Today's Tech Trivia #2

Re: Today\'s Tech Trivia #4

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lead acts as a lubricant. without the lead in the gas, the engine doesn't get all the lubrication it's designed to receive. i THINK there were some cases of people converting to mogas and then not making TBO because of valveguide problems dealing with insufficient lubrication. don't quote me.

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Urban Ledgend/Old Wives Tale Alert !!!!! /ubbthreads/images/graemlins/banghead.gif /ubbthreads/images/graemlins/banghead.gif

Not slaming you Roundout, but this myth needs to be corrected.

Tetraethel Lead is the antiknock additive added to unleaded gasoline to boost it's octane number up to 100. This is to decrease the amount of detonation in the cylinder

It has nothing to do with lubrication of the valves! The only thing it does to the engine is to foul the spark plugs.

Running a airplane engine on mogas can be safley done on certian models. Low compresion, small displacement engines aren't really prone to detonation. These don't need all that octane value. The EAA researched several STCs for common aircraft engines in the 1980s acording to the instructions the timing of the magnetos must be adjusted so that detonation dosen't occour. If this is properly done, a small four cylinder engine can run just fine on 92 octane auto fuel. I personally know a guy who flys pipeline patrol in a C-172 who usses auto fuel to keep his costs down. His engine has 7000 hrs on it, and shows no sign of any problems.



Now to the answer of my origional question.
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Why can't auto gas be used in high altitude aiplane engines? (even if the timing has been adjusted to prevent detonation)

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Auto gas has a lower vapor pressure than 100LL. At higher altitudes this can cause vapor lock.
 
Re: Today\'s Tech Trivia #4

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Why can't auto gas be used in high altitude aiplane engines? (even if the timing has been adjusted to prevent detonation)

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Most 'high altitude' engines are turbocharged and most available mogas STC's are for lower compression engines which are normally aspirated. But I am guessing that the reason you are looking for has to do with the vapor pressure of auto gas. At high altitudes, it will tend to evaporate due to the lower atmospheric pressure. This can cause all sorts of interesting fuel problems, but vapor lock would be the most common. Avgas is formulated to help avoid this, but it can be much more important to follow correct procedures when switching fuel tanks at higher altitudes. Some manuals specify using the electric boost pumps when switching tanks. At low altitudes it is usually no big deal if you omit this, but at high altitudes, you might get a suprise.


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You leave the mixture rich and turn the mag key to OFF. The engine keeps running, what's wrong?

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You either have a broken 'P-lead' or lots of carbon deposits in the combustion chambers.

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Where does the magneto get it's electricity?

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The magneto generates its own current. It is kind of a combined generator/distributor, which unlike a car, will allow the engine to run after a complete electrical failure.

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Who is ultimately responsible for the mechanical condition of the airplane?

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The owner/operator is responsible for keeping an aircraft in airworthy condition. The pilot is responsible for making sure it is safe for flight.
 
Re: Today\'s Tech Trivia #4

Hey anonaman, how about you throw some questions out there? I've tried to include some that will be informative to newer pilots, but I'm sure you can think of some I've missed. You obviously know your stuff.
 
Re: Today\'s Tech Trivia #4

[ QUOTE ]
lead acts as a lubricant. without the lead in the gas, the engine doesn't get all the lubrication it's designed to receive. i THINK there were some cases of people converting to mogas and then not making TBO because of valveguide problems dealing with insufficient lubrication. don't quote me.

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So much for not quoting you! This has some truth, but not in the example you mention, and not usually in aircraft engines. When leaded auto gas became unavailable there were some problems with farm equipment and some cars, due to the valve seats wearing out. Most of the farm tractors did not have high compression engines and did not need the octane, but the cylinder heads were somewhat 'soft' and the lead helped cushion the valves and prevent wear. Some cars also had this problem, and some of the older muscle cars had difficulty with the lower octane ratings. The fix for this was to machine out the valve seats and press in new hardened seats. These engines would have had cast iron cylinder heads that were fairly soft. Newer engines have induction hardened seats (a rapidly oscillating electric field is used to heat the metal in a localized area to heat treat the metal) or have aluminum heads with pressed in valve seats made of steel. This is the reason aircraft engines usually do not have this problem, they have aluminum heads with pressed in valve seats of sufficient hardness to avoid erosion.

You are more likely to have problems with valve guides from too much lead than not enough. If you run rich, you will have lead deposits build up on the spark plug electrodes and on the valve stems, since the engine is not running hot enough to burn it off. This lead build up can cause a 'sticky valve' which can be bad news.

As USMCmech says, lead causes as many problems as it solves. The only reason it is still around is because it is the only successful way they have found to raise the octane rating to the levels needed in some high performance engines.
 
Re: Today\'s Tech Trivia #4

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... The only reason it is still around is because it is the only successful way they have found to raise the octane rating to the levels needed in some high performance engines.

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I wonder about this. Automotive gasolines are available in unleaded formulations up to at least 104 octane. Of course, that's pump method (R+M)/2, so it probably doesn't square up exactly with aircraft lean/rich ratings.

I know there are other problems with automotive fuels (vapor pressure, alcohol, water, etc.) but it seems like the technology out to be out there for a no-lead aircraft fuel.
 
Re: Today\'s Tech Trivia #4

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... The only reason it is still around is because it is the only successful way they have found to raise the octane rating to the levels needed in some high performance engines.

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I wonder about this. Automotive gasolines are available in unleaded formulations up to at least 104 octane. Of course, that's pump method (R+M)/2, so it probably doesn't square up exactly with aircraft lean/rich ratings.

I know there are other problems with automotive fuels (vapor pressure, alcohol, water, etc.) but it seems like the technology out to be out there for a no-lead aircraft fuel.

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The octane numbers for auto and aviation gas are based on diffrent scales. So replacing 87 octane avgas (which isn't made anymore) with 87 octane unleded you get at your local 7-11 probably won't work.

There are several systems in development that hope to allow high performance engines to run on no lead avgas. The one that shows the most promise is an ignition timing system from GAMI in Oklahoma. They are the ones who make GAMIjectors which improve fuel injections systems on many aircraft engines.

I wish I had some money to invest with those guys, 'cause when they get it perfected and 100LL disapears they are going to be in very high demand.

Read this article about leaded gas, it's really informative!
http://www.avweb.com/news/columns/182149-1.html
 
Re: Today\'s Tech Trivia #4

This is a copy of a question Roundout sent me as a PM.

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hey there mr. Mech,

i have a quick question regarding octane ratings. my girlfriend (commecial applicant) called me regarding her systems homework. she was talking about 100 octane avgas being the leanest performance and 130 being the richest performance. first thing that struck me is "wtf is she talking about?" i had her read it to me straight out of the book, and sure enough, the book made some reference to lean and rich performance based on the octane rating of the gas you're burning. now i'm thinking "what is this book talking about?"

i consider myself decent at systems, but this flew completely over my head. i can get you a direct quote out of the book if you'd like it.

TIA for the help, if you have any idea what this book is trying to say, or if i'm blatantly missing something.




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No fair, you made me look stuff up! /ubbthreads/images/graemlins/cool.gif


Currently only 100LL is being produced, so it's really a dead issue. If anybody asks your GF about any other type, tell her to say "I don't care, since they aren't in production anyway."

There were two other grades for aircraft, but they have gone out of production.

80-87 octane (colored red). Mainly used in very old airplanes, WW1 era.

100-130 octane (colored green) This was basicly an expirment that didn't take off. It was an aromatic blend that had diffrent levels of additives (mainly Benzol) which would boost the "octane" rating.

100/130 is a diffrent way of describing the "octane" rating where there isn't any isooctane in the fuel. These are called the performance numbers

100 is lean best power, and 130 is rich best power.



100/130 avgas was very damaging to any rubber parts it contacted. It would eat away at rubber fuel baldders, and hoses. This is probably why it disapeared.



I think I will add this to the "Tech Trivia" thread with your permision. I think it will be informative to others.

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Re: Today\'s Tech Trivia #4

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Currently only 100LL is being produced, so it's really a dead issue. If anybody asks your GF about any other type, tell her to say "I don't care, since they aren't in production anyway."

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btw, that's a UND-type answer /ubbthreads/images/graemlins/smile.gif

edit: (as i sit in class wearing my UND epaulets and ID card holder...) double /ubbthreads/images/graemlins/smile.gif
 
Re: Today\'s Tech Trivia #4

Don't forget the mighty 115/145 octane Purple. It was what the WW2 fighters and big prop liners were made to run on.

USMC is correct, these are all actually 'Performance numbers'. Octane ratings are based on a fuels resistance to detonation. The highest actual octane rating possible is 100. This indicates a fuel with the same detonation resistance as a reference fuel consisting of 100% iso-octane. If a fuel gives the same detonation resistance as a reference fuel consisting of 80% iso-octane, then it is given an octane rating of 80. You get the idea.

In the 1930's much research was done on increasing the detonation resistance of fuels for use in aircraft. This increased during WW2, and had a direct effect on the war. The more compression or boost you can run, the more power your aircraft engines can make. This was obvoiusly a topic of great interest at the time!

Fuels were developed which had greater resistance to detonation that the standard fuel containing 100% octane, and the standard was changed to reflect this as a 'performance number'.

There are many variables that can cause detonation. Compresson ratio, Cylinder bore size, ignition timing, intake air temperature, engine temperature, etc. The list is probably endless. For this reason, a standard engine is used in a lab, they have a variable compression ratio and are operated under set conditions. In a previous post someone mentioned the (R+M)/2 method. This is the method used to determine the octane of pump gas for your car. It is actually the average of two methods. I have no idea if this is the same method used for aircraft fuel.

One of the variables that will affect a fuels resistance to detonation is the mixture. If a mixture is run rich, it will resist detonation because it will burn slower and slow the pressure rise in the cylinder. The same thing happens if it is run very lean. At the extremes, if you run too lean, you get a lean misfire, and if you run too rich, the engine will flood and quit.

Between these two extremes there is best power (slightly rich) and best economy (slightly lean). Since the fuel air mixture burns at different rates, depending on the mixture setting, and since this will affect pressure rise in the cylinder, which will affect how much compression you can run before detonation is encountered, the 'octane' rating is directly affected by the mixture. (That is a run-on sentence if I ever saw one!)

It is my guess that although 100LL does not list these numbers, they are probably included in the fuel specification. It is easier for everyone this way, and like you guys said, it is the only available avgas anyway.
 
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