The car engine moves with the fuel. Oxygen to burn that fuel If there is not enough oxygen with the fuel, the fuel does not burn properly. So, exhaust or unused fuel comes out with Xhost.
White smoke means there is a mixture of water wIthaca oil.
Gray means fuel and inner lubricating oil.
Blue wash means oil.
Black smoke means carbon and fuel.
Carbon and fuel refers to the lack of fuel (carbon remains) or unused fuel.
Now that the black smoke is being pulled out of the vehicle it means the fuel is not burnt properly. It may be due to lack of proper maintenance or loss of car engine. When the cylinder decay, the fuel will leak out. Again the oxygen is coming in which the line and clogd can be closed.
As a result, the fuel does not burn properly (no oxygen). Or there may be a problem with the fuel injection line. As a result, the fuel is injected more. This makes the air-fuel ratio more and less. More fuel is entering but not burning (because there is no air / oxygen compared to that). Black smoke may be due to some other reasons.