Separate names with a comma.
Discussion in 'Flight Following' started by Diana, Mar 8, 2005.
Is auto fuel really hard on airplane engines that are approved for auto fuel? If so, why?
Good question Diana,
My cherokee can use auto fuel and i have always wondered if it woukd be hard on it if i were to do that.
If you have an engine that has an STC for mogas (auto fuel) its actually better for the engine as there is no lead to foul the plugs or valves. Now, on the other hand you have to make sure there is no ethanol in the mogas because it will eat your hoses and seals. No free lunch it seems.
Auto fuel is not harmful for your engine, 100LL uses TEL (lead) to retard the burn rate of gas stocks used to make aircraft fuels. but auto unleaded can't, so the fuel is retarded by the use of benzene and tolulene.
each component is harmful in different ways.
Lead is good as a retardant but every thing else it does is bad. A very small amount of TEL will do the job well, so almost all of the fuel is energy.
Benzene and Toluene do the job, but it requires a much larger % to get the burn rate we require. So the fuel actually produces less energy per pound. because the additive has less BTUs per pound than the gas it displaces.
Lead causes spark plugs to foul, and it sticks to valve stems and causes the valve to stick open and will if left to fully develop cause the cylinder to fail.
Toluene/Benzene does not do this, BUT it will attack the resins in the rubber parts of the fuel system if it has not been up graded to modern rubber products.
Some of the Marvel Schibler carburetors used in the PA 28s and C-100 series have a rubber tip on the float needle and after a year or two on auto this will fail and cause a serious fuel leak after shut down.
Some auto fuels contain Alcohol, the law does not require the fuel dealers to label the pump if the mixture is less than 10%, so If you use auto, also use a alcohol detector kit. because for every 10% alcohol, you loose 20% horse power.
Also, If you have 10% alcohol you can also have 10% water hidden by the alcohol, and it will not show at the low point drains, now you have a load of fuel that will not produce the power you need to climb.
Auto fuel was at one time cheaper than 100LL, not so any more, so it really is not worth the troubles it can cause.
Very informative post. Thanks.
If you detect the alcohol, then, and reject that batch of fuel, that will also take care of the hidden water?
Well, I think Diana baited me on this one (she knows how I feel) but here goes anyway:
For me, any cost savings just isn't worth the problems. Assume you fly an airplane that burns 8 gph and that you fly about 100 hours / year. I'm paying $2.45 for 100LL and pay almost $200 for premium mogas. That's a savings of less than $500 / year. If the mogas use causes ANY mechanical issues (rebuilding your carburetor, flushing "varnish" out of tanks, getting stuck at some backwater airport because your engine just won't run right) your fuel savings will indeed look minimal.
With mogas, it's not just about the octane: "Besides octane, new unleaded fuels also have to meet vapor pressure, freeze point, energy content, component separation, storage stability, corrosion, conductivity, oil and water reaction and other specifications, as well as practical price and distribution requirements. 'Anything much different from 100LL has difficulty satisfying these issues,' Roehl said. 'The most viable fuel has the minimum change, otherwise it opens up all these issues.' - Tim Roehl GAMI Corp
Don't miss those items: Vapor pressure; freeze point; energy content; component separation; storage stability; corrosion; conductivity; oil and water reaction. In this article Roehl is discussing alternative aviation fuels. Mogas doesn't meet the requirements.
"Auto gas is not regulated as closely as aviation fuel. It is handled differently. The handling restrictions on auto fuel are more lax. A pilot doesn’t know what he is getting when he pulls into his favorite gas station to purchase fuel for his plane. Your favorite service station may have clean restrooms, clean pumps, clean window washing materials, but less than clean fuel that could have been contaminated by the carrier. Some auto fuel contains alcohol. If it does, it can slowly eat away at the rubber lines or polystyrene parts and bladders. Anything non-metal in your fuel system might deteriorate with the use of Ethanol fuels." - http://www.fly-low.com/feat0412/autofuel.html
(In an effort for balanced reporting, here's the follow up "Pro" auto fuel article: http://www.fly-low.com/feat0502/Autofuelii.html. Also not too positive IMHO)
I can't imagine being at 8,000' at night over a solid undercast (or inverted at 2,000') and trusting that the local Kwik-Stop didn't lift their mogas in the tank with water.
I have never spoken with a mechanic who believes running mogas is a good idea.
Absolutely, positively not.
I think that's a bit overstated Tom. Alcohol does indeed contain less energy per pound (20% less sounds about right), but if you have 10% ethanol and 90% gasoline, you would only lose 2% of the energy not 20%. Expanding your calculation to 50% ethanol says you'd have no horsepower at all, but assuming you got the thing started, I think you'd find that there is some power there after all. Worse yet "Gasohol" which is 85% ethanol would produce negative horsepower!
With the average price of autogas creeping above the $2/gallon mark vs Avgas at $3+ I think there's still money to be saved, but I agree that it may not be worth the hassles.
I have run a C-150 for 700 hr on MoGas (Auto fuel)
Also a AA5 for 500 hr
Also a C-182 for 500 hr.
I never had to buy a spark plug.
For the older low compression motors it is great.
MUCH longer spark plug life
Much less problem with sticking valves
Cleaner for the environment ... I wonder how much lead we put into the ground sumping fuel.
Safer for your body... Dripping 100LL on you is like eating lead paint
Auto fuel wont last as long as 100LL sitting idle. 1 year max. I bet 100LL could go 3 years.
Most A/Ps hate it & will tell you it is bad, bla. bla. bla.
People who haven't used it, think it will ruin you plane, Bla, Bla, Bla
It stinks if spilled on you.
I have never met someone who used it & didn't like it.
I have met MANY people who say their friend used it & it ruined this. that & the other thing (BS)
Old wives tail, just like the story of the local boy that buried his cat 1/2 way & ran it over with the lawn mower
How many times have you heard that one?
Has you car/tractor ever stalled from a bad fuel delivery? I worked as a auto mech for 12 years & the only bad gas problems I saw were from leaking fuel caps (Vetts & Jaguars with top caps), Old gas & once from a child filling daddy's car up with the garden hose
With you having auto fuel on the farm, it is a no brainer. Just make sure the auto gas has no alcohol in it.
Well, yeah, but not technically the same issue. Got a load of #2 diesel from the local station when the temps were in the 50s F. That night it plummeted to -20 F and stayed there for a week. The fuel gelled in the truck and took me a couple of days to get it running again. :cheerio:
Chip, thanks for your input and the references. Good reading. I'm still open to switching over to av gas.
My mechanic thinks it's OK to use auto fuel in my airplanes, as have all the other mechanics I've ever used. They all use auto fuel in their airplanes, as well. See why I get confused?
Thanks Eamon. We probably need to test what we put in our tank at home. They told us it didn't have any alcohol in it, but we should test it to make sure.
I'd put it at more like 3 months max. I don't know how small the passages and jets are in an airplane carb, but I do know that motorcycle carbs will gunk up in as little as 3 months.
If I let anything gas powered sit for more than a month, I treat the gas with Sta-il or similar and run it before storage. I have no idea if Sta-Bil is approved for use in aircraft, though.
Well that is also not the samr issue.
I delivered Propane in a Hino Diesel for 12 years. Every winter when the temps would be forcast to be below "0" we all added 20% Kero to the trucks so they wouldn't gel up. This is very common for diesel & has nothing to do with the gas station or delivery/handeling of it. Maybe you were used to buying at a station that had pre-treated the load with conditioner & this was your first time without it. In NY is doesn't get that cold that often so most stations sell unconditioned, but some do sell it allready conditioned.
Unleaded is MUCH thinner & will not gel up.
It gave me some (small) comfort to know that I wasn't the only one who got caught by surprise. The station is a major truck stop and they normally sell #1 or condition their #2 fuel when the temps are predicted to go low. This cold snap caught everyone off guard and there were trucks parked all over the freeways and highways changing fuel filters and trying to ungell the tanks.
Sorry for the thread hijack - diesel vs unleaded isn't even a close comparison...just got carried away here today.
Also consider that the STC that allows the Citabria to use mogas is a paper-only STC. That should tell you something, given how conservative the FAA generally is.
I could only imagine what #2 would do in a jet at FL350
I ran a VW rabbitt on straight Kero in the winter & it did fine.
Jet-A, Diesel, #2 home heating, kero.. All pretty close at room temp.
A friend of mine is past TBO on his 182 with no problems. He does say that every 4th tank or so he fills with AvGas to get some lead in the system, but I really don't know if that is necessary.
I think you'll find that most of the true experts would say that there is no benefit to lead in fuel beyond detonation prevention. Many folks claim that lead is needed to "lubricate" the valves, but AFaIK this is bunk.
As to the shelf life of auto fuel, I think it varies with the makeup, and definitely with temperature (deteriorates faster when hotter). I do agree that anything beyond three months should be a concern. I think I've had some varnish problems that occurred in auto fuel that sat for as little as a couple months in a vented tank, and the effects are cumulative. OTOH I keep an above ground 200 gallon tank at my house for the boat which gets filled about twice per summer plus it sits all winter and I haven't had any trouble with the gas going bad in there. There is a filter between the tank and the hose on the dock, but I don't know if that helps any.
I've always wondered if it has anything to do with a relatively tight storage container, in other words, I'm wondering about oxidation of the fuel. Fuel stored in a tight container with no air turnover seems to keep much longer than fuel exposed to air, such as in a carb float bowl.
Thanks Tom. Wonder where I can get an alcohol detector kit?
I would think you could take a glass beaker with the fuel in it and add some water. If the water is absorbed then the fuel has alcohol in it. Yes?
Ken, I'm not sure what that means.
Hmmm, that almost sounds too easy. I bet I could buy a kit for $125 that would do the same thing.
Great article Steve.
Thanks. I'll order one from Peterson.
"I think that's a bit overstated Tom. Alcohol does indeed contain less energy per pound (20% less sounds about right), but if you have 10% ethanol and 90% gasoline, you would only lose 2% of the energy not 20%. Expanding your calculation to 50% ethanol says you'd have no horsepower at all, but assuming you got the thing started, I think you'd find that there is some power there after all. Worse yet "Gasohol" which is 85% ethanol would produce negative horsepower!"
First of all, you won't have 10% alcohol and 90% gas, it will be closer to 10% Alcohol, 10% benzene, 10% tolulene, and 70% gas. And maybe some water thrown in for good measure.
I do believe the 20% numbers, but not because I do math all that well, but I did load a super cub with a load of 92 oct unleaded that tested 10 Alcohol, and it would not fly. (I had to see for my self)
I have a friend that owns N6988S a C-150 That I restored in the early 90s I built (from scratch) an 0-200 that has been on UL for all its life. no problems yet at 1200 hours. still on the same plugs, and pumps 78/80 all cylinders.
But I got a dose of stupid and filled it from ARCO (10% Alcohol) It would fly but, only show 150 feet per minute climb, (normal 500-+) and when you are 2 minutes from cold blue water in any direction a sick engine gets your attention.
Like I said I believe the 20%, Some times I don't believe the number crunch, but I do believe the VSI.
That's just my humble opinion you understand
I have an old set of bang compression gauges that you subsitute for the upper spark plugs and run the engine on the bottom row, and read direct BMEP.
What do you think the comparison readings are on 10% alcohol (ARCO 92 oct UL) to 100LL?
Or would you want to guess where I got the 20% number from.
That's my experiences with auto, 92UL in aircraft, I like it, but please don't think that you can run it with out doing your home work, and knowing what the hazards are.
Tom's comment about alcohol eating seals is correct. Back a number of years ago I bought a new 1986 Dodge van. The parting words from the salesman as we left were "don't use gasoline with ethanol. It will eat the seals.". A few years later California mandated ethanol in the gas. Guess what? I had a fuel leak that was running fuel over the block and onto the driveway. Did the state pay for my new carburator? Yeah, sure The new carb had seals that would tolerate ethanol, but that didn't pay for the repairs the first time.
I've wondered the same thing, and it seems logical if the degradation is an oxygen or nitrogen consuming process. Any Chemists out there with an answer?
Ask your self this,, What would happen in this test IF the alcohol was already saturated with water, and you added more?
The answer is, you would get back all the water you added.
This would be the worse case you could have, and never know it until the engine would not make power.
I see your point. One more reason to burn 100LL instead of mogas.