Idling is the stationary condition of the vehicle means when the engine is running but the vehicle is not in motion. At this time, no power transmits from the engine to the vehicle gearbox since there are disconnected from each other.
At this condition, the accelerator pedal or also known as the throttle pedal is not pressed by the driver.
There is only about 0 to 20 % load on the engine at this position of the vehicle. The throttle valve is open about 0 to 20 % (Nearly closed).
The throttle valve is nearly closed hence the airflow is reduced by the throttle valve and only few amounts of air comes into the engine.
Due to the low amount of air, the rich air-fuel mixture burns in the engine resulting the improper combustion takes place.
This phenomenon reduces the thermal efficiency of the engine and exhausts the hazardous unburnt elements in the atmosphere, which are causes many health problems such as Nausea, breathing problems, etc.
Usually, the engine is at idling position when the vehicle driver waits for someone as well as during the driver waiting in the traffic.