Research On Injection Strategy Of Marine Ammonia/Diesel Dual Fuel Low Speed Engine

With the extensive use of fossil fuels,Greenhouse Gas（GHG）emissions are increasing day by day,leading to the intensification of the greenhouse effect.In order to control GHGs emissions and curb global temperature rise,IMO and major countries have set strict emission reduction policies and medium and long-term decarbonisation targets,and the shipping industry is facing a very serious decarbonisation situation.In this paper,the injection strategy of marine ammonia/diesel dual-fuel low-speed engine under high pressure direct injection mode is numerically simulated to explore the effect of different injection strategies on the improvement of work capacity and the reduction of pollutants,and integrating the advantages of each strategy to develop an injection scheme that can achieve high power and low emissions.The main research contents are as follows:（1）A three-dimensional simulation model of a marine ammonia/diesel high-pressure dual fuel engine was established,and the model was validated with experimental data.Two common types of dual fuel nozzle arrangements were selected,and different pilot fuel share schemes were researched for each type.The results show that both arrangement types of nozzle can achieve the work level of the original engine.When using the arrangement of dual fuel nozzles adjacent to each other（Scheme I）,the impact on the air flow in the cylinder is greater,and the starting time of ammonia combustion is advanced,while the circumferential uniform arrangement of dual fuel nozzles（Scheme II）can obtain a faster combustion speed and stronger work performance.However,due to the late ignition,there is a sharp increase in pressure and heat release rate,increasing the mechanical load and heat load of the engine.As ammonia reduces part of NO_X to N₂,NO_X emissions in ammonia/diesel mode are significantly reduced compared with those in diesel mode.The N₂O emissions under each scheme are reduced with the increased share of pilot fuel,but they remain higher than those in diesel mode.The GHGs emissions are increased with the increased share of pilot fuel,but they are much lower than those in diesel mode.In addition,compared with scheme II,the emission performance of Scheme I with lower emissions of all kinds of species is better.（2）The effect of injection timing and multi-stage injection on the engine combustion and emissions were investigated based on 3%pilot fuel in Scheme I,and the combustion heat release and material evolution laws were summarised.The results show that advancing the timing of dual fuel injection synchronously improves the work performance of engine,although NO_X emissions are increased,they are always lower than those in diesel mode,and GHGs are significantly reduced compared to diesel mode.Advancing diesel injection timing increases work capacity and then decreases it,reaching a maximum at the injection timing of-4°CA,NO_X emissions will be further reduced,but GHGs emissions will be increased.Advancing ammonia injection timing can improve the work capability,but NO_X emissions will be further increased.Delaying ammonia injection timing can reduce NO_X emissions to below 3.4 g/k Wh,but the work capability is also decreased accordingly.Advancing or delaying ammonia injection timing can increase GHGs emissions,and the impact of advanced injection timing is greater.Increasing the pre-injection proportion of liquid ammonia can significantly improve the work capacity,NO_X and GHGs are also increased accordingly.Extending the pre-injection interval can reduce NO_X emissions,and GHGs emissions will be increased,but NO_X and GHGs will always remain below diesel levels in all schemes.Increasing the ratio of post-injection and extending the post-injection interval can reduce the work capacity,but the NO_X emission can be reduced to less than 3.4g/k Wh more easily with the post-injection strategy.（3）Strategies that can increase power and reduce emissions were coupled with separately to explore combustion strategies that can achieve both high power and low emissions.The results show that when using the advanced dual fuel injection timing coupled with ammonia partial post-injection strategy,under the-4°CA/-3°CA coupled with post-injection 10%scheme,IMEP and ITE are guaranteed,and NO_X emissions are reduced.Although GHGs emissions is slightly higher,the increment is negligible.When using the advanced ammonia injection timing coupled with ammonia partial post-injection strategy,under the-3°CA coupled with post-injection 10%scheme,IMEP and ITE are only slightly decreased,NO_X emissions are below3.4 g/k Wh,and GHGs emissions are the lowest.When using the ammonia three-stage injection strategy,both IMEP and ITE are increased in the pre-injection 30%coupled with post-injection20%scheme,with NO_X emissions below 3.4 g/k Wh.Comparing different coupling strategies,it can be concluded that if the GHGs emissions requirement is reduced,the performance of the pre-injection 30%coupled with post-injection 20%scheme is the best.If the NO_X emissions requirement is reduced,the-5°CA coupled with post-injection 10%scheme is the best.If both GHG and NO_X emissions are considered,the-3°CA coupled with post-injection 10%scheme is the most valuable for application.By researching the comprehensive performance of the engine under different injection strategies,this paper summarizes the influence law of fuel injection parameters on the combustion and emission characteristics of the engine.The research results can provide theoretical reference for the application of marine ammonia/diesel high-pressure dual fuel engine,and provide improvement ideas for engine performance optimization.