| According to IMO emission regulation,Tier III NOx emission standard carried out in North America from 2016 and will be carried out in global NOx emission control area from 2021.As the main share of marine ship power,the reduction of NOx emissionon two stroke low speed diesel engineis required.In order to study the possibility of EGR technique on low speed marine engine and the effect of using EGR for meeting Tier III standard,this thesis choose two stroke low speed marine engine with 6 cylinders and 340 bore as studying sample.This study using simulation method to get the influence about combustion and emission by EGR,and compare the influences by different EGR systems on low speed engine.This thesis build a three-dimensional numerical simulation model of flow and combustion in cylinder chamber to study the influence on the combustion and emission with different EGR rate and temperature,and get the curves of NOx emission,soot emission and fuel consumption.The conclusion is that higher EGR rate results in lower NOx emission but higher fuel consumption,and higher temperature results in higher NOx emission and fuel consumption.Then study different EGR systems operated on low speed engine base on one-dimensional simulation model.Regarding the fuel oil consumption rate among the three methods,the best one is high pressure EGR system with two turbochargers,low pressure EGR system takes second place and the last one is high pressure EGR system with one turbocharger.The cylinder by-pass which is benefit for the last one to avoid turbocharger surge at high EGR rate.The low pressure EGB system have a risk on turbine reliability that caused by recirculation gas,although it's less influence to the efficiency of turbocharger.The EGR system will make engine performance a little worser compare to the normal configuration,so the fuel oil injection timing,exhaust timing and EGR pipes should be optimised to decrease fuel consumption without NOx emission increasing. |
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