The second most attractive parameter is spray cone angle for spray characterization. Theoretical and experimental studies reported in the literature show that, spray angle is quasiconstant after spray developed enough and empirical equations describing the spray angle do not include a time variable. According to Hiroyasu and Arai cone angle is as follows:

Another important characterization parameter for spray development is its ignition delay. Related to defining ignition delay, different correlations which are based on temperature and pressure parameters can be found in the literature. Numerous correlations based on experimental data are in the form of Arrhenius equation, but the constants in correlations vary by several orders of magnitude [43–45]. The reason of this variety might be related to different experimental conditions. The most used form for defining the ignition delay is:

where τ is the ignition delay, A is a constant, P and T are the ambient gas pressure and temperature at the time of autoignition. Pischinger et al. enhanced the basic equation by changing denominator of the superscripts T with (T −T₀), where T₀ is defined as reference temperature that no ignition is observed. A reducing effect on ignition delay when injection pressure increased has been reported by Pischinger et al.

Since this equation is obtained from similar experimental setup with this study, it is expected to give similar results.