Rotax 914 Fuel Pressure

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?

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 

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.

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

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