What is idle speed referring to in engine operation?

[DeKimberly]

Idle speed refers to an operational state of the engine where there is no external power output. Purpose of idle speed: It overcomes internal resistance of the engine to maintain a non-stalling condition. At this point, the throttle opening is minimized, allowing the engine to operate smoothly without vibration in a relatively fuel-efficient state. Disadvantages of idle speed: In this state, the airflow velocity inside the engine is relatively slow, making it prone to leaving residual exhaust gases in the combustion chamber. This can reduce the fresh air-fuel mixture entering the cylinders, causing RPM drops. The engine control unit then compensates by increasing air intake and fuel injection to restore RPM. This continuous adjustment process is known as idle speed regulation, which is why some older vehicles exhibit minor RPM fluctuations visible on the tachometer during idling.