Why is the exhaust pipe black?

[Holden]

Insufficient combustion of automobile fuel, resulting in carbon deposits, is the root cause of the blackening of the exhaust pipe. Here is relevant information: 1. If the engine cannot fully combust, some of the unburned or incompletely burned fuel vapor will be carbonized when passing through the three-way catalytic converter. These carbon deposits accumulate at the exhaust pipe outlet, and over time, the exhaust pipe will turn black. 2. The blackening of a car's exhaust pipe is generally a normal phenomenon, because any oil-burning vehicle will inevitably have its exhaust pipe turn black after prolonged use. If the exhaust pipe of a new car turns black, it may be caused by incomplete combustion of gasoline, or it may be due to the addition of inferior gasoline or an incompatible fuel grade.