Fuel cut-off
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Overrun fuel cut-off is an intentional, temporary interruption of the fuel supply in an internal combustion engine when it is not supposed to deliver any power, but rather is being dragged by the vehicle mass in motion.
In overrun mode of an internal combustion engine used as a vehicle drive, it is not necessary to add fuel, even though there is air throughput, since the movement of the engine is maintained by the rotation imposed by the drive train. Only just above idle speed is it necessary to supply energy again by adding fuel so that the engine does not stop and die. An overrun fuel cut-off was first used in diesel engines, whereby the injection pump switches off the fuel supply when the speed controller is active and the engine speed is too high. This usually occurred when the accelerator was not pressed and the engine was being pushed by the vehicle. When gasoline engine , the fuel cut is in since 1980 electronic injection systems used. The fuel supply is switched off via the injection valves from an engine speed of approx. 1100–1400 / min (depending on the parameters engine temperature, speed tendency and throttle or accelerator pedal position ).
The main purpose of the overrun fuel cut-off is to save fuel. It comes into play when you have to reduce the speed and use the motor as a brake ( motor brake ). The equivalent is called overrun . You must not operate the clutch, as the engine is then disconnected from the drive train and consumes fuel again. It is then relatively little, but still significantly more than when using the fuel cut-off. Fuel savings can therefore be achieved through anticipatory driving style , in which the gas is completely withdrawn in good time and you brake as late as possible. This protects the brake pads and saves a little fuel. If a delay undesirable or their need for very long before foreseeable, it is advisable to further fuel savings, before the pushing operation, an idle sailing ahead increase (will z. B. in current version of the automatic transmission "DSG" VW or "S-Tronic" Audi automatically controlled ).
During overrun , the engine speed increases and that of all attached attachment systems such as the alternator and air conditioning compressor . The engine brake therefore does not reduce the braking energy solely through compression work in the internal combustion engine. An increased part is also recuperated in the battery and evaporator of the active air conditioning system , especially in connection with intelligent energy management , which later leads to the corresponding reduction in engine load with further savings.
In modern vehicles with a display of the current fuel consumption, the difference between idling and overrun fuel cut-off can be observed very well. When the overrun fuel cut-off is activated, the display should show 0.0 l / 100 km, while idling 0.5–2.0 l / h (converted to l / 100 km while driving). In some exceptional cases (such as automobiles from the manufacturers Fiat and Daihatsu) the display still shows 1.0 to 2.0 l / 100 km incorrectly despite the overrun fuel cutoff.
With a cold internal combustion engine, while the fuel supply is open and there is no decrease in output (e.g. during “warming up” or “coasting”), there is a latent risk that fuel will condense on the piston and cylinder surfaces and wash off the lubricating engine oil film. In addition to avoiding cold running without a decrease in performance, overrun mode is also suitable to prevent this and thus prevent premature engine damage.
See also
literature
- Jürgen Kasedorf: Motor vehicle engine test, gasoline engines. 7th revised edition, Vogel Buchverlag, Würzburg 1997, ISBN 3-8023-0461-6 .
- Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Vieweg, Wiesbaden 2005, ISBN 3-528-23933-6 .