Fuel System Design Review

[flyvans.com]

hi don,

appreciate your input!
this was in no way meant to be a rant...

another interesting point about the return line blockage / over pressure situation.
the fuel selector is a duplex valve, so any time the return line is blocked, there is no supply either...

of course it is not intended, but how do you expect the pump to behave if it is running and the fuel selector is switched off? i imagine this being a pretty unhealthy situation? the problem lying more on the suction side than on the overpressure side (as there is sufficient tubing for the fluid/pressure to spread).

regards, bernie
www.flyvans.com

[rickmellor]

Don, thank you very much for the feedback. I'm thinking that in this case I can have my cake and eat it too (when will I learn?! )...

If I revert to the original plans for having the relief valve return to the inlet then the air bleed-back issue and the over-pressure situations will go away (yes, the Andair will block both ports in the OFF position). It looks like I can then keep my purge line running back to the tanks via the returns and still get the complete fuel system purge capability that I want.

For the returns going high into the tank ... I was planning to put them low in the tank to get a smoother diffusion of fuel when it leaves the return tube. I don't know if it happens but I've seen comments that if the return fuel spills into a partially filled tank from the top that there can be foaming. Again, not likely that I'll have this problem with the AFP setup just purging back to the tanks but it's still something to consider.

-Rick

[Don at Airflow]

I have never done this but common sense says with the duplex valve set up with the valve in the off position on initial start up of the boost pump the fuel pressure would go up to 125 PSI (the internal relief on the pump) this might pop the breaker as the pump will pull in excess of 10 amps. But until the breaker shuts the pump off the pump will make pressure until the fuel is expelled from the suction side of the pump (it can pull about 1.5 PSI negative head), then the pump will run dry and burn itself up as fuel is what cools the motor. So the amount of time it runs depends on how long it takes to get rid of the fuel on the suction side of the pump. With the throttle at idle the flow rate is less than 2 GPH, so it could take a minute or so. With the pump relief returned to the inlet side of the pump there is more fuel in the loop, so running with the selector valve off won?t cause excess pressure and there is more time the pump can be run before the fuel runs out. If there was no fuel used then the pump could run quite a long time just re-circulating the fuel back to the inlet of the pump. Eventually the pump loop will heat up to boil the fuel on the inlet. We have done this and it takes 30 minutes of running or more before the pump gets too hot. In actual use there is always some new fuel being passed through the pump as the engine is running. Most situations don?t make you idle the engine for an hour, so the pump loop heating is not an issue.


Don

[dbuds2]

That photo is enough to cause me to stop thinking of ever putting on FI.

[rv6ejguy]

Originally Posted by dbuds2

That photo is enough to cause me to stop thinking of ever putting on FI.

That's a tinker toy compared to my EFI setup with 4 pumps and a header tank!

[gpiney]

Don,

It was my understanding that your system did NOT need a return like other systems. If this is not true, could you please explain?

Thank you

[az_gila]

Originally Posted by az_gila

If the flow meter is located as the diagram shows, don't you need a "negative" flow meter to subtract the fuel quantity that is returned to the tanks?

I presume the flow meter is also a fuel quantity gauge...

Originally Posted by dan

No. The purge valve is the only mechanism returning fuel, and it's only operating for 30 sec at a time. At 3.8 gph flow, you're only talking about 0.03 gallons flowing during the purge. Even if you purge 10 times between top-offs, you're still only talking about 0.3 gallons. And on average, realistically, how many purges are you really likely to do between top-offs?

Imho it's SO NOT WORTH BOTHERING to calculate the returned fuel in with your "fuel consumed" totalizer! Considerable complexity for negligible benefit.

....

OK Dan ... that makes sense... but if we are only talking about 0.03 gallons a purge, why bother with the duplex valve?

It's a lot more cost and a lot of extra plumbing/lines.

Always start on the left tank when the tanks are full, and just return the purge line to the left tank.

Any other time, no big deal that the purge return goes to the left tank.

This alone would save a few $100 and reduce the plumbing needed.... Less complex is usually better....

gil in Tucson

[RVbySDI]

Originally Posted by az_gila

OK Dan ... that makes sense... but if we are only talking about 0.03 gallons a purge, why bother with the duplex valve?

It's a lot more cost and a lot of extra plumbing/lines.

Always start on the left tank when the tanks are full, and just return the purge line to the left tank.

Any other time, no big deal that the purge return goes to the left tank.

This alone would save a few $100 and reduce the plumbing needed.... Less complex is usually better....

gil in Tucson

As one interested in the fuel injection systems I am very interested to hear responses to this question. As dbuds2 said all of that complicated plumbing sounds a little much for my taste. If I can simplify the FI installation I am very interested. I will anxiously look to read any responses to Gil's question on this.

[flyvans.com]

Hi,

our thinking really goes this way:

avgas is gonna go sooner than later...
even if the return line might only be used for purging at this point in time (current plans are for a tmx-iof-360), constant return systems will be required to run automotive gasoline. and the various fadec's and related systems coming to market all have their individual requirements as well.
how diesel/jet-a systems will look like is not yet quite clear, but chances are that they will need some form of return line as well. in any case, expect to see changes firewall forward within the next 10 years.
also, avgas is way more expensive over here in europe, so motivation to move to alternative fuels is even bigger when the time comes...

with our setup, we're pretty much ready for whatever may come down the road. at least no hassle in removing the wings, installing a return line etc...
it's a bit of extra cost (fuel selector $$$, very little added weight), but peanuts compared to the overall tab :-( the AFP FI setup we need anyway for the fuel injected engine.)

regards, bernie
www.flyvans.com

[RVbySDI]

Bernie,
Can you give more details on what all of that weighs? Also you posted a picture of the fuselage but I am not quite clear where your fuel systems will be mounted. Can you provide some more details on where exactly all of that will be mounted in the fuselage? Is the Andair fuel selector valve going to be in the same location as the standard Van's selector valve location? I would appreciate any additional information you can provide as I am looking to do the FI and am concerned about some of the same issues with running various fuel types you mentioned above.

Thanks,