Fuel return line

by in 912 / 914 Technical Questions 13 years ago

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Re: Fuel return line

by Isaac F » 13 years ago

Isaac F

14 19 11

Thanks Isaac

Most of are Rotax installations are in homebuilt aircraft and do not use the Rotax fuel distribution block. The system is made up using fuel hose and T-pieces. I would say that 70% do not have the return line and many have run for hundreds of hours without problems.

David, guess what? I also have seen a lot of instalation with hoses and T-Conectors and most of them does no have a Fuel Return Line and are flying without problems..........The same goes with Electric Fuel Pumps..........Rotax recomend to intall one but many people does not intall one and they are flying without problems.......... The only thing I can say is that Rotax know more than us, and they recomend these for some reason.

In my case, my 912 Rotax come factory with the fuel distribution block and I follow the Rotax manual.....I have a Fuel Return Line and a Aux Fuel Pump.

[quote]Can you please explain why you need to relieve some fuel pressure? Do Rotax install a mechanical pump that over pressurises their fuel system?[/quote]

I am not an expert but for example, if you are using a Electric fuel pump + Mechanical Pump, the fuel presure will go up and this fuel presure can override the float valve in the carb bowl and can make your engine to stop . The fuel return line relieve some of this presure.

In my Rans S-12XL, I can see were this fuel return line enter my fuel tank because only in that part I put like 2 inches of clear/transparent fuel tubing so I can see the fuel flow and during regular operation only with the mechanical pump, I can see a small flow of fuel returning........When I turn on the AUX electric fuel pump, I can see the flow is biguer.

[quote]How does the orifice prevent vapour lock? If there is vapour between the pump and the carb, it will vent into the space above the fuel in the carb bowl, which is in turn vented to the atmosphere. If the vapour is before the pump (which is most likely as this fuel could be at a lower pressure due to the pump sucking the fuel) the orifice does not enter into the equation[/quote]

In the Rotax Manual they dont say this will prevent vapor lock but this was explained to me by a Rotax Mechanic.

Isaac

Thank you said by: Michael Saunders

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Re: Fuel return line

by Robert Lavigne » 12 years ago

Robert Lavigne

0 4 0

Thanks Isaac

How does the orifice prevent vapour lock? If there is vapour between the pump and the carb, it will vent into the space above the fuel in the carb bowl, Dave

Hi .
Like most of 912 owners, i have( had ) a lot of vapor lock issues, specially from hot starts.
The vapor will not vent into the space in the carb. bowl, because the float( Jet neddle valve ) will close the arrival of gaz as it will be up du to full gaz in the bowl.
So with the heat, there will be a significant increase of pressure in the fuel hose from the pump.
Having a ''T'' in this line with a small restrictor ( Screw an idle jet in the Brass T jonction )
and just return the line on top of the gascolator or on the fuel line before the pump.
Most of installation do not have a way to return to the fuel tank.

You will then cut off most of the vapor lock problems.

Robert

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Re: Fuel return line

by itemprop='name' » 12 years ago

0 0 0

Sorry to be a complete pain in the a** with this, but if your float bowls are full of fuel, your engine should run? When the fuel level drops in the bowl, the valve will open and allow more fuel (and the vapour) into the bowl. The vapour should then vent out of the float bowl vent. On the carb side of the pump, the system is under pressure, and a bubble of vapour is not going to hold back .35bar of pump pressure pushing the fuel towards the carb. I am not saying there is anything wrong with the return line, but still have not had anyone explain how a vapour bubble between the pump and the carb will stop the engine running.
Dave

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Re: Fuel return line

by bill martin » 12 years ago

bill martin

10 757 0

Dave,

Vapor lock usually doesn't occur while the engine is running. Most of the time the vapor lock occurs when the hot engine is stopped and fuel in the lines close to the engine heats up and vaporizes. When you try to start the hot engine, the pump is trying to pump the vapor rather than fuel and with no return line, the vapor has no where to go. The engine will try to start and the fuel level in the bowl will begin to drop. The problem is that now the pump is pumping vapor into the carb bowl instead of fuel. eventually fuel will reach the bowl and bring the level up to where it should be, but starting is difficult until this happens. A return line gives this vapor somewhere to go when the engine is not running. It also allows you to pump the vapor out of the lines with an electric pump before you try to start the engine. With insulated fuel lines and a well vented cowling, vapor lock is not a problem, even without a return line. A return line is kind of like air conditioning. You don't really need it, but it sure makes life easier.

Bill.

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Re: Fuel return line

by jay white » 12 years ago

jay white

36 97 23

I have recently replaced the entire fuel system in my Kitfox Mod IV. The original configuration did not have a fuel return line. At 250 hours of trouble free operation, I had an engine failure that I could not easily identify. I decided the fuel system which had been installed 10 years before my purchase, needed redesign. Upon completion of the install, a fuel venting problem occurred and the carburators were spewing fuel from the vent tubes during low idle. After hours of troubleshooting, I am taking the advice of Kitfox Factory and Rotax Qualified Repair facility "ROTECH" and installing the return line. The placement of the small orofice in the return line is not optional. You can call Rotech and ask them which Jet they use for that addition. I bet there are a dozen guys on the forum who know the answer as well.

Lastly, I am also replaceing the 12 year old rubber vibration isolator system on my plane. Their capacity to minimize vibration is, by the book, obsolete and may be a major cause of my venting problem. Rubber mounts are supposed to be replaced every 5 years. The bottom line is, Rotax reccomends that fuel return line for a reason.

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Fuel return line

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