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.