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> There are two methods of measuring octane for auto fuel, the
> "Research" (R) method and the "Motor" (M) method. Research
> yields a higher
> number than Motor. Many years ago, the EPA decided to
> eliminate the dual
> numbers (forcing a marriage of apples and oranges) by adding
> the two and
> averaging them as [(R+M) / 2].
>
> 100LL is measured on the Research scale. When leaded
>
> auto gas was
> available, you could buy 100 to 103 octane (Research) at some
> gas stations.
> Currently available 93 [(R+M) / 2] octane auto gas would
> measure about 97
> octane, if measured on the Research scale.
>
From a lengthy explanation of fuel grades at:
http://www.cs.ruu.nl/wais/html/na-dir/autos/gasoline-faq/part1.html
Avgas is rated using the Motor method, not Research. See 6.19 below.
Motor method measures sustained heavy load. 6.5 below.
Research is for lightly loaded applications. 6.6 below.
With the typical spread of 10 between Motor and Research, 100LL is the
equivalent of 105 AKI (R+M/2, or what you see on the gas station pump)
or 110 Research. This is 13 points higher than the best premium
Autogas you can generally buy, so be careful where you use it. It works
fine where it is intended to be used.
*****************************
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.
6.5 What does the Motor Octane rating measure?
The conditions of the Motor method represent severe, sustained high speed,
high load driving. For most hydrocarbon fuels, including those with either
lead or oxygenates, the motor octane number (MON) will be lower than the
research octane number (RON).
Test Engine conditions Motor Octane
Test Method ASTM D2700-92 [104]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 900 RPM
Intake air temperature 38 C
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature 149 C
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - variable Varies with compression ratio
( eg 14 - 26 degrees BTDC )
Carburettor Venturi 14.3 mm
6.6 What does the Research Octane rating measure?
The Research method settings represent typical mild driving, without
consistent heavy loads on the engine.
Test Engine conditions Research Octane
Test Method ASTM D2699-92 [105]
Engine Cooperative Fuels Research ( CFR )
Engine RPM 600 RPM
Intake air temperature Varies with barometric pressure
( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity 3.56 - 7.12 g H2O / kg dry air
Intake mixture temperature Not specified
Coolant temperature 100 C
Oil Temperature 57 C
Ignition Advance - fixed 13 degrees BTDC
Carburettor Venturi Set according to engine altitude
( eg 0-500m=14.3mm, 500-1000m=15.1mm )
6.7 Why is the difference called "sensitivity"?
RON - MON = Sensitivity. Because the two test methods use different test
conditions, especially the intake mixture temperatures and engine speeds,
then a fuel that is sensitive to changes in operating conditions will have
a larger difference between the two rating methods. Modern fuels typically
have sensitivities around 10. The US 87 (RON+MON)/2 unleaded gasoline is
recommended to have a 82+ MON, thus preventing very high sensitivity fuels
[39]. Recent changes in European gasolines has caused concern, as high
sensitivity unleaded fuels have been found that fail to meet the 85 MON
requirement of the EN228 European gasoline specification [106].
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