Importance of Air/ fuel ratio

For a given type of fuel, air/fuel ratio controls the combustion and amount of POC.

Consider combustion of fuel of amount  with air   which produces POC  

Let 

Stoichiometric amount of air  

Theoretically  if

1. which means or  this situation leads to incomplete  combustion. POC will contain CO, smoke dust besides  etc.                      
2.  

This situations leads to complete combustion.

      3.   

Complete combustion but POC will contain excess  in addition to  and  etc
Theoretically CO will be absent.
Amount of POC depends on value of K. Increase in K beyond 1 increase the amount of POC