Simulation And Optimization For Low NOx Emission In A Medium Speed Marine Diesel Engine

This paper is about a study on the performance of a medium speed marine diesel engine.The simulation of diesel working process is operated and analyzed with AVL Boost.This thesis mainly contains:(1) According to the physical principle of working process in the diesel engine,a numerical model is built up.Based on this model the propulsion characteristic is simulated and compared with the experimental results.The results show that,the calculation can be in well accord with experimental results,and thus the validity of this model is confirmed.(2) Then the author simulates the diesel engine with a pressure-modulating-hole fuel pump,which is available to realize boot-type injection rate.It is found that, compared with original engine,the model with a strategy Boot1 of injection rate can obtain better economy and the one with a strategy Boot2 can receive a lower NOx emission.(3) Next,the thesis analyzes the effects of the injection timing on the engine performance.These results indicate that,using strategy Boot1,retarding the injection timing makes a remarkable improvement of NOx emission as well as a less than 3%percent increase in the fuel consumption.And the optimal injection timings under three strategies of injection are chosen according to optimization model and the analyses of trade-off relationship between fuel consumption and NOx.(4) A model of an external EGR system is built up based on the fluctuation effects of pressure lying in the intake and exhaust pipes.After analyzing the main parameters in the EGR system such as opening of back pressure valve,opening of EGR valve and EGR temperature,the conclusion could be summarized as follows:adjusting the opening of back pressure valve can realize a high EGR ratio and eventually a notable decrease of NOx.Under the same opening of back pressure valve,as the EGR temperature drops,NOx decreases and meanwhile,intake temperature decreases, lience,improvement of engine power output and fuel economy are attained.These analyses provide a qualitative guide for design of a real EGR system in future.