# Environmental and Economic Aspects of a Containership Engine Performance in Off-Design Conditions
## 5 Conclusions  
In this article, the goal was to investigate the influence of the two-stroke marine diesel engine variant on the selection of the most favorable slow steaming speed and its environmental and economic impact. This research was conducted on a case study of a Post Panamax containership with three different two-stroke marine diesel engine variants (six-cylinder, seven-cylinder and eight-cylinder engine). To select the sailing speed in slow steaming conditions, which would result in optimal correlation between reduced fuel costs, CO2 emissions, NOx emissions and increased operating costs in the frame of recent legislative measures to reduce GHG and NOx emissions, the propeller-delivered power and engine performance and emissions must be estimated with a high level of accuracy. The propeller-delivered power-engine speed function required for the engine selection was experimentally determined based on the hydrodynamic characteristics of the selected ship and propeller and it defined the required engine brake power and engine speed. The marine two-stroke diesel engine was simulated using a comprehensive thermodynamic 1D/0D tool to predict the engine performance and emissions in design and off-design conditions. Engine variants with six, seven and eight cylinders were analyzed, and the effect of slow steaming on engine emissions and operating costs was quantified for one typical Trans-Pacific route. For the analyzed containership and propeller, the maximum sailing speeds of 23 knots, 25 knots, and 26 knots can be achieved with the engine variants with six, seven, and eight cylinders, respectively. The case study investigated resulted in the following conclusions:  
* The application of the 1D/0D simulation model and propeller-delivered power function can be a useful step for the calculation of ship engine performance and exhaust gas emissions in off-design conditions typical for slow steaming. * The lowest engine speed, which the engine can safely sustain, varies between engine variants. The lowest achievable sailing speed for six-cylinder and seven-cylinder engines is 15 knots, and for the eight-cylinder engine, it is 17 knots. * The reduction in sailing speed by 35% (from 23 knots to 15 knots) for ship powered by the six-cylinder engine variant resulted in the reduction in CO2, NOx, and soot by 67.1%, 89.6% and 53.4%, respectively, and that engine variant provides the highest income per route. * The income for the analyzed Trans-Pacific route at the sailing speed of 15 knots is increased by 6.2%, while the annual income is decreased by 32.8% because a longer sailing time per route is required. * To satisfy the targeted 55% reduction in GHG emissions by 2030 under slow steaming conditions, the sailing speed should be reduced by at least 26% compared to the design speed. Such a reduction can be achieved with all engine variants at sailing speed of 17 knots, in which case the eight-cylinder engine variant ensures the highest reduction in CO2 emission (55%), NOx emission (87.4%) and the highest annual income decrease (23.7%). Future studies could be focused on the analysis of the environmental and economic impact of ship fleets, which includes different engine sizes, ship capacities and sailing routes. On the other hand, the application of alternative fuels such as natural gas, e-fuels and ammonia has great potential to reduce GHG emissions, especially under slow steaming, which could also be incorporated into the analysis. Conceptualization, M.S., R.T., I.M., N.D. and C.G.G. ; methodology, M.S., R.T., I.M. and N.D.; software, M.S. and R.T.; validation, M.S. ; formal analysis, M.S., R.T., I.M., N.D. and C.G.G. ; investigation, M.S., R.T., I.M., N.D. and C.G.G. ; resources, M.S., R.T., I.M., N.D. and C.G.G. ; data curation, M.S., R.T., I.M., N.D. and C.G.G. ; writing--original draft preparation, M.S., R.T. and I.M. ; writing--review and editing, R.T., I.M., N.D. and C.G.G. ; visualization, M.S., R.T., I.M. and C.G.G. ; supervision, N.D.; project administration, N.D.; funding acquisition, N.D. and M.S.