Idle speed (idling, or idle) is the rotational speed the engine runs on when the engine is uncoupled to the drivetrain and the throttle pedal isn't depressed (generally measured in revolutions per minute, or rpm, of the crankshaft) of a combustion engine. At idle speed, the engine generates enough power to run reasonably smoothly and operate its ancillaries (water pump, alternator, and, if equipped, other accessories such as power steering), but usually not enough to perform useful work, such as moving an automobile. For a passenger-car engine, idle speed is customarily between 600 rpm and 1,000 rpm. For buses and trucks it is approximately 540 rpm. In case of many single-cylinder motorcycle engines, idle speed is set between 1200-1500 rpm.