Fuel Pressure and Flow problem resolved

Fellow RV builders,

Some of you are already aware of the investigation I did into resolving a problem I saw with periodic deviations in fuel flow and fuel pressure in cruise flight.

The highly summarized lesson learned: Don't mount the fuel flow sender in Van's recommended location inside the tunnel. Mount it between the fuel controller and fuel spider as many other builders have done.

I captured data showing that I was on the verge of vapor lock, due the pressure drop and temperatures associated with the fuel flow transducer in the tunnel.

For a detailed report including the data I collected here's a link to my write-up on the full situation.

http://tcwtech.com/Fuel flow write-up.pdf


Bob Newman
N541RV
RV-10

No problems with stock position

After reading this post and report, I checked a couple long-flight logs for which I have data downloaded from my AFS. I don't see any such sags in fuel psi during cruise, and my red cube is in the tunnel at the stock location. I usually fly pretty high and my tunnel never gets hot, even in hot weather at low altitudes, but I will keep an eye on it. I do have my sample rate set at 5 sec, so I might increase it to 1/sec and check again, but I've not seen any of the other symptoms Bob mentions, like inability to run lean of peak or engine roughness. I've consistently operated on long flights at 9.5-10.5 gph, 30-90 deg LOP without engine roughness. I have a similar set up to Bob's except no gascolators in the wings, Aerosport Power IO540 engine and Silverhawk injection system. Thanks for the research and diligence, Bob, something for us to keep an eye on.

My red cube is also located in the tunnel. I have the tunnel insulated and the pump, filter, red cube, and all lines are insulated in fire sleeve. Since this subject matter cropped up again, I gave serious consideration to move it FWF but will leave it as is until I am flying to decide weather or not to move it. I do have 6" before and after the cube of straight fuel line. Some folks report problems and others don't.

I started a thread a few months ago. I was concerned about potential vapour lock due to high tunnel temperatures and was intending to fit an OAT probe to monitor. I was looking for data/opinions on what a critical temperature might be.

The overwhelming view was that, under cruise conditions, vapour lock was impossible due the the rate of flow through the tunnel. Simply, the fuel could not get warm enough, fast enough to cause a problem.

Now I am confused!

vapor lock considerations

A combination of heat and the pressure of the fuel cause vapor lock. So you have to look at the total installation. In all the cases no one had actually instrumented the aircraft to measure the absolute fuel pressure and temperature of the fuel at the inlet of the engine driven fuel pump, or to my knowledge the absolute pressure and temperature of the fuel in the tunnel before the flow meter transducer. It is a fact however that avgas boils at around 130 degrees F at 14.7 PSIA (sea level pressure). Since no one has measured the absolute pressure of the fuel it is only speculation as to when vapor lock will occur. In the case of Mr. Newman, he was obviously experiencing vapor lock with the way his installation was done. And the proof was that moving the flow meter to the pressure side of the system eliminated enough of the pressure drop on the suction side of the engine driven pump to eliminate the problem. The problem could also be solved by dropping the fuel temperature 20 to 30 degrees at the flow meter inlet as well as the rest of the fuel system. This solution was probably not practical. Flowing more fuel to the engine, which in turn will pull more heat from the components, may have eliminated the problem. Also not a practical situation in a cruise power setting.

Knowing the issues with vapor pressure for avgas and auto fuel and knowing what the pressure drops in the fuel system can help in the design of the fuel system in the plane. From our stand point we recommend installing the fuel flow transducer on the metered side of the fuel injection system and eliminating as much fuel volume in the engine compartment. All installations are different and fuel systems are different (carbureted, mechanical fuel injection and EFI), so these systems should be designed based on the manufacture?s specifications or proven out by comprehensive ground and flight-testing.


Don