Rob MidgleyAvgas - What is it?

deHavilland Moth Club

Fuels Avgas - What is it?

Mogas/UL how does they differ?

Vapour Lock awareness.

Match the oil with the fuel.

Future trends.

Avgas - What is it ? Narrow Fraction Distillate of crude oil. 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.

Ethylene Dibromide. Acts as a scavenger for the Lead.

Anti Oxidant. Prevents the formation of gums and lacquers

from the oxidation of Olefins in the fuel. Has a calorific values of circa 43.5 kJ/g, about

5% greater than for Mogas.

How do Mogas / Unleaded differ from

Avgas? Mogas/UL are wider fractions of distilled crude than Avgas.

They have different additives which are mainly aimed at improving fuel injector and carburettor chamber cleanliness.

They do not contain Lead (TEL). They have less specific energy than Avgas

100LL. Lower Octane number than Avgas. If measured

on the same system as Mogas/UL, Avgas would have an Octane rating of 108 RON.

Rich Mixture Octane Rating is 130, Mogas does not display this increase.

How do Mogas / Unleaded differ from

Avgas? Mogas has poor long term storage stability. Gums and lacquers form over time.

Mogas can contain alcohol, which is unstable when exposed to water.

Mogas is less controlled in its production and handling procedures. In the US alone there are 17 different specs.

The specs can change over time with no consideration to Aviation.

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

Fuel Distillation Curves

Fuel Distillation Curves

0

25

50

75

100

125

150

175

200

225

0 10 20 30 40 50 60 70 80 90 100

% Recovery

Tem

per

atu

re

Avgas

Mogas

10% Starting & Vapour Lock.

50% Carb Icing & Vapour Lock.

FBP. Fuel Distribution & Cylinder washing.

Reid Vapour Pressure

38oC 30oC 20oC0

20

40

60

80

100

RVP (kPa)

38oC 30oC 20oC

Temp

Avgas

MogasSummer

MogasWinter

Pressure at which stable vapours form.

Function of temperature, but also effected by altitude.

ISA pressure at amsl - 101 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 Private CofA aircraft must have an STC raised by

the airframe design authority approving both engine and fuel system.

Each individual permit aircraft must be assessed by the PFA (not just engine / airframe type).

Tank temp at take off must be less than 20 oC. Pressure Altitude must be maintained at less

than 6,000 ft.

Do I always use Aviation Oils ?

Aviation engines must run on Aviation Oils.

Automotive Engines must run on Automotive oils.

Some Automotive derived engines can run on automotive oils, but not all.

If using Avgas in these engines, avoid full synthetic oils and the latest modern mineral automotive oils.

Future Trends

Unleaded 82 UL specification official (ASTM D-6227). Shell to trial in Germany this year.

Unleaded high octane specification uncertain (5-8 years best guess)

Four diesel aviation engine programs underway (all on AeroShell oil)

Electronic engine controls New AeroShell oils under development.

Modern Diesels TCM / NASA GAP Engine

4 cylinder, water cooled, uniflow 2 stroke diesel. Single exhaust valve.

To use Jet fuel. 4 throw crank, opposed cylinders. Originally supercharged, now with

exhaust turbo. Running Shell Oil, but not yet flown.

Renault Sport / SMA Diesel 4 cylinder, 4

stroke, air cooled turbocharged diesel.

180 - 300 H.P. engines

To use Jet fuel. 30% lower fuel

burn. 3,000 hour T.B.O. Better altitude

performance. Due for

certification in 2001.

Running Shell Oil

Questions & Answers