Aviation Gasoline – Its 2003, why is there still lead in AVGAS?
The short answer to this question is because the engines require it. The long answer follows, with a bit of history just to make the story interesting.
The first engines used to power aircraft were based on automotive engines of the day and they were fuelled with automotive gasoline. The development of aviation piston engines was largely driven by the military’s requirement for high performance, small, lightweight, highly reliable engines and the development of automotive and aircraft engines quickly diverged. The fuel requirements for the two types of engines also diverged. The fuel and the engine can be thought of as a system – improvements in one of the components allow further improvements in the other component. The development of aviation engines and the fuels used in them was thus a leap-frog process: better fuels allowed engine designs that used the fuels more efficiently, therefore demanding further improvements in fuel quality.
The performance of an engine – its power output – is determined more by the engine design than by the fuel used. The more air an engine can process, the more power it can produce. There are several ways to increase the amount of air an engine can process: increase the cylinder volume or displacement, increase the pressure of the incoming air by turbocharging or supercharging or increase the compression ratio. Each of these design changes requires a fuel that burns more smoothly than would be required in an engine of lower performance. The tendency of a fuel to burn smoothly rather than explosively is characterised by its “knock” or rather “anti-knock” rating. The term “knock” comes from the noise made in the cylinder when the fuel burns explosively (detonates or autoignites) rather than in a smoothly, rapidly moving flame front moving outward from the spark plug.
The major challenge in the fuel development was to increase the antiknock properties of the fuel so that the engine performance would not be “knock-limited”. This was accomplished by tightly controlling the hydrocarbon composition of the fuel and by the use of an additive (tetraethyl lead = TEL) to improve the antiknock properties of the base hydrocarbon fuel.
Combustion is a very rapid series of chemical reactions between the fuel hydrocarbons and oxygen in the air. Factors that favour an increase in the rate of these reactions (higher temperature, higher pressure and more time after spark ignition) also favour autoignition and detonation. Tetraethyl lead is an effective antiknock agent as it interrupts the chain reactions that lead to autoignition.
The fuel’s antiknock properties are measured at two limits – lean mixture and rich mixture. A fuel that has good properties at one limit does not necessarily have good properties at the other limit. The lean mixture limit represents operation at cruise power, where the fuel flow has been adjusted to obtain best power at a lower rate of fuel consumption. The rich mixture limit represents take-off operation where maximum power is required.
The antiknock properties are expressed in terms of “octane number” or “performance number”. The higher the octane number or performance number, the smoother burning the fuel is and the greater the power can be obtained from an engine. A fuel with a designation “80/87” has a lean-mixture octane number of 80 and a rich-mixture octane number of 87. Today, only the lean-mixture octane number is quoted (See Table 1).
By the end of World War II, 6 grades of aviation gasoline were in use (see Table 1) and Allied production peaked at 25 million gallons per day. With the turbine becoming the engine of choice for commercial and military operations, jet fuel became the dominant fuel required for aircraft. As a result, by 1996, world-wide production of aviation gasoline had dropped to about 2.2 million gallons per day, and today there are only two grades of aviation gasoline in use, and one of them is now very difficult to obtain.
Currently, ASTM (American Society for Testing and Materials) specifies five grades - 80, 82UL, 91, 100, and 100LL (low lead). In practice only 100LL is widely available. Production of Grade 80 has generally ceased due to small demand. Grade 100 is not seen in the continental US. Production of Grade 91 stopped in the 1960s but was resumed in 2001 as a test fuel for engine development purposes only.
The specification for 100LL limits the amount of TEL added to the fuel to 0.53-ml TEL per litre of fuel, which results in a maximum lead concentration of 0.56 g/L. Grade 100 fuel can have up to twice this amount of TEL. TEL is added in a mixture with another compound called ethylene dibromide. When TEL is burned in the engine, it creates lead oxide, which is a white solid material that collects on spark plugs and intake and exhaust valves. The ethylene dibromide reacts with the lead oxide to create volatile lead bromide and lead oxybromide, which are exhausted with the rest of the combustion gases. Just enough ethylene dibromide is added to react
Moving on to the third page: if you have any questions that we might be able to help you answer, e-mail us or drop a note off at dispatch. We’ll see what we can do to find an ‘expert’ in the field! Meantime, check out Lisa’s web page at www.bluesideup.com for the most recent adventures of the ‘Flying Grahams’.
February PPL Ground School sold out!
From the Desk of the President
Firsts and Other Wonderful Achievements in January
OAS would like to congratulate these pilots!