Idle speed

Idle speed is the minimum operating speed (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.

Because an engine at idle may not produce enough power to overcome the inertia of the drivetrain, the engine must either be disconnected from the drivetrain by shifting to neutral or disengaging the clutch or through use of a fluid coupling whose slippage will allow the engine to run without moving the transmission gears.

If the engine is operating a large number of accessories, particularly air conditioning, the idle speed must be raised to ensure that the engine generates enough power to run smoothly and operate the accessories. Most air conditioning-equipped engines have an automatic adjustment feature in the carburetor or fuel injection system that raises the idle when the air conditioning is running.

Engines optimized for power at high engine speeds, such as auto racing engines, frequently have poor low-speed power, and tend to be very rough at idle unless the idle speed is raised significantly.

Effort has been made to reduce the amount of time engines spend idling, chiefly due to fuel economy and emissions concerns, though some engines can also be damaged if kept idling for extended periods. In the United States, about a billion gallons (3.8 billion liters) of fuel gets consumed by idling heavy-duty truck and locomotive engines each year [http://www.epa.gov/smartway/idle-questions.htm] . Many newer semi trucks have small auxiliary power units (APUs) to run accessories more efficiently while the truck is parked. Hybrid vehicles typically shut down their internal combustion engines while stopped, though some conventional vehicles are also including start-stop systems to shut off the engine when it would otherwise idle.