Fuel Flow Test - Pitch Angle

[robcaldwell]

Here's the remaining data for fuel flow taken from the locations discussed.

1. Fuel flow from the servo inlet, boost pump ON
2. Fuel flow from the servo inlet, boost pump OFF (Gravity)
3. Fuel flow from the engine driven fuel pump, boost pump off (Gravity)

schu I did see 32.8 GPH at the engine driven fuel pump inlet on gravity. So I exceeded your suggested 31 GPH by 1.8.

The results are not looking very favorable with neither of the two pumps online at the servo inlet. But I think I can lose either of the two separately and still be in good shape.

Will test further with engine running and boost pump off at higher RPM's. Will be looking for 25+ PSI there.

Thanks to all for the the input which pushed me to dive deeper into my testing.


Screen Shot 2021-03-13 at 2.07.10 PM.png

[schu]

Rob, the engine mechanical pump isn't designed to have unpressurized fuel flow through it, so it's a massive restriction without the boost pump running. It doesn't matter though because you must have fuel pressure to push your fuel through the injector nozzles anyway.

In other words, no injection engine will run without at least one fuel pump running, so all you really care about is that mechanical only pump operation isn't sucking (according this chart, it's not), and that you have 25PSI at 125% fuel flow with either pump. The last one is hard to test because ideally you would want to restrict the fuel flow to 125% then measure the pressure, but given that you are hitting 200% fuel flow, I suspect you will have enough pressure to make the injectors work.

[svyolo]

I wonder if the boost pump bypass is also much of a restriction. Hopefully it was designed to be free flowing with the BP off.

[robcaldwell]

schu I appreciate your input and collaboration on this. Very helpful. My final check will be with the engine running and boost pump off in both low and high power settings.

svyolo The boost pump is the free flowing type from Airflow Performance. https://airflowperformance.com/index...pump-assembly/

[zkelley2]

I think this video could be of some use. Tempest actually has pressure vs flow charts for their pumps. And they also show the inner workings answering some other questions asked.

https://www.youtube.com/watch?v=fxT4RAEssnw

[robcaldwell]

Great video.

"Not really an engine driven pump and more of a spring driven pump".

Also clears up the purpose of the drain line and why if fuel is seen passing through it is an indication of a diaphragm failure.

[schu]

I think this video could be of some use. Tempest actually has pressure vs flow charts for their pumps. And they also show the inner workings answering some other questions asked.

https://www.youtube.com/watch?v=fxT4RAEssnw

Good call.

[Battson]

Here's the remaining data for fuel flow taken from the locations discussed.

1. Fuel flow from the servo inlet, boost pump ON
2. Fuel flow from the servo inlet, boost pump OFF (Gravity)
3. Fuel flow from the engine driven fuel pump, boost pump off (Gravity)

schu I did see 32.8 GPH at the engine driven fuel pump inlet on gravity. So I exceeded your suggested 31 GPH by 1.8.

The results are not looking very favorable with neither of the two pumps online at the servo inlet. But I think I can lose either of the two separately and still be in good shape.

Will test further with engine running and boost pump off at higher RPM's. Will be looking for 25+ PSI there.

Thanks to all for the the input which pushed me to dive deeper into my testing.


Screen Shot 2021-03-13 at 2.07.10 PM.png

Those numbers look good to me Rob. Good on you for testing it.

With both tanks selected, your fuel pumps will have positive pressure at all times i.e. no suction. This reduces the risk of drawing air into the system as Whee explained earlier.

Test results with just one tank selected aren't meant to be as good, so that is normal.

A fuel injected engine doesn't want to run on gravity feed alone, if there is a mechanical fuel pump in the line. The best you can hope for is partial power (like fast idle) with both pumps off (failed). When my pump failed, I could not get the engine to run at all, unless the boost pump was running.

I will be interested to see your fuel flow at maximum power, you are not far away now. We see exactly 30.0 GPH at full power and should be generating 260hp. I wonder what the extra 15 hp are worth in terms of gallons. That said, I cannot say how much unburnt fuel goes out the exhaust pipe.

[robcaldwell]

Thanks for your comments, Jonathan.

We ran the engine and are seeing 33 PSI @ 1,500 RPM with no boost pump. About 8 GPH on the flow meter. Haven't ran it hard yet.

[Battson]

I trust the flow meter is not yet calibrated? That sounds rather high for 1,500 RPM. Good high pressure too.

[robcaldwell]

Yes. Need to calibrate the flow meter and everything else related to the Dynon HDX.