10.1 The myth of Triptane
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
10.1 The myth of Triptane
[ This post is an edited version of several posts I made after JdA posted
some claims from a hot-rod enthusiast reporting that triptane + 4cc TEL
had a rich power octane rating of 270. This was followed by another
post that claimed the unleaded octane was 150.]
In WWII there was a major effort to increase the power of the aviation
engines continuously, rather than just for short periods using boost fluids.
Increasing the octane of the fuel 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
[134]. A 12 cylinder Allison aircraft engine was operated on a 60% blend of
triptane ( 2,2,3-trimethylbutane ) in 100 octane leaded gasoline to produce
2500hp when the rated take-off horsepower with 100 octane leaded was 1500hp
[14].
Triptane was first shown to have high octane in 1926 as part of the General
Motors Research Laboratories investigations [135]. As further interest
developed, gallon quantities were made in 1938, and a full size production
plant was 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% improvement in fuel economy over iso-octane [14].
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 been used as an octane agent in leaded aviation gasolines.
Significant design changes would have been required for engines to utilise
the high antiknock rating.
As an unleaded additive, it was not that much different to other isoalkanes,
consequently the modern manufacturing processes for aviation gasolines are
alkylation of unsaturated C4 HCs with isobutane, to produce a highly
iso-paraffinic product, and/or aromatization of naphthenic fractions to
produce aromatic hydrocarbons possessing excellent rich-mixture antiknock
properties.
So, the myth that triptane was the wonder antiknock agent that would provide
heaps of power arose. In reality, it was one of 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 was not _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.
The blending octane number, which is what we are discussing, of triptane
is designated by the American Petroleum Institute Research Project 45 survey
as 112 Motor and 112 Research [52]. Triptane does not have a significantly
different blending number for MON or RON, when compared to iso-octane.
When TEL is added, the lead response of a large number of paraffins is well
above that of iso-octane ( about +45 for 3ml TEL/US Gal ), and this can lead
to Performance Numbers that can not be used in conventional automotive
engines [14].
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