Relief Valve Across Fuel Filter?

by geoffrey klestadt in 912iS Technical Questions 6 years ago

According to my interpretation of the latest fuel system diagram, Rotax is recommending a relief valve be fitted across the fine fuel filter with a cracking pressure around 8 psi. Am I correct?

Is this to allow for a suddenly blocked or collapsed fuel filter? What is the thinking behind this recommendation? Untidy plumbing is going to be needed for me to do this.

Re: Relief Valve Across Fuel Filter?

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 8k

It actually calls for 0.8 -1.2bar, which is a fancy way of saying 15±3psi.
The installation calls for a fuel pressure gauge to be installed between the Fuel Pump and the Fine filter.
This means that if the fine filter were to become restricted enough to raise the pressure at the gauge 15psi above the normal 45psi (60psi) the Relief valve would allow the fuel to bypass the filter.

Now if the restriction was caused by a sudden inflow of enough debris-filled fuel to plug the filter the bypass would allow all the crud to bypass the filter and pass directly to the fuel injectors unfiltered.

The other, more likely scenario is where the fine filter slowly becomes restricted over time and the operator continues to ignore the fuel pressure Gauge/Alarm as the pressure rises from 45 to 50 to 55 to 60psi and the valve finally bypasses the filter.

If you were to decide to NOT install the bypass valve, the pressure before the filter would increase as the filter became restricted.
However, the fuel pressure after the filter is all that matters to the engine and is controlled by the engines fuel pressure regulator at 45psi
The fuel pumps are capable of delivering fuel to the upstream side of the fine filter with a pressure as high as 100psi.
This would produce a 55psi pressure drop across the fine filter, and may or may not collapse the filter element but the engine would still be happy as it would still be seeing 45psi of relatively clean fuel.

Note that the Fuel Pressure gauge is NOT reporting the fuel pressure at the fuel rail of the engine.
It is reporting the Fuel Pressure in the intermediate hose before the Fine fuel filter and is actually displaying the condition of the fine filter in an indirect way.
No matter which way the fuel supply is plumbed, the wise man would take action as soon as the normal fuel pressure of 45psi was noted to be approaching 50psi. Long before it ever became a real problem.

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

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