In article .com,
wrote:

Charlie Springer wrote:
On Sat, 24 Sep 2005 23:45:43 -0700, Roger wrote
(in article ):

Alcohol which has a low octane rating although it keeps getting credit
for a high one, when added to gas up to 10% by volume will increase
the octane rating of the fuel. 10% seems to be the maximum amount for
increasing the octane rating.

Can you explain the octane rating? In my little world octane is a chain of 8
carbons with 18 hydrogens hooked on.


Memory fades but the standard test for octane rating goes something
like this:

A standard engine is set up and run on the fuel to be tested. Than
a standard additive (hmm, pure octane perhaps?) is added to the fuel
until the engine begins to kock. The more of that additive it takes
to make the engine knock, the higher the octane rating.

The rating posted on gas pumpps in the US is the average of a
bench test and a test on an engine installed in a vehicle.

Or something like that.

Something like that...

There are two "Octane" ratings systems, Research amd Motor.
Research octane rating compares the evaporation rate of the test fuel to
iso-octane's by dumping a measured amount from a tower and seeing how
much hits the ground.
In Motor Octane rating, a single cylinder engine with adjustable compression
ratio is run at progressively higher compressions until it knocks.
The result is compared to iso-octane.

The number on the pump in the USA is the average of Research and Motor
octane numbers.


Octane rating is an indicator of the fuel's resistance to
predetonation (knocking). It is not an indicator of how good
the fuel is in other repsects.

Very true. The gas companies used to advertise that their high octane
fuels had more power than their competitor's, or low octane fuels.
Both are untrue, and they were taken to task by the FTC in the 60's for
false advertising.
High octane fuels actually have slightly lower energy content than low
octane fuels, it's just that since they resist knock better than low octane
fuels, you can run a higher BMEP* and get higher power from the engine.
(*BMEP Brake Mean Effective Pressure - the combination of compression
ratio and boost pressure via turbocharging or straight supercharging.)
The lower energy content results from the extra additives used to increase
the octane. The additives have lower energy content than gasoline.

--
Harry Andreas
Engineering raconteur