Engine knocking

The knocking is an uncontrolled combustion or autoignition of the fuel in internal combustion engines. Regular operating an internal combustion engine requires a controlled combustion ( deflagration ).

A distinction must be knocking and ringing. Knock, right from the noise a hammer blow, arises primarily when accelerating under full load. Motor ringing, also called high-speed ring is accordingly evident at high speeds as a quiet whirring at constant full load. Both are harmful to the engine.

Effect of knocking

Too hot combustion chambers, the air-fuel mixture can ignite explosively itself. This is likely to occur when fuels are used with low octane number. The gases then burn uncontrollably at high speed.

Temperature and pressure rise abruptly to (very high pressure gradient ), and the pressure waves propagate at the speed of sound in the combustion chamber and impinge on the walls. The results are sound and mechanical loading of the combustion chamber limiting components. By reflection, there is a high-frequency vibration in the cylinder pressure curve.

By knocking combustion of the motor is loaded mechanically and thermally very high. It occurred to pressure spikes that can damage pistons, bearings, cylinder head, valves and spark plug.


Uncontrolled auto-ignition of the air -fuel mixture next to the actual flame front, which is triggered by the ignition spark, can be for the following reasons:

  • To high compression of the mixture
  • High temperatures of the cylinder inner wall ( at high engine power )
  • Glowing combustion (combustion and oil residues ) to the combustion chamber walls
  • False to earlier ignition timing ( older, adjustable ignition systems )
  • Fuel with too low an octane number (code for the knock resistance )
  • High temperatures in the outer areas of the cylinder by the radiation energy of an ignited flame front, which, however, has not yet been reached, the outer regions
  • Unfavorable nested combustion chamber shape without turbulence ( favors the formation of hot spots ) of the air -fuel mixture
  • Oil particles which can enter the air -fuel mixture from the crankcase gases

In any case, it is next to the combustive controlled flame front to a further explosive combustion ( see picture).

Demarcation knock / pre-ignition or spontaneous combustion / Super knocking

A knocking combustion, as shown in the animation, develops during the regular combustion of the fuel-air mixture.

Spontaneous combustion, ie irregular burns are not initiated by the spark of the spark plug, but prematurely by other sources such as coal particles ( combustion chamber deposits ) and / or oil ( from the crankcase breather as with 4- stroke engines common). The effects of pre-ignition usually correspond to the knocking combustion.


The consequences of self-ignition sometimes heard the detonation, and thus:

  • The perceptible as knocking noise generated in the cylinder head
  • Crucial: extreme pressure peaks in the cylinder ( high-frequency pressure increases ) lead to excessive material loading of cylinders, pistons, connecting rods, etc.
  • More heat

The effects are usually unintentionally. It is different with high-performance engines, which are partly conscious operated in the beating area in order to achieve higher performance by extreme pressures. However, these engines also have a correspondingly low life.

For maximum utilization of the combustion energy while avoiding the knock in modern engines, knock sensors are used which affect the ignition timing and thus stop the motor directly to the knock limit. This allows the maximum pressures in the spark-ignition engine, which in turn leads to the improvement of performance and efficiency ( see also: Otto cycle ).

Prevention of knocking

By the following process, it is possible to prevent such uncontrolled motor and damaging combustions:

  • Shift of the ignition point in the direction of late; moves the heat development in the expansion stroke, output power is reduced; realized in modern engines with electronic knock control
  • Decreasing the load, ie the mean effective pressure
  • Increasing the engine speed and as a result a rapid propagation rate of the flame front such that it reaches the rest of mixture before its ignition
  • Enrichment of the fuel supply to the cooling of the combustion chamber ( without a power consumption increases growth)
  • Efficiency- reducing limiting the compression ratio
  • Injecting cooling detonationsverhindernden substances (water, ethanol, propane, methyl tert -butyl ether ( MTBE), and others)
  • Subdivision of the combustion chamber by two spark plugs

Recognition of knocking

In order to prevent engine damage, many automobile engines are equipped with sensors which are able to detect the knocking. There are different principles by which these sensors operate:

  • ' Knock sensor ' ( a seismic mass ), the signal with the aid of filters, and a digital signal processor is tested for the typical tapping Spektralgehalte. One or more of these sensors to detect detonation in any cylinder. In some cases, lead ( mechanical noise of the engine components, such as system change the piston, closing the intake and exhaust valves, etc.) to false signals this acoustic method due to other mechanical influences.
  • Cylinder pressure sensor: It can be seen the pressure increase in the compression ratio, the ignition and, optionally, the reflecting pressure waves of detonation for each individual cylinder.
  • Ion Sensor: between firings is applied a DC voltage of about 400 volts is applied to the spark plugs. The measurement of current flow is the same information on pressure, temperature and ion density. A knock is reflected in ion current peaks that are filtered out are counted and evaluated if certain criteria as a knock indicators.

Diesel engine

Similar effects (called nails), also occur in the diesel engine, but with the self-ignition is the basic ignition principle.

Background can be:

  • Ignition delay in the combustion
  • Nozzle error when, for example, the atomization is not enough and, instead of a beam with a continuous distribution of the droplet sizes creates more large droplets
  • Unfavorable motor parameters ( eg cold-running, fuel with too low cetane number )

In these cases, the mixture of diesel fuel and air will not burn eingespritzem evenly through but large amounts of the injected fuel ignited simultaneously, so that an abrupt steep rise in pressure can be heard as a sound. This increase in pressure leads to high mechanical stress. The consequences of nailing are identical to those of knocking.

The so-called cold start nailing disappears with increasing temperature of the engine and is usually harmless.