AVGAS OR AUTOGAS

AVGAS OR AUTOGAS

My Cessna 150 has the STC for autogas.. but since I have owned it for the past 2 months I have only used 100 avgas.. I have been told t o never use auto gas .. and then I have been told to use auto gas over avgas. Can I get someones expertise on this subjects.. also I was flying yesterday and when pulling the carb heat to check it.. at 2300 rpm.. the engine sputtered a little.. I left it on but didnt correct.. so I pushed it off. I have been noticing it a little more often when fully pulled out.. could this be Water in fuel ??? what do you think??

Barry

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Re: AVGAS OR AUTOGAS

Nasty subject..Starts big rawls..  Sorry but this subject ought to wither.

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Re: AVGAS OR AUTOGAS

There are alot of opinions on this subject.  Here is mine.

I've owned three planes that have had the autofuel STC.

I flew around in my Piper Colt on purely autogas that was available at my home field.  Seems to work ok, no problems to speak of.

I then owned a Piper Tri-Pacer, again I started with autofuel but swicthed to 100LL in the winter because the plane seemed to be easier to start.  When spring and summer came I started to use a mixture of autofuel and 100LL.

Now I own a C172 and I only burn 100LL in it.  Autofuel is not available at my home airport.  There are a few fields that I visit from time to time that have autofuel so I usually have a half and half mix of autofuel and 100LL.

Since autofuel is not generally available at the fields I visit on cross countries that makes it easy to not use it.  And I'm way to lazy to haul it to the airport myself smile

     Dave

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Re: AVGAS OR AUTOGAS

Plug the subject line words into a search engine and you will have enough reading entertainment to last a long long time!
Glenn

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Re: AVGAS OR AUTOGAS

Barry:

When you pulled the carb heat on it richened the mixture significantly.  The engine may have been rough as a result of an overly rich mixture.

Auto fuel and Avgas produce almost the same BTUs of energy.  They result in the same HP output.  Auto fuel has a shorter latency period and as a result seems to burn faster (it really doesn't burn faster, it gets started burning quicker). This results in higher internal cylinder pressures and higher CHTs but lower EGTs.  For this reason, the use of autofuel narrows the detonation margin on your engine.  That is not really a big issue in the smaller engines like it is in the larger TCM and Lycoming engines.

Besides Mogas kinda stinks!  <g>

Walter Atkinson

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Re: AVGAS OR AUTOGAS

fuel and oil  is the least expensive part of flying, why scrimp here.

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Re: AVGAS OR AUTOGAS

What octane autogas are you guys talking about?

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Re: AVGAS OR AUTOGAS

The STC for autogas specifically states the required octane minimum,and also makes it very clear that there is to be NO alcohol in the fuel...That being said,I have the autofuel STC in my 150K,and have run it from time to time exclusively on EXXON mid-grade.The mid-grade complies with the STC,and the engine ran MUCH smoother and seemed to be happy with its fuel.This was done before the major...since major,Ive run exclusively 100LL(at the advice of my A&P) and have just now got about 52 hours on it.
One of the benifits I derived from the autofuel is NO carbon fouling,quick starts,and "well over factory specs" climb out.
Autogas is rough on O rings,and rubber hoses,and it indeed smells forever if you get it on you....There are likely 200 responses about mogas from previous posts on this forum,and it will likely come up again...I suspect youll get good advice from both sides of the issue.Pro and con......then do what you wanna do.

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Re: AVGAS OR AUTOGAS

Just to confirm the opinions of those who feel I'm opinionated:
  Autogas is for Autos and usually, but not always, works OK in them and other ground-based engines that have fresh fuel, and is good for parts cleaner if used outdoors..
  Avgas is for airplanes.  It works just like it's supposed to, stores a long time, causes no serious problems.
  I only use Avgas in my airplanes, and usually also use Avgas in my lawn mower, boat, tiller, power washer, etc. because it doesn't ruin the carburetors during long winter-month storage like autogas does.
  I've noticed a curious thing about autogas users.  They become very agitated, defensive, and/or aggressive when their use of autogas in airplanes is questioned.   Avgas users never do this.
I wonder why this is so.

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Re: AVGAS OR AUTOGAS

George...to drift only slightly from the subject...the Avgas in the mower, boat, etc.  do you have any idea how long it will store for?
I do know that the "newer" autogas does get "stale" quicker than it used to. I hadn't really given thought to Avgas in my boat..etc.
Michael

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Re: AVGAS OR AUTOGAS

Thanks guys.. Im going to stick with AVGAS. I feel the same way ..

Thanks

Barry

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Re: AVGAS OR AUTOGAS

Im assuming the comment left about aggressivness/defensiveness is directed at me,I have used autogas in my aircraft on many occasions,and it has indeed done me well.Whatever one chooses to use in his/her personal aircraft(as long as its legal) is their decision.
My comments on the matter were purely from experience(my own)......It appears that merely replying to a question now a days draws criticism...Its time to takethe COO site out of the computer...Tailwinds to all.....

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Re: AVGAS OR AUTOGAS

Alan, I dont think it was directed at you... alot of pilots feel avgas is for aircraft.. however my A&P Swears by AutoGAS and says the 0-200 were meant for the lower oct. fuel and will perform alot better.. he did tell me about the cons as well but said he perfers the autogas and directed me to use it also.. but the hassle of carrying 5 gal of gas to pour in the tanks are anoying.. When I bought the plane he used AV Gas so I guess unless it gets way out of range I will stick to AV..

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Re: AVGAS OR AUTOGAS

I have a customer from Connecticut that is paying the same for auto fuel as avgas.
I agree with George. I also use avgas in all my lawn and garden equipment; dont have to worry about the carbs gettin gummed up, starts right up every spring season.
Have you ever noticed that the octane rating of auto fuel offered at the pump is lower at higher elevations. I'm just guessing that there is a relationship between available oxegen and detonation. George or Walter could explain this.

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Re: AVGAS OR AUTOGAS

Del:

* Have you ever noticed that the octane rating of auto fuel offered at the pump is lower at higher elevations. I'm just guessing that there is a relationship between available oxegen and detonation.*

Detonation is a very poorly understood event and we have been doing a lot of research into the issue for the FAA and others.  We recently gave a program on this topic to the FAA engine directorate folks where we demonastated these issues in real time on the engine runningon the test stand.  We can WATCH the detonation occur that way.

The availability of O2 is not a factor in detonation... I have no idea why there would be an adavntage to a lower octane fuel used at higher elevations.  Airplanes certianly use *higher elevations* and we use 100 octane. <g>  (just a thought!)

The detonation margin is altered by the latency period of the fuel.  The latency period is the time it takes for the flame front to become organized and begin to burn after the spark event.  In auto fuel (lower octane) the fuel gets organized and burning quicker than in 100LL.  This makes the effective timing advanced and the peak internal cylinder pressures higher.  This makes the CHT's run higher.  Higher CHTs and higher ICPs result in a narrowed detonation margin.

So, by comparison to 100LL, auto fuel (lower octane fuels) run lower EGTs, higher CHTs, and have a narrower detonation margin.  In lower HP aviation engines this does not seem to present a problem and that's why they can get the STC for those engines.  In the higher output engines this is a BIG No-No.  Ag Operators who run Mogas in their 985s and 1340s find that retarding the timing one or two degrees offsets the advanced effective timing quite nicely.

When PRISM is available, altering the timing will be what will make us able to run these engines on lower octane fuels witohut lead.  We are able to operate the IO-540J2BD, arguably the most detonation-prone engine in the fleet on 95UL (100LL without the lead).

(Mogas still stinks!)

Walter Atkinson
Advanced Pilot Seminars

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Re: AVGAS OR AUTOGAS

Thanks for the reply Walter,

Walter wrote:
"The availability of O2 is not a factor in detonation... I have no idea why there would be an adavntage to a lower octane fuel used at higher elevations. Airplanes certianly use *higher elevations* and we use 100 octane. <g> (just a thought!)"

I agree with everthing you said above, but I'm a curious person, and have a few questions;
Is compression ratio a factor in detonation? Or, can it be a factor?
Do higher compression engines benifit from the use of higher octane fuels, to control detonation?

My reason for these questions is simply to learn. I know you'll correct me, if I'm wrong, and I hope you will. So, I'll go out on a limb here and say that there is not an "advantage" to lower octane fuels at higher altitudes, just that the higher octane fuels become less and less necesary the higher you go, because of internal cylinder pressure imediately prior to ignition.
Compression ratio is a constant. However, it would seem logical that the actual pressure of the compressed mixture in the cylinder would be directly proportional to the density altitude; that becomeing less and less as you gain altitude.
If you performed a pressure test on a cylinder, say, here in Mena at 1100 ft elevation, then did the same test on the same cylinder on Pikes Peak, wouln'd you realize a difference in the pressures inside the cylinder, the same as you would with the atmosphere outside the cylinder? And if so, wouldn't the likelihood of detonation deminish somewhat, thus less of a need to control the combustion.
Just some thoughts. Thanks for your time, and sharing your knowledge!
Del

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Re: AVGAS OR AUTOGAS

Del,
I don't have the ability to be overly technical on this subject, I'm tossin' in my 2 cents in worth anyway... 

As the compression ratio increases so does the octane rating.
If the octane rating is too low for the engine's requirements, you're just going to wreck your engine.  Higher octane than necessary is of no benefit, especially $wise.

You may be right about lower octane *requirements* at higher altitudes.  I saw 85 octane gas (auto) once, in Colorado.  Everyone was driving around at 5,000'.  Auto fuels are "blended" for regions...or at least, they used to be.

Our airplane engine fuel requirements were designed for operation at sea level..  You may not NEED 80/87 (or 100LL) to cruise at 5,000 or 8.000', but you DO NEED the octane rating to get you there.
Ever hear an "old clunker" car going up hill, a knockin' & a pingin'?  Often due to someone just too cheap to put in "hi octane".

You're right about the compression ratio remaining constand and the compressed  mixture being directly proportional, but not so with the compression test.  If you squish 80psi worth of air into an airplane cylinder or 3,000psi into a SCUBA tank, the pressure inside the cylinder will be the same inside that cylinder not matter what the altitude.  So pumping pressure into the cylinder as done in a compression check at different altitudes would not be relevant to the air that is "sucked" in during operation and compressed.

Michael

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Re: AVGAS OR AUTOGAS

Del:

You are right to a small degree, but the major factor in internal cylinder pressures and heat are the effective timing (ThetaPP).  While MP plays a role in that, I'd have to label it as a second-order effect.

Now, that said, if at high altitude in a car, lowering the octane could result in slightly better power as a result of this advance in the effective timing.  Auto engine's spark timing is differnt than airplanes, so that *might* work.  I gotta cogitate on it some more.  <g>

Besides, I don't have to consider the effects of the Mena Triangle!! <VBG>

Walter

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Re: AVGAS OR AUTOGAS

Thanks Micheal and Walter for your replies.

The Mena Triangle? Could be a very interesting book! Or a good article for aviation publications, to make pilots aware, who are unfamiliar with our "triangle".

Kind regards to everone,
Del Lehmann

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Re: AVGAS OR AUTOGAS

Is there a way to get Autogas fuel tested to see if it complies with an STC.

Im in Venezuela and here the Autogas rating system is a little different than in the USA. I have an STC for my 182, but I heve never used it because i dont know if the fuel complies.

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Re: AVGAS OR AUTOGAS

> The Mena Triangle? Could be a very interesting book!
> Or a good article for aviation publications, to make pilots
> aware, who are unfamiliar with our "triangle".

The Mena Triangle??? That must be that strange place in western  Arkansas where aircraft go in, and mysteriously emerge  later different colored.

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Re: AVGAS OR AUTOGAS

I recently acquired a C172K which is eligible for the EAA AutoGas STC, which I will be buying soon.  This is a form of insurance to allow me to legally use auto gas in the event of no availability of 100LL.  In an airport meeting the other night, one of the members (a professional charter pilot) said there is only one supplier of 100LL in the country.  If this is the case, I want to be ready to fly if 100LL is no longer available at any time.  My preference is still to use Avgas if it's available, especially since the savings vs the work and inconvenience of hauling Mogas to the airport isn't worth it now...also Mogas does stink quite a bit.  As an additional comment, to fuel the fires of both sides of Avgas/Mogas I will say my engine overhauler (at my last major OH at a good overhaul shop) said he had no problem with my using Mogas and there were no warranty issues.

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Re: AVGAS OR AUTOGAS

Is anyone aware of the difference in boiling points of AVGAS
and MOGAS??? Does VAPOR LOCK scare you?? MOGAS at
higher altitudes may be more subject to vapor lock. Anyone
know???

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