efficiently performed. MHI has developed a special scavenging port shape to improve scavenging efficiency. For this purpose, a simulation of scavenging flow has been used. Fig.3 shows gas replacement in the cylinder based on this method.
2) Fuel Spray simulation
Fuel spray simulation models how drops of fuel oil injected in the combustion chamber cylinder mix and spread with air in the cylinder, while taking into account the air flow from the scavenging simulation described previously. This method is used for selection of fuel injection valve specifications including the number and the angle of injection holes, considering factors such as fuel consumption, generation of NOx, temperature of combustion chamber material, etc. Unfortunately, the relationship between spray pattern and combustion (heat generation) is not accurately defined with the current level of technology. This method, however, will be very useful in forecasting generation of NOx in large marine diesel engines. Fig.4 shows an example of spray simulation.
3.2 Environmental technology
1) Stratified water injection
To meet current NOx regulations specified by the IMO, methods such as timing retard and engine tuning are available. In anticipation of even more stringent regulations in the future, MHI has studied water injection technology. The specified exhaust gas temperature for a 2-cycle marine diesel engine is relatively low for SCR equipment. MHI started development of the water injection device long ago as an effective measure for reducing NOx during ship navigation in the port, when significant load variation occurs. In cooperation with the Institute for Sea Training of the Japanese Ministry of Transport, MHI has installed this prototype on the main engine of the training ship "Ginga-maru" and confirmed its reliability during an around-the-world voyage. Consequently, the stratified water injection device was produced and 3 sets were installed on the medium speed KU engine for power generation. The devices have been operated for 3 years with favorable results. Figs.5 and 6 show the outline and results of the stratified water injection device.
3.3 Electronic technology
1) To optimize engine capacity, it is essential to apply an electronic technology. Electronic systems for the marine diesel engine have not been accepted because of problems related to reliability and backup function. Electronic systems are, however, fully used in automobile engines, and are being used for power generation engines together with the common rail fuel injection method. Electronic governors for marine diesel engines are now widely used. The reliability of electronic equipment for marine use has been improved.
The current engines are designed to produce maximum performance at a specified point (usually under normal load), but they cannot produce maximum performance away from this specified point. It is necessary to control the exhaust valve
