On the Origin of Pre-Ignition inside a Pre-Chamber Spark Plug – Gas Analysis

Pre-chamber spark plugs are widely used to increase the efficiency of internal
combustion engines in order to meet future CO2 emission regulations. Unfortunately,
pre-ignition at higher load operation points in combination with pre-chamber ignition
systems has been reported in the past [1]. Two main assumptions for pre-ignition
are (1) pre-ignition to be triggered either by hot pre-chamber surface components
or (2) by hot and still reactive residual gases present inside the pre-chamber from
previous combustion cycles. Assumption (1) was examined in a previous publication
[2] and not found to be the leading cause of pre-ignition. However, with the help of
optical pre-chamber spark plugs, it was discovered that the origin of the pre-ignition
is located in the upper part (around the insulator) of the pre-chamber. For this reason,
the gas composition in the upper part of the pre-chamber is to be examined in detail
and the CFD calculations thereby validated.
The objective of this study is to analyze the residual gas composition inside a
pre-chamber spark plug while running in an engine. For this purpose, a heavy-duty
natural gas engine, that is operated with a M14 pre-chamber spark plug, will be
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run purposely up to a point where pre-ignition occurs. A small gas probe is to be
sampled from the internal pre-chamber spark plug volume at various times during
the combustion cycle and gas analytical measurements are to be performed. The
gas is extracted from the area identified in the previous publication as the source
of the pre-ignition. The sampling is controlled by a fast-switching valve so that the
sampling time and duration can be selected precisely. In order to achieve the necessary
sensitivity for measurement of the low sampling volume flow, the sample is diluted
with nitrogen in a controlled manner.
In addition to carbon dioxide, unburned methane was also detected in the pre-chamber
spark plug throughout the entire combustion cycle. These findings corroborate the
hypotheses proposed in the previous publication, namely that the upper area of the
pre-chamber spark plug only undergoes combustion at a relatively late stage, which
allows for the presence of unburned gases and radicals that can lead to pre-ignition.
However, further measurements are necessary to obtain a comprehensive analysis.