Knock has historically been one of the main limitations on spark ignition (SI)
engine compression ratio and hence efficiency. The trend to downsizing or
rightsizing in recent years, driven by ever-reducing carbon dioxide
(CO2) targets, has increased the relevance of the knock limit for
typical engine operating conditions. Even for scenarios where an engine is run
on carbon-neutral fuel, thermal efficiency will always be fundamental in terms
of best use of scarce resources. Knock, therefore, remains a relevant topic for
current and future research.
Knock is typically quantified through analysis of high-pass-filtered cylinder
pressure signals. For SI engines, this is relatively unproblematic. A promising
technology for further combustion engine efficiency gains, however, is
prechamber ignition. It has been noted that prechamber combustion systems result
in significant high-frequency content on the cylinder pressure trace in the
bandwidth of interest for knock. It is therefore more difficult to accurately
determine the knock limit for such engines, which is necessary in order to make
a fair comparison to traditional SI systems.
There is relatively little detail on this key topic in the existing literature.
Accordingly, this study compares knocking experimental data from the same
high-performance single-cylinder research engine with both SI and prechamber
combustion systems. Established and new approaches of interpreting the knocking
data are examined, applying both high-frequency and low-frequency techniques to
cylinder pressure signals, complemented by statistical methods. The analysis
conducted demonstrates that for knocking prechamber combustion, three distinct
combustion stages are detected. The high-frequency content of the prechamber
pressure signal is also more complex and is analyzed in detail. A significant
gain in knock-limited (KL) combustion phasing is thus confirmed for the
prechamber igniter, at appropriate levels of knock, in comparison to the
standard spark plug (SP) system.
