912 ULS fuel pressure
Posted: Wed Apr 20, 2022 10:07 pm
I have a question for anyone who flies with a 912 ULS and a mechanical fuel pump. How much fuel pressure do you see in cruise flight with the mechanical fuel pump? What is the maximum that you see?
I have been having issues recently with high fuel pressure during cruise flight. I have an Evektor Sportstar and recently have seen typical fuel pressure of around 6.9 psi in cruise flight. On one recent flight the pressure reached as high as 8.2 psi for a minute or two. On the next flight it had an excursion to 7.9 psi. The Dynon EMS has the fuel pressure redline at 7.2 psi
We connected a mechanical gauge up and on the ground verified that the EMS was accurate. In this case the fuel pressure was doublechecked from zero to 5 psi which was the highest it went during our ground testing. Fuel pressure from the electric fuel pump alone with the engine not running is 2.5 psi. With the engine running the electric fuel pump has no effect on fuel pressure.
We swapped the mechanical fuel pump out with an old "AC" pump that was replaced a year and a half ago. During test running and a flight with this pump fuel pressure was steady at about 4 psi the whole time. I have been made aware that by design this pump runs at a lower pressure. I am not sure typically how much lower that is.
I have identified three suspects that might cause the problem. All three sound unlikely to me but here they are:
1. The fuel pump went bad causing higher pressure operation
It seems unlikely that a spring in the pump would get more rigid with time or the pump would gradually increase its output
2. The fuel return is overly restricted.
This seems unlikely but it is probably the next thing that I'll look at. From my understanding the spring in the fuel pump sets how high the pressure can get and the primary role of the return is to prevent vapor lock. It is not clear that the return with its restrictor orifice could have a significant effect on fuel pressure. It would be interesting to know what the pump will achieve pushing against a complete blockage.
3. The fuel pressure transducer is bad
The seems unlikely because it passed our accuracy test but maybe it is inconsistent or diverges from reality in the untested territory above 5 psi.
I have been having issues recently with high fuel pressure during cruise flight. I have an Evektor Sportstar and recently have seen typical fuel pressure of around 6.9 psi in cruise flight. On one recent flight the pressure reached as high as 8.2 psi for a minute or two. On the next flight it had an excursion to 7.9 psi. The Dynon EMS has the fuel pressure redline at 7.2 psi
We connected a mechanical gauge up and on the ground verified that the EMS was accurate. In this case the fuel pressure was doublechecked from zero to 5 psi which was the highest it went during our ground testing. Fuel pressure from the electric fuel pump alone with the engine not running is 2.5 psi. With the engine running the electric fuel pump has no effect on fuel pressure.
We swapped the mechanical fuel pump out with an old "AC" pump that was replaced a year and a half ago. During test running and a flight with this pump fuel pressure was steady at about 4 psi the whole time. I have been made aware that by design this pump runs at a lower pressure. I am not sure typically how much lower that is.
I have identified three suspects that might cause the problem. All three sound unlikely to me but here they are:
1. The fuel pump went bad causing higher pressure operation
It seems unlikely that a spring in the pump would get more rigid with time or the pump would gradually increase its output
2. The fuel return is overly restricted.
This seems unlikely but it is probably the next thing that I'll look at. From my understanding the spring in the fuel pump sets how high the pressure can get and the primary role of the return is to prevent vapor lock. It is not clear that the return with its restrictor orifice could have a significant effect on fuel pressure. It would be interesting to know what the pump will achieve pushing against a complete blockage.
3. The fuel pressure transducer is bad
The seems unlikely because it passed our accuracy test but maybe it is inconsistent or diverges from reality in the untested territory above 5 psi.