<VV> RE: BMEP was octane

[Jim Burkhard]

> Now generally we see a correlation between compression ratio 
> and octane  requirement. Higher comperssion requires higher octane but 
> what we are really saying is that higher BMEP requires higher octane (for
a given 
> chamber geometry - best is a spherical chamber with the spark in the 
> middle). So if we can limit the BMEP, we can also limit the octane 
> requirement because it is acting like a lower compression engine.

No, no, no!  Brake Mean Effective Pressure (BMEP) is just brake torque
(normalized for displacement).  Unless you want to go slow and take a long
time to get there, you do not want to limit BMEP.  You want to control the
PEAK pressure, not the mean pressure.  BMEP good. More BMEP, more good.
 
> Now this is also going to require precise control of the 
> ignition because hi-test burns slower than regular (and has a higher flash

> point - see also "flame propagation rate") 

The actually burn *rates* for high octane fuel really are not that much
different from low-octane.  Once the kernal is lit, things move at about the
same rate. The difference is in how easy (or overly easy) it is to get
things going in the first place.  Knock formation takes temperature and
time.  The precursor reactions for knock progress very fast in low octane
fuel, whereas they don't in the high octane stuff.  This is all prior to the
true burn.

> but the whole idea is to have max chamber pressure occur at about 7-10
degrees ATDC. 

Actually, that is rather over-advanced on most engines.  Typically loction
of peak pressure is around 13-15 deg atc.

Jim Burkhard