Internal combustion engines have dealt with increasingly
restricted emissions requirements. After-treatment devices are
successful bringing emissions into compliance, but in-cylinder
combustion control can reduce their burden by reducing engine-out
emissions. For example, oxides of nitrogen (NOx) are
diesel combustion exhaust species of notoriety for their difficulty
in after-treatment removal. In-cylinder conditions can be
controlled for low levels of NOx, but this produces high
levels of soot particulate matter (PM). The simultaneous reduction
of NOx and PM can be realized through a combustion
process known as low temperature combustion (LTC).
This paper presents an investigation into the manifestation of
LTC in the calculated heat release profile. Such a study could be
important since some extreme LTC conditions may exhibit a return to
the soot-NOx tradeoff, rendering an emissions-based
definition of LTC unhelpful. For example, in this study, increased
exhaust gas recirculation (EGR) levels at LTC injection timings
result in a slight, albeit small, increase in smoke concentrations.
As a result, this study is motivated by the need to observe some
other metric in defining LTC that fundamentally could be
independent of emissions observations. Specifically, this study
finds that the delay between start of combustion and start of
significant heat release increases substantially. Combustion
phasing is shifted significantly into the expansion stroke such
that the burn rate is slowed enough to create a reaction that is
characterized by lower temperatures than otherwise would have
occurred.
This study employed high levels of EGR and late injection timing
to realize the LTC mode of ordinary petroleum diesel fuel. Under
these conditions a two-part criteria was developed that identifies
the LTC classified conditions. The criteria are as follows: the
combustion event of conventional petroleum diesel fuel must show a
two-stage ignition process; and the first stage must consume at
least 2% of the normalized fuel energy before the hot ignition
commences.