## 4 Discussion  
Figure 6 shows the comparison of the experimental vs. simulation outcomes of the droplet ignition delay assessment.  
As can be seen from the juxtaposition of the experimental and simulation outcomes (Equation (36)), very good agreement was obtained between the ignition delay values for both DF and RME, including their blend. The greatest difference between the experimental and simulated values was obtained in a range of ambient temperature between 980 and 990 K for diesel fuel. This may be associated with the strong nonlinear character of the kinetic factor for diesel fuel and RME before reaching the inflammation temperature. Apparently, in the case of DF, the degree of influence of the kinetic factor on the process of ignition of the vapors surrounding the droplet is greater than that of RME (the process of combustion of the droplet occurs in a relatively slow diffusion mode). This work also provides the methodology for developing realistic droplet vaporization/ignition model with a simplified approach, which is not too complicated or oversimplified. The main physico-chemical characteristics of the test fuels, which were established in this study, are presented in Table 5. In this case, the disparity in values reaches 10-11%. For the remaining temperatures, the difference between the experimentally established values and the simulation result varies  
Figure 5: Dependence of the dimensionless coefficient A* on ambient temperature \(T_{\infty}\) (\(n\)–octane drop radius \(r_{0}\) = 2 mm).  
Figure 6: Comparison of experimental vs. simulation outcomes of droplet ignition delay assessment.  
in the range of 1-13%. The error range in single droplet combustion modelling can vary significantly, but, according to an overview of previously published works on a particular topic, a common range for deviations between model predictions and experimental results is within 15%. However, some studies report errors as high as 21% under specific conditions. The sensitivity of the simulation results for DF80/RME20 vis-a-vis experimentally obtained values for the same blend has shown the highest level of correspondence of \(r\) = 0.91-0.98 throughout the temperature range, as presented in Figure 6. It should be mentioned that all the data points generated by a model for \(T\) = 990 K demonstrated the highest discrepancy in values (\(r\) = 0.87-0.91) if compared to the experimental results. This phenomenon can be associated with a prolonged time of evaporation for DF and a slightly higher mass of the RME droplet due to its higher density. In terms of simulation accuracy evaluated for six different temperatures, the ignition delay values for different fuels arranged in decreasing order DF80/RME20 (\(r\) = 0.98 (2), \(r\) = 0.96 (2), \(r\) = 0.92 (1), \(r\) = 0.91 (1)) \(>\) DF (\(r\) = 0.99 (1), \(r\) = 0.96 (2), \(r\) = 0.95 (1), \(r\) = 0.92 (1), \(r\) = 0.90 (1)) \(>\) RME (\(r\) = 0.95 (2), \(r\) = 0.92 (1), \(r\) = 0.91 (2), \(r\) = 0.87 (1)).  
Based on the quasi-stationary model of the inertial heating of a hydrocarbon fuel droplet, an equation has been derived for calculating the ignition delay, which is applied to assess the ignition characteristics of combustible mixtures and predicting the possibilities of their use in the conventional industrial burners designed for premixed flames. It has been established that the calculations of the ignition delay period of diesel fuel and rapeseed methyl ester droplets using the quasi-stationary combustion model agree well with the results of experimental studies within the selected range of temperatures, up to 1010 K. This value exceeds the critical ignition temperature of the RME by 294 K. The described model can be applied to calculations in the case where the temperature values of the oxidizer surrounding the drop are equal to the ambient temperature and significantly exceed its critical ignition temperature, both.