Simulation And Optimization Of Combustion And Emission Characteristics For Diesel Ignited Gas Fuel Engine

With the critical energy crisis,environmental pollution and increasingly stringent emission regulations,technological innovation is urgently needed to achieve low carbon in short to medium term and zero carbon in the medium to long term for internal combustion engines.The combustion of carbon-neutral fuels is an effective technical means to ensure the performance of diesel engines and achieve green combustion,and natural gas and hydrogen are considered clean and effective alternative fuels for diesel engines.The problem of natural gas ignition difficulties can be effectively solved by diesel ignition.However,this dual-fuel engine has problems such as low load combustion difficulties and high unburned gas fuel emissions.Therefore,in this paper,the performance（fuel consumption,brake power and thermal efficiency）,combustion characteristics and emission characteristics of a diesel-ignited natural gas dual-fuel engine and a natural gas hydrogen blended tri-fuel engine at different loads are numerically simulated and optimized.The main research contents and findings are as follows:（1）Effects of different natural gas replacement ratios,pre-main diesel injection strategies with different pre-injection advance angles,and main-post diesel injection strategies with different post-injection start angles on the combustion and emission characteristics of the diesel natural gas（DN）dual-fuel engine at 25%,50%,75%,and 100%loads were simulated by the validated 1D and 3D models.The results showed that at 100%load,the combustion of DN dual fuel led to a decrease in peak cylinder pressure and in-cylinder temperature,with a greater tendency to decrease at a higher natural gas replacement ratio;at 25%-75%load range,the combustion of diesel-natural gas dual fuel led to an increase and then a decrease in peak cylinder pressure and in-cylinder temperature.Combustion of DN dual fuel reduces NO_x,CO₂ and soot emissions in all load ranges but significantly increases CH₄ emissions,especially at low loads.A pre-main injection strategy with appropriate pre-injection advance angles significantly increases peak cylinder pressure and in-cylinder temperature,reducing CO,CO₂,soot and CH₄ emissions but significantly increasing NO_x emissions.The main-post injection strategy reduces peak cylinder pressure and in-cylinder temperature and is accompanied by marked post-combustion.However,at100%load,there is an appropriate post-injection start angle to reduce NO_x,CO,CO₂,soot and CH₄ emissions simultaneously.（2）When the natural gas replacement ratio was kept at 50%and the natural gas partially replaced by hydrogen,the effects of different hydrogen addition ratios on the performance,combustion and emission characteristics of the diesel natural gas hydrogen（DNH）tri-fuel engine were investigated at four engine loads of 25%,50%,75%and 100%.The results showed that adding hydrogen to the DN dual fuel shortened the combustion duration and accelerated the flame propagation at all loads studied,and the higher the hydrogen addition ratio,the more pronounced the effect.Combustion of DNH fuel resulted in a significant increase in peak cylinder pressure and in-cylinder temperature,which tended to increase with the hydrogen addition ratio in the gaseous fuel,and significantly reduced CO₂,soot and CH₄emissions,slightly reduced CO emission,but resulted in a dramatic increase in NO_x emissions.（3）A combined RSM-NSGA II-TOPSIS optimization scheme is proposed for the trade-off between ISFC,NO_x and CH₄ emissions at low loads for DNH tri-fuel engines.The objective variables of the optimization are ISFC,NO_x and CH₄ emissions,and the decision variables are hydrogen addition ratio,ignition diesel injection advance angle,diesel spray angle and engine swirl ratio.The optimized results showed that with higher hydrogen addition ratio,ignition diesel injection timing concentrated at 15-20°CA BTDC,spray angle controlled at 90-100°,and swirl ratio around 0.6-0.8 or 1.2 during low loads of DNH tri-fuel engine,better engine performance and lower emissions can be achieved.