Application of a Multi-Zone Combustion Model for the Prediction of Large Scale Marine Diesel Engines Performance and Pollutants Emissions 1999-01-0227

In the present work a multi-zone combustion model, initially developed for naturally aspirated, high-speed, direct injection diesel engines, is used for studying the performance and emission characteristics of large scale, slow-speed marine diesel engines. Up to now pollutant emissions was not considered a problem in the field of marine engines, since no specific legislation existed. However, the International Maritime Organization (IMO) is forwarding a legislation that will be applicable in the next years concerning soot and nitric oxide (NO) emissions. This legislation will make it impossible for vessels to enter the native waters into countries where this legislation applies. Due to this fact, engine manufacturers are making serious efforts to design new engine builds with reduced soot and nitric oxide emissions using new designs and exhaust gas aftertreatment systems. The most promising of these techniques is the one referring to new engine designs and especially the development of new combustion systems. In the present work an attempt is made to transform an existing simulation code and to apply it to large marine engine designs. The model has up to now proved to give reliable predictions of both engine performance and emissions for high speed diesel engines, but large modifications have been made in order to simulate the slow-speed, marine diesel engine. At first the gas exchange process has been completely revised to include a scavenging model, while the simulation of the turbocharger and air cooler has been included since almost all engines of this kind are turbocharged ones. Also, modifications have been made in the combustion model and especially the jet model breakup and penetration mechanisms, because of the heavy fuel used and due to the relatively large time available for combustion. To validate the model an application is given in the present work concerning two types of marine diesel engines. For both engines tailpipe emission values are predicted for nitric oxide and soot and for one of these engines experimental values were available for nitric oxide at various engine operating conditions. By comparing the experimental with calculated values for both engines, it is revealed that the modified multi-zone model predicts with very good accuracy engine performance and with reasonable accuracy nitric oxide emissions. Also, results are provided revealing the evolution of the jet inside the combustion chamber at various time instants from injection and the distribution of thermodynamic properties and pollutants inside it. Observing these results and comparing them with previous ones obtained for high-speed diesel engines, it is concluded that the distribution of all parameters inside the jet is highly non-uniform, while the areas of soot and NO formation are clearly different. Almost all soot is formed towards the center of the jet, while nitric oxide is formed at the periphery. The present model appears to be promising, providing a good tool for the study of marine engines pollutant formation and performance. Its importance will be revealed in the near future when the previously mentioned new legislation will become applicable. Of course more validation is required for the model, especially concerning the prediction of pollutant emission. For this reason an effort is currently under progress to measure pollutant emissions from marine engines operating on the field, since very little such data is available from engine manufacturers.