Idle speed (or idle) is the rotational speed an engine runs at when the engine is idling, that is, 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 (rpm) of the crankshaft. 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.
Car, truck, and motorcycle engines
For a passenger car engine, idle speed is customarily between 600 and 1000 rpm. For medium and heavy duty trucks, it is approximately 600 rpm. For many single-cylinder motorcycle engines, idle speed is set between 1200 and 1500 rpm. Two-cylinder motorcycle engines are often set around 1000 rpm.
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 modified for power at high engine speeds, such as auto racing engines, tend to have very rough idle unless the idle speed is raised significantly.
In addition to this, setting the idle speed too slow can result in very low oil pressure when the engine is hot, resulting in faster wear and possible seizure.
Commercial aircraft descend with minimum thrust, that is, the engines are operating at idle speed. This situation happens when an aircraft is gliding or landing.
- Bennett, Sean (2013). Medium/Heavy Duty Truck Engines, Fuel & Computerized Management Systems (4th ed.). Clifton Park, NY: Delmar Cengage Learning. p. 368. ISBN 1111645698.
- Woodring, Kip; Love, Kenna (2006). 101 Harley-Davidson Evolution Performance Projects (2nd ed.). St. Paul, Minnesota: Motorbooks. p. 72. ISBN 0760320853.
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