o2 and EGT questions

Mark Shaw mark at wdc.sps.mot.com
Fri May 12 17:37:18 GMT 1995

> At the risk of further driving efi'ers up the wall, what oxygen 
> concentration in the exhaust corresponds to a stoichiometric mixture.  
> Naively, one would expect that a stoichiometric mixture having undergone 
> complete combustion would not have any residual oxygen.

A common misunderstanding about stoichiometric.  It is actually the condition
where the correct amount of air (containing the correct amount of oxygen)
is combined with the correct amount of fuel in order to provide for complete
combustion of all the carbon and hydrogen to CO2 and water.  In reality,
"complete combustion" never really happens mainly due to some fuel mixture
not being burned (i.e. caught in piston crevices or swept though the exhaust
valve due to overlap) and the "robbing" of oxygen from the process by oxidation
of the nitrogen at the high temperatures and pressures of the combustion. 

So some oxygen never gets used, and even for rich mixtures there may be 
0.2 to 0.3% of O2 in the exhaust.  The concentration at stoichiometric is
given by Bosch is around 0.7% O2 (by volume).


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