During an incomplete combustion, instead of Carbon Dioxide, Carbon Monoxide and Carbon are produced as well.

 

Basic Equation for an incomplete combustion

hydrocarbon + oxygen à carbon monoxide + carbon + water

• Key reason for incomplete combustion is due to insufficient supply of oxygen available to react with the hydrocarbons. As partial combustion occurs, hydrogen will still react and oxide to form water vapor. However due to the shortage of oxygen, carbon monoxide or carbon in the form of soot is produced instead.

Chemical Equation

2C₁₄ H₃₀ + 33O_{2   →} Heat Energy + 4C + 12CO + 12CO₂ + 30H₂O

Diesel+ Oxygen   →    Energy+Carbon(Soot)+ Carbon Monoxide+ Carbon Dioxide+ Water Vapour

OR

C₁₄ H₃₀ +11O₂  →  Heat Energy + 7C +7CO + 15H₂O

Diesel   + Oxygen → Energy + Carbon(Soot) + Carbon Monoxide + Water Vapour

The chemical equation above demonstrates the 2 different outcomes of incomplete and partial combustion where one still manage to produce carbon dioxide along with soot and carbon monoxide whereas for the second equation only demonstrates soot and carbon monoxide as by products. Carbon particles are usually seem as soot or smoke.

 

Reference from: http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/crudeoil/hydrocarbonsrev1.shtml