SA_F_2015_04_

110 SA Flyer

AOPA BRIEFING

is permissible in fuel standards.

Toluene and xylene are also fairly toxic,

but the aromatics have very desirable fuel

characteristics in that they have a high

energy density, due to the high proportion

of carbon to hydrogen. They also have fairly

low volatility and a very high octane rating

of over 100.

For that reason, aromatics like toluene

and xylene often make up nearly 30%

of most aviation fuels. These desirable

characteristics also put them in great

demand, thus making them more

expensive than many of the less desirable

hydrocarbons derived from raw materials

such as crude oil.

THE OCTANE RATING CONUNDRUM

One thing making life difficult for

pilots when choosing fuels is the different

published octane ratings and the methods

to determine them.

As mentioned, all laboratory

octane testing is done using a special

engine (known as a combustion fuel

research engine or CFR), n-heptane

as the zero-octane baseline fuel and

2-2-4-trimethylpentane as the 100-octane

marker.

But test methods vary, including

factors such as intake air temperature, oil

temperature, rpm and manifold pressure.

The different testing methods and resulting

octane ratings are known as research

octane number (RON) and motor octane

number (MON) for automotive fuels, and

then a different pair of test methods are

used for aviation fuel, giving ‘aviation rich’

and ‘aviation lean’ ratings.

Just to add to the confusion, many

countries such as the USA and Europe

use and publish the ‘anti-knock index’

or AKI of their auto fuels. AKI is simply

(RON+MON)/2 – the average between the

two Mogas rating methods.

In South Africa, the Mogas pumps are

marked in RON, with 93 and 95 RON being

the commonly available grades.

There is no direct correlation or

conversion factor between RON and MON,

but in general the MON will be about 8 to

12 points lower than the same fuel’s RON.

That means that Mogas sold as 95-octane

in South Africa could have a MON as low as

83, and not likely higher than 87.

The aviation lean rating is equivalent to

MON below 100-octane. But aviation fuels

may have a lean rating higher than 100, in

which case different test methods have to

be used. This is very important when using

motor fuels in aircraft, since local unleaded

Mogas marked as 95-octane should not be

considered to have an aircraft octane rating

of more than 83-octane.

Aircraft octane ratings are typically

given as two numbers such as 80/87,

91/96, 100/130 and for military and racing

applications even a 115/145 octane has

some limited availability. The second

number on these ratings is the ‘aviation

rich’ rating, which is tested using forced

induction, a rich mixture and high intake

temperatures on a supercharged test

engine.

These aviation ratings have largely

fallen into disuse because of the problem

of carrying several different grades of

fuels at airports. Since 1975, the industry

has standardised on 100LL Avgas as the

replacement for most of these grades.

100LL has an aviation octane rating of

100/130, but the specification for it restricts

the amount of TEL in it. The ‘LL’ stands for

low lead.

The reason for limiting the lead content

in 100LL is this: TEL burns to lead oxides

which cause build-up and fouling of cylinder

heads and spark plugs. Fuel additives like

ethylene bromide are added to leaded fuels

which then reacts with those hard lead

oxides to produce softer lead bromides.

Using a high-lead fuel in a low-compression

engine still leads to unacceptable fouling,

so the low lead of 100LL is something

of a compromise to keep fouling within

acceptable limits for engines intended to

run on 80/87 Avgas.

Although older aircraft mostly have

engines designed for 80/87 Avgas, these

engines are still in production today, as

are engines designed to feed on 91/96.

These engines usually run well on modern

unleaded Mogas with much less cylinder,

spark plug and oil fouling. There is the

caveat that Mogas has a higher volatility,

and even different volatilities between

summer and winter, so it is recommended

that Mogas only be used where it is

specifically approved by the manufacturer

or has been thoroughly tested for the

issuing of STC approvals.

Mogas refiners typically add butane to

their base fuels to adjust its volatility and

often add more in the winter months in

order to aid quick engine starting.

AVGAS GOING UNLEADED

With the long-imminent demise of

lead in aviation fuel, several fuel refiners

are ready to produce, or even have been

producing for years, unleaded Avgas

equivalents.

These fuels typically have to rely on

higher proportions of those expensive

iso-paraffins and aromatics to jack up the

octane ratings of their base fuels without

adding TEL to the mix.

Hjelmco Oil in Sweden has for years

been producing an unleaded equivalent to

91/96 which complies with the ASTM D910

standard and exceeds the specs for a 91/98

Avgas. It is approved by most engine and

airframe manufacturers and government

standards agencies for use in most aircraft

that normally use 100LL.

Other major refiners such as Shell have

announced that they will be following suit

in producing 100/130-octane unleaded

equivalents.

Several tentative standards for 82UL

and 85UL unleaded fuels have been

developed, but these seem not to have

gained broad traction among engine

manufacturers. They are therefore not in

production.

Other hopefuls have been GAMI (of

fuel injector fame) and Swift Fuel who have

proposed and produced limited quantities

for testing of their G100UL and 100SF

fuels. These two manufacturers also use

an aromatic called mesitylene to boost the

characteristics of their fuels.

THE ALCOHOL PROBLEM

Because of the banning of lead in

Mogas long ago and the constant search for

inexpensive additives to boost the octane

rating of automotive fuels, the addition

of alcohols and ethanol in particular has

become prevalent. Governments have also

often mandated the blending of ethanol with

Mogas in order to reduce dependency on

foreign crude oil imports.

Chemically, alcohols are very similar

to hydrocarbons, but differ in that their

molecules contain an oxygen atom

somewhere in their structure.

Ethanol on its own is an excellent

aviation fuel. It has low volatility and very

high octane ratings. Its main disadvantages

are that its low volatility makes for difficult

starting in cold weather and its low energy

content which requires minor modifications

to carburettors or fuel injection systems,

and results in higher fuel consumption and

reduced range. In fact ethanol fuel has

been widely tested in aircraft in the USA,

Brazil and even South Africa.

There are two ways in which the hard

starting has been dealt with: either the