How much 100LL to stabilize MoGas?

[Clark1961]

Help me with this. My point was that the lighter fractions will still evaporate faster than the balance of the mix, leaving you with a mystery fuel. Not true? I figured Boyle's law pretty much assured that. But I'm not a petro guy, so I defer to expertise...

Hydrocarbons are sort of "interesting" solutions. The way we model the behavior is to characterize the effect or interaction of one molecular component on another molecular component. Methane in particular has a tremendous influence on heavier components. The effect can be to the point of some of the heavier components crystallizing if methane is allowed to escape (vaporize) from the mixture. Conversely, the presence of heavier ends can help lighter ends to remain in the mixture at a particular pressure and temperature.

We do a lot of testing to characterize the make-up of solutions. Once we have identified the components and the relative quantities present in a mixture we can sort of predict the mixture's physical properties. I say "sort of" because the predictions are typically poor unless we have data from observed physical behavior to tune our models.

So with all that said, will the mixture result in an unknown solution. To a degree, yes. Will the mixture actually change much? no, not under normal storage conditions. The avgas will reduce the vapor pressure and the losses from the mogas.

 

[Capt. Geoffrey Thorpe]

FWIW

The previous owner of my ride used auto gas (like me) pretty much exclusively to the best of my knowledge. He also generally did not fly during the winter - but I should point out the he lived way down south by Indianapolis so he didn't have much winter to wait out. I don't know what (if anything) he did before giving up for the winter, but the airplane seems to have survived the "ordeal" just fine with no fuel system issues.

 

[PaulMillner]

I am working on the same basic set of criteria as you. Looking at running mogas(92 octane) in a O-320. The issues I'm coming up with boil down to two basic safety issues.

First is vapor pressure. I'm in TX and it gets hot here. My plan is to remove any/all 90deg fittings and go with curved pipe. Put the fuel pump right at the outlet of the tank so that there is always good head pressure behind the pump which will be the lowest pressure point of the fuel system. Run my lines directly to the mechanical pump and insulate any lines that are inside the engine bay, even if behind the baffle plate to protect them from heat. I'm considering running a circulating return line with a pressure regulator to keep a continuous flow of fuel going and return it to the tank.

Next is detonation margin. The O-320 comes in three basic comp ratios; 7:1, 8.5:1, and 9:1. The difference is in the pin to crown distance. I have the 8.5:1 pistons and the spec for those is 91/96 octane. I figure I'm within the limits of the detonation margin if I use pure 92 octane mogas, or a mix of 50:50 mogas and 100LL. To mitigate any problems I'll have my carb set so that full rich mixture is avail through all settings. I'm going to install a knock detector kit on the crankcase and one on the hottest head.(http://tunertools.com/gizzmo-electronics!!!-k!!lite-engine-knock-monitoring-alert-light.html) I'm considering an electronic ignition like the TEC3 http://www.electromotivestore.com/ setup with automatic retard in the event of detonation. If you have a lower C/R, pure mogas would be no problem.

<edit: I am not considering ANYTHING with Ethanol in it. The issues are just too great>

Random Doc,


True to your screen name, You are randomly mixing together four different octane number systems. You might want to do a little reading on that topic.


The 92 octane Mogas you propose is an average of a ~97 Research octane rating and a ~87 Motor octane rating. The 87 Motor octane rating is comparable to the 91 Aviation lean octane rating your engine is certified for... So you're only four octane numbers short, the difference between regular and premium in some parts of the country.


It might be OK, but you'll be a test pilot.


The 96 octane number in 91/96 is the aviation rich octane number... Probably only important if your engine is supercharged.


Paul

 

[docmirror]

Random Doc,


True to your screen name, You are randomly mixing together four different octane number systems. You might want to do a little reading on that topic.


The 92 octane Mogas you propose is an average of a ~97 Research octane rating and a ~87 Motor octane rating. The 87 Motor octane rating is comparable to the 91 Aviation lean octane rating your engine is certified for... So you're only four octane numbers short, the difference between regular and premium in some parts of the country.


It might be OK, but you'll be a test pilot.


The 96 octane number in 91/96 is the aviation rich octane number... Probably only important if your engine is supercharged.


Paul

Yup, I'm aware. As said, I'll use the tools avail to me to test it well before putting into service full time. I have two tanks so I can have a test profile and a safe profile fuel system. It's not as random as you might think as I'll be following some careful protocols for mixing, testing and taking plenty of measurements along the way. I have a data logger avail to me, and it'll be put to good use.

 

[denverpilot]

prolly as much as when you read IT or radio stuff...

IT stuff ON the radio... Be it commercial band broadcast or Amateur band, usually has me wanting to reach through the radio and strangle someone. LOL!

Pretty much makes one sad to hear just how bad end-users have it in a 90% Microsoft world. But hey... They bought the Nokia CEO by rescuing his sinking ship this week. That should make for some great customer focused solutions!!

ROFL...