Diesel engine manufacturers are currently intensifying their efforts to meet future emission limits that require a drastic reduction of NOx and particulate matter compared to present values. Even though several after-treatment techniques have been developed for tailpipe NOx reduction in heavy duty diesel engines, the in-cylinder control of NOx formation still remains of utmost importance. Various methods have been used to control NOx formation in diesel engines such as retarded injection timing and EGR providing each one of them very promising results. However, use of these techniques is accompanied by penalties in specific fuel consumption and exhaust soot. A promising technology for NOx reduction especially for heavy-duty diesel engines and mainly large scale ones is the addition of water to the combustion chamber to reduce peak combustion temperature that obviously affects NOx formation. This method has been applied mainly for marine and stationary diesel engines due to the requirement of minor modifications on engine infrastructure. However, up to now, its true potential for diesel engines remains uncertain due to the limited theoretical and experimental knowledge. For this reason, it is examined herein the possibility to use this method to reduce NOx emissions in direct injection heavy-duty diesel engines. Two different technologies are examined for the addition of water into the combustion chamber: use of a water-fuel emulsion or injection of water into the intake manifold. Various percentages of injected water are examined for both technologies. This is attained using a multi-zone simulation model appropriately modified to simulate the use of water/fuel emulsion or injection of water into the intake manifold. The analysis provides information concerning the actual effect of water on the combustion and pollution formation mechanisms. It is revealed a significant reduction of NOx with both techniques compared to conventional engine operation. The reduction is higher when using water-fuel emulsion compared to water injection in the intake manifold for the same water percentage. Information is derived concerning the penalty on fuel consumption, which is a dominating parameter especially in large diesel engines. Furthermore it is examined the effect of water usage on soot formation since in some cases a reduction of soot can be observed due to the dissociation of water, the production of OH radicals and improvement of the air fuel mixing process. Thus, the simulation model is used to estimate the correlation between water percentage, NOx relative reduction and the associating penalty on engine bsfc and soot. The results can be utilized to define an optimum water/fuel ratio, which will provide the maximum reduction of NOx at an acceptable fuel penalty. The results of the present investigation can be used as a pilot for the application of this method on different diesel engine types and sizes.

In the present work, the contemporary exhaust gas treatment systems (EGTS) used for SO_x, PM, and NO_x emission mitigation from shipping are reviewed. Specifically, after-treatment technologies such as wet scrubbers with seawater and freshwater solution with NaOH, hybrid wet scrubbers, wet scrubbers integrated in exhaust gas recirculation (EGR) installations, dry scrubbers, inert gas wet scrubbers and selective catalytic reduction (SCR) systems are analyzed. The operational principles and the construction specifications, the performance characteristics and the investment and operation of the reviewed shipping EGTS are thoroughly elaborated. The SCR technology is comparatively evaluated with alternative techniques such as LNG, internal engine modifications (IEM), direct water injection (DWI) and humid air motor (HAM) to assess the individual NO_x emission reduction potential of each technology. Detailed real data for the time several cargo vessels spent in shipyards for seawater scrubber installation, and actual data for the purchase cost and the installation cost of seawater scrubbers in shipyards are demonstrated. From the examination of the constructional, operational, environmental and economic parameters of the examined EGTS, it can be concluded that the most effective SO_x emission abatement system is the closed-loop wet scrubbers with NaOH solution which can practically eliminate ship SO_x emissions, whereas the most effective NO_x emission mitigation system is the SCR which cannot only offer compliance of a vessel with the IMO Tier III limits but can also practically eliminate ship NO_x emissions.