Mogas or avgas

Hi James,

In Australia we tend towards standardised terminology, when describing petrol  types.

By law, fuel sold with an ethanol content must be so described. A petrol with the letter E indicates an ethanol content eg E10 which contained up to 10% ethanol. Supposedly E 85 (for very specific vehicles) is available but I have never seen it.

Non ethanol fuels are simply 91 RON, 95 RON and 98 RON. (RON = Research Octane Rating) 95 & 98 RON are suitable for Rotax 912's

The above descriptors are standardised across all states (there are some geographically specific fuels)

It would seem that in the USA you have all sorts of descriptors, (clear, non ethanol, mogas, marine, winter/summer, gas, ??)  which seems to lead to some confusion. Added to this is the winter/summer blends, causing further complication.

Ethanol has a higher knock/Octane rating than petrol but a lower energy content. So octane rating unlikly to be an issue however users of ethanol blends, will find that their engine delivers slightly less power than non ethanol fuel.

Rotax allow the use of E10 in their engines. To take advantage of this, the whole of the fuel supply system, to the engine, must be ethanol compatible. I do not know of anyone using E10 in their aircraft.

"Is 92 octane clear exactly the same as 92 octane mogas except without the ethanol"

Speculation:

If its 92 Octane, with or without ethanol,  it can be used in your Rotax. If it containes ethanol, you need to check with your aircraft maker for the fuel supply system compatibllity.

If in doubt, avoid using ethanol fuels.😈

Hi all

First, there seems to be confusion about fuels when it comes to what is really important.  Yes octane is one of those, the requirement is not an option, too low and you can have engine detonation.  We however must not confuse this with the ease of starting the engine, that was the original question.  

Fuels fall into two broad categories.  The first is automotive fuels, some call them mogas.  These are generally blended from many parts of the petroleum family and can have various materials in them.  The most controversial is alcohols.  With the worldwide banning of lead additives in the fuel in the 1980s other chemicals were used and blends developed.  Methanol, made with petroleum alcohol, (MTBE) was found to work very well and was thought to be the natural replacement to maintain octane levels and allow less expensive fuel.  The issue however came up that MTBE when it was spilled, or fuel with MTBE, on the ground the MTBE would contaminate the groundwater.  Because of that we had to stop that as an additive.  The replacement for that was alcohol additives, by varying degrees, of Ethanol, a plant based alcohol.  One major driver of this in some countries is the availability of ethanol.  In Brazil for example it is readily available from sugarcane.  Brazil has so much it is a net exporter of sugarcane based ethanol.  In the USA corn sugar is the most common way to get ethanol.  The US based lobby from farmers to sell their surplus corn (yes they have that much) pushed for the government to adopt ethanol additive to fuels.  The current E10, E for ethanol and 10 for the percentage used, is the most common auto fuel in use.  Higher levels are marked as E15 and some even E85.  The only reason you don't buy 100% ethanol is because it has such a high octane rating that the engines must have special high compression and they need some part of the fuel to have a lower octane to start.  In countries with 100% ethanol cars and trucks they have engines tuned much differently with special high compression and even some have a small tank of "gasoline" just to start the engine.  

By now perhaps some will start to understand that high octane fuels are actually much harder to start that low octane fuels.  This is because the burn inside the engine, the flame front as it is ignited by the kernel of spark from the plug, travels much slower the higher the octane.  

In some countries, such as Canada and the USA, winter presents some problems to start the engine in cold temperatures.  Fuel companies in these areas blend the fuel differently depending on the prevailing weather conditions they know it has to be used in.  Normally auto fuel will be somewhere around 7 PSI Reid Vapor Pressure (RVP), this controls the volatility of the fuel.  That number is great as it is far less likely to vaporize off in higher temperatures.  The problem is they add high levels of Butane to the fuel to allow for faster starts, this is called winter blend.  The butane is cheap and winter fuel prices are always lower than summer, partly do to this fact.  A major effect however is that this causes the RVP to jump a lot, as much as double to 14 PSI.  (never run winter fuel in hot weather, that equals almost certain vapor lock or bubble problems to your fuel system.  Best advice is never store winter fuel for use in summer. 

Ethanol as we know is very high in octane however it creates some issues.  It is a solvent so you have to be careful of the type of fuel system materials you use.  Additionally it lacks the same energy per lb as pure gasoline, so high % can create lean burn.  For that reason Rotax has approved on 10% as a max to be safe.  The engines lack the complex computer/sensor systems as you have in modern automotive systems.  The Rotax fuel injection has some of that but not to the extent of new cars or trucks.  To allow for aviation leaded fuels for example we do not use Lambda sensors, lead would destroy them very quickly.  

Aviation fuels are called just that, Avgas.  The primary difference is it has to meet a worldwide standard, not regional.  They do not allow the oil companies to tinker with the RVP or change the blends seasonally.  They have to be blended to a strict standard.  ASTM D910 for example is what avgas 100LL is blended to meet.  It is the same in each country and in each month.  There are other avgas standards now such as UL 94 and soon we will see UL 100.  The UL in this case is "unleaded" and the 94 and 100 are the octane.  Be aware that this octane is much higher than auto fuels.  

Testing octane numbers are one thing you need to know.  There are several tests, RON, research octane number.  This basically is run on a 600 RPM test engine with variable compression ratio to know where it will knock.  This however does not tell the whole story as there is another test MON, motor octane number.  The major difference is it is done at 900 RPM.  There are some other differences but this is the easy one to remember.  In Canada and the US they use both and average them for AKI, anti knock index.  RON + MON / 2 = AKI, this is the value you would see on the pumps in Canada and the US.  Most of the world uses only RON but this is not the whole story so here we do the average.  Check where you are to know what test they are telling you it is rated under. Avgas is rated to the MON test value.  In general terms the RON value is always much higher than the MON on the same fuel.  For example RON 95 and MON 87, add together and divide by 2 gives you 91 AKI. 

sorry for the long text, its complicated.  To answer the original question, use auto fuel, fresh, in the winter.  For long term storage avgas is better.  (it has up to 20% tolule that keeps the system clean and less varnish also) 

Cheers