Numerical Simulation Of Combustion In Ammonia/Diesel Dual Fuel Low-speed Marine Engine With Pre-Chamber Jet Ignition

With the rapid development of the shipping industry,CO₂ emissions from low-speed marine engines,which are the main power of ocean transportation,keeps increasing.With the trend of low-carbon and zero-carbon in the global shipping industry,low-speed marine engines are facing serious challenges to reduce carbon emissions,and the application of low-carbon and zero-carbon fuels in marine engines is a critical way to reduce carbon emissions.Ammonia is considered to be a promising fuel to achieve the goal of zero-carbon emissions of internal combustion engine.However,it has some bottlenecks such as high ignition temperature,high ignition energy,and slow combustion rate.Pre-chamber jet ignition is an effective method to improve ignition stability and combustion rate.Therefore,in this paper,the combustion process of ammonia/diesel dual fuel low-speed marine engine with pre-chamber jet ignition is simulated by coupling the chemical reaction mechanism with computational fluid dynamics（CFD）models,and the control strategy to improve the thermal efficiency of ammonia/diesel dual fuel combustion in low-speed marine engine is investigated.Firstly,a chemical kinetic mechanism of ammonia/diesel dual-fuel composed of97 species and 498 reactions is proposed and it can predict well with experiment results including the ignition delay time,laminar flame speeds and important species mole fraction.Then the mechanism is coupled with a three-dimensional CFD models of a low-speed marine engine.The effects of compression ratio,scavenging pressure,scavenging temperature,and pre-chamber parameters on the ignition and combustion process of ammonia/diesel dual fuel marine engine are studied.The results show that properly increasing the compression ratio improves the ignition stability of the dual fuel marine engine.When the compression ratio is 14.5,high thermal efficiency can be obtained and the in-cylinder peak pressure can be controlled within a reasonable range.Appropriate reduction of scavenging pressure can reduce the air intake mass and improve the equivalence ratio of ammonia/air mixture.When scavenging pressure is reduced to 3.20 bar,ammonia is completely consumed and higher indicated thermal efficiency can be obtained.Increasing the scavenging temperature effectively increases the mixture temperature in the cylinder and shortens the combustion duration obviously.Increasing the volume of pre-chamber can advance the ignition timing of pilot fuel.When the volume ratio of pre-chamber is 1.5%and the pilot fuel mass is 180 mg,the engine can obtain better fuel economy.At compression ratio of 14.5,the strategies to improve the thermal efficiency of ammonia/diesel dual-fuel low-speed marine engine are investigated further.It is found that ammonia injection pressure has a big influence on the distribution of velocity and mixture concentration.When the ammonia injection pressure is 10.6 bar,it has the shortest combustion duration and the best indicated thermal efficiency of 50.21%,but the NOx emissions is increased due to the higher combustion temperature.Increasing scavenging pressure with more air intake mass and pilot fuel mass with higher jet flame energy simultaneously is beneficial for combustion stability of thinner mixture.It is an effective way to improve indicated thermal efficiency and reduce NOx emissions at the same time.In this paper,indicated thermal efficiency of 50.17%is achieved by increasing scavenging pressure from 3.29 to 3.43 bar and increasing pilot fuel mass from 135 mg to 180 mg,while NOx emissions are reduced.