Just tried MOGAS, didn't go well

[rv7charlie]

I know it's unlikely, but any chance you got 'old' gas? How often does the mogas supply cycle at your field? I've run 91 octane for many years (carb'd engine), & the only glitch I've had was when I was (stupidly) running winter blend gas on a 100+ degree day, after the engine/cowl heat soaked on a ramp.

Charlie

[Fred.Stucklen]

Originally Posted by dweyant

How can a 90 turn cause air bubbles? Are they somehow coming out of solution?

-Dan

Auto gas, especially winter blended fuel, has a lower vapor level rating. The sudden turn in flow will cause the fuel to "boil", especially at a low fuel pressure like you have before the mechanical fuel pump. ANY type of restriction to flow between the tank and the mechanical pump will make the situation worse because the fuel pressure will be lower.

[N526JS]

Years ago I had a Piper Cherokee with an auto gas STC. On a warm day one February I had some "stumbles" climbing out and during taxi after landing I could hear the electric fuel pump running furiously and not pumping up pressure as prior flights (no fuel PSI gauge so this was subjective). I attribute this to "winter gas" and was the last time I used auto gas in the plane. A carbureted engine but still a data point. You said that the OAT was 25 C. Pretty warm for this time of year.

[N526JS]

Unless you meant 25 F.

[krw5927]

Like Bob said, this is purely a function of the Reid Vapor Pressure (RVP) of your winter-blend mogas. This means at any given temperature and pressure, it will be more susceptible than avgas to boil in the fuel pump and/or fuel lines and cause the symptoms you described. In other words, the gas boils at a lower temperature than Avgas.

Avgas has a tightly controlled (and relatively high) RVP. Mogas, even that which is ASTM-spec, has a wider tolerance band for RVP, the center of which changes by season and location. There used to be a commonly available RVP measurement device one could purchase, similar to a brake bleeder, to measure what came out of the local gas pump.

Anecdotally, I ran mogas in my RV9A for the first 20-30 hours. One day, after about an hour break on the ground, the outside temperature had climbed a bit and I hopped back in the airplane to head for home. About 600 feet off the runway the engine started doing exactly what you described. Quite the attention-getter when you're close to the ground. Lowering the nose a bit and pulling some power out reduced the under-cowl heat enough to get things working well again. That event sparked my research into RVP and my subsequent decision to only fly 100LL in this airplane.

Research the issue, keep testing SAFELY, and you'll find what works for you. My guess is you'll have a much different (better) experience with summer blend mogas.

[grubbat]

Instead of putting 100% mogas in one side, try a 50/50 mix or maybe 75/25 mix for the mogas side. It doesn't take much avgas to "help" mogas burn well.

cj

[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.

[Fred.Stucklen]

Originally Posted by Elwell81

There are no 90 degree fittings in the engine compartment. There is only a about a 2' run from the AUX pump to the mechanical pump with nothing in between.

I understand the fact that excess restriction can cause issues and ran my fuel lines accordingly. The only thing different in my fuel system is an Andair gascolator (for easily servicable filter) between the tank selector valve and Aux. pump.

Temperature also seems like it wouldn't be an issue because OAT was 25 degrees. Running on 100LL this summer with 90 plus degrees OAT, I never had any hint of an issue.

Is the difference between auto gas and 100LL that substantial, that I have no issues 100LL but can hardly pump 91 octane?

So measure the fuel pressure (which will be negative) and fuel temps at the input to the mechanical fuel pump. That will be the indicator to what is causing your issue...

[1001001]

Originally Posted by dweyant

How can a 90 turn cause air bubbles? Are they somehow coming out of solution?

-Dan

To answer this specific question--

As others have mentioned, it is a function of pressure drops in the system and the vapor pressure of the fluid being moved. The bubbles in the system caused by this mechanism are not air, but fuel being vaporized due to the low pressure at the discharge of the fitting.

Cavitation can be a serious problem in any kind of fluid system--in my professional experience it has been seen mostly in water pump systems with disastrous results for pump impellers and other components. Here's an example of what cavitation can do:
This is a result of tiny bubbles forming and collapsing at the impeller surface due to the local static pressure being below the fluid's vapor pressure at the operating temperature. When the microbubbles collapse they can create very powerful shock waves that can damage metal surfaces. The picture is of the rotor of a water turbine on the discharge (low pressure) side, but the same type of damage happens to pumps on the suction (low pressure) side.

Here's a link to a brief description of cavitation.
http://www.engineeringtoolbox.com/cavitation-d_407.html

And another (the source of the above linked image):
http://authors.library.caltech.edu/25019/1/chap6.htm

Cavitation in our fuel lines isn't likely to result in terrible damage like this (unless it happens in the fuel pump itself, perhaps), but the same principle is behind vapor bubbles forming in fuel lines near restrictions that induce pressure drops.

[rv7charlie]

quote: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.
unquote

And, it's an extra restriction, even though it should be a small one. And, from the previous description, it's prior to the boost pump; does that mean that the boost pump is FWF, as well? That would be an *additional* restriction in a heat soaked area when it's not running, and another source of heat to the fuel (the pump gets heat soaked, too).

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.

To me, it still sounds like a problem who's cause is more than simply the vapor pressure of mogas. There are too many successful systems flying for it to be that simple. Bad gas, restriction in the line, gascolator, configuration, etc etc. Yes, mogas vapor pressure is higher, but not that much higher. My problem happened with winter blend in OAT of 100+ F, and only after flying, landing, and heat soaking.

Charlie