Dear Aviators

I trust you all had a peaceful and relaxing Christmas and New Year. I am sure this will be anotherexciting and prosperous year for all those out there in the Aviation Industry in Australia.

In this the first edition for 2008 - Tech Talk issue 20, our good friend Rob Midgley discusses whymany in the Aviation Industry prefer to use Aviation Gasoline (AVGAS) as opposed to MotorGasoline (MOGAS). Rob’s discussion covers aspects around the constituents, composition,properties, storage, quality, handling and Octane rating of the two fuel types. Rob highlightsaspects that are often overlooked e.g. the shelf-life. I trust you find the article interesting andbeneficial for the future.

I once again throw it out to you the reader, if you would like any aspects of this issue’s topicexplained further or there are points you need clarification from past articles, please drop me anemail and I will happily get our Technical experts to respond to you. Similar if there are topicsyou want cover in future issues, my details are below – please drop me a note with the topic.

Happy Flying

Craig RudolphRegional Account Manager - Oceania Aviation Lubricants and SpecialitiesMob: +61 0401677301 Fax: +61 2 9897 8211Email: craig.rudolph@shell.com

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octane number (MON) method,which uses higher enginetemperatures and high loadconditions in the test.

As the MON test is moredemanding, typical fuels havelower MON than RON values.Unleaded Mogas fuels tend to becontrolled in the range 95 - 98RON but can range anywherefrom 82 - 90 MON. Avgas ismeasured on lean mixture (similarto MON). The lean mixture ratinghas a specification minimum of99.5 octane (15 octane higherthan the comparable 85 MONtypical of unleaded Mogas), but inpractice has actual productionvalues around 104, so hasconsiderably better detonationresistance than Mogas.

But for low-octane demandengines, surely it is acceptable touse a lower octane fuel? This isthe thought behind the Mogassupplementary type certificates.

For low-powered aircraft,Petersen Aviation Inc in the USAand others, such as theExperimental Aircraft Association,have a test program agreed withthe Federal Aviation Authority thatqualifies a given engine oraircraft combination to run onunleaded Mogas. Theseapproval methods have alsobeen accepted by other aviationauthorities, such as those inEurope and Australia and involverunning the engine in the aircraftand looking for fuel flow anddetonation problems whilstburning a typical Mogas.

Research and DevelopmentSignificantly, these tests accept theengine conditions seen during thetesting and do not look at theengine’s whole operatingenvelope. This is the problem. As part of our own Research andDevelopment programs, Shell

Aviation has a low-octane demandLycoming engine – one apparentlysuitable for use with unleadedMogas – on a test stand. Duringsome related combustion studywork, Shell conducted some testswithin this engine’s operatingenvelope, but with higher thannormal cylinder head temperatures(CHTs). In spite of using a highoctane, 98 RON Mogas, ratherthan the normal 95 RONunleaded fuel covered by thetypical supplementary typecertificates, the engine sufferedsevere detonation under moderateload at only 1,200 rpm,somewhere close to what mostpeople would use as a groundidle condition. Reducing the CHTsto a more typical level allowed thefuel to work throughout the powercurve, which is why many of theaircraft do not experienceproblems under the Mogasapproval program, but it is proof

that even high-octane Mogas doesnot give an adequate detonationboundary throughout the operatingenvelope of the engine.

Aircraft fuel for aircraft enginesIt is not too hard to imagine ascenario: a hot day with a quickstop for fuel, a long hold on theground, or a rapid turn-around –all would allow the engine toheat up to higher than averageCHTs. Our engine bench testingwork shows that having Mogasin the tanks at this pointrepresents a much greater riskthan choosing to use Avgas andadds weight to the position bythe oil companies and enginemanufacturers that aircraftengines should use aircraft fuel.

What do I fly with? Avgas 100LL.

Happy flying.

No matter how reliable the fuel supplier, road fuels are designed only to be used in road vehicles 3

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Even where the use of Mogas is allowed bythe aviation authorities, Shell Aviation, othermajor oil companies and the enginemanufacturers have always taken the positionthat Mogas should not be used in aircraftapplications - in particular the common air-cooled Textron Lycoming and TeledyneContinental engines. The cynical may thinkthat this is due to an effort to sell moreAvgas, but in fact it is more to do with thesafety and suitability of the fuel.

There are several areas where thecomposition and control of Mogas differsfrom Avgas and this can result in safetyconcerns with Mogas even if the rules andrestrictions are observed.

The main areas of difference betweenAvgas and Mogas are:

• Less regulation of which constituents areallowed when making the fuel;

• Mogas routinely changes compositionand properties between winter and summer;

• Mogas is not designed to be stored forlong periods;

• Mogas does not have the same qualityand handling restrictions.

ConstituentsMogas is allowed to contain a number ofcomponents which are known to be aproblem with aircraft systems, or can evenreduce power. One such item is alcohol,which can be aggressive to seals,carburettor components, fuel tank linings.This can lead to leaks, engine failure dueto poor mixture control and it has evenbeen the documented cause of in-flightfires. There is also no assurance that onesupply source of fuel will have a consistentcomposition, so always buying from one

forecourt is no guarantee that the fuel’scomposition is always the same.

Composition and propertiesWinter grade Mogas is more volatile,which is intended to assist with enginestarting in cold weather. This means that ithas a lower initial boiling point and willform a vapour more easily; a problemwith aircraft fuel systems as a bubble ofvapour in the fuel line will prevent fuelfrom flowing and cause engine failure.Why is this a problem for aircraft and notcars? The significant difference here is thatcars don’t get up to flight levels. Thereduction in pressure with altitude causesvapours to form more easily even atmoderate temperatures. This can becompounded by the fact that aircraft tendto be used less frequently, meaning thatthere is a greater risk of winter gradeMogas still being in the tank on hot springand summer days, making the problem ofvapour lock more of a risk.

StorageWithin the automotive world Mogas isgenerally burned within a few weeks ofproduction so storage stability is not aconcern. However, kept for longer periods,it can form sticky lacquers and gums thathave the potential to result in fuel injectoror carburettor malfunctions. Avgas’scomposition is much more tightlycontrolled, allowing fuel to be kept formonths without deterioration. This issignificant in aviation as it is notuncommon for an aircraft to be in thehangar for several months with fuelremaining in the tank.

DESPITE THE FACT THAT SUPPLEMENTARY TYPE CERTIFICATES ARE IN PLACE TOALLOW SOME LIGHT AIRCRAFT OR ENGINE COMBINATIONS TO USE MOTORGASOLINE (MOGAS), MANY IN THE AVIATION INDUSTRY PREFER TO USEAVIATION GASOLINE (AVGAS).

Refuelling with Mogas – it's not just the process that is less controlled, so is the product

AVGAS: THE RIGHT CHOICE

Rob Midgley examines the benefits of Avgas

Continued on page four

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Restrictions on quality and handlingAvgas’s quality is guaranteed bythe use of dedicated manufacturingand storage vessels, road tankersthat are only used for Avgastransportation and dedicatedairfield storage. Furthermore, wateris removed and the fuel testedthroughout the delivery system,almost totally eliminating the risk ofcontamination. Upon delivery tothe aircraft Avgas is cleaned usingfilters so fine that they would becapable of separating red andwhite cells from blood – the result isthat you receive very clean, dry, onspecification fuel.

None of these quality restrictionsare in place for the Mogas supplychain, which involves numerousareas where there is a chance ofcross-contamination. These rangefrom the obvious, such as non-dedicated vehicles, to the less obvious, such as distributingMogas large distances bypumping it down multi-productpipelines along with otherproducts, ranging from diesel fuelto heating oil are also beingmoved. Remember there isequivalent to the edge of the road at 3,000 feet.

Octane ratingThere is one more fundamentaldifference where Avgas out-performs Mogas – that of octanerating. Octane is themeasurement of a fuel’sdetonation resistance. All fuelswill automatically combust beforethe flame reaches them if thetemperatures and pressures arehigh enough in the yet-unburnedgas. This leads to explosivecombustion of the remaining fuelin a phenomenon known asdetonation. Severe detonationcan destroy an engine in a fewseconds. Octane rating measures

how resistant a fuel is todetonation; the higher the octanerating, the more the fuel or airmixture can be compressedwithout detonation happening. To make this clear, octane ratingis not a measure of the amountenergy in the fuel but measuresits resistance to detonation. Theadvantage of higher octane fuelsis that a higher compression ratioor supercharging ratio can beused, which then leads to ahigher engine cycle efficiency,which in turn means more poweroutput for a given fuel burn.

There are several things thatdetermine an engine’s octanedemand of an engine (in otherwords things that encourage thefuel to detonate in a givenengine). They include compressionratio, supercharger pressure, inlettemperature, cylinder walltemperature, ignition timing, load(or power setting), engine speedand cylinder bore. In aviation, wegenerally have engines which dopretty much everything badly from

an octane demand viewpoint:low engine speed, air-cooled(high cylinder wall temperature),large cylinder bore, superchargedwith no intercooler (high pressureand high temperature inlet air),fixed ignition timing withmagnetos and relatively highaverage power setting.

Once we know the engine’soctane demand, how do wemeasure the fuel’s performance? Afuel’s octane performance ismeasured in the lab using singlecylinder test engines in which thecompression ratio can be alteredby screwing down the cylinderhead, therefore increasing the

compression ratio. However, theway in which this engine isoperated – with varyingtemperature, load and speed – results in different types ofresponses, leading to differingoctane numbers for a given fuel.

Road fuels tend to be measuredon a research octane number(RON) scale, in which the testengine is run at low cylindertemperature and low load. This isintended to replicate a liquid-cooled car engine on the openroad and so is commonly used asa reference point for Mogas fuels.Aviation fuels are measured usingthe more demanding motor

Engine Test Cell: There’s a Lycoming 360 in there somewhere!

The aircraft referred to in thisarticle are principally thecommon category A lightaircraft, which include Pipersand Cessnas, rather thanaircraft in the microlight andULM categories, such asRotax, which seem to operate

well on unleaded fuel.However, while this preventsthe problem of lead fouling,some of the compositionalconcerns outlined in thisarticle still apply and shouldbe considered if deciding touse Mogas.

Light aircraft

“Refueling with Mogas - it’s not just the process that is less controlled, so is the product”.

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