Diesel engines are almost exclusively used for propulsion of marine vessels. They are also used for power generation either on vessels or power stations because of their superior efficiency, high power concentration, stability and reliability compared to other alternative power systems. However, a significant drawback of these engines is the production of exhaust gases some of which are toxic and thus can be a threat to the environment. The most important toxic gaseous pollutants found in the exhaust gas of a marine diesel engine are NOx (NO, NO2 etc), CO and SOx. Particulate matter is also a major pollutant of diesel. Currently CO2 is considered to be also a “pollutant”, even though not being directly toxic, due to its impact on global warming.

In the Marine sector there exists legislation for marine diesel engine NOx emissions which is getting stricter as we move on towards Tier III. This brings new challenges for the engine makers as far as NOx control and its reduction is concerned. Towards this effort of NOx reduction, modelling has an important role which will become even more important in the future. This is mainly attributed to the large size of marine engines which makes the use of experimental techniques extremely expensive and time consuming. Modelling can greatly assist NOx reduction efforts at least at the early stages of development leading to cost reduction. As known NOx emissions are strongly related to engine performance and thus efforts for their reduction usually have a negative impact on efficiency and particulate matter. Modelling can play an important role towards this direction because optimization techniques can be applied to determine the optimum design for NOx reduction with the lowest impact on efficiency.

At present an effort is made to apply an existing well validated multi-zone combustion model for DI diesel engines on a 2-stroke marine diesel engine used to power a tanker vessel. The model is used to determine both engine performance and NOx emissions at various operating conditions. To validate model’s ability to predict performance and NOx emissions, a comparison is given against data obtained from the vessel official NOx file and from on board measurements conducted by the present research group. On board performance measurements were conducted using an in-house engine diagnostic system while emissions were recorded using a portable exhaust gas analyzer. From the comparison of measured against predicted data, the ability of the model to adequately predict performance and NOx emissions of the slow speed 2-stroke marine diesel engine examined is demonstrated. Furthermore, from the application are revealed specific problems related to the application of such models on large slow speed two-stroke engines which is significantly important for their further development.

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