Posted for Gary Casey <glcasey@adelphia.net>:


 One issue is the definition of "theory."  Otto described a  theoretical
process and predicted that the highest efficiency  came  with all the
combustion at TDC, but of course his "theory" neglected  a few real-world
concerns, namely that combustion can't occur  instantaneously and there are
heat transfer, leakage and friction  concerns.  Create a theoretical model
with all those considerations  (and some that I neglected) taken into account
and you will have a  best economy and best power point with the thetaPP at
about 16ATC.   One can then say that theory and practice match; it's just that
one  person's theory may not be the same as another's.  Ott's was very
 simplified and he said "compress the charge, add heat(burn) and then  expand
the charge."
 
 One very good point that George makes is that while the BSFC is not
 particularly sensitive to changes in thetaPP the peak pressure and  thermal
load on the engine go up dramatically when the thetaPP is  advanced.  I'm also
concerned that with increases in peak pressure  the shape of the pressure
curve also becomes sharper, inputing more  of a pulse to the engine structure.
 This could have a bad effect on  crankshaft torsional vibrations (sometimes
called "harmonics"), so  for engine durability I have a suspicion that we
should favor the  retarded side of the peak.  True?
 
 Regarding the RSA system, it works by balancing the fuel pressure  drop
across a fixed orifice with the air pressure drop in a fixed  venturi.  There
is a small offset spring, but at high fuel flows this  is a good
representation of how it works.  Both orifices operate on  the turbulent-flow
equation of mass flow being proportional to the  square root of rho times DP.
 A simplification that helps keep it  simple and doesn't hurt accuracy too
much is that the air is  incompressible, the mach number being relatively low.
 A very short  explanation is to note that the air changes density while the
fuel  does not and since the densities are inside the square root the error
 will be proportional to the square root of the air density change.   To use
more math, one can create the equation for air/fuel ratio(mass  flow of air
divided by mass flow of fuel), leaving out the fixed  coefficients and
assuming that the DP of the air is the same as the  DP of the fuel (a good
assumption).  Now air/fuel ratio is  proportional to the square root of the
ratio of density of air  divided by the density of fuel.  The fuel does not
change density so  that value can be lumped in with all the other constants
and what you  have left is that the air/fuel ratio is proportional to the
square  root of air density.  I'm no good at all the equation symbols in text
 format, so I hope this makes sense.
 
 On the subject of the LSE system I assume that the timing scheme is constant
with rpm and varies linearly with manifold pressure,  advancing by about 20
degrees with a 20-inch drop of manifold  pressure.  Those of you with his
system and with the display option,  is this what you see?
 
 Gary