Mixture and Ignition ...

[Ed Lansinger]

Jim Conforti wrote:

>Does a richer mixture burn faster or slower than a leaner mixture ...
>
>I *think* that (up to a certain point) that a richer mixture burns
>slower, but produces more power ...

Quoting from Heywood:

"Both flame development and burning angles show a minimum for slightly rich mixtures 
([phi is approximately] 1.2) and increase significantly as the mixture becomes 
substantially leaner than stoichiometric.

..

Faster burning engines (which have higher turbulence) are less sensitive to changes in 
mixture composition, pressure, and temperature than are slower burning engines (which 
have lower turbulence)."
-Heywood, John B., _Internal_Combustion_Engine_Fundamentals_,  McGraw-Hill, 1988, p. 395

In other words, richer (up to a point) apparently burns faster (smaller burn angle), not 
slower.  The minimum burn time appears to be right around the max power air/fuel ratio 
(phi about 1.2).

That doesn't necessarily tell you not to bother advancing the spark after adding fuel.  
Spark advance may have been chosen based on factors other than power, such as expected 
gasoline quality or anticipated combustion chamber deposit build-up, all of which would 
require the engine to run retarded from MBT in the first place.

>(Yes, I need to read Heywood ... I know!)

Run, do not walk, to buy his book.

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Ed Lansinger
General Motors Powertrain
Powertrain Control Center
Premium V Software & Calibration Group
Milford Proving Ground, Milford, MI
elansi01 at mpg.gmpt.gmeds.com  8-341-3049  (810) 684-3049
The opinions expressed above are my own, not my
employer's.
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