FI Fuel Return Circuit
Posted: Thu Oct 14, 2021 1:02 pm
by pschwenn
As with many/most fuel injected engines, Rotax 912i, 914 & 915T require a return circuit of excess gasoline - not delivered by the injectors. The excess fuel returns to the fuel tank. This circuit requires a backflow cutoff - a one way valve - to prevent fuel coming from the tank into the return circuit. Rotax recommends such a circuit but does not detail one or provide one.
When I look at schematics of examples of such circuits I can’t fully understand them because it’s not just a single path: source, fuel line, one-valve, fuel line, tank. There’s more going on, some kind of parallelism in lines of the circuit.
Who knows of a explanatory example? And of a particular type/capacity/… of valve to use?
[Related: I’m going to route the return thru the overflow space (small metal box at the top of the firewall) that contains the tank neck and then directly thru that neck not too far below the gas cap, on the theory that feeding it back lower in the tank risks putting it below the surface of the fuel and thus providing a back pressure. On the other hand, routing as I am means that return fuel will be running down the non-liquid gas above the fuel. Since the tank has a breather this means that highly aromatic gasoline may be constantly mixing with some air and thus either degrade (e.g. lose oxygen or additives) or increase the fire risk at the breather.]
Thank you,
Peter Schwenn
Re: FI Fuel Return Circuit
Posted: Thu Oct 14, 2021 3:40 pm
by GordonTurner
I’m running mine to a steel bulkhead fitting around the middle of the firewall (vertically and horizontally, but not intended to be exactly centered), through the one way check-valve which will be just inside of the firewall, then through stainless braided Teflon line to the top port on the back of the tank intended for the sight gauge.
I wouldn’t worry about back pressure. The fuel pump will never know the difference.
Re: FI Fuel Return Circuit
Posted: Fri Oct 15, 2021 8:37 am
by Kai
Peter,
I know nothing about the efi system Rotax supplies with their engines, but I have seen their diagrams. However, I have been flying my Sonex behind aftermarket efi systems since 2011- and my present 120hp EP915ECI is no different: they all require a fuel return system to the tank.
Like you’re planning, I have my return system dumping the fuel in the tank neck coming out in the fuel spill tray. I arranged this with an apprx 4” long brass pipe routing the fuel out of the bottom of the neck, and spilling it into the fuel. The neck is the highest point in the whole system, and in the AN6 return line between the fuel pressure regulator and the neck, there is nothing- and certainly no non return valve.
So far this has worked out according to expectations: no snags whatsoever.
However, you should be aware of another point: the return fuel gets heated. Warm enough, and especially running on bioethanol mogas, the stuff starts producing vapour. These vapour bubbles must be removed from the fuel before it gets back into the pump, and dumping it into a larger, colder quantity certainly helps, going into your main tank, where your fuel supply gradually gets hotter. You can under no circumstance allow the temperature to get up to 40 centigrades, so get back on the ground with at least 15l in the mains.
Re: FI Fuel Return Circuit
Posted: Fri Oct 15, 2021 9:01 am
by GordonTurner
That’s a very interesting observation Kai. Do you have a fuel temperature probe? It seems like it should be easy enough to install one.
Re: FI Fuel Return Circuit
Posted: Fri Oct 15, 2021 9:04 am
by GordonTurner
This duscussion raises another question in my pea brain. If the return flow is flowing through vapor at the top of the tank or cascading down the plastic interior of the tank is there any potential being built up that could under the wrong conditions cause a static discharge spark?
Re: FI Fuel Return Circuit
Posted: Fri Oct 15, 2021 11:40 am
by Kai
GordonTurner wrote:That’s a very interesting observation Kai. Do you have a fuel temperature probe? It seems like it should be easy enough to install one.
No- but I had one!
During the testing phase of the system we experimented with as little fuel as possible in the main tank- say something like one gallon: barely enough to avoid pump cavitation. We found that by just running the fuel through the system against the pressure regulator with the engine at standstill, the fuel temperarure increased from 6 centigrades to 23 centigrades in 20 minutes. With additional heat from a running engine this would of course be worse.
Now, with the system up and running, when flying I occasionally put my hand against the bottom of the tank: believe me, it’s warm- though not as warm as 40 centigrades. So, as stated previously, my personal limit for being back on the ground is 15l fuel remaining in the tank- I need it’s cooling!
Finally, bear in mind that your tank is atmospheric- unpressurized! The fuel in there is producing vapour just on its own, splashing around in there during flight. The effective way to avoid fuel gas in the tank would be to pressurize it- just like it’s done in your car. As far as I can understand, that is a possibility we do not have!
The small amount of additional fuel gas vapour coming out of the return line orifice would in my opinion have little bearing on the overall vapour situation in the tank: since 2011 and more than 300 hrs behind efi engines, I don’t seem to recall any tank explosions……..
Re: FI Fuel Return Circuit
Posted: Fri Oct 15, 2021 1:24 pm
by pschwenn
I might add a simple cooling means to the return line, such as an externally finned section, careful routing away from hot things, maybe via a cool inlet - whatever presents itself.
It occurs to me - warm fuel, like warm intake air, may not be best for power. But its a tiny effect with fuel compared to intake air, so safety seems to be the important thing.
Re: FI Fuel Return Circuit
Posted: Fri Oct 15, 2021 7:02 pm
by Skippydiesel
Kai wrote:Peter,
I know nothing about the efi system Rotax supplies with their engines, but I have seen their diagrams. However, I have been flying my Sonex behind aftermarket efi systems since 2011- and my present 120hp EP915ECI is no different: they all require a fuel return system to the tank.
Like you’re planning, I have my return system dumping the fuel in the tank neck coming out in the fuel spill tray. I arranged this with an apprx 4” long brass pipe routing the fuel out of the bottom of the neck, and spilling it into the fuel. The neck is the highest point in the whole system, and in the AN6 return line between the fuel pressure regulator and the neck, there is nothing- and certainly no non return valve.
So far this has worked out according to expectations: no snags whatsoever.
However, you should be aware of another point: the return fuel gets heated. Warm enough, and especially running on bioethanol mogas, the stuff starts producing vapour. These vapour bubbles must be removed from the fuel before it gets back into the pump, and dumping it into a larger, colder quantity certainly helps, going into your main tank, where your fuel supply gradually gets hotter. You can under no circumstance allow the temperature to get up to 40 centigrades, so get back on the ground with at least 15l in the mains.
Hi Kai,
As a long time diesel freak, I know that high pressure common rail fuel injection systems do heat the fuel (pressure = heat) much more so than the old injector systems. Most if not all such automotive applications have a cooling system on their return fuel line. It usually consists of a convoluted serpentine pipe surrounded by fins. I am not familiar with common rail petrol systems but would assume that the same principals apply. You could probably source a fuel cooler from the local car wreckers, failing that an aftermarket automatic transmission oil cooler wold probably do the job for you.
Re: FI Fuel Return Circuit
Posted: Sat Oct 16, 2021 12:29 am
by Kai
Well, yes,
But I was reluctant to suggest this- there is this deltaT thing!
In moderately warm summers a fuel cooler could perhaps be useful, but if you have something really hot like the more than 100F’s experienced a lot of places, the fuel cooler could turn into a fuel heater.
You would have to run a lot of tests before you have enough data to thrust its reliability and usefulness.
Re: FI Fuel Return Circuit
Posted: Sat Oct 16, 2021 2:53 am
by Murray Parr
I have Rotax 912ULS with Rotec TBI and is primarily gravity fed with a boost pump for back up. Gravity fuel flow tests are fine even when almost empty so the boost pump might not have been necessary. There is a check valve just before a Tee which goes through the firewall and straight down the the gascolator (all downhill to there then all uphill to TBI. I don’t have a lot of gravity head pressure so the boost pump branches off the gravity line then back to the Tee that goes through the firewall. The check valve mentioned earlier prevents boosted pressure flowing directly back to the tank. When fuel pressure is boosted, excess fuel passes through the TBI and through a restricted orifice and back to the gravity feed line through a check valve then to a Tee from there it can circulate back to the boost pump and/or return to the tank. Might not be perfect at removing air bubbles from lines as some bubles can circulate back to the pump.
As can be seen in the PDF attachment, the lower downhill line with the check valve is the gravity system and the boost pump loop can be seen above that. The return line from the TBI can be seen passing through the firewall, it has the red coloured elbow then its check valve. you can just make out where it TEE’s back into the system just after the fuel flow meter and just before the TEE that feeds either gravity or boost pump.
Re: FI Fuel Return Circuit
Posted: Sun Oct 17, 2021 6:50 am
by Skippydiesel
Kai wrote:Well, yes,
But I was reluctant to suggest this- there is this deltaT thing!
In moderately warm summers a fuel cooler could perhaps be useful, but if you have something really hot like the more than 100F’s experienced a lot of places, the fuel cooler could turn into a fuel heater.
You would have to run a lot of tests before you have enough data to thrust its reliability and usefulness.
Naa - not going to happen. If your fuel has been heated, due to high pressurisation, it will likely be above ambient - can only be cooled down. You will not get the fuel temperature to rise above ambiently using a cooling device and lets face it if you are dedicated/crazy enough to be flying in 40C or above temperatures you will already have warm/hot fuel in your tank.
In the Australian summer (up to 47C in the shad last summer), I plan all my long flights to commence near first light (cool of the dawn). Stop for a wee break about 2 hrs later - late morning. May fly again, if cool enough but will not take off above 35C - wait until mid afternoon and fly for another 2-3 hrs.
Re: FI Fuel Return Circuit
Posted: Thu Oct 20, 2022 10:46 pm
by lakespookie
Kai,
Do you have any pics of your installation i am curious as i plan on flying behind an EFI engine and have not really considered how i will solve the return line problem.
Re: FI Fuel Return Circuit
Posted: Fri Oct 21, 2022 3:02 am
by Kai
Hm…
You need to be cautious here: all efi systems are not created equal!
Mine is an Edge system- as long as the return fuel is dumped back into the tank above fuel level, there is no need for any oneway valve in the return line. How this is with other systems, I have no idea.
With the Edge system, fuel is returned from the pressure regulator in a ø3/8in (AN6) line to the top of the tank filler neck- there is perhaps a liquid column of 12in rise in there. All lines in the comlplete efi system are fire sleeved.
I go into the tank filler neck from the underside of the protective box. The connection is made by drilling a 3/8 hole through the top of the filler neck, through the box, and attaches to the fuel line. A pipe is angled 90 degs down into the tank (fuel), and has a collar/gasket silver brazed onto it just before the bend to prevent fuel leaks out of the tank. The line fitting I had to make myself, as I could not find a proper bulkhead fitting to my liking.
Works like a charm, perhaps a little crude and on the heavy side- but it has been sitting here since 2011.
Re: FI Fuel Return Circuit
Posted: Fri Oct 21, 2022 12:48 pm
by Kai
Skippydiesel wrote:Naa - not going to happen. If your fuel has been heated, due to high pressurisation, it will likely be above ambient - can only be cooled down. You will not get the fuel temperature to rise above ambiently using a cooling device and lets face it if you are dedicated/crazy enough to be flying in 40C or above temperatures you will already have warm/hot fuel in your tank.
In the Australian summer (up to 47C in the shad last summer), I plan all my long flights to commence near first light (cool of the dawn). Stop for a wee break about 2 hrs later - late morning. May fly again, if cool enough but will not take off above 35C - wait until mid afternoon and fly for another 2-3 hrs.
Yes- good point!
I am glad I don´t have to try out flying in temperatures like that myself. A couple of years back down south in Wagga Wagga I experienced 49C- the soles of my shoes were sticking to the asphalt!
Hot fuel in an efi system becomes a problem on the pump suction size. Any significant pressure drop here and fuel vapour will form in the line- rail fuel pressure will drop. The ecu will react as it always does when it detects an anomaly: more fuel! A lot of it! A typical pump will circulate something like 160 l/hr, so make sure the feed lines are of sufficient diameter, and have only long radius elbows.