"Ed Huntress" wrote in message
...

"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.


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.

What I should have said there is that autoignition is a close analog of
detonation, because at least two of the measures of octane do indeed base it
on propensity to detonate. It's because they're close analogs that cetane
and octane ratings aren't *exact* opposites. But they're close.

--
Ed Huntress