bannedagain":2xq3lfb6 said:
CB
We all are aware of the BTU deal, I was just trying to understand your comment about the octane being lower in ethanol. The numbers on the pumps don't show what you are suggesting I was just wondering.
This is not a simple answer as technically ethanol has no octane. This is from one of my blending sources maybe it will explain it better. It increases the antiknock properties of gasoline. Gasoline is rated in octane. Normal octane has a -10 octane rating as a Iso-Octane (trimethylpentane) has 100
The designation "octane" on gasolines is not what you might think. As you
know, octane is a hydrocarbon with the formula C8H18. You also know that
this is not the same thing as ethanol. You may suspect that there is no
simple chemical reaction that converts ethanol into octane, and you are
quite correct.
The octane rating of gasoline relates to how it behaves as a motor fuel.
You may have noticed, at some point in your driving experience, that a
gasoline engine will occasionally "knock" or "ping" under a load, such as
when you try to accelerate up a hill in a high gear. This occurs because
fuel is igniting too soon in the cylinder, under compression but before the
spark is fired. This reduces the vehicle's efficiency, and also damages
the engine. (In a gasoline engine, that is. In a diesel engine, the fuel
has to ignite under compression because there is no spark.)
Different hydrocarbons have different tendencies to ignite in this way.
One that is prone to ignition under compression is heptane, C7H16. One
that resists early ignition, and thus makes a very nice gasoline, is
2,2,4-trimethylpentane, C8H18. This is a highly-branched isomer of octane,
and is commonly called "isooctane." A gasoline engine burning heptane will
begin to knock under a light load; burning isooctane, it will be
knock-free under much higher loads. If it burns a mixture of heptane and
isooctane, the load under which it begins to knock will be somewhere in
between.
If you are buying gasoline to put in your tank, you want to have some idea
of how "good" it is for your engine, and how well your engine will perform
with it. This is why the octane rating was developed. This is a method
for comparing a new gasoline formulation to a heptane/isooctane mixture.
The rating is made this way: run a standard engine on the new fuel, and
apply an increasing load to it. Record the load under which the engine
begins to knock. Then find the heptane-isooctane mixture that causes the
engine to knock uner the same load. The octane rating of the new gasoline
is the percentage of isooctane in the isooctane/heptane mixture that causes
engine knock at the same load as the new fuel.
But wait, there's more! You may notice that on the pump, where the
gasoline's "octane" is displayed, it usually says something like "(R +
M)/2." This is because different engines may give different octane ratings
to the same fuel. The fuel industry couldn't agree on a single standard
engine, so there are two: "R" and "M." The octane rating reported for the
gasoline is the average of the rating from these two engines.
Given all that background, now I can answer your question. There are many
compounds that can be added to gasoline to increase its octane rating, and
ethanol is one of them. THAT is how ethanol increases the octane. It's
just a matter of knowing what is meant by "octane."
Richard E. Barrans Jr., Ph. D.
Chemistry Division