Why is a diesel engine also called an self-ignition engine?

A diesel engine is called a self-ignition engine because it does not rely on a spark plug to ignite the fuel. Instead, the fuel is injected into highly compressed, hot air inside the cylinder. The high temperature from the compressed air causes the fuel to ignite spontaneously or "self-ignite." This is different from gasoline engines, which use a spark plug to start combustion. Understanding this helps recognize that diesel engines work on the principle of compression ignition, making them more efficient in certain situations.