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Idle refers to the operation of a system or machine without it doing the work for which it is intended. Systems often require a certain start-up time until all parameters reach the desired values ​​(e.g. temperature, also known as operating temperature).


Since starting up often consumes a lot of energy and also increases wear and tear, under certain circumstances it is more economical to run systems without load than to shut them down completely and start them up again later. Avoiding unnecessary idling, however, has the potential to save energy and avoid noise.

Internal combustion engines

Internal combustion engines come to a standstill at a minimum speed of the crankshaft . For most motor vehicle engines, this speed is between 700 min −1 and 900 min −1 . In order for the engine to maintain the speed, a small amount of fuel-air mixture (conventional gasoline engine ) or fuel ( diesel engine ), the so-called idle gas , is fed to it. Some types of vehicles, e.g. B. Cars with automatic transmissions and small motor boats can take up low speed with the engine engaged in idle mode (which is then no longer idling). In intake manifold to a separate comes speed controller used. Carburetors are equipped with a separate idle jet. The idle speed is usually set a little higher than absolutely necessary so that fewer vibrations occur and a sudden change to load operation does not lead to the engine going out. On some vehicles, such as tractors , combine harvesters and some passenger cars (such as the Mercedes-Benz W 120 ), the idling speed can be continuously adjusted with a idle lever.

The consumption of fuel when idling (when the vehicle is stationary) is the idle consumption. With car engines it is between 0.8 and 1.5 liters per hour.


In automobiles that have been optimized for the lowest possible fuel consumption , the engine is not operated in idle with the help of a so-called start-stop system ; When the clutch or accelerator pedal is then depressed, the engine is automatically restarted. The biggest disadvantage of this possibility of saving fuel, however, is greater wear and tear, which is particularly noticeable in the engine in the crankshaft. The reason for this is their almost wear-free mounting in hydrodynamic slide bearings , which however requires oil pressure and the movement of the shafts, which means that increased abrasion due to mixed friction occurs with each new start-up .

Motor vehicles with automatic start-stop require a more stable starter and a larger-sized car battery ( AGM starter battery ), which are designed for frequent starting processes. This increases the manufacturing and maintenance costs of the vehicle.

Metaphorical meaning in ethology

Based on the technical meaning, the ethologist Konrad Lorenz introduced the metaphor idling action into behavioral biology in the 1930s . Within the instinct theory he advocates, it describes those instinctual movements that were set in motion by an innate trigger mechanism without the observer being able to prove a triggering key stimulus.


  • Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology . 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2001, ISBN 3-8085-2067-1
  • Hans Jörg Leyhausen: The master craftsman examination in the automotive trade part 1 . 12 edition, Vogel Buchverlag, Würzburg, 1991, ISBN 3-8023-0857-3

See also

Individual evidence

  1. Dieter Lohse and Werner Schnabel: Basics of road traffic engineering and traffic planning : Volume 1 , Beuth Verlag, 2011, ISBN 9783410172710
  2. TÜV Nord - vehicle optimization
  3. Klaus-Dieter Pannes: Analysis and classification of unsuccessful product innovations using the example of the automotive industry and deriving recommendations for action: Using a selected future product concept ., 2003, ISBN 9783832466862 , p. 54 ff.