Fuel --- Regular, Premium, Racing

[iaw4]

 

I learned the "easy" way that the Rotax flies on regular gasoline, too: unbeknownst to me, my wife decided to buy some regular last time, so I flew with what probably now amounts to 90 Octane in the tank, half full.  The Rotax engine was quite happy with it.  Alas, I am less happy with it.  so I wonder whether, with half a tank still full with the current gasoline mix now, I should go to a racing supply store and purchase 100-octane racing fuel to mix in.  PS: All regular CA gasoline is ethanol-blended E10.

100-octane racing fuel costs...$10/gallon here now.  yikes.  but if money was no issue, would 100-octane racing fuel be the fuel of choice for Rotax engines?

Incidentally, I even see 110-octane racing fuels on the web, but they seem to have lead, too...maybe.  not sure.

/iaw

[bitten192]

I use 91 octane no ethanol car gas. My Rotax loves it. Only problem is late spring when it starts getting warm out but  before they change to summer blend I get vapor lock so I use 50% 91 octane and 50% 100 LL.

[FlyingMonkey]

Don’t overthink it.  Use 91+ octane mogas or 100LL.  Race fuels are pretty exotic blends, and there is no way to know if the compounds in the fuel are fully compatible with our engines.

Fill up with premium, squirt a gallon or two of 100LL in it if it will make you feel better about octane.  The tiny amount of lead won’t hurt a thing.

[iaw4]

 

mille grazie, both of you.

 

[Mike Koerner]

Octane (the fuel rating system, not the hydrocarbon) is a measure of a fuel’s resistance to detonation.

Normally the combustion process starts at with the spark at the tip of the plug and rapidly propagates along the periphery of an expanding ball of flame. This causes the combustion pressure, and resulting force on the piston, to increase progressively.

Detonation occurs when the spark starts in other places as well, such as a hot edge of a piston or valve. The result is multiple flame fronts within the cylinder and a much quicker increase in pressure than intended. The combustion process becomes more like an explosion than a gradual burn. This is not good. The result is increased stresses and higher internal temperatures (the temperatures at the surface of the valves and pistons, not the coolant or oil temperature). Detonation will damage an engine.

With a car you can sometimes hear the resulting banging sound, hence the common names for detonation are knock and ping. But I’m not sure you can hear it with an uncorked engine like our airplanes… or with a headset on.

The tendency toward detonation increases with internal engine temperatures and compression ratio. Modern gasoline cars have low-compression engines and thus can operate fine on lower octane fuel. They also have electronic ignition systems which can detect ping and advance the spark timing in response to eliminate it (at a slight cost to operating efficiency). Diesels have high-compression engines that are designed for auto-ignition.

Carbureted Rotax engines (I can’t speak for the fuel injected models) have a relatively high compression ratio and electronic ignition systems that are not so sophisticated as to detect and compensate for detonation. You need a fuel with sufficient octane to avoid it.

Using a higher octane than specified accomplishes nothing.  In fact, the reduced volatility of higher-octane fuels may reduce the power output a bit as well. Racing cars that use higher-octane fuels either have higher compression ratios or are running at higher internal temperatures.

Mike Koerner

[FlyingMonkey]

5 hours ago, Mike Koerner said:

Using a higher octane than specified accomplishes nothing.  In fact, the reduced volatility of higher-octane fuels may reduce the power output a bit as well. Racing cars that use higher-octane fuels either have higher compression ratios or are running at higher internal temperatures.

Mike Koerner

THIS.  I tell people all the time that higher octane fuels have less fuel energy ("resistance to detonation" is another way to say "lower combustibility"), but many people don't get it.  Best power is achieved with the *lowest* octane that avoids detonation in a particular engine.  Now, we're talking very small power differences, but it's still a thing.

Most of the premium fuels where I live are blended to 93 octane, but this is Georgia and has both high heat and high humidity for most of the year.  DA on the ground here is typically 1000-3000ft above field elevation.  But in most circumstances, if the manufacturer specifies 91 octane, running a higher octane has zero benefit.

Running lower octane is a recipe for disaster.  By the time you hear the detonation, the damage is done and you can expect a very expensive bill shortly.  You might get away with it on a cool day, but on a hard climb out on a warm day those extra few octane points are the difference between normal operation and detonation.  The ethanol-free gas in my area is 90 octane, so I don't run it and run the ethanol 93 instead.  There is no sense in tempting fate.

iaw4, you got away with running a slightly under-spec fuel for a bit.  Now just fill up with 91 octane and call it good.  As I said, a gallon of 100LL might be appropriate for peace of mind in making sure the blended fuel in the tank is all 91+ octane.

[EB3]

Good info in this thread, thx!