Unleaded Avgas

General aviation is entering a period where the piston engine fleet is bracketed with aircraft that can run perfectly acceptably on unleaded avgas.  It is worth noting that many historic aircraft such as Tiger Moths were flying well before the pursuit of power drove the demand for increased resistance to detonation, which lead to the development and introduction of leaded fuels (if you pardon the pun) while at the other end of the time line to the current day, VLAs and current aircraft powered by modern technology engines are being designed to run on unleaded petrol.

‘100LL avgas is by far the major fuel for GA aircraft still’ says Alex Murphy Head of Aviation for Gulf Aviation; ‘but there is an increasing interest and demand for UL avgas, and there are sound operational reasons for this interest, particularly with owners of historic aircraft’ Murphy continues. ‘Historic aircraft, especially those with original engines and design, can suffer from long term degradation in addition to possible risk of engine cut-out due to what is termed spark plug fouling’ (simply put, spark plug fouling is a consequence of earlier engines operating at a relatively lower temperature range, so the Tetra Ethyl Lead – or TEL – is retained in the combustion chamber and through attraction to the metal surfaces builds deposits which in turn can prevent the spark plug from igniting the fuel, causing the engine to cut out) Alex notes the problem of materials compatibility, ‘some original components are susceptible to lead contamination, for example, bronze components which were common on very old engines is quite easily attacked by TEL, reducing the life of the engine, or requiring an increase in maintenance, either way they’re both expensive options.’ ‘of course they ran on an unleaded fuel then, although refining technology wasn’t as developed so the fuel was not as high octane as it is today,’

At the other end of the scale modern aircraft designers and engine manufacturers want to ensure they’re not constrained by limiting to one fuel, so many of the latest designs are certified to run on both leaded and unleaded gasoline. ‘then that opens up other challenges’ Murphy elaborates further, ‘the market for ul avgas is small but growing, however if airfield operators want to satisfy both markets and be seen as progressive, the logistics to achieve this start to become a factor; the obvious item being a requirement to have adequate storage for two grades of aviation gasoline, associated infrastructure is also a requirement to ensure the high standards of product quality can be maintained.

So the obvious question is why not just use unleaded from the local petrol stations? Unleaded fuel made for automotive use, and despite the similar descriptor, the product is quite different in many important areas.

To the layman, it is easy to think that avgas is simply a type of ULG that has dye and lead added,  and while both are made of hydrocarbons by refinery processes, that is where the similarity ends. First; both fuels have their respective specifications to which they must comply (ULG must meet with EN228 while Avgas meets ASTM D910 and Def Stan 91-90) it is no surprise that the avgas one is much more stringent and to meet this restricts refiners to only a select few refinery processes. Furthermore ULG will contain a percentage of bio-derived components (typically ethanol) which brings its own particular challenges and will be explained later.

To help better appreciate the inherent quality of avgas a comparison of two of the parameters is provided here, there are many variables but these serve to ‘scratch the surface’ and provide insight of the technical demands on the fuel.

Octane

Fuel is drawn into the engines’ combustion chamber along with the air at the chemically correct air/fuel ratio to produce combustion, after the mix is compressed it is ignited by firing of a spark* from a spark plug which generates a rapid flame burn, the resultant increase in temperature is accompanied by an increase in pressure which serves to push the piston down to provide the motive force to drive the engine. If the fuel is of poor quality then rather than a smooth flame, the fuel detonates (the ‘pinking’ noise sometimes heard in cars), this has the potential to destroy the engine. The measure of the resistance to this detonation is by determining the octane number of the fuel, the higher the number the higher the resistance, this determination is by an industry standard test method which compares the quality of the fuel sample against a pure chemical called 2-2-4 Trimethylpentane, thankfully it is also called iso-octane and it is from this we get the octane reference. Avgas is typically at least 10 octane numbers higher than even super unleaded fuels when measured by the same test, thus providing a greater margin of protection to detonation.

*except diesel which uses a different process for combustion 

Volatility

Avgas has a tightly controlled volatility and vapour pressure range to accommodate it operating in the wide temperature and pressure variation that can be experienced in flight, whereas  ULG volatility is altered throughout the year to better suit seasonal variations; the vapour pressure can vary from 45kpa in summer to 100kpa in winter, when road fuel has a more volatile characteristic; meaning it boils at a lower temperature range to assist the vaporisation process which in turn aids cold starting.

Composition

However, the most significant change in recent years has been the use of bio-content in ULG; specifically the inclusion of ethanol. Ethanol may be found in ULG at concentrations up to 5.75%,   

The inclusion of ethanol affects the performance and pilots should be made aware

1. Ethanol has a propensity for water, (technical term is hydrophilic) which it can draw from the atmosphere at lower temperatures this can drop out and form ice in the system.

2. Ethanol also has less energy content that normal hydrocarbon molecules (ethanol has approximately 2/3 the energy content to gasoline) therefore requiring more fuel to be carburetted into the engine to produce the same power, which affects range as there is less ‘bang for the buck’ as it were. An important factor when planning longer journeys.

3. It is corrosive to certain materials so can affect the entire fuel system, from pipes to carburettors, therefore materials compatibility is also a consideration, I’m not saying that aircraft cannot use ULG but it’s not simply that issues solely relate to burning it in the engine.

The other considerations not delved in here relate to the storage stability, composition of the fuel, and energy content, and control over what additives are permitted in the fuel.

Modern engines are often approved to use retail unleaded gasoline which is reputedly of higher quality than that first used by historic aircraft, and with so many flights of shorter duration and mainly undertaken in summer why bother with another grade?   

Murphy responds, ‘there are several reasons why this grade is valuable, starting with consistency’ he elaborates further ‘It’s not easy to know whether petrol uplifted from a retail forecourt contains ethanol, or what additives it has, whether those additives are suitable or even approved for aviation fuel; nor how old it is. This is where aviation leads; the manufacturing process and attention to batch traceability ensures that aviation grade products can be followed back to where they were made, this doesn’t just relate to the fuel quality on production, but right through the distribution chain, both in bulk and in drummed quantities for private owners, taking account of the transportation and final storage. All this providing assurance to the operator that their fuel is of the highest quality and designed for the job.’ 

The response becomes more philosophical, ‘Operators entrust the repair of their aircraft with approved maintenance outfits, employing trained competent personnel using OEM equipment, yet there is potential to fill up with fuel which is not optimised for flight, may be transported via non approved vessels, and filled by inexperienced users. As a responsible supplier, the question is more why wouldn’t we bother?’

Alex finishes with this comment ‘…So there you are, hopefully I’ve helped explain the difference between unleaded petrol and avgas, most pilots realise there are differences and it’s important to consider all factors to fully appreciate three key aspects. Why they differ in composition; why pilots should use the correct fuel; and why we are looking to support the market in this…..’

For More information contact us on aviation@gulfaviation.co.uk