Two-stage ignition is characterized by an initial cool-flame reaction followed by typical hot ignition. In traditional combustion conditions the ignition is fast such that the cool flame is not observed. By controlling initial conditions (pressure, temperature, and composition) in a rapid compression machine, for example, the creation and duration of the cool-flame event is predictable. This study focuses on linking the results from the rapid compression machine to those of low-temperature combustion behavior in a medium-duty compression ignition diesel engine. A correlation between cool-flame duration, nitric oxide concentration, and engine control settings allows the postulation that a sufficiently long cool-flame reaction results in a combustion event that can be classified as low-temperature combustion. A potential method for identifying low-temperature combustion events using only the rate of heat release profile is theorized which utilizes the cool-flame reaction duration as a metric.