What color is the exhaust smoke when the engine is judged to have incomplete combustion?

When an engine is experiencing incomplete combustion, the exhaust smoke typically appears black. This occurs because the fuel is not burning completely, leading to an excess of fuel in the combustion chamber. As a result, unburned fuel particles are expelled through the exhaust system, producing a black smoke that is often visibly thicker than other types of exhaust.

The presence of black smoke often indicates that the engine is running too rich, meaning there is too much fuel compared to the amount of air in the combustion process. Monitoring exhaust color is crucial for diagnosing engine performance and ensuring efficient operation. Proper combustion is key to engine performance, efficiency, and minimizing harmful emissions.