Unburned hydrocarbons (UHCs) are the hydrocarbons emitted after petroleum is burned in an engine.
Any fuel entering a flame will be reacted. Thus, when unburned fuel is emitted from a combustor, the emission is caused by fuel "avoiding" the flame zones. For example, in piston engines, some of the fuel-air mixture "hides" from the flame in the crevices provided by the piston ring grooves. Further, some regions of the combustion chamber may have a very weak flame, that is, they have either very fuel-lean or very fuel-rich conditions and consequently they have a low combustion temperature. These regions will cause intermediate species such as formaldehyde and alkenes to be emitted. Sometimes the term "products of incomplete combustion," or PICs, is used to describe such species.