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Revolutions Per Minute (RPM) refers to the revolution per minute in which the crankshaft turns.

Idle speed is the rotational speed an engine runs at when the engine is idling, that is the condition when the engine is uncoupled from the drivetrain and the throttle pedal is not depressed. 

  • In combustion engines, Idle speed is generally measured in revolutions per minute of the crankshaft. At idle speed, the engine produces enough power to run reasonably smoothly and operates its ancillaries but usually not enough to perform optimal work. 
  • For a typical passenger car, idle speed ranges from 600 rpm to 1000 rpm.

If the engine is operating a large number of accessories, for example, air conditioning, the idle speed must be raised to ensure that the engine produces enough power to run smoothly and operate its accessories. Most of the air conditioning equipped engines to have an automatic adjustment feature in the carburetor or fuel injection system that enables to raise the idle when the air conditioning is working