As the air pollution has been rather issued, IMO has promised to enforce and to strengthen gradually the regulation on the NOx emission from marine diesel engine. Accordingly, the efforts of measuring the emission for existing engine have been tried. But it is hard to get directly the data of emission from existing ship because of technical and economical problems. So the prediction of NOx emission from marine main diesel engine for existing ship, could be useful method for the purpose.
Generally, it has been known that excess air ratio and combustion temperature are important factors to control the NOx emission. Two zone model to calculate the burned gas and unburned gas temperature in combustion chamber could be adopted. However, the condition of combustion should be changed by excess air ratio variation in cylinder during combustion period. But it was difficult to know the variation of excess air ratio.
In this study, the simulation program was developed in order to predict the emission from main diesel engine of existing ship during operation. Modified two zone model was newly proposed. It was to calculate the combustion temperature by the rate of heat release obtained from measured cylinder pressure. The temperature of unburned region was calculated under the assumption of adiabatic process. And the excess air ratio was investigated to compare the NOx emission between measured and calculated results.
As the results, it was confirmed that NOx emission is well predicted by fixed excess air ratio for pratical use. It was discussed to be adopted the fixed excess air ratio with comparison of calculation results. In order to apply modified two zone model, more studies were required.