Fuel return line.
 

All the production planes I have flown with FI have a return line to the fuel tanks.

My F1 doesn't and has had no problems.

What exactly are the benefits of the return line and what are the dangers/problems associated with not having one?

Mornin' Milt
 

As I recall, most FI systems provide way more fuel flow than is needed, so it's returned. On my Cessna 310, it was returned to the tips so you had to be sure and run enough fuel out of them or the returned fuel would go overboard if they fill up while on the mains.

The IO-520's we ran in the Agtrucks also returned and they had a hot-start problem. You could lean the mixture, run the fuel pump, returning the heat-soaked warm/hot fuel in exchange for cooler fuel from the tanks to ease hot-starting woes.

BTW, are you about ready to fly the Rocket again?

Not sure how your non-return system is plumbed.

Regards

Probably have 4-5 months to go but I am at the point of running the plumbing to the tanks before I close the wings. Thus my question. My concern is my engine failure was due to vapor lock or more correctly fuel vaporizing from heat and a sudden reduction in fuel pressure during throttle back.

Currently in my F1 and in the Radial the fuel system is an AFP electric pump system at the fuel tank sump to the engine driven pump then into the AFP fuel controller.

I am wondering if I could have avoided the problem by teeing the fuel line at the controller and running the fuel past the engine instead of just into it.

My 414 Cessna was plumbed like your 310 and sop was boost pumps on "lo" all the time.


The F1 has the standard Multiport FI. On the Radial I just replaced the pressure carb with the AFP single point fuel controller.

The M14 community with pressure carbs is split as to need for a return line some have it some don't. No problems have been reported by either group. Unfortunately there are only a few of us running the AFP system and I do not know what they are doing. Mostly in competition aerobats.

Anyway once I am back in the air the Louisville Huddle House is high on my early trip list.

There is good stuff in the archives about this.

Don R. at Airflow Performance sums it up well here.

The thread from which the above came is here.

The answer to the original question in this thread is, it depends on which system.

Thanks Alex,

I have spent almost an entire day reading those threads and others and they really did not answer my question. Most of the discussion is on routing, placement of components of the system not muh substantive on purpose of the system.

My F1 Rocket has an AFP, no purge valve, no return line so doesn't fit Don's categorizations. The Radial is a different animal. I talked to Don about it on the first install and he was ambivalent about the need.

Perhaps I worded my initial query inncorrectly.

What is the purpose/theory behind return lines? Are these theoretical "nice to haves", convenience items, or absolutely essential to life requirements?

Or, if you will,wif they are nice to haves, what is the benefit derived from their presence? If they are absolutely must haves, why?

My understanding for the return line is to maintain a constant pressure to the engine and to help reduce the chance of vapor lock when idling down. Our return line comes off the pressure monitor and returns back to the tank it comes from through a six-port fuel selector valve.

FWIW, I know zero about FI systems for aircraft...but I deal with them alot on marine engines so the principals should be similar. That said, the choice of a bypassing system or non-bypassing system is usualy determined by the fuel pump design. Some are designed to be run to a dead head, however most require the use of a bypassing regulator so that the excess fuel is constantaly circulating through the system. It really doesnt matter what motor the system is feeding...the injector servo or rail really doest care as long as it has the proper PSI and flow volume. In theory, the advantage of a bypassing system is that by placing the bypassing regulator as close to the motor as possable, the fuel has spend the least amount of time in the hot area of the engine bay absorbing heat. Also, fuel is always flowing through the fuel pump at a high volume so its is not sitting stagnet in the pump, being heated up, when fuel demand is low. In our marine systems, our pumps typicaly can pump twice the necessary volume at WOT, and at idle the majority of the fuel is being bypassed.

Fuel return line
 

The benefits of a preventive method are hard to quantify.
We do know that a great many engine failures cannot be explained for lack of evidence.Vapor lock is exactly one of the failures that leaves no evidence
behind.
You may fly without a return line for 98% of your flying without a problem but one day you will be idling when temps are 100F while waiting for take off and cook the almost stagnant fuel in the overheated engine compartement.The ensuing take off will be the most likely point and time for the engine to spudder. Just when you need it most!

A constant flow fuel return line ensures that a constant flow of cool fuel is introduced to the fuel system and thus prevent the fuel from boiling.The fuel return line is more a function of the fuel system rather than a component of a certain kind of fuel injection system.
My 8 has an AFP injection system and since it already has a return line for the purge valve it was very easy to plumb a constant flow return line into this line. I teed off the engine driven fuel pump with a #70 orifice which returns 9GPH .(as tested a@25psi) The outflow from the Fuel pump is plumbed back behind the fire wall where fuel flow and pressure is measured just before entering the fuel meter.
I believe vapor lock can be prevented

OK it all makes sense. I really think vapor lock or boiling fuel is what knocked me out of the air last year and I really do not want to go through that again..

A couple of Murphy Moose operators have teed off the engine driven fuel pump thru a number 30 drilled orifice into an inverted metal gascolator the back to the tanks. Does this sound like a reasonable thing to do? (bearing in mind it is a pressure carb or single point FI not multiport FI)

Fuel return with FI
 

Milt,

I think they used a .030 orifice to run the return fuel back to the tank not #30 (which is .128 dia.). With a .030 orifice teed into the outlet of the engine driven fuel pump, this should circulate around 6-8 GPH at 25 PSI fuel pressure and 8.5-10 GPH at 50 PSI through the pump in addition to what the engine is using. The additional 6 GPH should take a fair amount of heat out of the pump, and should help keep the engine driven pump from vapor locking if the boost pump is not on and the engine is at low power. There should be no problem with pump capacity at high RPM?s but you will have to make sure the engine driven pump can keep up the pressure at idle, since you will be siphoning off an additional 6 GPH of the pump capacity at idle. Obviously you will now have some fuel management issues to deal with if you are returning the fuel to one tank and you have multiple tanks in the aircraft. This should not be a problem if you know what?s going on. Email me off line at our address if you have more questions on this subject.

Don