Flame radiation has been recorded in an experiment within an operating D.I. diesel engine and the flame temperature has been estimated (a grey-body radiation has been assumed in interpreting the two-color method results). An indirect comparison of measurements has been done by an analysis of the Θ-number ratio of the mean bulk gas temperature Tg and the flame temperature Tf. Regression analysis of the author's experimental data revealed a linear Θ-Tg relation. More conclusive and physically grounded appears the Θ/Bo - time correlation (Bo - Boltzmann number). The similarity in shape of the Θ/Bo-time curves for different engine operating conditions provides facilities for the quantitative specification of the correlation searched.