[Vwdiesel] Diesel octane or why can you run a diesel engine lean?

[Roger Brown]

Bryant Baecht wrote:
> 
> Hi all,
>      Been thinking about how little diesel engines drink and as I was
> reading the Bentley manual I read that diesel engines run on so much less
> fuel than gas engines.  The thought I took away was that you can run
> diesel's lean and not have problems.  If you run gas engines lean, you'll
> destroy them.  So, what about the compression combustion process makes them
> able to run lean and survive?  I'm contemplating it's the octane of diesel.
> Since I run air cooled VW's, everyone has told me the higher the octane, the
> lower the combustion temp, thus running air cooled, run the highest octane
> you can find.  Is it that diesel has such a high octane, that you can run
> them lean and not cause a problem?
>      Things I've been pondering lately...

Gasoline has octane (measures resistance to ignite), diesel has cetane (measures ability to ignite).  Spark ignition engines
have an explosive mixture of air and gasoline that is compressed and ignited with a spark.  If the octane rating is too low or
the mixture too lean, the mixture can detonate (ping) as it is being compressed.  Diesel engines inject fuel into the cylinder
that has only hot, compressed air.  It starts to burn as soon as it is injected and continues to burn until the fuel is used
up.  In a gas engine you have almost a constant volume combustion, initiated by a spark, flame front travels about 50,000
ft/sec.  In a diesel engine, it is a slower burn, so is more a constant pressure process and the flame front only travels about
50 ft/sec.  You need excess air in the engine becase if it runs rich, you'll get soot formation and it would fill up the
cylinders in short order.  And since you are shooting pure fuel into pure air you can't count on absolutely every molecule of
air and fuel getting together to burn, since the fuel/air is not pre-mixed as in a gas engine.  So generally a diesel runs up to
about 20:1 air/fuel ratio at full throttle and maybe 100:1 to 200:1 at idle.  

I have a few links to info. on diesel engines on my web page:
	http://www.4crawler.com/Diesel/index.shtml#TechInfo

--
   Roger