3 Knocking Combustion
ARIADNE Knock Control 7
3 Knocking Combustion
Researches on knocking combustion in spark-ignited engines started in the early 1920
th
years,
at the beginning of engine optimization and was already discovered quite short after the in-
vention of the SI engine itself. After some time it became evident that knocking is caused by
auto ignition of the air-fuel mixture in front of the flame propagating across the combustion
chamber when the temperature and pressure conditions of the end gas have reached a critical
level. In addition to annoying noise emissions, which gave the name “Knocking”, the engine
performances are reduced and the engine itself can be destroyed by this uncontrolled combus-
tion process. The main factors acting on the knock level are: compression ratio, cylinder fill-
ing, spark advance, intake pressure and temperature, air-fuel ratio and fuel gas properties.
During a normal combustion (↑ Figure 2 Normal combustion, cylinder pressure without
knocking) a turbulent flame is propagating, changing the air-fuel mixture into burned gas.
This reaction releases energy which leads to a certain pressure increase in the combustion
chamber. The pressure is considered homogeneous in the cylinder since the propagation speed
of the flame front is rather small (10-50 m/s) and the combustion does not create pressure
waves of high amplitudes. The propagation of the flame front needs a certain time during
which the end gas is submitted to increasing pressure and temperature conditions which can
reach the auto ignition critical level. The auto ignition of a part of the air-fuel mixture has for
result its combustion at almost constant volume or at least at very high combustion speed (the
mixture is instantaneously transformed into burned gas) . This kind of detonation leads to a
high and quick local pressure increase, followed by vibrations of the whole cylinder filling
and progressive equalization of the pressures in the cylinder. The pressure oscillations have a
fundamental frequency range of 2000-8000 Hz, mostly depending on the gas temperature in
the combustion chamber and the cylinder diameter (↑ Figure 3 Uncontrolled combustion, cyl-
inder pressure with heavy knocking). The abrupt characteristic of the phenomenon can be
explained by the high propagation speed of the pressure waves (500-1000 m/s), compared to
the normal propagation speed of the flame front (10-50 m/s). Knocking combustion leads to
higher heat transfers between burned gas and cylinder walls and consequently reduces engine
efficiency. The higher mechanical and thermal stress on the cylinder walls (vibrations and
heat transfers) can be destructive specially on already high loaded engines (erosion or destruc-
tion by fatigue of piston, cylinder head, liner, piston compression rings, pre-chamber …).
After 80 years of research, it is clear that the question today is not how to basically suppress
knocking combustion as such, which is illusory, but how to best deal with this remaining
threat in the future engine developments, making a reliable knock control system a major key
for increased engine performances.
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