Numerical Study On Low-speed Marine Diesel Engine To Meet Tier Ⅲ Regulation With EGR Route

The increasingly stringent emissions regulation has posed more requests on the development of engine technologies.It is a big problem and challenge to reduce NO_x emissions and improve the fuel economy for diesel engines.Among combustion optimization technologies in the cylinder,EGR technology is one of the key approaches to achieve high efficiency and clean combustion.In the current study,numerical studies have been conducted to resolve the trade-off relationship of NO_xISFC and propose two road maps to meet the IMO Tier Ⅲ regulation.In the first part,the 3D model of the low-speed two-stroke marine engine was established.Multi-dimensional numerical simulations were conducted to explore the potential of EGR and intake humidifying coupled with optimized injection strategy for improving the trade-off relationship of NO_x-ISFC in this marine engine.The results show that,the dilution and thermal effects of EGR are the main reasons for the lower combustion temperature.However,the oxygen concentration reduces as EGR increases,thus resulting in lower combustion rate,prolonged combustion duration and retarded combustion phasing,in which lead to deteriorated fuel economy.The intake humidifying shows miner effect on dilution and the following combustion process when the water added is relatively small.With higher humidification ratio,thermal effect is enhanced and the combustion temperature is reduced due to high heat capacity of water vapor.NO_x can be reduced by 90.5% with minor penalty on fuel economy using 30% EGR combined with 1.5 W/F(water to fuel ratio)and slightly retarded SOI timing(1 °CA after the top dead center),which meets the Tier Ⅲ emission regulations.On this basis,Miller cycle through retarded exhaust valve closing timing,high compression ratio coupled with EGR(Exhaust Gas Recirculation)and WEF(Water Emulsified Fuel)was studied to improve the trade-off relationship of NO_x-ISFC(Indicated Specific Fuel Consumption).The results show that the EGR ratio could be reduced combining with WEF to meet the Tier Ⅲ emission regulation.The penalty on fuel consumption with EGR and WEF could be offset by Miller cycle and high geometric compression ratio.NO_x can be reduced by 81% without penalty on fuel economy by using retarded exhaust valve closing timing(20 °CA later)combined with 20% EGR and 40% W/F(the ratio of water and fuel),which can meet the Tier Ⅲ emission regulation.Two promising road maps to meet the IMO Tier Ⅲ regulation have been proposed in this study.Through EGR,the main technology,coupled with other optimizing technologies,the trade-off relationship of NO_x-ISFC is able to be improved.This may also provide technical support for the increasingly stringent emissions regulation.