idle

Idle is the rotational speed at which an engine runs when the throttle is closed and no load is applied. For a piston engine, this is typically between 600 and 1000 rpm for automotive applications, though marine and stationary engines often run higher. At idle, the engine produces just enough power to overcome internal friction and keep itself running; it cannot propel a vehicle or drive external equipment. Idle speed is a fundamental operating point that every internal combustion engine must maintain reliably.

The idle speed is controlled by a throttle stop or idle control valve that meters the amount of air and fuel entering the cylinders when the accelerator is fully released. Carburetted engines use a mechanical idle stop screw; fuel-injected engines typically employ an electronically controlled idle air control valve (IACV) that adjusts airflow based on engine temperature, load, and other variables. Modern engines also trim the fuel injection duration and ignition timing to hold the target idle speed steady as the engine warms up or accessories like the alternator cut in.

Idle speed stability is critical because a rough or unstable idle causes vibration, poor driveability, and rough engagement when selecting a gear. If idle speed creeps too high, engine wear accelerates and fuel consumption rises; if it drops too low, the engine stalls. Idle quality problems often stem from vacuum leaks, carbon buildup in the intake valves or ports, contaminated fuel injectors, or a faulty idle control valve. Diagnostic technicians check idle speed with a tachometer and may adjust the idle target via computer tuning or mechanical trimming.

Idle running significantly affects tailpipe emissions because the engine operates at a rich air-fuel ratio and low combustion efficiency when unloaded. Regulations in many markets specify maximum idle-speed emissions limits; some jurisdictions require engines to automatically shut down after a set idle period to reduce unnecessary emissions. The idle control system must balance fuel economy against smooth idle behavior and cold-start reliability.

Idle speed also matters for diesel engines, though the range differs. Heavy-duty diesel engines may idle at 600 to 800 rpm, while some small diesels run as low as 400 rpm. Unstable diesel idle is often caused by air in the fuel system, clogged fuel filters, or worn injection nozzles. Unlike petrol engines, many diesel engines do not use electronic idle control and rely on mechanical fuel-pump governors to hold a fixed idle speed across a range of operating temperatures.