Complete & Incomplete Combustion
The main difference between the theoritical and actual combustion of fuel is that in a theoritical combustion prosess the process is complete, while in an actual combustion process the process is incomplete. A complete process means that all of the combustible components of a fuel are burned to completion as seen in the following example (Chemical Energy by Steven S. Zumdhal)
C2H6 + 9/2 O2 -> 2CO2 + 3H2O + O2
In an incomplete process the combustion products contains unburned fuel or components such as carbon, dihydrogen, CO and OH :
C2H6 + 5 O2 -> 2CO2 + 2H2O + O2 + 2OH