Formation of CO:
Carbon monoxide (CO) emissions from internal combustion engines are controlled primarily by the fuel/air equivalence ratio. Figure 11-20 shows CO levelsin the exhaust of a conventional spark-ignition engine for several different fuel compositions. 27 When the data are plotted against the relative air/fuel ratio orthe equivalence ratio, they are correlated by a single curve. For fuel-rich mixtures CO concentrations in the exhaust increase steadily with increasing equivalence ratio, as the amount of excess fuel increases. For fuel-lean mixtures, CO concentrationsin the exhaust vary little with equivalence ratio and are of order 10-3 mole fraction. Since spark-ignition engines often operate close to stoichio metric at partload and fuel rich at full load, CO emissions are significant and mustbe controlled. Diesels, however, always operate well on the lean side of stoichiometric;CO emissions from diesels are low enough to be unimportant, therefore, and will not be discussed further.
The levels of CO observed in spark-ignition engine exhaust gases are lower than the maximum values measured within the combustion chamber, but arehigher than equilibrium values for the exhaust conditions. Thus the processes which govern CO exhaust levels are kinetically controlled. In premixed hydrocarbon-air flames, the CO concentration increases rapidly in the flame zoneto a maximum value, which is larger than the equilibrium value for adiabatic of the fuel-air mixture. CO formation is one of the principal reaction steps in the hydrocarbon combustion mechanism, which may be summarized by
Where R stands for the hydrocarbon radical. The CO formed in the combustion process via this path is then oxidized to CO, at a slower rate. The principal CO oxidation reaction in hydrocarbon-air flames is
CO increases rapidly as the inlet mixture becomes richer than stoichiometric ratio. And also improved cylinder-to-cylinder fuel/air ratio distribution has become essential. In addition to this, it is necessary to enrich the fuel-air mixture when the engine is cold since CO emission is higher at engine warm up.