Study On EGR System Of 6EX340 Low-speed Two-stroke Diesel Engine

The problems of envirment become more and more serious,and the Tier III for NOx emissions of marine diesel engines has come into force since January 2016.Exhaust gas recirculation(EGR)technology has become one of the key technologies to achieve the Tier III because of its enormous potential to reduce NOx emissions.Based on the 6EX340 engine simulation system,this paper carries out the EGR system design,turbocharging system matching research and analysis of the Tier II and Tier III mode switching control strategies under different load,in order to ensure that the marine diesel engine achieves different emission standards in different areas and achieves the smooth switching of different modes.In this paper,the one-dimensional simulation model of two-stroke diesel engine is set up and calibrated,the design scheme of EGR system is studied,and the brief design of each component in EGR system is completed,in addition,the main design parameters are given,such as EGR pipeline,EGR cooling system,washing system,valve,EGR blower and so on.Results show that the diameter of EGR pipe is 265 mm,the heat dissipation of EGR cooling system is 1621 kW at 100% load,and the washing system is composed of venturi scrubber and packing tower scrubber.The total pressure drop of EGR system is 48.5kPa and the power of EGR blower is 68 kW.After the EGR system is introduced,in order to solve the turbocharging system matching problem of exhaust gas recirculation applied on the marine low-speed two-stroke diesel engine,effects of the turbocharger matching,cylinder bypass,EGR on engine performances under Tier III and Tier II restrictions are made and the turbocharger matching strategies are obtained.Results show that in the Tier III model,reducing turbine equivalent area can effectively restore the boost pressure,but the flow rate first increases and then decreases at the same time,which lead operating points close to the compressor surging line;the introduction of cylinder bypass can effectively increase the compressor flow and solve the surging problem.In the Tier II model,the EGR and EGB schemes can effectively solve the high charging pressure caused by the decreased turbine equivalent area.The comparison results show that NOx emissions are low and BSFC is high in the EGR scheme;BSFC is low and the NOx emissions are high in the EGB scheme.To fulfill a better fuel economy performance under the Tier II NOx restrictions,the EGR strategy should be adopted at high loads with the EGB strategy applied at medium to low loads.The valve control strategies under different operating conditions are determined based on the research of Tier III and Tier II mode.Turbine equivalent area,EGR rate,and EGB rate are optimized based on the coupling of Isight and GT-Power to get the lowest BSFC and NOx.The 6EX340 transient simulation model is established,and used to determine the valve status in different modes.Based on the simplified method of EGR blower and EGR valve switching at the same time,the cylinder bypass valve and the exhaust gas bypass valve switching at the same time,switching control strategies of different modes at 75% load with the boost pressure and the explosion pressure as the optimization standard are analysed.Results show that,when Tier II and Tier III switch between each other,we should adjust the EGR pathway at first and adjust the bypass valve after a certain time's delay.The reasonable delay time are determined in the simulation,which can effectively eliminate the fluctuation of pressure during switching.