petehowell
Well Known Member
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)
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!
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!