Question

why does carbonaceous fuel undergo incomplete combustion? how can a fuel undergo complete combustion?

Answer 1

Insufficient oxygen, an improperly stacked fire, a poorly designed furnace; all of which can limit egress of oxygen to the centre of the pile of fuel. Note that sometimes you want incomplete combustion, and wood is burnt under conditions that promote formation of charcoal

Answer 2

Carbon fuels undergo incomplete combustion in many way like not getting proper oxygen a bad fuel like coal and the dunk cakes so many reason for incomplete combustion.