something similar.  Now in 3 years and 150 hour of flying; I have
never seen a single drop of water at the sump drain.  And if I check the
sump before each flight I'd say it is essentially impossible for there ever
to be a quart of water in the sump that could result in water getting to the
engine. I see no need for other water traps, or other complications to the
sump to avoid the water issue. OK, maybe that's just me.

 

FWIW,

 

Al

 

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.tvbf.org/mailman/private/reflector/attachments/20090407/c9b8e3ca/attachment.htm>