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Everyone thinks we all know that Autogas is better than 100LL for the Rotax so here I am trying to be more specific as to WHY. This will be an educational Blog for Rotax engines in general. Any comments, thoughts, corrections or additions are welcome. Trying to get the best collective knowledge on the good and the bad. Here you go:

It is common knowledge that Rotax 912 engines like Autogas/Mogas better than 100LL Avgas.

The problem with the 100LL-Avgas for the Rotax engine is that it has lots of lead. Rotax calls it “leaded gas”. The Rotax engine was designed and built for unleaded gas or mogas-Autogas.

Here are some of the advantages and disadvantages for both 100LL-Avgas and MoGas-Autogas:

ROTAX LINE SERVICE MANUAL

Auto/Mogas, or unleaded fuel is better for the Rotax engine since it does not have lead. This becomes obvious in the Rotax line maintenance manual 05-20-00 SCHEDULED MAINTENANCE CHECKS specifically:

  • Change the Spark plugs at 200 hours if leaded (100LL) fuel is used more than 30% of the time. With Mogas/Autogas the spark plug replacement is every 400 hours, twice as long.
  • Change oil and filter every 50 hours for Leaded fuel (100LL) and clean the oil tank every 50 hours. Using Auto/Mogas change the oil and filter every 100 hours and clean the oil tank every oil every 100 hours. Again, twice as long. Typically the industry standard is to change the oil every 50 hours for Autogas (which I do) and leaded gas (100LL) every 25 hours.
  • Check the gearbox overload clutch every 600 hours using leaded fuel (100LL/Avgas) and 1000 hours using Autogas.

GENERAL STATEMENT

It has been said that engines will “last longer” on Autogas/Mogas because less gumming up of the gearbox and general condition of the engine. The lead tends to build up on the valves and valve openings allowing the compression to lower faster with 100LL leaded Avgas. Generally the lead creates more wear in the engine components.

LONG TERM STORAGE

100LL is better for long term storage (say over the winter) since it does not gum up the carburetors or lose it’s octane rating over time. Typically, auto gas deteriorates after 3 weeks of storage and should be drained and replenished. The amount of storage time the Autogas will lose its octane depends on the storage median, vented less or sealed container more, but generally Autogas loses its octane over time.   In my decades of interaction with students and customers, some of the manufacturers recommend 100LL. Why is this? After talking with well know technical service/company owners/manufacturers the party line was 100LL was more reliable. Digging deeper after ruthless grilling and presenting the facts, the top experts of the manufacturers finally admitted that they were getting so many service calls from customers that the engines would not start or running badly after leaving them for the winter or longer with Mogas/Autogas. It became company policy to recommend 100LL to avoid consistent and bothersome service calls. So, the extra maintenance and reduced engine life is less of a problem than the annual calls after winter storage for the manufacturers. Simply, 100LL is better for long term storage.

VAPOR LOCK

100LL has less chance of vapor lock. Vapor lock is when the fuel temperature is high and the pressure low to allow the fuel to boil and create an air bubble in the lines. BAD. This is where some suggest a 50/50 mixture to avoid flying at high altitudes and hot temperatures. I use 100% Auto/Mogas in my Sling and WSC trike with the 912iS fuel injected engine and never had a problem with vapor lock typically at high density altitudes 12,000 feet and above. Thousands of hours.  I did have a slight sputtering one time with the 912ULS carb engine with low fuel that had been heated buy the oil cooler and a very hot day. I switched on the auxiliary electric fuel pump and it cleared that up right away. It depends on the installation and if there is allot of heat around the fuel lines that are not ventilated well creating a greater chance of vapor lock. Now I fly with the fuel injected engine and very little exposure of the low pressure before it hits the fuel pump which pressurizes it to 60 to 70 PSI before the fine fuel filter and 40 PSI after the fine fuel filter. I believe in the fuel injected engines,  the chance of vapor lock is before the fuel pump. That is why from the fuel tank to the fuel pump the course fuel filter should be clean to avoid any pressure drop from the course fuel filter to the fuel pump.

FUEL SUPPLY

100LL generally has better quality getting from fuel supply to the aircraft fuel tank. It typically does not transfer via a separate fuel tank, thus, less chance of being contaminated. The Autogas/Mogas many times will go into separate five gallon tanks or larger allowing more possible contamination. I have seen some pretty shabby, old five gallon fuel tanks used for this. This fuel transfer can be controlled by the operator with a filtering funnel or simply using a known clean filtering transfer tank. Additionally, there is a chance the fuel supplier has old contaminated fuel. The simple procedure is to use fuel from a high volume quality gas station and clean 5 gallon tanks. Use a filtering funnel if you really want to be diligent.

GENERAL OPERATION

The amount of lead is directly proportional to the amount of 100LL used. When I fly cross country or get low on fuel, I do not think twice about going to the local airport fuel island, fuel farm or what ever you call it and adding 100LL. But generally I try to keep this to a minimum.

OIL USED FOR 100LL AND AUTOGAS

For those ROTAX people reading this, skip this paragraph since it may offend you. This is just my opinion from various reliable sources that will not be named.  Rotax has brilliantly developed the “Rotax approved” oil for use in the 912 series. This special formulation is to adapt the oil for use with 100LL fuel. It does the job well. However, the Mobil 1 motorcycle oil works great and I notice a lower oil temperature on my 5000 foot climbs on hot days.  

ALCOHOL IN THE AUTOGAS

Lastly, the myth that the Rotax engine should not use alcohol/ethanol, simply not true. This myth started when the old fuel systems had fittings and tanks that were not compatible with alcohol/methanol mixtures and would dissolve and/or deteriorate in some form. Typically now, the aircraft using Rotax engines have fittings and tanks compatible with alcohol mixtures just like your car. The manufacturer of the aircraft should warn against alcohol in the fuel for their specific aircraft, it is not for the Rotax engine. The Rotax 912 series has no problem with 10% tested alcohol mixtures and pilots are running 15% in some countries with no known problem.

IN CONCLUSION

Each fuel, 100LL or Autogas has its advantages and disadvantages for specific operations and hopefully this will help you determine which fuel is best for you.

 

 

  • Re: 100LL-Avgas versus MoGas-Autogas

    by » 6 months ago


    Hi Paul,

    I have been flying behind a Rotax 912 ULS for about 14 years now and have never had a ULP problem with my engine.

    I live in Australia where very few airfields have ULP supply on site.

    "..........Typically, auto gas deteriorates after 3 weeks of storage and should be drained and replenished". 

    ULP stored in an airtight container at a minimum of 75% full will maintain quality for at least 6 months.

    While ULP will start to deteriorate, in a vented (aircraft fuel tank) container, within a few weeks (depending on temperature and tank level) it can be brought back to almost full standard, by adding a quantity of fresh fuel. As for the amount of "fresh", without any research, I usually work to a minimum ratio of about 1:3 - works for me.

    Note:

          I would not do this for fuel that was say 3 months old - this would be drained & used in the mower.

          I do not refuel after a flight, so when I refuel before the next, the amount of "old" fuel may be very low, giving a fresh to old ration much        greater than the above.

    "100LL generally has better quality getting from fuel supply to the aircraft fuel tank. It typically does not transfer via a separate fuel tank, thus, less chance of being contaminated. The Autogas/Mogas many times will go into separate five gallon tanks or larger allowing more possible contamination. I have seen some pretty shabby, old five gallon fuel tanks used for this. This fuel transfer can be controlled by the operator with a filtering funnel or simply using a known clean filtering transfer tank. Additionally, there is a chance the fuel supplier has old contaminated fuel. The simple procedure is to use fuel from a high volume quality gas station and clean 5 gallon tanks. Use a filtering funnel if you really want to be diligent."

    At home I have 10 x 20L plastic fuel containers filled with 98 RON petrol from the nearest reliable service ("Gas") station. I refuel using a 12V petrol pump to decant the containers and always filter in, using a Mr Funnel or similar.

    On long trips I carry 2 x 20L collapsible fuel bladders & the above 12 V refueling pump (powered from aircraft start battery). When I need to refuel, I beg or pay for a lift to the nearest suitable fuel service station to fill my bladders OR on very rare occasion have refueled with AvGas.

    In short, I know that the quality control for ULP is not as for AvGas, so it behoves me to do my best to reduce the chances of putting contaminated fuel in my tanks.

     


    Thank you said by: Paul Hamilton

  • Re: 100LL-Avgas versus MoGas-Autogas

    by » 6 months ago


    Sean,

    That is interesting as to how long the Avgas can stay good.Vented verses unvented is new to me. Yes On cross country I use 100LL since it is easier.


  • Re: 100LL-Avgas versus MoGas-Autogas

    by » 6 months ago


    I just did a complete edit on the main blog based on some new topics and Sean comments.

     


  • Re: 100LL-Avgas versus MoGas-Autogas

    by » 6 months ago


    Hi All

    First thanks for a discussion on fuels Paul, while it is not new information but many on the forum are new and have not seen past posts. 

    A bit about storage.  AVGAS is good for at least 1 year  however that is if it is in a metal container and sealed.  (think fuel drums stockpiled in Alaska for example)  Sealed keeps in the lighter volatile elements and AVGAS has no oxygenates (no ethanol) that can promote corrosion. The only issue with leaded AVGAS is the lead.  We can deal with the lead if you use a fuel treatment with TCP. "TCP stands for tricresyl phosphate which is an effective tetraethyl lead scavenger. During the combustion process TCP chemically converts TEL to lead phosphate, which is less conductive, thereby reducing spark plug fouling."  This quote is from Alcor, the makers of Alcor TCP fuel treatment.  Another TCP fuel additive is Decalin RunUp fuel additive.  Alcor mets a certification of the FAA and Decalin is for experimental and not certified.  They both work when used correctly.  

    In the Rotax the issue is colder cylinder heads (liquid cooled) and this may result in heavier lead depositess than you might see in an air cooled engine.  The problems with lead "packing" in the overload clutches and sprag clutches are pretty much solved within the last 10 years by Rotax making oil slinger holes in both of those parts for some time.  Reduced oil change intervals as recommended by Rotax solve the major build up of lead deposits.  The use of a correct oil also makes a big difference.  Stay away from automotive oils and use the Rotax recommended brands.  In the near future we should see the new fully synthetic approved for all the engine models once field testing confirms dynamometer tests.  It has been designed to reduce lead buildup and still have good wear protection. 

    Auto fuels are varied as everyone has noted and we see even at the fuel pumps.  Aside from octane we have a lot of issues with storage.  MOGAS is the common name for all auto fuels.  You should break them into groups, no ethanol and with ethanol, they vary substantially. Looking at fuel without ethanol the fuel companies will tell you they are good for 6 months.  Exactly where they start that 6 months is another guess we have to make.  It would be safe to assume that is from the time it leaves the bulk station to the supplier.  Since we do not really know that time frame most should work on using your fuel faster and not storing it for long periods.   A major concern with blended fuels, ethanol in particular, is the fact that ethanol contains as much as 30% oxygenate.  This when in high amounts of moisture can lead to corrosion.  While fuel separation from saturated fuel is rare it might be a concern if you try to store MOGAS with ethanol in any vented tank.  (in you aircraft wing for example) Fuel companies state that the storage time for ethanol blended fuels be cut in ½ down to no more than 3 months in ideal conditions.  

    We can't consider fuels without looking at Reid Vapor Pressure, RVP.  This is the measure of fuel volatility at 100F by a prescribed method that determines how much vapor is released by the fuel.  (ASTM D323 standard) In many countries this is not normally an issue as the year round temperature spread is not enough for the fuel supplier to change the blending for seasonal changes. It is however a major issue in the USA and Canada.  The RVP can double with winter fuel (more volatile) The EPA in the USA mandates 7 to 8 PSI for summer fuels.  Higher numbers also are responsible for higher environmental releases of volatile gases.  AVGAS is fixed and is the same year round.  This also explains why AVGAS starts so bad in the winter in cold environments.  Better get the plane in a hanger or heat the engine if you want to start it in Montana in winter.  To solve this the fuel supplier switches to a more volatile fuel blend by adding Butane.  This is easy to produce and actually lowers the cost of the fuel.  most of us are aware that winter fuel is always less cost than summer and that is the reason. Summer fuel, mostly mandated by EPA to switch over in the Spring will have about 7.8 psi RVP, the winter fuel however can be as high as 14 psi.  (almost double the vapor lost) Here is the bit issue.  If you try to use winter fuel in summer, hot conditions, you will most likely experience vapor lock.  By the way another dirty secret about winter fuels is the fuel companies can get exemptions to increase the RVP 1 point when they add ethanol.  This means winter fuels with ethanol can have up to 15psi RVP.  Why you ask?  Because they argue that the oxygenates are 30% of the ethanol they add and this vapor loss is not as bad !  Read all this messy stuff at the EPA websites.  They also host all the information on which states and even to the county level exemptions for sale of winter gas in summer happens.  One might ask also how is this allowed?  The bulk of transportation using auto fuels now runs with engines that are computer controlled with anti knock solutions within the engine designs.  They basically have no room for our issue with aircraft.

    Another solution is to us a new AVGAS that is unleaded.  They are developing fast and from users within the USA seem to be a solution to lead and an approved AVGAS with enough octane to work with Rotax.  Swift fuel UL94 is one good example of a fuel that seems suitable.  The RVP is low enough to be safe in both winter and summer.  

    I can go on but this is far too much as it is so let me summarize what I believe is important. 

    First, AVGAS is not a bad thing if you address the lead with something like TCP and frequent oil changes.  Try to be sure that your cylinder head temperatures are normal and not running excessively cold.  

    Second, MOGAS without ethanol and at the right octane for your engine is great but personally I would want to use it up within 3 months, ½ the time the fuel supplier says.  Don't use winter fuels in hot environments.  (don't ever save winter fuel) 

    Third, MOGAS that has ethanol is fine but not over 10% by volume.  Storage time should be ½ of the claims it will last so no more than say 6 weeks.  Personally I like to see it gone it 4.  Never use winter fuel in summer. 

    Lastly, RVP is critical to prevent vapor lock.  (as well as a good fuel system with boost pump, return line, etc.) If you are unsure of the blend you purchased and concerned about winter fuel or having stored it longer than you want...do what Paul and Sean recommend.  Blend it with some AVGAS.  if it is a small amount just burn it up in your car in small amounts.  (don't dump it on the grass) 

    If you suspect fuel issues try some AVGAS mixed into your tank.  it may save you a lot of troubles later.  

    Cheers


    Thank you said by: Sean Griffin, Paul Hamilton

  • Re: 100LL-Avgas versus MoGas-Autogas

    by » 6 months ago


    More juicy details I will have to digest. Really appreciate the time and effort. With a more recent event, the MOGAS is blamed for corrosion, or some debree (unknown what at this time) in the fuel tank.  After 2 decades and over 7000 hours of MOGAS, in multiple aircraft, my fuel tanks are super clean using MOGAS, just looked inside 2 where I can see the bottom fuel drain. As the ethanol absorbs the water, can the dissolved water create corrosion in typical fuel tanks? Second, I have heard many praise TCP for 100LL to get rid of the lead, and I think you explained it above, it only helps the spark plugs or gets rid of the lead problem all together? Is the Rotax suggested half oil changes still needed with TCP. I am generally not that up on TCP.


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