Third fuel pump on 912 iS engine installations

by james porter in 912iS Technical Questions 6 years ago

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Re: Third fuel pump on 912 iS engine installations

by Glenn Martin » 6 years ago

Glenn Martin

3 281 794

Hi James,
I do not want to sound like a pedantic old scientist.....but I am a pedantic old scientist. I have read your info and maybe i missed it but I believe you do not know you have fuel vaporization problems. You do know that under certain conditions with certain temps and fuel you have a stumble and even an engine stoppage. Anything from there on is speculation. Sometimes making an assumption can head you down a path that may not be correct.

List the info you have that are demonstrable facts ie hot temps, Mogas, when you turn a pump off, low fuel pressure etc.

keep these separate from the assumptions and conclusions.

In soe ways the facts are more important than the reasons or the assumptions. Clearly an injected engine will not run with 4 psi fuel pressure, so seek to cure that. There are many possible reasons, concluding it is vaporization on the suction side without being able to confirm that is not best scientific practice.

I hope you find a simple solution

Good luck]

Thank you said by: james porter

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Re: Third fuel pump on 912 iS engine installations

by Paul Simmonds-Short » 6 years ago

Paul Simmonds-Short

4 25 7

Rotax do mention vapour lock a lot in their manuals, so I don't think it is unreasonable to look at the symptoms of the phenomena. Maybe James is making things 'fit' to suit this diagnosis....step away from the confirmation bias :-)

Anyhow, after that interlude of nonsense, one thing I have noticed is the use in your aircraft of 7mm hose instead of 7.5mm. I know it doesn't sound a lot but that is a difference of 5.7mm2 of area.....which translates to quite some difference in volume over the length of the pipes. Do you know why it was built with smaller hoses than those required by Rotax? They make a specific point of saying what things are good for the 912iS fuel system and what things are bad. That naughty list includes hoses with an internal diameter of less than 7.5mm. It would probably be a pain in the butt replacing them with hoses the correct size but I think it would be on my job list.

Thank you said by: james porter

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Re: Third fuel pump on 912 iS engine installations

by james porter » 6 years ago

james porter

14 78 70

I know it gets hot there, but do you fly with auto fuel on those 115 degree F days?

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Re: Third fuel pump on 912 iS engine installations

by james porter » 6 years ago

james porter

14 78 70

Thanks for the advise.
Yes I think I said that I suspected vapor lock.
The data collected from the engine computer provides a great deal of information.
The Rotax Installation Manual provides guidance for diagnosis.
Everything combined and considered- probably vapor lock.

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Re: Third fuel pump on 912 iS engine installations

by james porter » 6 years ago

james porter

14 78 70

Thanks Paul,
I am well aware of the requirement.
The manufacturer installed 7mm hose probably because they usually sell the carb. equipped engines.
I kept the 7mm hose because the fuel selector has a both position which is used.
Obviously two 7 mm hoses combined provide sufficient capacity.
Sorry, maybe that wasn't obvious in my original post.
Again, as mentioned in my original post, my fuel system flows 20 g/p/h at the fuel drain with gravity flow.
The fuel drain is the standard spring loaded type. The fuel passes through the sump course filter into the sump and then through a small ID outlet. The outlet OD is approx. 5/16" so the ID is much smaller. The fuel is then routed to the spring loaded drain valve via a 5/16" ID hose.
As you can see, the fuel flow through these restrictions is far less than the fuel available to the fuel pumps.
I only used this for testing the fuel flow because it was much easier and didn't require disconnection the fuel line at the fuel pump.
If the fuel flow through these fittings is 20 g/p/h then the fuel available to the fuel pumps is at least that and obviously more.
By the way that 20 g/p/h rate was regardless of fuel selector position. R or L or Both didn't change the flow.
The installation manual references a minimum fuel supply of around 15 G/P/H.

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Third fuel pump on 912 iS engine installations

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