# Environmental and Economic Aspects of a Containership Engine Performance in Off-Design Conditions
## 3 Methodology  
Once the propeller curve was determined, the two-stroke marine diesel engine was selected regarding the speed range and maximum brake power. The Wartsila Sulzer RTA-96C engine was selected based on the required brake power at 22 knots, as shown in Table 3. The measured data of engine performance and emissions declared by the engine manufacturer are defined according to ISO standard reference conditions. Engine variants with six, seven, and eight cylinders were considered a suitable choice based on the available data in the literature for containerships of a similar size. For the selected case study, the smallest variant of the selected engine type can propel the ship up to a speed of 23 knots. Usually, the design speed of containerships of similar size is around 25 knots. On the other hand, the engine with eight cylinders allows for sailing speeds up to 26 knots. The 1D/0D simulation model of a six cylinder engine variant was made in AVL Boost(tm) [31] using the available geometry data from the manufacturer. The calibration of the simulation model was performed at 4 design operating points declared by the engine manufacturer, as illustrated in Figure 4. The simulation results of BMEP (brake mean effective pressure) and BSFC (brake specific fuel consumption) were compared with the reference engine data, achieving the deviation of simulation results within \(\pm\)5% compared to the reference data of engine manufacturer. Figure 4: Definition of off-design engine operation and positions of calibration operating points. R1-R4 declared by engine manufacturer. Figure 3: Self-propulsion test at the speed corresponding to 22 knots in full scale. At design operating points R1 and R3, the emission sub-model of nitrogen oxides was calibrated. The threshold of specific NO\({}_{\text{X}}\) emissions is defined with Tier II regulation, which imposes that specific NO\({}_{\text{X}}\) emissions should not exceed 14.4 g/kWh for engines with maximum speeds below 130 rpm. The additional calibration of the soot emission sub-model was made by simulating the engine load thatvaried from 5% to 100%. The 1D/0D simulation models are sensitive to the values of calibration parameters. Therefore, the reference engine performance and emission data should be used in the calibration steps. To correctly predict the engine performance and emissions in off-design conditions that are typical for slow steaming, the simulation model was calibrated at the design operating area. By considering the engine performance defined by the manufacturer for different numbers of cylinders (from 6 up to 14), it is evident that BSFC and BMEP at R1 and R3 operating points are independent of the number of cylinders. Therefore, the engine performance with seven and eight cylinders was not simulated using the 1D/0D simulation model, but it was calculated by multiplying the engine brake power at each analyzed operating point with a factor that represents the relative increase in the number of cylinders compared to the power of an engine with six cylinders. It is assumed that transition periods of ship acceleration and deceleration are very short compared to the total travel time, which means that instantaneous engine brake power is equal to the engine brake power required by the propeller. In other words, the instantaneous engine operating point lies on the propeller curve shown in Figure 4. The required power, ship speed, specific fuel consumption, and emissions can be evaluated for an arbitrary engine speed using linear interpolation. The analysis of the effect of slow steaming on emissions of CO\({}_{2}\), NO\({}_{\text{X}}\) and soot is performed for a typical Trans-Pacific route (4503 nm) for six-, seven- and eight-cylinder engines. The calculated operating costs, income per route and annual income represent the economic aspect of the application of slow steaming. Finally, the overall results of emissions for the seven- and eight-cylinder engines, costs and income are compared to the results for the 6-cylinder engine operating at the speed of 102 rpm.