Subject: Triptane revisited
Date: Mon, 04 Oct 93 03:49:53 GMT
Over the last week there has been a discussion on sci.chem about octane
boosters, and finally someone brought up triptane as the highest octane
fuel available. I added a summary of the discussions here some months ago,
and Bruce Hamilton responded with a very complete article which, in brief,
says that triptane is most notable for its extremely high sensitivity to
tetraethyl lead, and only has an octane (performance number) of 112 by
itself. I thought y'all might be interested, and a few of you might not
regularly read sci.chem, so here is Bruce's article:
In Article <28ftgcINN3jl@zephyr.grace.cri.nz>
I apologise in following up on my own post, but I wandered over
to our library and checked out triptane. I thought you might
like the references.
[ all previous triptane data deleted ]
>If people start to introduce performance numbers, they should be very
>careful to ensure the numbers do not accidently include the extremely
>high sensitivity to lead ( known as the "lead response" of the
>compound) unless they are specifically discussing addition to
>formulated leaded gasolines such as Aviation Gasolines ( except the
>100 Low Lead grade ). Note that alcohols can have negative lead
Well, my guess was right, triptane ( like many other iso-alkanes )
does have a high lead response, and it was evaluated during WWII
as an avaition gasoline additive. I should perhaps recap the octane
ratings here to thoroughly bore you all. Remember that octane ratings
are a measure of the ability of the fuel to prevent the undesirable
self-ignition of unburnt gases ahead of the spark-initiated flame
1. Research Octane Rating ( RON ) -appropriate for general law-abiding
driving around town.
2. Delta 100 RON - the volatile fraction of gasoline, necessary for
good acceleration/high loads on carburetted engines where manifold
separation of volatiles can occur.
3. Motor Octane (MON) - sustained high speed driving, went out of fashion
in the 1950-60s, but came back after Germans experienced engine
damage during their high-speed autobahn runs.
4. Road Octane - the actual octane requirement of the engine under
the test conditions. Determined on the road by using different
octane blends until knock appears.
5. Blending octane - this is the amount the octane will increase by
when a certain % ( usually 20% ) is added to a base fuel
( usually an unleaded refinery stream or 60:40 iso-octane/n-heptane
All of the above use n-heptane = 0 and iso-octane ( 2,2,4
trimethylpentane ) = 100. Today many countries use RON+MON/2 as
the pump rating of motor gasoline. The difference between RON and
MON is known as the Sensitivity, because it indicates the sensitivilty
of the fuel to changes in operating conditions. The RON, delta RON,
and MON are are rated on single cylinder, variable compression ratio
CFR engines. Details in ASTM D2699, and D2700 for the curious.
Generally the RON is about 5 - 8 units higher than the MON
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 decades ago. This uses a superch
Aviation Number = An obsolete method, now replaced by the Motor
method used for motor gaslines, which includes a correlation
table that allows aviation numbers to be derived from MON
( they are numerically very similiar ).
Aviation gasoline PNs are rated used variations of mixture strengh
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 stoic ) and
minumum boost ( weak about 16:1 stoic ) before knock.
The aviation fuels are rated by two numbers eg 100/130 and 115/145
The first is the Aviation Number (essentially the Motor Octane Number)
and the second the Performance Number ( supercharge method ) They
are also known as the "lean" and "rich" ratings respectively.
Well, if you've waded through all that, then you might wonder where
triptane ( 2,2,3 trimethylbutane ) fits in.
Well, in WWII there was a major effort to increase the power of the
aviation engines an increasing octane had dramatic effects on engines
that could be adjusted to utilise the fuel ( by changing boost
pressure). There was a 12% increase in cruising speed, 40% increase in
rate of climb, 20% increase in ceiling, and 40% increase in payload
for a DC-3 if the fuel went from 87 to 100 Octane, and further
increases if the engine could handle 100+ PN fuel.
A 12 cylinder allison aircraft engine was operated on a 60% blend of
triptane in 100 octane leaded gasoline to produce 2500hp when the
rated take-off horsepower with 100 octane leaded was 1500hp.
Triptane was first shown to have high octane in 1926 as part of the
General Motors Research Laboratories investigations (1). As further
interest developed gallon quantities were made in 1938, and a full
size production plant completed in late 1943. The fuel was tested,
and the high lead sensitivity resulted in power outputs up to 4 times
that of iso-octane, and as much as 25% fuel economy over iso-octane.(2)
All of this sounds incredibly good, but then, as now, the cost of
octane enhancement has to be considered, and the plant producing
triptane was not really viable. the fuel was fully evaluated in
the aviation test engines, and it was under the aviation test
conditions where mixture strength is varied that the high power levels
were observed over a narrow range of engine adjustment. If turbine
engines had not appeared then maybe triptane would have use an octane
agent in leaded aviation gasolines, however significant design changes
would have to be made to engines to utilise the high anti-knock
As an unleaded additive, it wasn't that much different to
other isoalkanes, consequently the normal manufacturing processes for
avaition gasolines are alkylation of unsaturated C4s with isobutane
to give a highly iso-paraffinic product, and/or aromatization of
naphthenic fractions to produce aromatic hydrocarbons possessing
excellent rich-mixture anti-knock properties.
So, the myth that triptane was the wonder anti-knock agent that
would provide heaps of power arose. In reality, it was probably
the best of the iso-alkanes ( remember we are comparing it to
iso-octane which just happened to be worse than most other
iso-alkanes), but it wasn't _that_ different from other members.
It was targeted, and produced, for supercharged Aviation engines
that could adjust their mixture strength, used highly leaded fuel,
and wanted short period of high power for takeoff, regardless of
economy. If your street car fits that description, and you want to
use triptane regardless of cost-effective octane enhancement...
(1) Lovell, W.G., Campbell, J.M., and Boyd T.A., Ind.Eng.Chem.
v 23 p26 (1931)
(2) The effect of the molecular structure of fuels on the power and
efficiency of internal combustion engines. Charles F. Kettering.
Ind.Eng.Chem v36 n12 p1079-1085 (1944)
(3) Knocking Characteristics of Pure hydrocarbons
ASTM Special technical Publication 225 ( about 1960 )
This lists values that are more relevant to modern automobile
gasolines. Triptane = 112 RON, 112 MON,
I highly recommend (2), not just because of it's illustrious author,
but because it details the history of triptane, with performance
graphs. Why is life so short that I never get time to read the early
scientific journals, they're more full of knowledge than todays
jargon-filled, expensive specialist publications. :-(
Carl Ijames email@example.com
[I might add to this excellent summary that Gen Dolittle of Tokyo
bombing fame was the guiding force behind 100 octane avgas becoming
the standard military fuel for WWII. Many historians call this
decision one of the greatest strategic decisions of the WWII air war
because of the performance and range boost it gave our planes.
Every hotrodder should observe a moment of silence for Gen Dolittle
who died last week, for most of what we enjoy today in high
octane fuels was researched and developed in WWII under his tutelage.