Combustion chamber heat

[Robert Harris]

Any particular mechanically efficient engine combination has a sweet spot
of advance, generally at the onset of detonation, where maximum power is
made. If they are not close, such as the case with early FE heads, it means
you have either room for more mechanical improvement (e.g. more
compression)
or your chamber has gotten crapped up - dome shape etc.

Richening a mixture "cools" a mixture because more low evaporative
temperature
components vaporize prior to combustion, absorbing more latent heat from
the
air and cylinder, thus favorably changing the time/pressure/temperature 
detonation (ping/knock/big bang whatever) relationship.

Air cooled engines run hotter cylinders and heads, thus because of
temperature,
not mechanical compression, run closer to detonation than an identical
engine
that is water cooled.  Richening the mixture again cools the mixture,
allowing
more mechanical compression prior to detonation and thus more power out of 
the engine.

Aluminum heads absorb more heat during compression, keeping the mixture
cooler, allowing higher compression than iron heads.

Both air cooled and aluminum heads have the disadvantage of after ignition
of
rejecting more heat to the cooling system and thus are somewhat less
efficient
than iron heads.  Changing to aluminum heads from same design iron heads
generally involve adding a few degrees lead to compensate for the
additional
cooling of the charge by increased cooling system loss's.

Ceramic's work by vaporizing more fuel during compression, but more
importantly
retaining significantly more heat within the cylinder to make working
pressure
after combustion.  Lower cooling system loss's directly translate to power
at the
flywheel.  Ignition must be retarded several degrees to compensate for the 
higher initial charge temp, and faster flame spread because of reduced heat
loss's.

All of the above comparisons are based on the impossibility of mechanically
identical engines except for the changes noted. Please note an earlier
writer's
statement that a highly turbo'd mega output engine made peak power at 18
degrees advance - about half of what aluminum headed street V-8's like.
IMHO,
the more mechanically efficient (chemical energy converted to mechanical
energy) an engine is, the less total lead it will take to make maximum
power.

Just keep in mind that firing the mixture before top dead center, i.e.
spark 
advance means initially building pressure OPPOSING the pistons upward 
movement to build maximum pressure about 15 degrees past top dead center.


-----     Cut here - personal comment follows -----
Q: What's the difference between Jane Fonda and Bill Clinton?
A: She had the balls to go to Viet Nam
  
Robert Harris <bob at bobthecomputerguy.com>


----------
> From: talltom <talltom at teleport.com>
> To: diy_efi at coulomb.eng.ohio-state.edu
> Subject: Combustion chamber heat
> Date: Thursday, September 26, 1996 1:45 AM
> 
> 
>   I've noticed a little discrepency here in relation to combustion
chambwer
> heat.
> It seems that some say it's necessary to go over rich to cool and prevent
> preignition/detonation. Others say that thermal barrier coatings are the
> neat trick,