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Fuel / Vapor Return Lines for FI Lycomings
I'm planning to install a fuel injected Lycoming in my 7 and am beginning to think about the fuel system. I haven't closed the QB fuel tanks yet but that time is fast approaching.
I've seen some discussion around fuel /vapor return lines to the tanks. It looks like there are two options: 1) dump this stuff overboard (not really appealing at all) or 2) return it to the tanks. I know that the Egg Subie guys use a 6 port Andair selector to ensure that the fuel return goes to the tank that the fuel is coming out of to prevent over-filling the tanks. Their needs are slightly different than ours because of the fuel rails, etc., but it still seems like a good practice in any event. Is this something that folks have done for the FI Lycomings as well? The Egg installation is pretty well documented ... is it reasonable to adapt this installation to the Lycoming? If anyone has links to information on this I'd appreciate it. I need to read up on the high pressure pumps, return lines, etc. I'd also appreciate some insight from those of you who have already braved these waters. I've seen some of the previous discussion on this topic, but I'm getting stuck on the "how do I know that I even need to do this?" question. Thanks. -Rick |
The type of return system you require in the aircraft depends on the type of fuel injection system you install on your engine.
1) Bendix/Precision. No return system required. 2) Airflow Performance. A return line is required from the purge valve to one of the tank feed lines before the selector valve. A dedicated return line to the tank is not required. The purge valve feature eliminates hot start problems and aids in a clean shut down with no fuel bleed off into the engine after shut down. You can add the purge valve feature to a Precision fuel injection system if you like. There is no fuel returning once the engine is started. 3) ECI fuel injection. A dedicated return line to the tank is required with this system. To eliminate fuel management problems a duplex fuel selector valve should be used on this system so that the return fuel is being returned to the tank that is selected. 4) FADEC. Aerosance system. I could be wrong on this but I think this system requires a return system to the tanks. Couldn?t find information on their web site on the fuel system plumbing. |
Thanks Don
Alright ... I think I'm getting it. :)
I'm planning on an XP-361 with Airflow Performance FI, so I'll be going with option #2. From what I understand, it looks like I'll end up with the following:
Does this sound right? Is the purge valve a separate component or is it built into the injector body? Also, it sounds like there is only a small amount of fuel purged by the system so I don't have any fuel management issues ... hence no need for a duplex valve. I'm happy to save this $300. ;) Is there any vapor that can get purged as well? If there were some vapor in the system it seems like running a line from the purge valve back to the tanks would be optimal. Does bleeding off the pressure on shutdown via the purge valve prevent vapor locks to begin with? Do you have links to recommended fuel system layouts for the purge valve that I can access? |
not quite
I have recently fired up my motor using the Airflow Performance system.
The purge valve it situated right next to the distribution block...in other almost at the end of the FI system, right before the injector lines themselves. The reason it is there is to bleed as much vapour/hot fuel form the hoses witin the hot cowl. The purge is only used during startup/shutdoown so the quanity of fuel purged is measured in ounces. The standard Vans valve will work just fine as there is no need to choose which tank to purge to....just plumb a #4 fitting one of the tanks and connect the discharge from the purge valve to it. Frank 7A...No mechanical fuel pump, electric pump in each wingroot to avoid sucking on fuel...intended for autofuel use. |
Two return lines?
Thanks Frank.
Your configuration makes more sense to me because I thought most vapor lock issues were from heated fuel lines within the cowl. If there were any vapor in the fuel lines, it seems that you'd want to return this to the tanks so that you don't feed it back to the injectors. I was looking at Van's catalog for their high pressure pump and they talk about using a swivel tee to hook the pump to the andair fuel selector. I'm not sure what this does since any vapor will be produced forward of the pumps. Where does one find out this stuff? :confused: |
More thoughts
Need to clarify a couple of points for you.
What most folks refer to as "Vapour lock" is not vapour lock at all. VL is when the fuel pump sucks on the fuel (that has been warmed and/or is at a low enough pressure) causing it to boil at the inlet of the pump. This is relatively deadly...I.e it can kill you! This is most likely to happen whenever the fuel is hot, at low pressure (high altitude) and is being sucked upon. Autofuel is even worse because it has a higher vapour pressure which means it is more prone to boiling. Without getting too involved in this, think about the mechanical fuel pump location.....In a hot cowl. and sucking from the tank...Not good!..Thats why my fuel pumps are in the wingroots which is much cooler than the cowl and its not sucking a long way from the tanks either. Mind you with a 3/8ths line you could suck it a afair distance without causing an issue. Being an engineer that designs pumping systems for high VP liquids I wanted them in the hydraulically correct place. Anyway, lots of folks call localised boiling in the injector lines as vapour lock....This is not correct. The only reason it happens is when you shut down the fuel lines get heat soaked and because the pump is not making pressure (hot fuel won't vapourise when its under significant pressure) the fuel boils...When you got to start the engine has to be cranked to pump all the hoses free of vapour and get wet fuel in there. Hence the classic hot start...Crank crank crank... The purge valve is intended to take care of most of this. Turn on the electric boost pump (I believe the mechanical pump has an internal bypass) and open the purge valve...This will continue to pump the vapours through the lines and replace with fresh (cool and pressurised) fuel. Once you have done this for however long it takes, close the purge and this will prime the engine. In modern automotive engines there is a permanent bypass that continually returns fuel and any vapours back to the tank. So if you have a modern Soob say you need a selctor valve that both selects the feed and returns the waste fuel back to the selected tank...hence the six port Andair valve. The airplane is not setup this way. The airplane motor is really setup with no return at all but the AFP system provides the purge that can be actuated just before start to flush the system...Or indeed to provide a shutdown method. Not sure why you would want a "swivel tee"??? The boost pump (incidently the Rotary guys have been using a NAPA aftermarket pump..:)..) should be fed from the selectro valve. As Don said you can feed the purge return to the inlet of the pump so I can see how you could use it. Personally i want the purged fuel back in the tank so I added a port to do that. Its probably not a big deal due to the small quantity of fuel that gets purged. As I intend to use autofuel after break in I want to be safe and not have warmed fuel back in the inlet of the pump but thats more of a personal choice. Anyway, Hope this helps. If you buy your system from AFP and stick with the standard setup you should end up with everything you need. Frank 7a, inspection Thursday. |
AFP plumbing option
Rick,
Another option for plumbing the fuel return line from the purge valve is to 'T' into one of the fuel vent lines on either tank rather than the feed lines. I did this on my -6 and it works fine. This saved installing extra line all the way back to the fuel valve area. Rather you can 'T' into the vent line just aft of the firewall area. The amount of fuel pumped back into the tank is minimal so it doesn't throw off the fuel computer much. I also installed my gascolator (Vans) in the wing root to keep the fuel as cool as possible until it entered the engine compartment. Once in the engine compartment, the fuel line goes directly to the engine driven fuel pump. The purge valve can also be used to shut the engine down easily, which also helps in saving wear and tear on the injector body. :) Contact me if you'd like a schematic on how I installed the system on my -6. Joe Blank RV-6 IO-320-D1A N6810B jeblank 'at' molalla.net |
Thanks guys. I guess this means I need to run a control cable back to the panel to actuate the valve and this needs to be added to the startup checklist. I still don't get what Van's is talking about with their swivel tee thing, but I guess I'll see when the fuel pumps arrive.
-Rick |
swivel tee
probably to plumb the purge return into the inlet of the boost pump....many are plumbed this way but I took mine back to the tank which is a bit more plumbing work.
Frank |
Joe,
Are there any issues with fuel purging overboard when you tee into the vent line? I like the idea of saving a line run to the tank, and I much prefer this over purging back into the intake lines. I just don't want to spit gas out on the ground everytime I start the engine. -Rick |
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