## 5 Conclusions  
A simulation study regarding the influences of using different injection rate shapes on the performance, efficiency and pollutant emissions of an agricultural tractor diesel engine fuelled with diesel fuel and biodiesel B20 and operating at maximum brake power and maximum brake torque test conditions has been conducted. In this sense, a simulation model in the AVL-BOOST software was developed, using the AVL-MCC combustion model, which was calibrated for the reference ROI shape (Top-Hat) using the experimentally recorded data. Even though there are some specific limitations associated with the 1-D simulation model, concerning the prediction capacity of the differences existing between the values of the ignition delay when using different ROI shapes, the conclusions of this study can be summarized as follows:  
* The variation of performance, efficiency and emission indicators with the ROI shape shows similar developments for both tested fuels (diesel fuel and biodiesel B20) and on both engine operating conditions (maximum brake torque at 1400 rpm, and maximum brake power at 2400 rpm);
* The group of injection shapes presenting a fast increase of the injection rate at the beginning of the process (RT, TR1 and BO1) produces higher values of NO\({}_{\mathrm{x}}\) emissions due to superior peak in-cylinder temperatures achieved by the development of the premixed rapid combustion. The group of shapes showing a fast increase of the injection rate at the end of the process (TR3 and BO2) produces smaller values of NO\({}_{\mathrm{x}}\) emissions due to the development of the diffusion gradual combustion associated with moderate peak in-cylinder temperatures;
* The triangular ROI shapes (TR1, TR2 and TR3) concentrate a large amount of fuel up to the self-ignition time, which enhances mixing and determines faster burning rates, generates lower Soot emissions, better engine performance and efficiency for both fuels;
* Compared to the reference ROI shape (Top-Hat), the triangular TR3 shape produces only a slight increase of NO\({}_{\mathrm{x}}\) emissions, but a significant decrease of Soot emissions, a higher engine brake output and a lower brake-specific fuel consumption;
* There is a quasi-linear correlation between the amount of fuel injected during the initial combustion stage and the values of NO\({}_{\mathrm{x}}\) and Soot emissions. The correlation shows that, the greater the injected fuel quantity, the higher the NO\({}_{\mathrm{x}}\) emissions, and the Soot emissions are lower;
* The engine operation with the optimum injection rate shape (BO2-where the minimum surface calculated between the SOI and MFB10% events is registered) involves a  
Figure 15: NO\({}_{\mathrm{x}}\) and Soot trendlines variation relative to SOI–MFB10% surface area. simultaneous reduction of NOx and Soot emissions with 17%, respectively 9% for diesel fuel on R1 test condition and with 28%, respectively 12% on R2 test condition. The replacement of diesel fuel by biodiesel B20 produces similar reductions of NOx and Soot by 11%, respectively 4% on R1 test condition and by 22%, respectively 7% on R2 test condition. The results obtained by this study show that an optimum shape of injection rate as the BO2 applied to a diesel engine may ensure the simultaneous reduction of NOx and Soot emissions without scarifying engine performances and efficiency even when the original diesel fuel is replaced by biodiesel B20. Conceptualization, R.C. ; methodology, A.L.N. and R.C. ; software, A.L.N. and R.C. ; validation, A.L.N., R.C. and A.R. ; formal analysis, A.L.N., R.C. and A.R. ; investigation, R.C. and A.R. ; resources, R.C. and A.R. ; data curation, R.C. and A.R. ; writing--original draft preparation, A.L.N., R.C. and A.R. ; writing--review and editing, A.L.N., R.C. and A.R. ; visualization, A.R. ; supervision, R.C. ; project administration, R.C. ; funding acquisition, A.L.N. and R.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.