Knocking (internal combustion engine)
When knocking uncontrolled is combustion of the fuel in gasoline engines , respectively. The temperature and pressure rise suddenly, creating more ignition nuclei. These then hit each other at the speed of sound. Pressure peaks occur that can damage pistons, bearings, cylinder head, valves and spark plugs. Reflection results in a high-frequency oscillation in the cylinder pressure curve, which is audible.
A distinction must be made between knocking and ringing. Knocking, similar to the sound of a hammer, occurs primarily when accelerating under full load. Engine ping is a sound that indicates improper ignition timing . The fuel-air mixture is ignited due to glowing carbon residues in the combustion chamber before the spark plug ignites. This error occurs in two operating circumstances:
- as acceleration bells, for the sake of transition in short-term operating conditions and usually clearly audible,
- than high-speed bells, which can last much longer and, disadvantageously, are far less audible. The high-speed bell ringing is capable of severely overheating and ultimately destroying an engine . If the bell rings for too long, too much thermal energy goes into the moving components of the combustion chamber, the valves and the pistons. The piston can burn out or a valve stem can melt and break. The too high pressure can also damage the connecting rod bearings . At the beginning of 1975 the second stage of the gasoline lead law came into force, the lead content ( tetraethyl lead ) in fuel was limited to 0.15 g / l. With the use of the new super fuel , it started with the older engines, which required a higher octane rating for high-speed ringing and increased engine damage. A lead substitute additive had to be added to every tank filling. All German automobile manufacturers (the first was Opel ) had to adapt their engines to the guideline, which was associated with a lower compression and a slight reduction in power with the same cubic capacity. As of January 1, 2000, leaded petrol was finally withdrawn from circulation in the European Union , and modern petrol engines no longer needed lead to avoid engine bells.
High-performance engines are sometimes deliberately operated in the knocking area in order to achieve higher performance through extreme pressures. However, these motors also have a correspondingly short service life.
Uncontrolled, usually premature self-ignition of the air-fuel mixture, i.e. flame fronts that are not triggered by the ignition spark , can occur for the following reasons:
- too high compression of the mixture
- high temperatures of the cylinder inner wall (with high engine power)
- glowing burn-off (combustion and oil residues) at hot points of the combustion chamber (spark plug area, exhaust valves, piston recess)
- ignition timing too early (older, adjustable ignition systems)
- Fuel with an octane number that is too low (code for knock resistance )
- Unfavorable combustion chamber shape and fuel mixture flow without swirl and turbulence favor the formation of hot spots
- Lubricating oil particles that can get into the air-fuel mixture from the crankcase gases
- Incorrect heat value of the spark plug , so that it either becomes too hot and thus becomes a glow starter itself or that it remains too cold to burn off residues (coking), which in turn trigger glow ignition.
Knock control is used in today's engines to maximize the use of combustion energy while avoiding knocking. This consists of one or more knock sensors , signal evaluation, knock detection algorithm and ignition angle control with adaptation. It enables the engine to operate close to the knock limit and, if necessary, shifts the ignition point towards retarded.
Other ways to prevent knocking
- Reducing the load, i.e. the effective mean pressure, and thus the output power
- "Late" ignition
- Enrichment of the mixture for internal cooling of the combustion chamber (increases consumption, is used to increase maximum output)
- Using knock solid fuels ( gasoline with high octane number as leaded gasoline or premium gasoline, methanol , ethanol , etc.)
- Injection of cooling substances that prevent detonation (water, ethanol, propane , methyl tert-butyl ether (MTBE) and others)
- Use of two (or more) spark plugs to ignite the mixture evenly
- Construction of the most compact combustion chambers possible; jagged combustion chambers tend to knock more often
Alternatives to the knock sensor
- Cylinder pressure sensor: You can see the pressure increase of the compression, the ignition and possibly the reflecting pressure waves of a detonation for each individual cylinder . Unlike the knock sensor, this sensor requires a bore in the cylinder.
- Ion sensor: A DC voltage of around 400 volts is applied to the spark plugs between ignitions . The measurement of the current flow provides information about pressure, temperature and ion density at the same time. Knocking is indicated by ion current peaks, which are filtered out, counted and, if certain criteria are met, evaluated as knocking indicators.
Similar effects (designation: nailing ) also occur in diesel engines , in which, however, compression ignition is the basic ignition principle.
The cause can be:
- Ignition delay during combustion
- Nozzle failure, for example when the atomization is no longer sufficient and instead of a jet with a continuous distribution of the droplet sizes, more large droplets are produced
- Unfavorable engine parameters (e.g. cold running, fuel with too low a cetane number )
In these cases, the mixture of injected fuel and air does not burn through uniformly, rather larger quantities of the injected fuel ignite at the same time, so that a sudden, steep rise in pressure is audible as a noise. This increase in pressure also leads to high mechanical stress. The consequences of nailing are comparable to those of knocking.
The so-called cold-running nailing disappears when the temperature of the engine rises and is usually harmless. In older diesel engines up to around 1980, idling nailing arose because they were pre-chamber engines . The noise can be taken literally and was typical for this type of construction. There is still no combustion in the antechamber itself because the temperature is initially too low, especially on very cold winter nights. The idle speed could be too low after the cold start, depending on the outside temperature, which is why you could regulate the speed with a control button on the dashboard. When the engine speed is increased in the load range, the nailing disappears suddenly. Once the engine has reached operating temperature, it becomes significantly quieter when idling, but does not go away completely.
- Jan Drummans: The car and its technology. 1st edition, Motorbuchverlag, Stuttgart 1992, ISBN 3-613-01288-X .
- Peter A. Wellers, Hermann Strobel, Erich Auch-Schwelk: Vehicle technology expertise. 5th edition, Holland + Josenhans Verlag, Stuttgart 1997, ISBN 3-7782-3520-6 .
- Kurt-Jürgen Berger, Michael Braunheim, Eckhard Brennecke: Technology automotive engineering. 1st edition, Verlag Gehlen, Bad Homburg 2000, ISBN 3-441-92250-6 .
- Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag GmbH, Braunschweig 2000, ISBN 3-14-221500-X .
- RWTH Aachen University: Final Report of the Collaborative Research Center 224 Engine Combustion Chapter 3: Otto Engines , Section 4: Knocking , November 28, 2000, accessed on August 11, 2010
- Karl-Heinz Dietsche, Konrad Reif, Robert Bosch GmbH: Kraftfahrtechnisches Taschenbuch. Springer Vieweg, Wiesbaden 2014, ISBN 978-3-658-03800-7 .