From Air BP - Fuels
We supply aviation gasoline (avgas) at many of our locations.
- ASTM D910 in the US
- DEFENCE STANDARD 91/90 in the rest of the world.
This is very dangerous as these explosions can cause loss of power, blast metal from the piston crown, or at worst, result in total engine failure. Engine design relies heavily on fuel octane quality. High power to weight ratio, large piston diameter, high compression, and supercharging, all call for fuels of high octane quality.
AVGAS has a much higher octane quality than other aviation gasolines such as Mogas. The best quality Mogas in the world today is about 88 Motor Octane Number, MON being a measure of octane quality (0 for very poor, 100 excellent). The minimum for performance AVGAS, 100/100LL, is 99.5 MON. But this is not the end of the story because there is an even more severe test Mogas does not face: Supercharge. Supercharge is a unique test requirement for AVGAS. The test was developed to ensure AVGAS would perform under the most demanding of conditions in high performance, high power output engines. Basically, the AVGAS is run in a single cylinder, fuel injected, supercharged engine by adjusting the air fuel ratio to give maximum power. The pure chemical 'iso-octane' which gives 100 MON, can only achieve 100 performance number for Supercharge. The specification minimum for AVGAS 100/100LL is 130 performance number.
Mogas volatility is set to meet the requirements of ground based vehicle operation for particular regions and seasonal conditions. AVGAS volatility must meet far more stringent criteria for aviation use as atmospheric pressure and temperature can change dramatically during normal operations. For example, fuel warmed to 70° F on an airfield can, in a matter of minutes, experience a pressure drop of 17% as an aircraft climbs to 5,000 feet. The fuel must not cause vapor lock. Similarly, an aircraft descending from 10,000 feet to sea level can experience an almost 36° F swing in temperature. The cold fuel must still be sufficiently volatile as atmospheric pressure increases.
To meet these demanding criteria, AVGAS distillation and vapor pressure are tightly controlled. The result is a fuel which can perform in many different and severe conditions from the equator to the arctic, from sea level to many thousands feet.
Mogas is manufactured using many different refinery components to meet the demands of high volume production. Components often include reformate and alkylate, high octane materials, cracked spirit - an olefinic material - possibly oxygenated materials such as ethers (e.g. methyl tertiary butyl ether), alcohols (e.g. ethanol) and a host of other possibilities. If some of these components were used for AVGAS there could be disastrous consequences as discussed below.
AVGAS can only be made from a limited set of components of the highest quality. The octane and volatility requirements generally limit the choice of alkylate, isomerate and possibly reformate. As a result, only a limited number of the world's refineries have the capability to manufacture AVGAS. No ethers or alcohols are allowed in the blend as these have a low energy content and would reduce the range of aircraft. This is also captured in a minimum energy specification for AVGAS - there is no such specification for Mogas.
AVGAS, and other aviation fuels, are very carefully controlled at the refinery and in the distribution system to ensure no contamination by other products. Red, green and blue dye, respectively, are added to distinguish between AVGAS 80, 100 and 100LL for instance. Quality control follows every batch ensuring it is clean and on specification, ready for use. Overall, AVGAS is the highest quality gasoline a refinery can manufacture.
All members of the industry - engine, aircraft, oil and regulatory bodies, meet regularly to ensure their customers needs are met and flight safety is maintained through the specifications. Many different specifications exist for Mogas across the globe to meet different climatic, environmental and economic requirements. Similarly to AVGAS, the specifications are agreed between the industry members. The aviation industry has no influence over these specifications, while the automotive industry has no influence over AVGAS specifications.
Mogas is of very much lower octane quality than AVGAS 100/100LL and this level of quality can be achieved without necessarily resorting to octane enhancement additives such as lead. AVGAS is of such high quality these additives must be used, but only one, 'TEL-B', is permitted.
Some engines benefit from the use of lead to help protect vulnerable engine valve seats. These could otherwise rapidly wear away, leading to loss of compression/engine failure. However, it must be said the use of lead can also give rise to problems such as spark plug fouling and attack on certain valve metallurgies.
The industry is highly aware of the environmental pressures on lead additives and is actively seeking an unleaded replacement for AVGAS. This is a challenging program and any product must meet stringent criteria to ensure flight safety and performance is maintained. BP is actively participating in this program, working with regulatory authorities, engine and aircraft manufactures. BP is seeking to ensure customers receive safe, performance products for their aircraft.
Any new additive for AVGAS must be stringently tested to ensure a safe and satisfactory performance.
The detergent additives in Mogas have not been approved in aviation fuels/engines which operate under different conditions to ground based vehicles.
In addition, AVGAS must pass a severe storage stability test to ensure that prolonged airfield/aircraft storage does not result in excessive gum deposits which may block fuel lines and filters. High levels of gum are often linked to the detergent additives and cracked spirit used in Mogas.