[petehowell]

Well I do at times........

Winter to summer mogas changeover season has always been a challenge - the winter mogas on a warm day would lead to some issues, like fuel boiling in the fuel line, and carb, leading to an over rich condition on take off with a corresponding stumble after a hot turn on the ground. It can also lead to rough running when taxiing on a hot day and dieseling after shutdown. The RVator and some old Sport aviation articles ("Fuel System Vapor Lock" October 1995) discuss adding a vapor return line to help remedy the situation. This same problem vexes classic car guys that run carbs as seen in this video.

So with the help of some great people here, I added a line that Tees off at the fuel line to carb fitting and is plumbed back to the left tank via a valve. A bit of work, but not that hard. The tee at the carb has an 0.040 restrictor fitting that limits the flow of fuel to about 4GPH - low enuf that if it is left on, it will never starve the engine of fuel. The valve is located on the left gear weldment and is easily actuated in flight.

Here is the tee at the carb

Here is the valve with a thermistor attached. Designed and 3d printed the custom valve mount. Yup, shorty sox, too......

I used the thermistor to get temp data on the fuel flowing thru the vapor/recirc line. Utilized the Horton LM34 with multimeter set up to capture data manually - I might set up an arduino multichannel data rig here soon to allow me to get carb and fuel pump temps as well.

Here is the data captured on short cruise around the twin cities tonight.

Hi Fuel Temp = max temp seen after opening the valve
Low Fuel temp = temp seen after opening the valve until the temp stopped dropping, Usually about 1 min (except for the hot soak - valve and pump were run for 3min)

Conditions​	OAT​	Alt​	​	​	​	​	​	​	​
​	75​	2000​	​	​	​	​	​	​	Post shutdown​
Flight time​	Start up​	5 min​	10 min​	15 min​	20 min​	25 min​	35 min​	Taxi in​	10 min hot soak/3 min recirc​
Hi Fuel temp​	75.8​	95.1​	101.7​	105.3​	106.7​	107.3​	108.1​	115.1​	118.7​
Low Fuel Temp​	75.8​	90.4​	97.5​	100.7​	102.1​	102.2​	103.4​	111.1​	99.2 still dropping​
Fuel Pressure​	4.5​	4​	3​	3​	3​	3​	3​	3​	6psi bled down to 0 when valve was opened​
Oil temp​	76​	163​	178​	186​	189​	191​	191​	191​	188​

Observations:

- Fuel gets hot quickly FWF
- Recirculating the fuel in flight cools the temp a bit
- The big benefit is after shutdown and hot soak

After the hot soak, the fuel pressure (with no pumps on) rose to 6psi b/t the engine pump and the carb. That leads me to believe the fuel was boiling in that line. Opening the valve immediately bled that pressure down to zero as expected. Turning on the boost pump pushed that hot gas/vapor back to the tank and replaced it with 20 deg cooler liquid fuel.

I did a hot turn after refueling and recirculated fuel on the taxi from the pumps to just before taking the active - there was no stumble and no drama on takeoff. I need to get more data and I need to test on a hotter day, but this mod seems to work with observable benefits.

More to come!

 

[Dan 57]

Interesting stuff @petehowell
As mentioned in this thread, I have a T off the carb inlet and return line to the selected tank. Still had a few instances of probable hydraulic lock, e.g. fuel boiling in the carb bowl. I now received T strips which will be applied to the carb, EDP, and gascolator during my next inspection.

Of course, installing a thermistor as you did, is a much better way, and I'll be following this thread as well. Ideally thermistors placed on those 3 T critical items should yield some interesting results.

 

[DanH]

Go Pete, go!

Want to borrow my Dataq 1100? Simultaneously log four temperature signals to your laptop. https://www.dataq.com/products/di-1100/

 

[blaplante]

How are you feeding back into the tank?

 

[Mikeyb]

Thanks
I love it when VAFers put the effort into taking and sharing data.

 

[petehowell]

How are you feeding back into the tank?

I put an AN4 fitting into the tank cover plate. Let me know if that is what you were asking.

 

[docjjrmd]

Well I do at times........

Winter to summer mogas changeover season has always been a challenge - the winter mogas on a warm day would lead to some issues, like fuel boiling in the fuel line, and carb, leading to an over rich condition on take off with a corresponding stumble after a hot turn on the ground. It can also lead to rough running when taxiing on a hot day and dieseling after shutdown. The RVator and some old Sport aviation articles ("Fuel System Vapor Lock" October 1995) discuss adding a vapor return line to help remedy the situation. This same problem vexes classic car guys that run carbs as seen in this video.

So with the help of some great people here, I added a line that Tees off at the fuel line to carb fitting and is plumbed back to the left tank via a valve. A bit of work, but not that hard. The tee at the carb has an 0.040 restrictor fitting that limits the flow of fuel to about 4GPH - low enuf that if it is left on, it will never starve the engine of fuel. The valve is located on the left gear weldment and is easily actuated in flight.

Here is the tee at the carb

Here is the valve with a thermistor attached. Designed and 3d printed the custom valve mount. Yup, shorty sox, too......

I used the thermistor to get temp data on the fuel flowing thru the vapor/recirc line. Utilized the Horton LM34 with multimeter set up to capture data manually - I might set up an arduino multichannel data rig here soon to allow me to get carb and fuel pump temps as well.

Here is the data captured on short cruise around the twin cities tonight.

Hi Fuel Temp = max temp seen after opening the valve
Low Fuel temp = temp seen after opening the valve until the temp stopped dropping, Usually about 1 min (except for the hot soak - valve and pump were run for 3min)

Conditions​	OAT​	Alt​	​	​	​	​	​	​	​
​	75​	2000​	​	​	​	​	​	​	Post shutdown​
Flight time​	Start up​	5 min​	10 min​	15 min​	20 min​	25 min​	35 min​	Taxi in​	10 min hot soak/3 min recirc​
Hi Fuel temp​	75.8​	95.1​	101.7​	105.3​	106.7​	107.3​	108.1​	115.1​	118.7​
Low Fuel Temp​	75.8​	90.4​	97.5​	100.7​	102.1​	102.2​	103.4​	111.1​	99.2 still dropping​
Fuel Pressure​	4.5​	4​	3​	3​	3​	3​	3​	3​	6psi bled down to 0 when valve was opened​
Oil temp​	76​	163​	178​	186​	189​	191​	191​	191​	188​

Observations:

- Fuel gets hot quickly FWF
- Recirculating the fuel in flight cools the temp a bit
- The big benefit is after shutdown and hot soak

After the hot soak, the fuel pressure (with no pumps on) rose to 6psi b/t the engine pump and the carb. That leads me to believe the fuel was boiling in that line. Opening the valve immediately bled that pressure down to zero as expected. Turning on the boost pump pushed that hot gas/vapor back to the tank and replaced it with 20 deg cooler liquid fuel.

I did a hot turn after refueling and recirculated fuel on the taxi from the pumps to just before taking the active - there was no stumble and no drama on takeoff. I need to get more data and I need to test on a hotter day, but this mod seems to work with observable benefits.

More to come!

Love it! Solving problems with data! I suspect this may become a standard feature for builders using Mogas.

 

[blaplante]

I put an AN4 fitting into the tank cover plate. Let me know if that is what you were asking.

Did you end up pulling the tank to do that? Thinking about doing a similar recirc system and pulling the tank, etc. has me a bit put off.

On another note, I'm really surprised how warm the fuel is given it looks like you have fire sleeve on all the lines. And I'm assuming it wasn't a very hot day.

 

[petehowell]

Did you end up pulling the tank to do that? Thinking about doing a similar recirc system and pulling the tank, etc. has me a bit put off.

On another note, I'm really surprised how warm the fuel is given it looks like you have fire sleeve on all the lines. And I'm assuming it wasn't a very hot day.

Nope, just pulled the cover, drilled a hole, prosealed the fitting in, and sealed her back up. Socket head screws on the tank cover make this a bit easier.

It was about 78 degF on the ground when I tested

 

[pjc]

Pete,
Excellent data. Was this testing done with 100LL, Mogas or something else ?
pjc

 

[petehowell]

Pete,
Excellent data. Was this testing done with 100LL, Mogas or something else ?
pjc

pjc,

About a 50/50 mix for the test

 

[petehowell]

Here is a short video showing the vapor return line bleeding off pressure after shutdown and a short hot soak. Sorry about the quality.....

I opened the return valve, and could hear the vapor bleed back into the tank. Watch the Fuel Pressure in the lower right quadrant of the Dynon.

 

[akschu]

My build (not an RV) has an aluminum manifold on the firewall. It has:

Fuel IN from mechanical pump and upstream boost pump (for an injected engine)
Fuel OUT to fuel servo
On the very top a -3 vapor return line with a rivet crushed in it and .040 hole
On the very bottom a fuel pressure sensor
A fuel temp sensor.

I also have the fuel pump shrouded and a 1" scat tube to it.

I'm not flying yet, but my hope is that passing 6-8gph through the fuel pump all of the time will cool it and give vapor a place to go since it's the highest point. The goal is to run mogas with injection. If it doesn't work, I can always put 100LL in it as well as remove the manifold and revert to the standard setup.

I'm still plumbing so not everything is sleeved or connected yet.

 

[Fred.Stucklen]

I'm running 93 oct E10 gas in an injected IO-360-B1B engine. Almost 2300 Hrs now. My system is to cool the fuel pump with a Shroud & 1" air tube off the baffles. I have never had to rely on a gas return line back to the tank.. But my fuel system doesn't have any 90 degree fittings after the boost pump (located in the cockpit). All fuel hoses have fire shields. To prevent any vapor lock issues, my boost pump has an electronic circuit with independent low pressure sensing at the fuel servo input, that automatically turns ON the boost pump (just like in your car!) when a low pressure event occurs. What I have found is that if the fuel servo fuel shutoff is operating properly, no fuel will seep into the servo during shutdown (full lean mixture) so there is no need to bleed off the indicated fuel pressure.

Yes, fuel is left in the distributor and line to the injectors after shutdown. When it's very hot outside, this will cause the engine to be in a rich condition on re-start. But this know condition is easily overcome by using the proper hot re-start procedures. I always assume a hot start will be necessary unless the engine has been sitting for multiple hours..