Fuel by-pass valve

by Phill Hooker in 912iS Technical Questions 8 years ago

Hi folks.The installation manual of the Is engine shows a by-pass valve in the return line prior to fuel selector. Can some one explain why it is there and what it does? Seems like in a multi-tank system it does it matter what tank it feeds back into and what if other tank selected?. There is no explanation in the manual what it is for. What is the danger of not having it installed? Thanks Phill

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Re: Fuel by-pass valve

by Phill Hooker » 8 years ago

Phill Hooker

7 17 0

sorry, i mean't to say 'Check Valve"

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Re: Fuel by-pass valve

by Bill Hertzel » 8 years ago

Bill Hertzel

13 1.95k 7.98k

Man! You come up with some good ones! :)

Let's consider the fuel pumps first.
They are positive displacement pumps, meaning that they will attempt to pump 30 gallons per hour no matter what tries to stop them.
The fuel regulator normally returns all unused fuel back to the fuel tanks in the process of holding the pressure at ~45 psi.
Similar to a hydraulic pump, very similar, if the outflow of the pump is restricted, the pressure will continue to rise until something gives.
The fuel pumps are likely capable of producing well over 100psi in a blocked situation.
Fuel hoses are likely to burst or get blown off their fittings with the pumps now spraying fuel all over the engine compartment at 30gph. Very Bad!

Now, let's consider the fuel selector valves in a two tank system.
When tank #1 is selected the fuel return line must also select tank #1.
If the fuel does not return to the supply tank, the non-supply tank would quickly be overflowing as the engine returns fuel to the wrong tank at ~30gph (0.5 gal. per minute).
The fuel tank's vent line would be dumping fuel overboard very quickly. Again, Bad!!

Now, let's consider that the fuel selector valve is placed in the OFF position.
Engine running or not, if the fuel pumps are energized, fuel is being pumped.
In the first few seconds that the valve is closed the fuel pumps will pull fuel from the fuel filter and the fuel lines.
The vacuum in the lines will cause the fuel in the filter to boil allowing the pump to pull a few ounces of fuel from the filter and the intake side fuel lines.
A few seconds later the pump will be sucking vapors and the fuel lines might even collapse from the suction.
No real damage has been done at this point.

The problem is on the pressure side of the fuel pump.
The pump may be able to draw a couple of ounces of fuel before it encounters the vapor on the intake side.
It is only an ounce or two, but that fuel needs to go somewhere or the pressure is going to climb to failure levels.
The fuel regulator can't dump it because the fuel return valve has been closed.
The Check valve that bypasses the fuel return valve will save you by bypassing the fuel in the return line as soon as it sees more than ~15psi in the return line that is not supposed to have any pressure in it at all.

You soon realize and correct your mistake and no damage has been done.

Note:
The Check-Valve shown in the Single Tank System with the SINGE Valve Option looks like a definite mistake to me!
It isn't bypassing anything, it is just an unnecessary restriction.
Page 7 of the manual specifies less than 10psi in the Return Line at full fuel pump flow, Engine OFF.
The check valve is going to introduce a 15psi restriction from the start. OOPS! :ohmy:

73-00-00 page 7 also has a typo.
0.5 Bar is not 10.15psi
0.5 Bar = 7.25psi ... 10.15psi = 0.7Bar
So which is it!

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Phill Hooker

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Re: Fuel by-pass valve

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 251

I agree the check valve in the single tank system has a mistake; it should not be spring loaded.
You can use a simple no-bleed check valve in this system to prevent reverse flow during maintenance and in case of a breach/leak.

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Fuel by-pass valve

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