what _IS_ a good cruise A:F ??

[Tom Cloud]

>On Tue, 25 Mar 1997, Tom Cloud wrote:
>
>> I've been told by someone that my term "detonation" is incorrect,
>> that "pre-ignition" is the term I should be using.  Wish I knew  8^)
>
>I've got, what I think is a pretty damn good, definition and distinction
>between the terms on my Web page at:
>
>	http://gtravis.ucs.indiana.edu/Engines/octane.txt


interesting read ...


        * * * * * *  [ snip ]


>Lean running primarily causes the following:
>
>	1.  The mixture takes longer to burn (assuming normal combustion).
>
>	2.  The mixture burns hotter for the given amount of fuel.


Greg, you may be right, but my understanding of what happens
differs slightly from what you describe.  I understand
that the hottest burning mixture is stoich, since all the 
components combine and burn.  That way, leaner and richer
both burn cooler since less is burnt.  The problem with leaner
is as you describe below, it burns slower, so unless the timing
is adjusted, the exhaust valve will burn.

>With extremely lean mixtures, it's actually possible for the charge to
>be still burning as the exhaust valve begins to open.  This raises the
>temperature of the exhaust valve SIGNIFICANTLY.  The hot exhaust valve,
>in turn, can become a source for preignition.

I've read that closed chamber designs and flat-topped or dished
pistons lessen the likelihood of detonation whereas open
chambers and/or domed pistons tend to aggravate the problem.

Tom Cloud <cloud at peaches.ph.utexas.edu>