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Hello All,

While I have been reading on this forum for many years this is my first post, so I apologize if I made any mistake!

I have been flying with Rotax engines for about 20 years now (mainly 912 UL/ULS) but recently switched to an airplane equipped with a 914, so as we all know many parameters are different.

I have been reading a lot regarding the fuel pressure on the 914 and the importance of it. My understanding is that no values are given by Rotax for the fuel pressure but instead it must be checked and adjusted against airbox pressure. The normal value is 0,25 bar above airbox pressure and this is clearly explained also in the video sections of the 914 on how to adjust the fuel pressure.

During the first flight I noticed that the fuel pressure was around 0,45 bar (29 MAP @ 4800 RPM) with both pumps on. also different power settings will not make much difference. The engine is running perfectly and smoothly but being a meticulous person I decided to investigate more. After further testing on the ground I confirmed the fuel pressure readings and started to adjust the screw on the bottom of the regulator. According to the video as well as the 914 installation manual the fuel pressure should be around 1,60 bar at full power (40" InHg airbox pressure = 1,35 bar + 0,25 bar = 1,60 bar). Of course considering the tolerances you could go from 1,50 bar up to 1,70 bar but still these are values that are far away from 0,45 bar I was reading. In order to achieve these higher pressure you need to adjust the screw on the regulator at a position that it will be impossible to re-install the safety nut and the cup nut because the screw will be almost entirely in with only a couple of threads left. So I decided to replace the fuel pressure regulator to exclude any failure with it but the new one is exactly the same. Also, I verified that the fuel system has the correct needle jet (0,35) installed on the fuel pressure banjo as suggested by the Rotax manual.

After that I started to talk to different people flying the 914 and discover that nobody is actually flying with the correct fuel pressure suggested by Rotax but with much lower values similar to mine. The video as well as the manual suggest that the regulator comes pre-set by the factory but that does not seems to be the case.

Can anybody shed some light on this issue? Any input will be much appreciated as I would like to fly with the correct values for a piece of mind.

Thank you in advance!

Matteo 

  • Re: Rotax 914 Fuel Pressure

    by » 5 weeks ago


    Do you have a differential pressure sensor that automatically calculates the number that represents fuel pressure above airbox pressure?

     

    What is the reading with one pump running?

     

    What is the reading with both pumps running?

     

    How large and how long is your fuel return line?

     

     


  • Re: Rotax 914 Fuel Pressure

    by » 5 weeks ago


    Hello,

    No, I do not have a differential fuel pressure sensor since it it installed with a Dynon D100 and does not support the differential type sensor but only a regular sensor. 

    Pressure while idling with one pump is 0.40 bar and 0.55 with two pump. Also I forgot to mention that both pumps are brand new.

    Fuel pressure hose is about 18" long and the standard banjo size of 5/16". Please note that while testing with the analog fuel pressure gauge I used a shorter hose to avoid pressure loss but there was no difference.

    Thank you,

    Matteo 


  • Re: Rotax 914 Fuel Pressure

    by » 5 weeks ago


    I don't think that the bare fuel pressure reading is meaningful. I think the only meaningful fuel pressure reading is the pressure over and above airbox pressure.

     

    I would recommend rigging a system for measuring fuel pressure above airbox pressure. I believe this is covered in the videos on this website, as well as the Rotax 914 installation manual. Since the only thing that matters is fuel pressure over airbox pressure, you need to measure that directly to know whether or not you have a problem.

     

    The fact that your fuel pressure increases when you activate the second pump indicates that the pumps might be connected in parallel rather than in series, as Rotax recommends. Compare your fuel pump plumbing to that which is specified in the 914 Installation manual.

     

    I was asking about the fuel return hose, not the pressure hose. Because in the 914 the return fuel hose is high flow, in order to function properly, the return hose must be very low resistance. Ideally it would be the same size as the pressure hose.

     

    The fuel pressure regulators are indeed factory adjusted, and I have been told by Rotax experts that it is very unusual to have to mess with that adjustment.

     

     


  • Re: Rotax 914 Fuel Pressure

    by » 5 weeks ago


    Yes, you are right about the absolute fuel pressure value not being meaningful and that is why as I explained in my first post I performed the test in accordance with the video on this website. But if you set a fuel pressure at 0.25 bar above airbox pressure during the test the reading that you will have during flight it must be the same as the one while on ground and therefore at or about 1,5 bar. I confirm that the fuel pump are installed in parallel due to an older installation (at the time of installation Rotax recommended fuel pumps to be in parallel) and also to airframe restriction to upgrade to the in series installation as suggested in the latest revisions of the manual; this will not effect the fuel pressure, first at all at the time the video was made (2011) the fuel pumps were still installed in parallel and so they must work like that with the correct fuel pressure and second the fuel pumps installation was modified to avoid issues in case of a pump failure blockage and not to raise the fuel pressure.

    I was referring to the fuel "pressure" return hose being a 5/16" hose (same size as the pressure hose) so that can be excluded. Sorry for the misunderstanding.

    Yes, the fuel pressure regulator usually do not require adjustments but again I wanted to understand people here in the forum flying the 914 what fuel pressure they have since all the people I talked to with engines installed by the manufacturer do not have the fuel pressure at a value near the recommended value. Some of them are flying a brand new Magni AutoGyro with the 914 at 0.35 bar fuel pressure. That is over 1 bar below the recommended value.

    Thanks again

     


  • Re: Rotax 914 Fuel Pressure

    by » 5 weeks ago


    My fuel pressure is rock solid at 3.6 psi. It does not deviate, regardless of throttle position.

     


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