# Environmental and Economic Aspects of a Containership Engine Performance in Off-Design Conditions
## 4 Results and Discussions  
### Fuel Consumption and Emissions  
Once the simulation model of the selected two-stroke marine diesel engine was calibrated, it was applied to simulate the off-design operating range. Topographic diagrams of BSFC, NO\({}_{\text{X}}\) and soot emission are given in Figures 8-10.  
Ten engine speeds in the range from 22 rpm to 120 rpm are simulated, and at each engine speed, seven loads are analyzed, corresponding to 2.5%, 5%, 10%, 30%, 50%, 70% and 100% of the full load referenced at each engine speed. It means that 70 operating points were simulated, representing the overall engine operating range that includes design and off-design conditions.  
BSFC is calculated so that the total fuel flow is divided by the engine brake power. When the engine power is lower, the mechanical losses become dominant in the total energy balance and therefore the BSFC increases significantly.  
Figure 7: Validation of soot emission results at 102 rpm.  
Figure 8: Diagram of BSFC for design and off-design operating range.  
Figure 10: Diagram of specific soot emission for design and off-design operating range.  
Figure 9: Diagram of specific NOX emissions for design and off-design operating range.  
The engine power required by the propeller is also shown in Figures 8-10. Since the transitional times of the propeller usually last for short periods while changing rotational speed, the engine operating points are positioned on the propeller curve. In other words, the equilibrium state between the engine and propeller is assumed.  
The maximum values of specific emissions of \(\mathrm{NO_{X}}\) are achieved at maximum engine loads when the maximum peak in-cylinder temperatures are reached. The \(\mathrm{NO_{X}}\) model is calibrated so that the specific \(\mathrm{NO_{X}}\) emissions at R1 and R3 operating points do not exceed values of 14.4 g/kWh, as limited by Tier II regulation standards for ship engines with rotational speed below 130 rpm. Since the two-zone combustion model formulation is used in this study, the main parameter responsible for the calculated \(\mathrm{NO_{X}}\) emissions is the burned zone temperature. It can be seen from Figure 9 that a reduction in engine speed below 70 rpm will reduce \(\mathrm{NO_{X}}\) emissions more than twice compared to the operating point at 102 rpm, where the maximum sailing speed of 23 knots will be achieved for the six-cylinder engine variant.  
The specific soot emissions, shown in Figure 10, are calculated after the soot emission model is calibrated with reference data at 102 rpm, as already shown in Figure 7. The main characteristic of a marine two-stroke diesel engine is that soot emissions increase as the engine load decreases. Since the two-stroke marine diesel engine considered has unidirectional scavenging that mainly depends on the intake port pressure, an increased soot emission at a low engine load is caused by the increased residual gas concentration and lower in-cylinder turbulence intensity (lower flow velocities at intake port sectional area). The applied soot emission model used in this study does not include details about residual gas concentration and turbulence quantities and therefore the calibration of the soot formation parameter was required.