An overview of systems supplying water into the combustion chamber of diesel engines to decrease the amount of nitrogen oxides in exhaust gas

Abstract

The paper analyses legal requirements for the composition of exhaust gas emitted by ships, marine engines including. The background for this paper is the increasingly stricter limits set for the emission of the toxic exhaust gas components by marine engines. Legal requirements force designers to search for new constructions of marine propulsion systems and constantly improve the existing ones. One of the solutions is supplying water into the combustion chambers of diesel engines—the solution widely known for many years n ow wins favour and, according to the authors, has a chance to gain competitive advantage over alternative constructions. A special attention has been paid to the allowed amount of nitrogen oxides and sulphur oxides in exhaust gas resulting from the International Convention for the Prevention of Pollution from Ships. So far, one, global evaluation of brand new and retrofitted marine propulsion system designs presenting this issue thoroughly has not been done. The authors’ interest in the issue derives from this fact and has been confirmed by many papers. We showed the mechanism of decreasing nitrogen oxides in exhaust gas by means of water supply into the combustion chambers of diesel engines. We presented an overview of designs which might be used to retrofit vessels already in operation or introduced at the stage of vessel construction in the shipyard. The paper also contains an evaluation of the described designs. The following systems have been discussed: continuous water injection into the scavenging air, humid air motor, direct water injection with the use of combined nozzles, water-cooled residual gas system and fuel–water emulsion supply system using emulsifiers or devices of high-pressure water injection into fuel. We have made a comparison of the effectiveness of different methods used to reduce the emission of nitrogen oxides. Advantages and disadvantages of supplying water into the combustion chambers of diesel engines have been shown together with the comparison of the range of changes in their construction. The authors have indicated potential opportunities derived from injecting Brown’s gas into the combustion chamber in order to change the composition of the exhaust gas. As a consequence, it will also affect the natural environment where vessels operate.