## 4 Results
### Pollutant Emissions Evaluation  
For what concern the polluting emissions of the reference ship, the method suggested by [PERSON] [6] has been followed, which is based on the use of polluting emission factors. In particular, the emission factors depend on the quality of the fuel [62] and on the combustion system, and are expressed as gpollutants/kgFuel. Therefore, the calculations of pollutant emissions can be carried out once the optimization procedure has been completed.  
In this approach, the SO\({}_{x}\) emission factor depends only on the sulfur content of the fuel. Therefore, considering all the sulfur compounds in the exhaust gases as SO\({}_{2}\), the emission factor can be calculated as reported in Equation (45):  
\[EF_{SO_{x}}=20\times\%S_{Fuel}\left[\mathrm{g_{pollutants}}/\mathrm{kg_{Fuel }}\right] \tag{45}\]  
The average percentage of sulfur content considered in the calculation is 2.7% and 0.1% respectively for HFO and Marine Gas Oil (MGO), consistently with [63]. Consequently, the SO\({}_{x}\) emission factors calculated using Equation (45) are 54 gSO\({}_{x}\)/kgFuel for HFO and 2 gSO\({}_{x}\)/kgFuel for MGO.  
The NO\({}_{x}\) emission factors are based on the quality of the fuel (HFO or MGO) and the combustion mode (ICE or Oil Fuel Burner-OFB). In the present study, emission factors propose in [64] have been used.  
Therefore, once the total fuel quantity (in tons) has been determined, Equation (46) allows us to evaluate the emissions of all types of pollutants:  
\[PE_{h}=EF_{h}\times Fuel\times 1000\ [\mathrm{g}] \tag{46}\]  
where \(PE_{h}\) is the h-th pollutant emission, \(EF_{h}\) is the h-th emission factor of pollutant reported previously and \(Fuel\) is the total amount of fuel burned in the ICE, or in the OFB.  
The efficiency of the abatement devices (SCR and scrubber) must be taken into account, if they are present, obtaining the following two new equations:  
\[PE_{NO_{X}}=EF_{ICE\_NO_{X}}\times Fuel_{ICE}\times(1-\eta_{SCR})\ [\mathrm{g}] \tag{47}\]  
\[PE_{SO_{X}}=EF_{ICE\_SO_{X}}\times Fuel_{ICE}\times(1-\eta_{scrubber})\ [ \mathrm{g}] \tag{48}\]  
where \(EF_{ICE\_NO_{X}}\) is the specific NO\({}_{x}\) emission factor of the ICE, \(EF_{ICE\_SO_{X}}\) is the emission factor by using HFO, \(\eta_{SCR}\) is the abatement efficiency of the SCR, assumed equal to 85% and \(\eta_{scrubber}\) is the abatement efficiency of the scrubber, assumed equal to 97%. The latter perform also an important reduction of PM, which can be regarded of about 75%. For what concerns the emissions produced using MGO, the abatement efficacy values equal to 75% of those considered for the HFO have been taken into account, as a precautionary measure.  
On the basis of the identified coefficients and abatement efficiencies, the total emissions have been calculated making reference to the ship's consumption obtained by the optimization procedure. The results obtained for the winter/summer cruises (ID 210/ID 18) are shown in Figures 17 and 18 for the main pollutants, SO\({}_{x}\) and NO\({}_{x}\) respectively, also considering different options for the use of fuel and abatement devices, in the various phases of the cruise. In particular, 4 different strategies for the use of fuels have been compared:  
1. HFO for ICE and boilers in all cruising conditions,
2. HFO for ICE in all cruising conditions and MGO for OFB,
3. HFO for ICE in navigation and maneuvering and MGO for ICE in port and OFB,
4. HFO for ICE in navigation and MGO for ICE in maneuvering and in port and for OFB.  
All the cases obtained with the optimized designs allow us to obtain total values of the emissions during the cruise, well below the regulatory limits, both for SO\({}_{\text{x}}\) (SECA lim. in Figure 17) and for NOx (NECA lim. in Figure 18).