Just tried MOGAS, didn't go well

[airguy]

Originally Posted by rv7charlie

Some alt engine guys are now putting all the pumping operations (electric pumps only; no mech pump) on the cool side of the firewall, so only pressurized fuel is exposed to engine heat.
Charlie

Not just the alt engine guys - many of us with Lycosaurs (myself included) have done this.

[Elwell81]

Originally Posted by aerhed

Isn't 18psi low to begin with? Isn't 23-27 most normal?

This got me thinking. I went back through all of my G3X touch data-logs since first flight (April 2015) and noticed a trend. The fuel pressure started at 24 psi in cruise, then in the early August time frame began to drop slowly (slowly enough I never noticed a difference) to 18 psi, where it is now. I have changed the gascolator filter since then so I'm confident its not a filter issue but I'm going to check it again. I'm also going to make up a test hose to bypass the gascolator and AUX pump to make sure I'm not getting to much restriction through them. I have doubts that this is the case but have to check to be sure. But definitely need to find out what happened to 6 psi of fuel pressure. The autogas just magnified the issue.

Originally Posted by ChiefPilot

If the gascolator is forward of the firewall as most are, it's a pre-warmer for the fuel. It soaks up heat, then heats the fuel, which increases the potential for the fuel boiling in the lines. The presence of a gascolator may be at least contributory to the symptoms reported.

I installed my gascolator in the cabin, under the tank selector valve for that very reason.

[krw5927]

Originally Posted by Elwell81

This got me thinking. I went back through all of my G3X touch data-logs since first flight (April 2015) and noticed a trend. The fuel pressure started at 24 psi in cruise, then in the early August time frame began to drop slowly (slowly enough I never noticed a difference) to 18 psi, where it is now. I have changed the gascolator filter since then so I'm confident its not a filter issue but I'm going to check it again. I'm also going to make up a test hose to bypass the gascolator and AUX pump to make sure I'm not getting to much restriction through them. I have doubts that this is the case but have to check to be sure. But definitely need to find out what happened to 6 psi of fuel pressure. The autogas just magnified the issue.

Fill a tank with 100LL and see if the pressure returns when running on that tank.

[BillL]

You might get a sample of the fuel and test it for boiling point. A hand vacuum pump from HF and the adapter can do this. You must know the temperature of the fuel, but put a sample in the small clear container and reduce pressure with the vacuum pump until it "boils". Do the same with the 100LL for reference. Let us know the results.

I don't use mogas (yet) but the above is a comment from another VAFer that does and he tests each batch to ensure it is suitable for use. It eliminates guessing and hoping.

Lycoming recommends a maximum of 2 psi pressure drop for fuel into the mechanical pump. It would have to be measured with appropriate flow. I think they say 16 gph for the 180 hp 360.

EDIT: Go to the December 2013 issue and read our PeteHowell's writeup on mogas.

[scsmith]

Most of the suggestions here are right, such as:

1) your mechanical pump pressure is a little low to begin with -- mine runs 24 psi. So might be a marginal mechanical pump

2) you probably got winter gas, which does have high "front-end volatility" and will easily vaporize/cavitate

But here is an experience I had a few weeks ago that rings similar.
During condition inspection, I removed the Airflow Performance fuel filter for inspection and cleaning. I did not turn the fuel selector off first, as I probably should have, so with the line open at the filter, the fuel lines drained back to the tanks. After reassembly, and doing some other inspections, I did a test run.
I found that the mechanical pump was only providing 10 psi and the engine wouldn't run smoothly above idle. I turned the boost pump on, and the pressure came right up to 25 psi (normal). I turned the boost pump off, pressure dropped back to 10-11 psi. Boost pump on, pressure came right up. Boost pump off, pressure dropped again. Then, I put the boost pump on, increased engine RPM to 1900, let it run there for a minute, then turned the boost pump off. Pressure held normal. Dropped RPM to idle, pressure normal (on mechanical pump still).
At first I was really puzzled. Seems like even with the lines drained, turning the boost pump on was able to 'prime' the mechanical pump, so why did pressure fall again when the boost pump was turned off?
My belief is that there was a large slug of air in the fuel filter (not fuel vapor, but air). The boost pump was able to suck fuel past the large air bubble in the filter and achieve normal pressure, but the mechanical pump could not, it essentially lost prime because of the large air bubble. When I increased fuel flow by increasing RPM, plus just more running time, I eventually sucked that air bubble out, got the lines full, and everything is normal.
Just to be sure it wasn't a coincidental failure of the mechanical pump, I have been watching it like a hawk. About 5 hours of flight time, and it is acting normally.

So, consider that by draining your tank, you got a bunch of air in your line from the fuel selector to the tank. Then, same as me, running on that tank un-primed the mechanical pump. Like me, it may be that your boost pump was able to pull enough fuel past the air bubble to maintain pressure, but your mechanical pump couldn't. (it would have pulled the bubble through the small line, into the filter or gascolator)
It may be that by the time you ran off 3/4 of a tank with the boost pump on, that bubble was gone, and if you had turned the boost pump off at that point, the mechanical pump may have worked fine with the mogas.

Then, you mention seeing similar symptoms when you put 100LL back in. Hmmm, but you didn't drain the tank again, did you? Just topped it up with AVGAS? So this may kill my theory --suggests that it was fuel cavitation somewhere, just less so because of the avgas mix reducing the volatility some. But something to think about.

PS I will continue watching my pump pressure closely

[erikpmort]

I also tried mo gas and read everything I wanted to read - all positive. Only ran auto gas at cruise. And guess what it did nothing good. Power surges, and finally a power outage when I switched tanks.

It's not worth it. I could have died.

[The Wizzard]

My 92 octane Marine fule runs fine. No detectable difference. NONE! I have one tank with avgas and the other with 92 octane. NOTHING DIFFERENT! NO 90 bends, no sharp turns in the fuel lines and yes it does cavitate if you have them. Just put a piece of clear plastic tubing on the end of the fuel line before it goes in the throttle body/ carb and you will see them!

[The Wizzard]

18 psi seems quite low! my low pressure alarm is at 19 psi!

[David-aviator]

Originally Posted by Elwell81

This got me thinking. I went back through all of my G3X touch data-logs since first flight (April 2015) and noticed a trend. The fuel pressure started at 24 psi in cruise, then in the early August time frame began to drop slowly (slowly enough I never noticed a difference) to 18 psi, where it is now. I have changed the gascolator filter since then so I'm confident its not a filter issue but I'm going to check it again. I'm also going to make up a test hose to bypass the gascolator and AUX pump to make sure I'm not getting to much restriction through them. I have doubts that this is the case but have to check to be sure. But definitely need to find out what happened to 6 psi of fuel pressure. The autogas just magnified the issue.



I installed my gascolator in the cabin, under the tank selector valve for that very reason.

I have no gascolator, don't believe it is necessary with Fi. A large filter, yes, but no gascolator.

Your experience with MOGAS is unusual. It should be ok with winter temperatures, lots of guys use it all year round. You may have a pump pressure problem, I see 30 psi with electric on, 28 with it off. 18 psi is very low for Lycoming FI.

[Pittsartist]

Some thoughts - not always directly related, but hopefully worth knowing.

1) Did you know that you can cheaply & easily test fuels for ethanol content yourself ?

- Get a tall, thin graduated glass container. Half fill with water, then top up to full with your chosen fuel. Mark the intersection line of the 2 fluids with a sharpie pen. Leave overnight and check for a movement of the intersection against the line. If movement is observed it is because the Ethanol in the fuel has absorbed some of the water. More movement = more ethanol.

2) The key problem with Mogas is its lower vapour pressure. In practice this means that you are MUCH more at risk of fuel vaporisation than with 100LL. Heating the fuel is the enemy and should be controlled at all costs.

3) Ethanol in motor fuels absorbs water (see "1"). Water is corrosive, Vans RV's have aluminium fuel tanks and fuels lines.

4) When you leave your aeroplane in the sun the fuel in the tanks is being warmed. Solar Absorptivity for Black = 0.94, Pure White = 0.12

5) Back to Vapour Pressure. If you dilute 100LL with mogas by even 5% the vapour pressure of the mixture becomes virtually the same as pure mogas.

(Apologies to the many on here who probably know all this)

My personal experience is that my RV was specified and built from the outset for (currently illegal) running on Mogas. For me this meant

- Use of ethanol tollerant components (In particular the Polymer fuel system parts)
- Purchase of a 160hp engine with an 8.5:1 compression ratio and latest generation cylinders
- A "2 tank" strategy - 1 always pure 100LL, 1 sometimes Mogas / Mogas Mix
- Always use "Shell V Power" Mogas (a UK brand) - Shell spec is nil or max 5% ethanol, 99 RON octane. It lacks the additives found in 100LL.
- Andair electric boost pump & Lyco Mech Pump, F.I. & no gascalator.
- Never run @ > 24" manifold or > 1250f EGT on mogas
- Never Run Mogas @ less than 1500' AGL
- Never take off or Land on Mogas
- Always run at least 5 mins on 100LL every time out (Valve stem lube in 100LL)
- I have minimised the amount of fuel "in the system" to make changeover of fuel types as fast as possible - I estimate that switching tanks takes about 20 seconds (0.3l system volume @ 45 LPH flow rate) to be effective ... provided the engine keeps running of course !

By and large this has worked well for me for about 60 hours. However, I have experienced fuel vapourisation problems on hot (at least by our standards) days.

MOGAS is in my opinion not without risk - you should only take risks that you fully understand and can afford to loose. (for instance - flying over water, adverse terrain, high ambient temps, high altitudes etc)