Rob Midgley Avgas - What is it? deHavilland Moth Club

Fuels n Avgas - What is it? n Mogas/UL how does they differ? n Vapour Lock awareness. n Match the oil with the fuel. n Future trends.

Avgas - What is it ? n Narrow Fraction Distillate of crude oil. n T.E.L. (0.56 g/l)  Acts as an anti detonant, lubricates the upper cylinder and controls the speed of the flame front during combustion. n Ethylene Dibromide.  Acts as a scavenger for the Lead. n Anti Oxidant.  Prevents the formation of gums and lacquers from the oxidation of Olefins in the fuel. n Has a calorific values of circa 43.5 kJ/g, about 5% greater than for Mogas.

How do Mogas / Unleaded differ from Avgas? n Mogas/UL are wider fractions of distilled crude than Avgas. n They have different additives which are mainly aimed at improving fuel injector and carburettor chamber cleanliness. n They do not contain Lead (TEL). n They have less specific energy than Avgas 100LL. n Lower Octane number than Avgas. If measured on the same system as Mogas/UL, Avgas would have an Octane rating of 108 RON. n Rich Mixture Octane Rating is 130, Mogas does not display this increase.

How do Mogas / Unleaded differ from Avgas? n Mogas has poor long term storage stability. Gums and lacquers form over time. n Mogas can contain alcohol, which is unstable when exposed to water. n Mogas is less controlled in its production and handling procedures. In the US alone there are 17 different specs. n The specs can change over time with no consideration to Aviation.

Lead in Avgas n Avgas 100LL contains 0.56g/l of TEL. BS4040 Mogas contained 0.075g/l. n Oil companies want to remove Lead. High cost, poor availability and separate refining required. n UL Grades would allow use of catalytic converters to reduce CO, NOx and SOx. n Avgas volume < 0.5% of Mogas volume.

Fuel Distillation Curves n 10% Starting & Vapour Lock. n 50% Carb Icing & Vapour Lock. n FBP. Fuel Distribution & Cylinder washing.

Reid Vapour Pressure n Pressure at which stable vapours form. n Function of temperature, but also effected by altitude. n ISA pressure at amsl kPa, 8,000ft - 75 kPa, 15,000 ft - 55 kPa.

Vapour Lock - What Causes it? Vapour forms in fuel lines, restricts the flow of fuel and causes fuel starvation. Mogas/UL are wider fractions of distilled crude. Therefore it has a higher proportion of light distillate, which can form a vapour at: a) higher pressures (i.e. lower pressure altitude) b) lower temperatures

Use of Unleaded Mogas This is now permitted in UK for some Private Cat CofA but not Public Transport CofA aircraft. However n Private CofA aircraft must have an STC raised by the airframe design authority approving both engine and fuel system. n Each individual permit aircraft must be assessed by the PFA (not just engine / airframe type). n Tank temp at take off must be less than 20 o C. n Pressure Altitude must be maintained at less than 6,000 ft.

Do I always use Aviation Oils ? n Aviation engines must run on Aviation Oils. n Automotive Engines must run on Automotive oils. n Some Automotive derived engines can run on automotive oils, but not all. n If using Avgas in these engines, avoid full synthetic oils and the latest modern mineral automotive oils.

Future Trends n Unleaded 82 UL specification official (ASTM D-6227). Shell to trial in Germany this year. n Unleaded high octane specification uncertain (5-8 years best guess) n Four diesel aviation engine programs underway (all on AeroShell oil) n Electronic engine controls n New AeroShell oils under development.

Modern Diesels TCM / NASA GAP Engine n 4 cylinder, water cooled, uniflow 2 stroke diesel. Single exhaust valve. n To use Jet fuel. n 4 throw crank, opposed cylinders. n Originally supercharged, now with exhaust turbo. n Running Shell Oil, but not yet flown.

Renault Sport / SMA Diesel n 4 cylinder, 4 stroke, air cooled turbocharged diesel. n H.P. engines n To use Jet fuel. n 30% lower fuel burn. n 3,000 hour T.B.O. n Better altitude performance. n Due for certification in n Running Shell Oil

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