"Bruce in Bangkok" wrote in message
...
On Mon, 14 Apr 2008 08:33:05 -0400, "Ed Huntress"
wrote:


"Bruce in Bangkok" wrote in message
. ..

snip

Cetane is the measurement of the time taken for a fuel to start
burning. Usually measured from the time of injection, on a diesel
engine, until the start of combustion.

Actually, it's measured as either the time to build up cylinder pressure
to
a certain value, or the cylinder pressure value at a specific time (13
milliseconds after injection of a unit volume of fuel was what I saw in
one
of the tech papers.) So it measures both the time to ignite and the
propagation speed of combustion

It is a measure of how long the period is from injection to
combustion. The method of measurement (which you are referring to)
varies from institute to institute. It can be as you describe above,
or it can be a different measurement, but in any case it is just a
measurement of time required for combustion to be initiated.

Octane numbers measure the resistance to detonation.

Autoignition. It may or may not result in detonation. Autoignition is the
process by which diesels ignite their fuel, so cetane and octane are said
to
be opposite values.


Auto ignition is not how a diesel ignites its fuel. A compression
ignition engine compresses air to above the ignition temperature of
the fuel and then injects the fuel into the preheated air causing the
fuel to ignite.

That's what autoignition is, Bruce. It doesn't have to be in a homogeneous
mixture. Regular stratified-charge diesels ignite by autoignition, too.


I hate to be argumentive but I worked in the Indonesian government
Petroleum Labs for a couple of years and I didn;t hear any of the
people from the French Petroleum Institute (who were the main
consultants) compare Octane to Centane ratings. They measure two
different things that apply to two different types of engines.

Maybe they don't read the literature. The French sometimes refuse to read
English. g

Go to the SAE site and search on some of the terms we've been using. Before
you get to 200 abstracts or so, I assure you that you'll see references to
cetane and octane being opposite measures of almost the same property

As for applying to two different types of engines, that's all gotten fuzzy
with the development of HCCI and related concepts. If you spend some time
looking, you'll see every kind of hybrid you can imagine.


Octane applies to gasoline powering a spark ignition engine. Centane
applies diesel fuel powering a compression ignition engine. You can
interpolate all you want but I can assure you that in an engine lab
they do not use the compression ignition engine to test for octane
rating nor the spark ignition engines to test for centane rating.

Of course not. That's "cetane," by the way.

--
Ed Huntress