Fuel system woes

Sean,

Of course a return line is always a good idea for all the reasons that you detailed, but it won’t solve this problem.  This is simply the fuel that is stored in the float bowls percolating up into the intake manifold when the engine is shut off.

Mark,

You did not mention an electric boost pump in your initial description so I assume there is not one.  So if you run the fuel out of your carbs during shut down that should solve the problem of heat soak flooding, but you still may have to crank the engine some for the mechanical pump to refill the bowls. I’m intrigued by the idea to opening the throttle after shut down. Not sure if it would help but the reasoning is sound…give it a try.  And as suggested by Michael, if you have an oil door opening that after shutdown may help.  

Hi Jeff B,

The fuel vaporisation issue in Rotax 912, is inherent in the design. As such can not be "solved" without a complete redesign, only managed.

Yours & Marks focus on one element of the fuel supply, float bowls, does not address the rest of the upstream fuel system, that is equally vulnerable to fuel heating/vaporization.

Without liquid fuel in the float bowl, the engine will not run as it should. 

Your focus should be on how to get liquid fuel into the float bowl, after the fuel system has been largely filled with vapour.

Any fuel vapour that may be in the induction system/chambers is of little consequence, as this will be cleared with but a few revolutions of the engine.

I have made my arguments, for the fuel return line + boost pump, as the primary (not exclusive)e best fuel vaporisation management tool, for Rotax 912 (carby models) currently available.

I predict that any success you have, in yours/Marks approach, will be marginal at best, most likely have nil effect.

I await with great interest, Markes follow up report, on the effect of his/your proposed system😈

Hi all

in general i have stayed out of this never-ending conversation.  The issues are real, the solutions are also already known.  

One item I saw however needs some comment, the idea that perhaps opening the throttle butterfly will reduce or at least give vapour somewhere to bleed off.  It must be understood that the Bing CV carb butterfly is not connected to the fuel at all, it does not move the slide up and down or mechanically connect the float area to the venturi of the carb.  The activation of the throttle slide is the low pressure created when actively running will lift the slide and needle from the fuel float bowl jetting.  Given this opening the throttle would have no effect on this issue.  

Fuel vapour must be allowed somewhere to go if you are in those conditions and use the type of fuel that is prone to this problem.  

In certified aircraft there are prescribed tests to verify and note the conditions where this is a problem.  The most likely is in a fuel-soak that occurs just after engine shutdown after flying.  The hot parts of the engine raise the temperatures to where the fuel boils and bingo, vapour lock.  You must remove the vapour or suffer the problem.

Cheers

I totally agree with the need for a return line, even if it doesn't go all the way back to the tank/header tank its better than none.

An interesting thing that hasn't been mentioned in all this long discussion is that the Rotax installation manual for my 912 ULS specifies a 0.35 mm (0.014") orifice jet in the return line. This is very small and calculates to a flow rate of only 0.48 gph at the typical running fuel pressure of 3.5 psi. It is quite interesting to me that this very small flow rate is enough to do the job that we have been discussing. I know that some builders install a much larger orifice (someone on this thread mentioned his flow rate at 7+ gph) which is fine I guess, but apparently unnecessary according to Rotax. 

Hi James

"(someone on this thread mentioned his flow rate at 7+ gph)"

Not sure where you got the above from - my aircrafts return line flow rate is aproximately 7L/hr (1.85Gal/hr) when pressurised by the Boost pump alone. My Boost pump delivers up to about 5 psi (pre start check).

"......specifies a 0.35 mm (0.014") orifice jet in the return line. This is very small ...."

When you consider the return lines primary function is to draw off fuel vapour, this jet size is not so small. Its secondary, important function, is to facilitate fuel circulation, so that cooler fuel can mix/replace hot & the fuel lines also cooled, minimising the tendency for fuel vaporisation to occur.😈