Why is diesel smoke black?

Diesel smoke turns black when the ratio of air to fuel in the engine is out of balance. This happens when there's either too much fuel added to the mixture or too little air supplied to burn the fuel.

The black smoke contains a lot of particulates that are essentially large diesel particles that would be burned as fuel if the engine were operating efficiently. Black diesel smoke is usually caused by faulty injectors or injector pumps; a malfunctioning air filter, which can prevent the proper amount of air from being supplied; a faulty EGR valve, which can cause the valves to become clogged; or a faulty turbocharger.