Fuel System Design Review
 

Hey guys ... I've finally got the wings attached to the fuselage so it's time to start routing all the fuel lines. Before I commit to my planned system I figured I'd post it up here to see if anyone notices something blatantly wrong. I believe the configuration is a pretty standard Vans/Airflow Performance fuel injected setup with just a couple minor tweaks. The details are:

RV-7A, Superior IO-360
Andair duplex fuel selector
Van's standard FI kit (AFP parts)
Dynon fuel flow meter (floscan)
Dynon fuel pressure sender
AFP FI engine components

I'm thinking it'll be good divert from Van's plan for a tee fitting for the pressure relief valve returning to the fuel supply line, and from AFP's callout for the purge line to return to the tank vent line (or supply line) and just run them both all the way back to the currently selected tank. This should give me the ability to completely circulate fuel through the system with no risk of returning vapors to the supply lines.

What do you guys think?

-Rick

Imho, put the flow meter in the metered fuel line. That is, inline in the hose between your fuel servo and the flow divider (or purge valve in your case).

There are numerous advantages to having it there...not the least of which is that you will be less prone to vapor lock. The fewer things between the fuel supply and the engine driven fuel pump, the less opportunity for pressure drop on the suction side of the engine driven fuel pump.

Just nit picking the diagram, but the fuel pressure sensor is not inline with the flow, it's kind of an "offshoot." Again, just being nit pickety. Great diagram!

Good points!
 

Dan, thanks for the feedback! Did you end up moving your floscan up to engine compartment? I was planning to copy your installation location near the fuel pumps, but I was going to see if I could find a way to avoid any blisters on the cover.

With the floscan up front, I suppose I'd want to mount the floscan on the firewall and run hoses to/from it as appropriate. It could be a bit of a pain getting the 6" straight hose length prior to entering the meter. I'll think on it a bit more ... just seems a bit more complicated.

Btw, were you at Hayward Exec the weekend before last? There was a 7 blasting out of there that looked just like yours.

-Rick

Fuel flow to tank...
 

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...

gil in Tucson

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.

Rick,

I didn't move my fuel flow transducer, because it's "working fine" for me where I have it. I have the K factor dialed in and it's accurate to well within half a gallon on 30+ gal top-offs. Good enough for me.

As far as vapor lock potential is concerned, it's something I'm aware of given where it's installed, but I haven't had any issues. I'm not trying to endorse the way I did it...just saying I'm comfortable with it there...knowing full well that it would be BETTER off installed in the metered fuel line. My next airplane will of course have fuel injection as well, and that airplane will definitely have the fuel flow transducer installed inline with the metered fuel hose.

I'm basically saying "do as I say, not as I do." :rolleyes: I would have done it differently with the knowledge that I have today (I didn't know diddly when I built my plane).

I'm wondering how folks install the FF transducer in the metered line. The mounting instructions from FloScan are sparse but the one thing they DO mention is mounting it with the wires pointing up. The reason is to allow any air in the system to escape the transducer. If the transducer gets air in it, the readings are affected dramatically because the FF transducer is an optical device.

I can't imagine how one could mount the floscan transducer in the metered line and have the wires pointing up. I would love to see some pictures.

I did.
 

Mounted between the rear baffle and the distributor. I have only test ran the engine, but it is working. This is where JPI recommended and an AP. Lots of cooling air in the plenum too.

Fuel system schematic comment
 

Since you are running dedicated return lines to each tank and the relief valve from the boost pump is plumbed to these returns through the duplex selector valve, make sure the returns in the tank are BELOW THE LEVEL OF FUEL in the tanks when that tank is selected. The reason: the relief valve in the boost pump has a #76 bleed hole in it to dump the fuel pressure when the pump is shut off. If the return line is exposed to air when the boost pump is not running there is a possibility of sucking air into the feed line. This is not a good thing.

That?s why we keep it simple and just return the relief valve flow to the inlet side of the pump. No way to get air in the system as long as there is fuel in the feed line.

Don

Thanks for this important info
 

Thanks Don for this important piece of information!
Too bad that wasn't documented anywhere...

in this case, we will have to replumb our setup. grrml :(

as it feeds the pressure relief valve directly into the return line as in Rick's setup... And the return lines are put in high in the tanks, as probably with most return line setups...

Also, we plan on installing a check valve in the return line where it penetrates back through the firewall. That way there is no chance for any reverse flow from the return line.

regards, bernie

P.S. Any other comments/constructive criticism regarding our setup? Anyone?

The mount (with the intended position for the return check valve (not yet installed):

Fuel System Comment
 

One more thing I just thought of is if you have the selector in the off position and turn the boost pump on you will put 125 PSI on the fuel system, another not good situation. This is because your relief valve is dead headed by the selector valve. I assume that in the off position the return ports are also shut off.

Its hard to anticipate everything you guys can do here. Our manual is pretty complete with approved methods on how to install our system that we know works, but that doesn't mean that that's the only way to do it. It's best to contact us for customer support before straying from the manual or getting "expert opinions". This can save you a lot of extra work and $.


Don