Why do plane engines smoke when they start up?

Plane engines may produce smoke during startup due to the combustion of oil or fuel that has accumulated in the engine or exhaust system. This can happen if the engine has not been run for a while, allowing oil to seep into the combustion chamber. Additionally, during the initial stages of engine startup, the fuel mixture can be rich, leading to incomplete combustion and visible smoke. Typically, this smoke dissipates quickly as the engine reaches normal operating conditions.