# Environmental and Economic Aspects of a Containership Engine Performance in Off-Design Conditions
## 4 Results and Discussions
### Off-Design Conditions  
To determine the suitable engine variant under the slow steaming for the analyzed containership, an analysis of \(\mathrm{CO_{2}}\), \(\mathrm{NO_{X}}\), soot emissions, operating costs, annual incomes, and incomes per route was conducted. Figures 11-13 provide an overview of the simulation results for \(\mathrm{CO_{2}}\), \(\mathrm{NO_{X}}\) and soot emissions for simulated engine variants and engine speeds which correspond to sailing speeds from 10 to 26 knots. \(\mathrm{CO_{2}}\) emissions correspond to BSFC and fuel consumption, which depends on the engine load and in-cylinder conditions. Therefore, the \(\mathrm{CO_{2}}\) emissions decrease with sailing speed up to a certain speed, at which point the trend changes. The six-cylinder engine variant could not provide the required power at sailing speeds of 25 and 26 knots nor could the seven-cylinder engine variant at the speed of 26 knots. Each engine variant has minimum \(\mathrm{CO_{2}}\) emission at different sailing speeds. For a six-cylinder engine variant, the minimum of 508.2 g/kWh is achieved at 10 knots (42 rpm). The minimum for a seven-cylinder engine variant (509.23 g/kWh) is achieved at 12 knots (52 rpm), and for an eight-cylinder engine variant (509.86 g/kWh), it is also achieved at 12 knots. The minimum CO\({}_{2}\) emission is achieved with a six-cylinder engine variant because more favorable in-cylinder conditions and higher load conditions at low engine speeds are achieved. At sailing speeds below 12 knots, lower CO\({}_{2}\) emissions are achieved with engine variants with a lower number of cylinders, but above 12 knots, the trend changes. At higher speeds, favorable conditions for more efficient combustion, i.e., lower BSFC values, as shown in Figure 8, are achieved in engine variants with a greater number of cylinders. The NO\({}_{\text{X}}\) emissions decrease with the decrease in sailing and engine speeds, which is expected because NO\({}_{\text{X}}\) emissions are highly dependent on in-cylinder temperature conditions, as shown in Figure 12. The minimum value is achieved at the lowest sailing speed (10 knots) with the eight-cylinder engine variant (0.57 g/kWh). At that operating point, the eight-cylinder engine variant has the lowest load and the lowest in-cylinder temperature. With the reduction in sailing speed, the soot emissions increase (see Figure 13) because the unfavorable in-cylinder mixing conditions caused by lower boost pressure and in-cylinder turbulence intensity are achieved. An indicator of a less favorable combustion behavior, which could lead to higher repair and maintenance costs, is a large gradient in the specific soot emission. Keeping that in mind, the sailing speed for six-cylinder and seven-cylinder engine variants could be limited to 12 knots (engine speed of 52 rpm) and that for the eight-cylinder engine variant to 15 knots (engine speed of 62 rpm). The confirmation for such criteria is that the relative increase in specific soot emissions of 95.5% in the case of the six-cylinder engine variant is observed in the sailing speed reduction from 15 to 12 knots. For the seven-cylinder engine variant, the relative increase in specific soot emission of 97.7% is also observed in the sailing speed reduction from 15 to 12 knots. For the eight-cylinder engine variant, the relative increase in specific soot emissions of 91.2% is observed in the sailing speed reduction from 17 to 15 knots. In addition to emissions, slow steaming also affects the operating costs. During slow steaming, the fuel costs are reduced and, due to longer sailing time, the crew costs are increased. Changes in operating costs consequently change the ship's income. The effects of sailing speed on operating costs, income per route and annual income are shown in Figures 14-16. The costs and income were analyzed for the Trans-Pacific route (4503 nm). According to [36], the average contract freight rate for a 40-foot container or 40-foot equivalent unit (FEU) on the route from Asia to North America is 2580 USD/FEU.