6.19 Are aviation gasoline octane numbers comparable?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
6.19 Are aviation gasoline octane numbers comparable?
Aviation gasolines were all highly leaded and graded using two
numbers, with common grades being 80/87, 100/130, and 115/145
[109,110]. The first number is the Aviation rating ( aka Lean Mixture
rating ), and the second number is the Supercharge rating ( aka Rich
Mixture rating ). In the 1970s a new grade, 100LL ( low lead =
0.53mlTEL/L instead of 1.06mlTEL/L) was introduced to replace the
80/87 and 100/130. Soon after the introduction, there was a spate of
plug fouling, and high cylinder head temperatures resulting in cracked
cylinder heads [110]. The old 80/87 grade was reintroduced on a
limited scale. The Aviation Rating is determined using the automotive
Motor Octane test procedure, and then converted to an Aviation Number
using a table in the method. Aviation Numbers below 100 are Octane
numbers, while numbers above 100 are Performance numbers. There is
usually only 1 - 2 Octane units different to the Motor value up to
100, but Performance numbers varies significantly above that eg 110
MON = 128 Performance number.
The second Avgas number is the Rich Mixture method Performance Number
( PN - they are not commonly called octane numbers when they are above
100 ), and is determined on a supercharged version of the CFR engine
which has a fixed compression ratio. The method determines the
dependence of the highest permissible power ( in terms of indicated
mean effective pressure ) on mixture strength and boost for a specific
light knocking setting. The Performance Number indicates the maximum
knock-free power obtainable from a fuel compared to iso-octane =
100. Thus, a PN = 150 indicates that an engine designed to utilise the
fuel can obtain 150% of the knock-limited power of iso-octane at the
same mixture ratio. This is an arbitrary scale based on iso-octane +
varying amounts of TEL, derived from a survey of engines performed
decades ago. Aviation gasoline PNs are rated using variations of
mixture strength to obtain the maximum knock-limited power in a
supercharged engine. This can be extended to provide mixture response
curves which define the maximum boost ( rich - about 11:1
stoichiometry ) and minimum boost ( weak about 16:1 stoichiometry )
before knock [110].
The 115/145 grade is being phased out, but even the 100LL has more octane
than any automotive gasoline.
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