108 SA Flyer

Last month we explored

the basic characteristics

of fuels for aviation

piston engines: energy

content, volatility and

octane rating. In this

part, we will look a

little deeper into those

basic needs for our

engines and consider

some of the issues that

may affec t us in the

future.

HE main driving force

for changes in aviation

fuels is the environmental

imperative that harmful

compounds, mainly

tetra-ethyl lead (TEL), are

removed from aviation

fuels. We have known that this is coming for

a long time, and the days of leaded Avgas

are now nearly over.

Many aircraft have been able to switch

over to ordinary petrol (which us yers

call Mogas) with little or no modication

required. But even that switch comes with

concerns. Particularly the issue of ethanol

being blended into our automotive fuels.

REVISION

Just to recap, the energy content of our

fuel, measured in joules/litre or joules/kg,

is an important factor in aviation. We want

to carry the least volume and weight of fuel

that will carry us the farthest.

Volatility of fuel must be carefully

balanced so that, on the one hand, it

vaporises well enough to mix well with air

for complete combustion. But on the other

hand, it must not be so volatile that under

higher temperatures and lower ambient

pressures it turns into a gas in the aircraft’s

fuel lines, pumps, carburettors and fuel

injection systems, causing vapour locks.

And nally, that so often misunderstood

thing called octane rating, which basically

means that a fuel has a high enough octane

rating to prevent it from igniting under high

temperature and pressure before we want

it to.

GETTING TO GRIPS WITH OCTANE

Although the testing of a fuel’s octane

rating in the laboratory is pretty much

the same for each kind of fuel, there are

different methods and different types of

rating systems. This is where the confusion

arises. Aircraft operators and pilots need

to take great care in understanding this

since, if the octane rating is too low, it

can catastrophically and expensively

disassemble your engine.

Using a high octane fuel in an engine

that does not require such high octane

ratings generally will not hurt it. But the fuel

will certainly cost more and, as a general

rule of thumb, higher octane fuels contain

slightly less energy than lower octane fuels,

so fuel consumption will be marginally

higher, plus it will cost more in the rst

place.

Fuel testing in the laboratory is done

using a special single-cylinder test engine.

The engine is built so that its compression

ratio can be varied. The fuel under test is

run in the engine and the compression ratio

is increased incrementally until the engine

begins to ping or knock from pre-ignition

or detonation – a subject we explored last

month. The compression setting of the test

engine is compared to that of two base-line

fuels that have an ‘octane rating’ of 0 and

100.

It was arbitrarily decided a long time

ago that the baseline ‘zero-octane’ fuel

would be pure heptane and ‘100-octane’

fuel would be pure octane. Or would it?

Unfortunately, although heptane has

the chemical formula of C7H16 (in other

words seven carbon atoms and sixteen

hydrogen atoms joined into a chemical

compound) and octane has the formula of

C8H18, the story does not end there.

The baseline 0-octane fuel is actually

called n-heptane and the 100-octane

reference fuel is an iso-octane known as

2-2-4-trimethylpentane.

So, we will need to dig a bit deeper into

the chemistry of these hydrocarbons.



AOPA BRIEFING - CHRIS MARTINUS

PART TWO

PRESIDENT AIRCRAFT OWNERS AND PILOTS ASSOCIATION – SOUTH AFRICA

& THE FUTURE

Hjelmco Oil in Sweden has been making

unleaded Avgas for many years.

www.sayermag.com

HYDROCARBONS AND THEIR ISOMERS

Hydrocarbons, as the name implies,

are naturally-occurring or synthesized

compounds consisting mainly of carbon

and hydrogen atoms. There are literally

millions of different ways these two

elements can be hooked together, resulting

in millions of different hydrocarbons which

have wildly different characteristics.

Without delving too deeply into

basic chemistry, a carbon atom has four

‘valencies’, which simply means it can

chemically bond to the valencies of other

atoms up to four times. Hydrogen has

only one valency, so it can only make one

chemical bond.

The simplest basic hydrocarbon is

methane, which has one carbon atom

and four hydrogen atoms bonded to

each of carbon’s four valencies. A simple

representation of methane looks like this:

Ordinary octane, or more correctly,

n-octane looks like this:

N-octane has eight carbon atoms

strung in a line, each joined by one of its

valencies. Hydrogen atoms then bond to

the remaining valencies.

However, there are actually many

ways in which eight carbons and eighteen

hydrogens can be arranged, all bonded

together into one compound. These

compounds are known as isomers of the

basic compound. The following depiction is

just one of isomers of C8H18 and would be

known as an iso-octane:

There are many isomers of octane,

but this is the special one called

2-2-4-trimethylpentane – the hydrocarbon

used as the reference ‘100-octane’ fuel in

the laboratory. Nevertheless, each of these

isomers have exactly the same number of

carbons and hydrogens as n-octane.

Although this one’s name is something

of a mouthful, it is called a ‘pentane’

because it has ve carbons in a row, but

has three ‘methyl groups’, two of them

bonded to carbon number 2, and one

methyl group bonded to carbon number 4.

As a general rule, the more carbon

atoms there are in a hydrocarbon, the

lower the octane rating and the lower the

volatility. But isomers of the same basic

compound change that: n-octane has an

‘octane rating’ of far below zero, lower than

n-heptane. But the iso-octanes have much

higher ‘octane ratings’, up to 100 for the

2-2-4-trimethylpentane variety.

This group of hydrocarbons which

includes fairly well-known substances

such as propane (C3H8), butane (C4h10),

hexane (C6H14) and so on are known

as parafns. There are several other

groups of hydrocarbons with very different

structures and with names such as olens,

naphthenes and so on.

One other hydrocarbon group that

is worth mentioning here is known as

‘aromatics’. The most basic aromatic is

benzene, which looks like this:

Note the ring structure of six carbons

with alternating double bonds. Other

aromatics used in fuels are toluene and

xylene. They have the same basic ring

structure, but toluene has one of the

hydrogens replaced with a methyl group,

i.e. one carbon and three hydrogens, while

xylene is also based on the benzene ring,

but with two methyl groups attached.

Aromatics got their name because they

have a fairly pleasant smell and benzene

was used as the base for after-shave lotions

and other cosmetics. That was until it was

discovered that benzene is highly toxic and

a known carcinogen. Benzene is therefore

banned in all fuels and only a trace amount

The Embraer Ipanema is the only

aircraft certified straight from the

factory to run on pure ethanol.

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 difcult 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 specication 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

specically approved by the manufacturer

or has been thoroughly tested for the

issuing of STC approvals.

Mogas reners 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 reners

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-parafns 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 reners 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 difcult

starting in cold weather and its low energy

content which requires minor modications

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

www.sayermag.com

addition of a small amount of Mogas, around 15%, or by priming the

engine with Mogas or Avgas to get it started. Ethanol containing 15%

hydrocarbon fuel has been marketed with limited success as E85

(85% ethanol), particularly for high-compression sports car engines.

An aviation-spec E85 was briey marketed in the USA under the

name AGE-85.

Baylor University in the USA also obtained

STCs for several aircraft types to run on

straight ethanol, using a separate priming

tank for starting.

However there is still, after

many years, only one aircraft

that is certied straight from the

factory to run on pure ethanol,

and that is the Brazilian

crop-spraying aircraft, the

Embraer Ipanema. High-

ethanol fuels are widely

available in Brazil, which

has a large ethanol industry

producing it from sugar

cane.

Ethanol compensates

to some degree for its lower

energy content by having a

high evaporation coefcient

which signicantly cools the intake

temperatures, resulting in greater

thermodynamic efciency. Since ethanol

has a very low freezing point and in itself is an

antifreeze, carburettor icing is eliminated because the

water vapour is absorbed by the ethanol.

Ethanol has been blamed for being corrosive and attacking

some plastic and rubbers found in fuel systems. South African

research over a period of several years indicates that this is not the

case. It is not the ethanol that damages these materials, but other

by-products of ethanol distilled from fermented biomass. These

other fermentation by-products are mainly ketones such as acetone

and MEK, which wreak havoc on some materials. South African

ethanol comes mainly from Sasol which uses a completely different

process to produce a very high purity ethanol as a by-product from

its coal-to-fuel process.

Mixing ethanol with hydrocarbon fuels does have a few

downsides of which pilots ying on such fuels must educate

themselves.

The main issue is that water does not

mix with Mogas or Avgas, but is readily

soluble in ethanol. Should an ethanol-

blend fuel be contaminated with

water, the water will be absorbed

by the ethanol – up to a point.

Once that saturation point is

reached, the ethanol-water

mix suddenly separates

from the hydrocarbons in

a phenomenon known as

‘phase separation’.

Unfortunately, the

lower the concentration

of ethanol in a blended

fuel (usually 5-10%), the

earlier it will experience phase

separation. The separated fuel

will cause serious engine surging

as the engine alternately takes in

hydrocarbon fuel and an ethanol-water

mix. Indeed, Baylor University managed to

destroy an IO-540 engine in the test cell while

exploring this phenomenon.

Another thing is that mixing an ethanol-blend fuel with other

fuels can radically and unpredictably change the volatility of the end

product.

In summary, ethanol blended fuels are not the end of the

world for aviation, and are even approved by some aircraft engine

manufacturers. However, care should be taken to avoid water

contamination or mixing with other fuels.