Question about the MAP limits in SB-912-079 R1

Brett Herrick wrote:

Rotax Wizard, thanks for your response.  

Fortunately in my case Van’s Aircraft provides a recommended propeller pitch my Sensenich ground adjustable propeller on my 912 ULS in my RV-12. This provides a great starting point.  

It fully makes sense that the propeller should be pitched to run at or slightly above the maximum continuous operating rpm when operating at WOT during cruise.  Also, that this is complicated by the impact of different environmental conditions (i.e., altitude, barometric pressure, temperature, etc.) and that the faster one operates the engine the easier it is to overspeed it when pitching the nose of the airplane down. 

This how I previously pitched my propeller.  

So, what about my MAP being above the RON 95 (i.e., 91 AKI) line shown in figure 1 of this Service Bulletin only during takeoff roll and initial climb?  Does this limit apply to my fixed pitch application?

I ask because it determines if I can switch from 100LL to 91 AKI unleaded.

Thanks. 

Brett

Rotax Wizard;

In my previous response I asked the following question and haven’t seen any responses.

So, what about my MAP being above the RON 95 (i.e., 91 AKI) line shown in figure 1 of this Service Bulletin only during takeoff roll and initial climb?  Does this limit apply to my fixed pitch application?

I ask because it determines if I can switch from 100LL to 91 AKI unleaded.

Any response or suggestions on how to get an answer?

Thanks. 

Brett

Hi Brett

The AKI limit for auto fuels is indeed a limit.  When the engine is highly loaded, such as climb WOT, it may go into detonation.  To solve your worry I would select a fuel of higher grade, say 93 or 94, if you want to avoid 100LL.  The cost is still lower and it is more likely to give you a buffer that is needed.  Remember this engine has almost 11 to 1 compression ratio.  We can't depend on auto fuels to be as advertised on the pumps given they are subject to so much influence by mixing and handling methods from the fuel suppliers.  

AKI is a average of 2 values, research octane number or RON and motor octane number or MON.  As such that is fine however understand that the 100LL is like the MON value, it is much higher octane than the auto fuel.  If we were to have the 100 LL tested to RON and MON and average the values it would be more like 106 race fuel in AKI.  Think about that one when you consider using 91 AKI.  The test values reflect the ability to resist detonation as the fuel is harder to ignite and less likely to burn from hot spots or glowing carbon inside the chamber.  Items like your intake air charge (too hot) and high exhaust resistance in the muffler for example, these also can fail even engines with 100LL.  

If it was mine and auto fuel was my desire then pick the higher octane fuel not the lower one.  It is still going to be less cost I am sure.  

Cheers

Rotax Wizard, thanks for your clarification on the octane requirement for the 912 ULS engine.

To me the Rotax documentation is confusing about what the octane requirement is.  Service Instruction SI-912-016R17 Selection of suitable operating fluids for ROTAX Engine Type 916 i (Series), 915 i, (Series), 912 i (Series), 912 and 914 (Series) section 5.3 states that the minimum RON is 95 and for the US standard it is a minimum of AKI 91.  The Operators Manual for ROTAX Engine 912 Series Issue 4 Revision 1 states the same.  Whereas, Service Letter SL-912-016R2 Essential information regarding engine behavior, performance and manifold pressure data for ROTAX Engine Types 912 and 914 (Series) and Service Bulletin SB-912-079 R1 Essential information regarding engine installation, operation, and maintenance for ROTAX 912 (Series) Aircraft Engines shows the octane requirement as function of engine speed/manifold pressure and encourages using 98 RON fuel.

I would strongly prefer to burn nonethanol unleaded fuel rather than 100LL.  My desire to avoid ethanol is due to its shorter storage life than non-ethanol unleaded.  I can’t control how frequently I fly due to weather, when the local airport finally clears the ice needed to open some of the taxiways, unscheduled maintenance, personal schedule and other unforeseen issues.

Unfortunately, only 90 AKI non-ethanol unleaded is available locally (i.e., $4.99 / gallon).  Further, unleaded is rarely available when flying to other airports and when I have seen it, it lacks definition as to what it is other than automotive fuel.  Finally, it is challenging to know what vapor pressure one is getting when purchasing automotive unleaded.  As an example, last summer the EPA allowed a refinery near Chicago to produce/sell in the winter grade gasoline all summer long due to production issues it was having.

It’s a bummer with me located near the headquarters of Swift Fuels, that Swift 94UL is essentially unavailable for retail purchase.

So, 100LL with Decalin and 25-hour oil changes is my plan until unleaded with sufficient octane becomes available.

Brett

Hi Brett

I feel your pain.  You are in a very difficult area, Chicago area, as they swing between 10 to 15% ethanol a lot in that region.  Don't buy your fuel in the city area as they mandate that level based on emissions so summer is perhaps the worst.  Vapor pressure is a problem there as well given the swing in temperature.  The national call out from the EPA (at least as long as there is one in the US) is for winter fuel to be introduced by mid September.  Summer fuel, with the desirable lower vapor pressure, should be out end of April.  This however is a crap shoot since politics get  in the way of what is good for the engine and the oil companies get exceptions and release the winter stuff well past that date in many areas.  (Even Texas and California)  Only the market, FBO people at the airports, will drive Swift being available at your field.  

Using 100LL and Decalin, even up to 50 hours, is fine however you do have options.  If getting the auto fuel is not a problem I would perhaps consider a mix of 50% 100LL and your 91 Mogas.  This will address 2 things, the lead concentration is cut in half and the octane will climb to put you into a comfortable range.  (it is not a lineal mix formula but will work fine and all fuels will blend together) This is a go to formula (not authorized but extensively used in the field) by many Rotax operators.  The Avgas lower RVP will cut the Mogas high levels of RVP to a better blend.  

Cheers

RW,

The manifold pressure restrictions when using 91 AKI fuel as published in SL-912-016R2 don’t seem to apply to the 912iS engine. There is a statement in SL-912-016R2 saying that these restrictions mostly apply to carbureted engines because the injected engines can adjust based on their onboard sensors. That makes sense for the 915 and 916 engines, but it’s my understanding the 912iS does not use its knock sensor.  Have you seen anything published showing similar limitations on the 912iS?