100LL-Avgas versus MoGas-Autogas
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.
