| With the coming of implement of International Maritime Organization (IMO) Tier II regulation on harmful emissions from ships, the research activities on reducing NOX emissions from marine diesel engines are drawing more and more attention. To keep the advantages of low BSFC and to reduce the costs for equipment retrofits are of vital important for NOχemission control of marine diesel engines. In this dissertation, the 6PC2-6/2L type diesel engine was selected as the research object to investigate the practical and inexpensive methods of reducing NOx emission. Comprehensive numerical and experimental researches on the fuel injection and combustion processes relating to the reduction of NOχemission were performed.To reduce the NOX emission and improve the fuel economy of 6PC2-6/2L diesel engine, a new fuel supply system, including a new fuel pump with a pressure modulating hole and intersect hole nozzle, was developed. In the new fuel pump, the enlarged plunger and the configuration of pressure modulating holes lead to lower initial fuel injection rate and higher fuel injection rates at medium stage. By this way, the boot-type injection rates, which will be profitable for decreasing NOχemission, are achieved. The holes of intersect hole nozzle are formed by the intersection and convergence of child holes at inner side. Each hole has only one outlet. The sprays of intersect hole nozzle are fan-shaped, which will improve mixture formation and mixture distribution, thus improve the combustion and emissions of the diesel engine.To clarify the spray characteristics of intersect hole nozzles, high speed photography of the spray in high pressure constant volume vessel was conducted. The analysis of spray images indicates that the structure of intersect hole has strong influences on the shape, penetration and spread angle of the spray. When the center line of the children holes intersect on surface of the nozzle tip, the spray penetration is longest, and the front spray angle is less than the side spray angle; when the center line of the children holes intersect in the nozzle valve body, the spray penetration becomes shorter, and the front spray angle is larger than the side spray angle; when the center line of the children holes intersect out of the nozzle tip, the spray penetration is shortest, and the front spray angle is much larger than the side spray angle. To understand the working process of 6PC2-6/2L type diesel engine, the 3D CFD package AVL FIRE was employed to simulate in-cylinder air motion, fuel injection, combustion and emission formation processes. The simulation model was validated by the pressure trace and NOχemission data of the E3 work cycle experiment of 6PC2-6/2L diesel engine. The performance of two kinds of boot-type injection rate curves produced by the two fuel pumps with pressure modulation hole, named as Bootl and Boot2, and the injection rate produced by the primary fuel pump, named as Base, were investigated for 100% and 50% load working conditions of 6PC2-6/2L diesel engine. And for each kind of injection rates, five injection timings were studied. The computational results indicate that, both of boot-type injection rates can obtain lower NOχemission than the primary engine. Particularly Bootl can get better NOχ-Soot trade-off relationship in both loads, whereas Boot2 can get better Soot-ISFC trade-off relationship in two loads working conditions. To balance the fuel consumption and emissions at high load and low load, the Bootl injection rates with an injection timing of-8°CA ATDC were selected as the optimized matching. Compared with the base engine, the engine with new injection system and optimized injection strategy has NOχemission reduction of 17% at 100% load and 20% reduction at 50% load; the Soot emission reduction of 24% at 100% load and 2% at 50% load; the relating ISFC increases 2% at 100% load and 0.3% at 50% load.To validate the effects of the new fuel supply systems on reducing NOχemission and improving fuel economy of marine diesel engines, performance test of 6PC2-6/2L diesel engine was carried out on test bench. The experiment results show that, with the application of the intersect hole nozzles with a specification of 18×0.65 mm, NOX emission was reduced by 9%, while the BSFC reduced by 1%. Moreover, by the use of these nozzles and new fuel pump, the NOX and BSFC are simultaneously reduced under all loads of the E3 work cycle. The best experiment results were 14.3% of NOX emission reduction together with 1.5% BSFC reduction.
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