Unleaded avgas

FORANE

FORANE

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FORANE
After reading the synthetic oil thread I wondered what the implications of removing lead from avgas would have.

I googled the benefits of lead in fuel and quickly found some purported benefits:
reduce engine knocking, boost octane ratings, and help with wear and tear on valve seats

I know some are using mogas. Other than the bureaucracy, whats to stop just removing lead from avgas? What real effect is likely? I have heard from some who run mogas that the plane runs better due to less spark plug fouling.
 
Some fuel systems wont run it - mine won't - hydraulic lock. Some High Performance engines won't run it either. And those engines make up most of the 100LL used.
 
EdFred said:
Some fuel systems wont run it - mine won't - hydraulic lock. Some High Performance engines won't run it either. And those engines make up most of the 100LL used.
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So now that Ed has given an unhelpful post, let me try to be helpful.

Ed, your plane will run unleaded fuel. It just won't run MoGas, which is what you mean. If you took 100LL and removed the lead today (which produces around a 94 rating), your plane would run it just fine. Lead doesn't have anything to do with vapor pressure (you aren't having hydraulic lock - that's when your cylinders are full of fuel and they physically can't move through the compression stroke).

Certain engines need the antiknock properties of 100LL. As Ed correctly points out, those engines make up most of the AvGas consumed. Navajos, 421s, Malibus, Cirrus... basically all of the turbocharged or high performance engines would suffer detonation under certain conditions and not work with a fuel that didn't match the anti-knock properties of 100LL.

The people who are running MoGas with better results than 100LL are in the lower horsepower range. Yes, those engines run MoGas today just fine (provided the airframe fuel system can handle the vapor pressure aspects, which some can and some can't). If I threw MoGas in the 414, I would pretty quickly have cylinders come off the side of the engines. For this reason, the FAA has (intelligently) stated that an unleaded fuel needs to meet or exceed the anti-knock properties of 100LL.
 
I have 10:1 pistons in the Lancair. I know that means it requires high octane fuel, but is that a problem for unleaded?
 
FORANE said:
I have 10:1 pistons in the Lancair. I know that means it requires high octane fuel, but is that a problem for unleaded?
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I would only run 100LL or a fuel with equivalent anti-knock properties with 10:1 pistons.
 
My Mooney has low compression cylinders but still has no STC for Mogas. Not a big worry really, getting the stuff without booze hereabouts isn't all that facile. Got gas for $3.99 today and was quite happy about it.
 
The guys who did the mogas STC are the ones that claimed the Comanche fuel system has hydraulic lock not vapor lock hydraulic lock those are their words not mine
 
The necessity of lead is largely illusive in most engines. You need the appropriate octane and as pointed out the vaporization characturistics of AVGAS. Believe that 100LL will be gone in a few years when the FAA moves along (and they are moving) with the alternative unleaded fuels.
 
EdFred said:
The guys who did the mogas STC are the ones that claimed the Comanche fuel system has hydraulic lock not vapor lock hydraulic lock those are their words not mine
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They used the incorrect terminology.
 
flyingron said:
TBelieve that 100LL will be gone in a few years when the FAA moves along (and they are moving) with the alternative unleaded fuels.
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The FAA has been moving ever since I got into aviation in the 90's. I believe they will, but you'll forgive me if I don't hold my breath.
 
FORANE said:
After reading the synthetic oil thread I wondered what the implications of removing lead from avgas would have.

I googled the benefits of lead in fuel and quickly found some purported benefits:
reduce engine knocking, boost octane ratings, and help with wear and tear on valve seats

I know some are using mogas. Other than the bureaucracy, whats to stop just removing lead from avgas? What real effect is likely? I have heard from some who run mogas that the plane runs better due to less spark plug fouling.
Click to expand...

It's a lot more complex than octane and valve seats. Any replacement fuel must also be harmless to the rest of the fuel system components. They also must pass environmental muster and be readily available.

I think it may be the availability of TEL (or lack of same) that ultimately kills leaded avgas. Last I heard, there was only one supplier left.

Also, avgas is such an insignificant percentage of total gasoline production that most fuel companies only spend a few refinery-days every year producing it. It also requires downtime after production to (literally) "get the lead out" of the works before the refinery equipment can be used to produce other gasolines.

Rich
 
According to the US Energy Information Administration, the US consumed approximately 175 million gallons of avgas last year. That figure has been declining by about three percent per year for decades, with an occasional swing upward, and an occasion big dip downward. I'm not sure what the wholesale price of avgas is, maybe $2.50 per gallon? It's still a good chunk of change for the couple of suppliers that still make it, but it's a declining market, and I'd wonder how many refineries would be willing to make the lead free version. Certainly, it would be less of a hassle to them than the current leaded stuff, since everything leaded fuel touches has to be decontaminated before it can be used for an unleaded product, which is everything else they make.

Avgas usage is very seasonal, in the summer months consumption typically goes up by 50% over the winter months, with spring and fall somewhere between. The seasonality makes me wonder if the majority of avgas goes into the higher powered aircraft, I'd think they'd be out year round.
 
I'm loving my low HP Continental that burns pure mogas. 'Course, it's a PITA to haul in small quantities.
 
Ted DuPuis said:
...Certain engines need the antiknock properties of 100LL. As Ed correctly points out, those engines make up most of the AvGas consumed. Navajos, 421s, Malibus, Cirrus... basically all of the turbocharged or high performance engines would suffer detonation under certain conditions and not work with a fuel that didn't match the anti-knock properties of 100LL.

The people who are running MoGas with better results than 100LL are in the lower horsepower range. Yes, those engines run MoGas today just fine (provided the airframe fuel system can handle the vapor pressure aspects, which some can and some can't). If I threw MoGas in the 414, I would pretty quickly have cylinders come off the side of the engines. For this reason, the FAA has (intelligently) stated that an unleaded fuel needs to meet or exceed the anti-knock properties of 100LL.
Click to expand...

To supplement Ted's post above, the efficiency (net mechanical work produced per unit of fuel energy input) of a spark ignition (Otto cycle) engine is directly related to its compression ratio. The higher the compression ratio the higher the engine efficiency. As air is compressed it heats up. The higher the compression ratio the higher the temperature rises. What places a practical upper limit on the compression ratio is the potential for autoignition - the fuel/air mix in the cylinder ignites too early from the heat of compression, before the flame front from the spark plug (this is engine "knock").

Higher compression ratio engines are more efficient, important in aviation, but as Ted notes these higher output engines need a higher octane fuel like 100lLL to avoid the autoignition regime, and mogas isn't generally capable of achieving that. TEL is not only an octane booster but also "lubricates" the exhaust valve seats. When lead was phased out of automotive fuels (right during my ill spent youth hot rod days) the valve and valve seat materials had to be upgraded, with hard alloys like Stellite used on the valve seat facings. The removal of TEL from avgas may require similar adaptations in our engine heads, and this may be just one of numerous challenges to phasing out 100LL.

Regardless, just taking out the lead won't work as the remaining fuel without some other additive octane booster does not have enough octane to be used in high compression aviation engines.
Maybe the FAA & EPA, in their infinite wisdom, will require us to overhaul with low compression pistons :mad:
 
RJM62 said:
...I think it may be the availability of TEL (or lack of same) that ultimately kills leaded avgas. Last I heard, there was only one supplier left.

...
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Unfortunately, that "miracle economy" called China is producing and putting on to the global market considerable amounts of TEL illegally. When I lived in the Persian Gulf a few years ago none of the gasoline in the region was unleaded, and of course none of the cars sold into the local market had catalytic converters. It is this way in most of the developing nations, which have been some of the fastest growing automotive markets in the world.
 
IIRC, removing TEL from 100LL would give you 94UL. If you had to use that in any of the high compression or turbocharged engines that require 100LL, you'd have to derate them, and that would severely limit the utility of those aircraft.

Long term, I question the likelihood that avgas of any kind will exist. I do think it would be wise for general aviation to take a two pronged approach, with a mogas derived fuel for the 200 hp and under folks, and some sort of compression ignition engine for the high powered set. I'm concerned that the oil companies will have much interest in making avgas when it becomes an even lower volume product than it already is. But, that could be 20 years from now.
 
We've been using ethanol free mogas in all the Rotax LSAs and my brother's Luscombe for years. It works great as long as you don't let it sit for more than a month or two. It's getting harder to get it in quantity since an update to Georgia law now prohibits dispensing fuel into containers more than 8 gallons. We will probably switch to Swift Fuel 94. Works great in the lower HP and non high compression engines. It also doesn't degrade like mogas either (2 + year shelf life) so the classic car guys love it.
 
GRG55 said:
Maybe the FAA & EPA, in their infinite wisdom, will require us to overhaul with low compression pistons :mad:
Click to expand...

The problem would be that low compression pistons (and the resultant reduction in horsepower) would completely decertify all airplanes since it would change the performance characteristics.
 
Who else thinks we'll have the same discussion in 20 years and the same exact aircraft will still make up the bulk of the fleet.
 
cowman said:
Who else thinks we'll have the same discussion in 20 years and the same exact aircraft will still make up the bulk of the fleet.
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Pretty likely.
 
N659HB said:
I'm loving my low HP Continental that burns pure mogas. 'Course, it's a PITA to haul in small quantities.
Click to expand...

I would have put this in to my thinking if I could readily get gasoline without booze in it. But I can't, so I won't.
 
I learned to fly in my dad's 1956 C172, with an O300, STCed for mogas. We had valve guide problems for 2 or 3 annuals. Went back to 80 octane (RIP), and no more guide problems. Maybe the older engines need the lead more than the newer versions.
 
I learned to fly in my dad's 1956 C172, with an O300, STCed for mogas. We had valve guide problems for 2 or 3 annuals. Went back to 80 octane (RIP), and no more guide problems. Maybe the older engines need the lead more than the newer versions.
 
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