Application of a multi-zone combustion model to investigate the NOx reduction potential of two-stroke marine diesel engines using EGR

The most promising solution for propulsion of marine vessels currently and for the near future is the two-stroke low-speed diesel engines. Despite its advantages, as far as specific fuel oil consumption, power density and reliability is concerned, it suffers from increased NOx emissions mainly because of its low rotational speed. But the upcoming NOx emission legislation (Tier III) in the marine sector requires considerable reduction of NOx emissions towards levels which have not yet been commercially achieved using primary methods. This creates new challenges for the development and application of innovative techniques that could reduce engine’s NOx emissions with the lowest possible fuel penalty. This is important considering current and future fuel prices. The large size of the 2-stroke marine engine makes the use of experimental techniques, to investigate this potential, expensive and time consuming. Modelling can significantly contribute towards this effort and result to reduction of research and development cost. Among the most effective in-cylinder techniques for NOx reduction is EGR, a proven technology for smaller engines used in the transport sector. In the present study, it is investigated via modelling, the potential to reduce NOx emissions of two-stroke marine engines using EGR. Despite the technological difficulties resulting mainly from the use of Heavy Fuel Oil (HFO), which makes the application of conventional EGR techniques difficult, it is worthwhile investigating the NOx reduction potential, since significant progress has been achieved towards the development of new EGR gas cooling techniques and scrubber technologies for removal of sulphur species from the exhaust gas. The present investigation makes use of an existing well validated multi-zone combustion model, initially developed for high-speed DI diesel engines. The model has been successfully applied in the past, to investigate heavy duty diesel engine NOx reduction via EGR, providing favourable results. Currently it is modified and applied on a two-stroke marine diesel engine using EGR. Model evaluation is based on experimental data acquired from the international literature, due to lack of experimental data for 2-stroke engines. The analysis of derived results reveals model’s ability to predict both engine performance and NOx emissions but most important the ability to predict the overall effect of EGR on NOx emissions in a qualitatively correct way. The results also reveal the strong potential of EGR to control NOx emissions of 2-stroke engines with relatively low fuel penalty compared to alternative techniques.