Research On The Fuel-rich Combustion Reaction Mechanism In Pre-chamber And Performance Optimization Strategy For A Marine LP-DF Engine

Under the current carbon peak and carbon-neutral global decarbonization energy background,the research on low-carbon fuels and near-zero emission internal combustion engines has attracted more and more attention,which puts forward higher standards and requirements for marine power plants.The use of marine LNG low-pressure dual-fuel（LP-DF）engine technology is a realistic and feasible solution that can gradually meet the ship industry’s goal of reducing carbon emissions by 40%by 2030.However,this engine has some problems related to methane slip and high NOx emissions from the pre-combustion chamber（PCC）.This is the current LP-DF engine difficulties in independent R&D and product upgrades.Therefore,this work focuses on methane slip and NOx emission problems from the PCC of LP-DF marine engines.A special combustion reaction mechanism and simulation model suitable for LP-DF engines were constructed from the perspective of combustion chemical reaction kinetics.Besides,the influence of pre-chamber inflow characteristics on engine NOx emissions and combustion characteristics was studied.Moreover,methane slip and NOx emission control methods for LP-DF engines were investigated to provide technical approaches for the design and performance optimization of near-zero emission pre-chambers for marine LP-DF engines.Based on the above,the following works were carried out in this paper:（1）By referring to the WinGD GTD database and sorting out relevant experimental data,a one-dimensional model and a three-dimensional simulation model for the working process of the marine LP-DF engine were constructed.The DRGEPSA method was used to simplify the detailed mechanism of n-heptane and natural gas.Aiming at the high-pressure and high-residual exhaust gas combustion environment of the non-scavenging PCC,the n-heptane rich combustion-natural gas wide-equivalent ratio combustion reaction mechanism was constructed.The flame stagnation period and laminar flame velocity were verified for the new mechanism.Moreover,the new n-heptane-natural gas combustion reaction mechanism was coupled with a CFD three-dimensional simulation model for verification.The optimized combustion chemical reaction kinetic mechanism could well predict the internal combustion and emission process of the low-pressure dual-fuel engine.（2）The working characteristics of the gas model of the marine LP-DF engine and the occurrence process of the inflow of the pre-chamber were studied.In addition,the influence of the inflow characteristics on the combustion and emission characteristics of the LP-DF engine was analyzed.The equivalent air/fuel ratio of diesel/natural gas dual fuel was defined and the minimum air-fuel ratio(λ_min)in PCC was used as the quantification parameter of the pre-chamber inflow characteristics.Besides,the evaluation of the influence factors of different pre-chamber inflow was given.The research results showed that the continuous high-temperature combustion reaction of the PCC was the main cause of NOx emissions.Pilot fuel combustion contributed a lot to the rapid increase in the temperature of the PCC and was an important factor that caused higher NOx emissions inside the PCC.The nozzle diameter ratio of the PCC and the GAV position had a greater impact on the inflow characteristics of the PCC.Moreover,there was a close relationship between the inflow and the jetflow of the pre-chamber.The characteristics of the inflow affected the state of the working fluid in the pre-chamber,which in turn affected the combustion reaction rate of the pre-chamber.Furthermore,theλ_min in PCC at the moment of ignition had a great influence on the jetflow intensity.（3）Four factors including the GAV position,the NG spray angle,the diameter ratio of the nozzle hole of the PCC,and the natural gas injection timing had been studied.The research results showed that the inflow of the PCC increased with the increase of the position of the GAV.When the GAV position was higher,the distribution area of OH radicals in the cylinder increased,the fuel combustion process speeded up,the indicated thermal efficiency of the engine decreased,and the amount of methane slip increased.As the NG spray angle increased,the uniformity of the mixture gradually improved.The amount of NOx emissions under the 45°spray angle scheme was the lowest but the combustion deterioration was serious.Moreover,the minimum air-fuel ratio in PCC first decreased and then increased with the increase of the nozzle hole diameter ratio of PCC（ω_r）while the peak compression temperature in PCC decreased.Furthermore,the indicated thermal efficiency was the highest under theω_r=2%scheme but the pressure fluctuation in the cylinder increased.When the natural gas injection timing was advanced,the mean in-cylinder pressure peak and NOx emissions increased,and the engine’s indicated thermal efficiency first increased and then decreased.（4）Low-pressure EGR technology was used to control the methane slip and NOx emissions from PCC of the marine LP-DF engine.Besides,EGR intercooling technology,pilot fuel timing adjustment,and fuel reforming technology were used to make up for the shortcomings of EGR technology to ensure the control of methane slip and NOx emissions while improving engine combustion efficiency.In addition,a study on the influence of natural gas components on engine performance was carried out and the technical direction for improving engine performance from the fuel point of view was given.The results showed that the engine’s indicated thermal efficiency first increased and then decreased with the increase of the initial intercooling temperature while NOx emissions increased.As the EGR rate increased,the average pressure peak and pressure rise rate of the main combustion chamber decreased and the combustion duration was prolonged.EGR technology can suppress the intensity of premixed combustion and reduce the amount of methane slip.As the fuel reforming rate increased,the maximum combustion pressure in the cylinder and the peak instantaneous heat release rate increased at the same time,the stagnation period was shortened,the total heat release of fuel increased,and the combustion duration shortened significantly.When the pilot fuel injection timing was advanced,the engine’s indicated thermal efficiency first increased and then decreased while NOx emissions increased.Furthermore,with the increase of ethane and propane content,the maximum combustion pressure of the main combustion chamber and the peak of combustion heat release rate increased,and the indicated thermal efficiency and NOx emissions of the engine increased.By adopting the-5°CA-E24-R2%optimization scheme,the indicated thermal efficiency of the engine could be increased by about 2.7%and the amount of methane slip could be reduced by about 24%.Finally,the 3-D simulation work was carried out by constructing a new combustion reaction mechanism of a marine LP-DF engine.the influence of inflow characteristics on engine combustion and emission performance was quantitatively analyzed and a coordinated control strategy that can improve engine thermal efficiency and reduce methane slip and NOx emissions was obtained.The results can provide a theoretical basis and engineering design reference for the optimization design of the PCC on a marine LP-DF engine and the further optimization of the engine using EGR technology.