Fuel Return Line

The fuel return in my Sonex with 912ULS just goes back to the fuel intake line about a foot from where the fuel line leaves the main fuel header tank. Plenty of cool fuel mixing in with this returned fuel. 

Although it has only 46 hours on this system, I am happy to report it has worked fine even in hot conditions, and I haven't had any vapor symptoms to date.

I also doubt the logic behind fuel vaporization as any vapor is free to leave via the top of the float bowls in the carbs and as long as you have good pressure, vapor isn't really possible inside the pressurized fuel lines.

If the fuel gets heat soaked upon shutdown, just turn on the boost pump until full pressure is maintained and that too will vent any vapor out the carb float bowls and starting is back to normal.

My automotive Tech teacher taught us years ago that vaporization is just an old wives' tale and is not possible if your fuel pump has good pressure and noted the other reason as I mentioned above. Lots of experience over the years has proven his theory to be correct.

Gee Murray, 

"Plenty of cool fuel mixing in with this returned fuel. "

This is just a variation on the gascolator/return line concept. No one has said it doesn't work, just that it's not best practice ie not the most effective system.

"....haven't had any vapor symptoms to date."

So?  Proves nothing, especially when you take into consideration that Goulburn is a relativly cool part of Australia.

"...doubt the logic behind fuel vaporization as any vapor is free to leave via the top of the float bowls"

When the mechanical pump is unable to deliver liquid fuel you have vapour in the line/pump. When the electric boost pump sounds like its pumping air (rattle), not liquid you have vapour in the line/pump.

If all that is being delivered to the float bowls is vapour, your carburettor won't deliver the correct fuel:air mixture to sustain smooth engine running (if it starts at all).

"...vapor isn't really possible inside the pressurized fuel lines."

The point at which a liquid turns to vapour, is dependent on temperature & pressure, so you may be partly correct IF your lines remain at a constant pressure & temperature.

When you stop your engine/turn of your boost pump the fuel pressure drops - vapour  may form.

When you start your pumps the pressure on the suction side of the pump will fall - vapour may form.

On a hot day, when you park your aircraft, engine off / hot, the rise in fuel temperature, due to radiation & convective heat from the engine - vapur likly to form.

"If the fuel gets heat soaked upon shutdown, just turn on the boost pump until full pressure is maintained and that too will vent any vapor out the carb float bowls and starting is back to normal."

Some vaporised fuel may be vented from the float bowl and this may be sufficient to get your engine to run reliably but the main scavenger of hot fuel/vapour will be a correctly fitted return line

In my (limited) experience, with a 1999,  912ULS, retrofitted with a home designed fuel return line (to tank) this is what happened;

Hot day, heat soaked engine, aircraft parked. Pilot stretches legs for say 20 minutes. Returns to aircraft. Boost pump before start, sounds "rattly",  pressure gauge erratic indication. Pump sound stops and pressure stabilised.

I suspect fuel vaporisation. I normally turn off the boost pump to start the engine however with the possibility of vaporisation, I leave it on.

Engine starts normally, but within a few seconds runs rough & stops.

Fuel pressure gauge may returned to erratic indication.

After several efforts to get sustained engine running  - all good.

Taxi to holding point and extended Run Up time - all good.

Have plan for aborted Take-0ff / Climb Out.

Line up. Full power. Engine looses power at about 20 m ground role.  Abort take off. Engine running normally at reduced throttle.

Back track. repeat  Run Up - This time everything works as expected/normal.

So what may have happened -

There was both liquid and vaporised (bubbles) fuel in the system.

Prestart, boost pump running, the return line vents vapour as does the float bowl. Float bowl fills with liquid fuel and the float needle/valve shut of the flow. Pressure rose to normal & delivered a normal engine start BUT the hot fuel, now at minimal flow, continued to heat, boil/vaporise, leading to engine failure & loss of power.

The Rotax recommended fuel return (including restrictor jet) to tank would likly have cleared the hot fuel & vapour more effectively, than the home designed one. I don't assume the Rotax design would be 100% efective, so caution is the way to go.

You have chosen to modify the Rotax concept/design of fuel return - its performance is unknown. You may never have a problem, but then again......???????

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