Study On Multi-Strategy Collaborative Optimization Of Combustion And Emission For Marine Low-Speed Engine

With more and more attention paid to environmental pollution problems,the international Maritime Organization（IMO）has launched a stringent Tier III emission standard,which poses great challenges to the optimization of the combustion performance and the improvement of the emission performance of marine low-speed engines.The effects of fuel injection strategies,Miller cycle,exhaust gas recirculation（EGR）and intake air humidification on the engine performance at 75%load were investigated using CFD software CONVERGE.The simulated results agree with the experimental data,demonstrating that increasing injection advance angle and injection pressure can improve combustion.And different fuel injection rate shape can affect the combustion process and emissions of low-speed two-stroke marine diesel engines significantly.The NOx emissions and fuel consumption rate can be reduced simultaneously by adopting appropriate main and pre-injection interval and pre-injection rate.Miller cycle reduces NOx emissions significantly.Additionally,a 30%EGR alone could reduce NOx emissions to 2.519g/kWh.And NOx emissions were decreased with increased water concentration,with a 2:1 water-to-fuel mass ratio yielding an emissions reduction of 21.4%relative to the reference case.An optimal emission reduction strategy was determined to be the M5E25W2.0P9injP900（which corresponds to the following:exhaust valve closure that is 5°CA larger than the reference case,an EGR ratio of 25%,mass of water that equates to 200%of the fuel mass,a main and pre-injection interval of9°CA,and a fuel injection pressure of 900 bar）,as it was proven to satisfy Tier III emission requirements and avoid high fuel consumption.On the basis of RT-flex 50DF dual fuel low-speed engine,the effects of fuel injection timing,EGR and reforming technology on the performance of nature gas/diesel dual fuel low-speed engine were studied.The results have shown that appropriate increasing fuel injection advance angle could improve combustion,but the NOx emission increases.When the fuel injection time is 12.5°CA BTDC,engine has the highest thermal efficiency.The increase of EGR rate could reduce NOx emission significantly and alleviate the roughness of low-speed engine,but it would increase fuel consumption.The application of fuel reforming technology in low-speed engine can improve the combustion performance of low-speed engine.A potential optimization strategy for dual fuel low-speed engine was determined to be the 12.5 BTDC_E10₂.25%(which corresponds to the following:fuel injection timing is 12.5°CA BTDC,an EGR rate is 10%,andγ_reformingeforming is 2.25%),as it was proven to satisfy Tier III emission requirements and reduce fuel consumption.This shows that multi-strategy collaborative optimization is a promising technical route to achieve economy improvement and emission reduction.