## 4 Conclusions  
In this study, an experimental investigation was carried out to assess the influence on the maximum ethanol fraction and combustion behavior of typical calibration parameters (rail pressure, injection pattern and EGR) and the geometrical parameter (nozzle holes number) employing ethanol in the DF combustion mode. All the tests were performed, with imposed limits on combustion noise and COVIMEP. The main outcomes can be summarized as follows:  
* At lower loads, higher injection pressures, up to 1000 bar, and multiple injection strategies did not influence the maximum FSR rate significantly. On the contrary, at higher loads, a multiple injection pattern and higher injection pressure led to a higher FSR value, up to 0.8. * With EGR, a reduction of the maximum FSR was observed in both test points. For lower loads, the lowest ethanol fraction (FSR equal to 0.2) was reached for more advanced combustion phasing. By retarding the CA50 an increasing trend was observed with FSR values until up to 0.65. Similar FSR values were reached at higher loads for retarded combustion phasing. * The ethanol addition was generally very effective in terms of PM reduction due to the high FSR values achieved, and we expect an abrupt variation to be obtained for more advanced combustion phasing. For higher loads, despite the decreasing trend of PM emission, a higher specific value could be observed, and in the range of 0.03-0.25 g/kWh, compared to the no EGR case. * A higher number of nozzle holes allowed us to reach higher FSR at lower loads, and over the whole area of investigation. Conversely, for higher loads, no influence on FSR was observed. In this case, despite the HC worsening, the combustion efficiency maintained acceptable values within the CA50 range considered. At lower load, the 9-hole nozzle shows an improvement in combustion noise, 3 dBA on average, and a reduction of the combustion duration compared to the 7-hole. Increasing the load insensitivity to a different hole number was observed. In terms of engine-out PM emissions, no relevant differences could be drawn by adopting a different nozzle hole number. The effect of ethanol addition on PM reduction was more predominant rather than the effect of nozzle holes. At lower loads, the 9 hole case showed lower CO2 emissions compared to the 7 one. For a higher load, no relevant differences in CO2 emissions could be noticed. Based on the above results, it can be stated that the use of ethanol in the dual-fuel mode led to a consistent decrease of engine-out PM and CO2 emissions compared to the diesel one. These benefits were obtained at the cost of higher emissions of HC and CO, but this worsening might be acceptable due to lower PM tendency and to lower carbon nature of the ethanol. Moreover, with the same diesel quantity injected, it was suggested to use lower nozzle hole numbers to reduce the HC emission. The near-zero PM emissions, especially for lower load case, suggested further experimental investigations with a different nozzle hole number to evaluate the effects of a greater amount of direct fuel that can be injected. * Ethanol in CI engines as a GHG and PM emissions enabler. * Maximum fuel substitution ratio and GHG and PM emissions by increasing the injection of the nozzle hole number injector were analyzed. * Improved atomization by increasing the hole injector led to a higher ethanol fraction injected, allowing for the reduction of GHG and PM emissions. * Significant reduction of PM and CO2 in the dual-fuel combustion mode. **Author Contributions:** Data curation, R.I.; performed the experiments, R.I. and M.S. ; methodology, G.D.B. ; supervision, I.D., C.B. and G.D.B. ; writing--original draft, G.D.L. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Conflicts of Interest:** The authors declare no conflict of interest.