## 6 Conclusions  
In the present study, an analysis of the combustion process, performance, and emissions of a single-cylinder engine was carried out, using biodiesel blends formed by the blend of palm oil and sunflower oil residues.  
The results of the diagnostic model show that the pressure curves in the cylinder chamber decrease as the biodiesel content in the fuel increases. For the modes of operation considered, the maximum pressures were found in the range of 16-57.58 bar, 17-60.68 bar, 18.1-65 bar, 19.1-68 bar and 20-72.32 bar for the PB10 SB5, PB55B5, PB10, PB5, and diesel, respectively. This decrease in pressure was attributed to the lower calorific value of biodiesel compared to diesel. Similarly, the results showed a decrease in the rate of heat release for biodiesel blends compared to diesel for all tested modes of operation. It was observed that in the highest engine operating mode, the HRR curves showed a maximum of 1.14 J/deg, 1.18 J/deg, 1.22 J/deg, 1.26 J/deg and 1.30 J/deg for the PB10 SB5, PB55B5, PB10, PB5, and diesel, respectively. The higher viscosity and lower calorific value of biodiesel blends are considered to cause this behavior. However, the addition of a percentage of sunflower oil residues does not cause a large difference in maximum pressures, and heat release rates, when compared to biodiesel produced only with residual palm oil.  
The higher viscosity of the biodiesel tested also causes a reduction in the combustion rate, decreasing the heat release of the combustion process. This fact is reflected in the accumulated heat release curves, which were larger for fuels with a higher biodiesel content. This effect is reflected in the thermal efficiency of biodiesel blends. It was observed that the maximum thermal efficiency was 29.4%, 30%, 30.6%, 31.2% and 31.8% for PB10 SB5, PB55B5, PB10, PB5, and diesel, respectively.  
The addition of the percentage of residual sunflower oil in biodiesel caused an increase in BSFC. On average, the PB10 SB5 and PB55B5 biodiesel increased the BSFC by 15.5% compared to the PB10 and PB5 biodiesel, respectively.  
The emissions analysis showed that PB10 SB5 and PB55B5 fuels show a reduction in CO, CO\({}_{2}\), HC, and smoke opacity emissions of approximately 14-23%, 23.3-28.1%, 15.58-19.5% and 7.9-4% in comparison with pure diesel, respectively. The above-mentioned results are mainly attributed to the higher oxygen content in this type of biodiesel, contributing to a cleaner and complete combustion.  
The temperature values in the combustion chamber showed that the addition of sunflower oil residues causes a rise in the maximum temperature. This temperature increase facilitated the production of NOx emissions. The results show that on average, the PB5 SB5 and PB10 SB5 increase NOx emissions by 8.3% compared to biodiesel blends of residual palm oil.  
In general, biodiesel with the percentage of residual sunflower oil does not cause a significant change in the combustion process and engine performance, when compared to biodiesel that includes only residual palm oil. Despite the increase in NOx emissions, biodiesel blends with the addition of residual sunflower oil allow CO, CO\({}_{2}\), HC, and smoke opacity emissions to be reduced. Therefore, biodiesel produced by mixing palm oil and sunflower oil residues could be used to replace up to 15% diesel, allowing the reduction of highly polluting waste and the production of a cleaner and more renewable fuel.  
All the authors of the present research contributed equally to conducting and writing this paper. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Acknowledgments to Universidad del Atlantico, Universidad Francisco de Paula Santander, to Sphere Energy Company and Colombian Institute for Scientific and Technological Development (COLCIENCIAS) through the \"Convocatoria Nacional para Estudios de Doctorado en Colombia año 2012\" for the support provided.  
The authors declare no conflict of interest.