Simulation Of Combustion Characteristics In A LNG/diesel Dual Fuel Engine

Natural-gas fuel power system is becoming the one of the greatest countermeasures to cope with the increasingly stringent emission standards and the goal of achieving sustainable development in the shipping industry.Natural-diesel dual fuel engine system has been widely applied.The initial conditions and the injection strategy of these two fuels have a significant influence on the combustion processes in cylinder.Hence,the influence rules of these parameters are needed to be analyzed to provide a theoretical basis for the performance optimization of NG-diesel dual fuel engines.During combustion processes of the dual fuel engine,the combination of the flow of the mixtures in-cylinder and chemical reaction kinetics and the uneven distribution of temperature and fuel concentrations in the spatial lead to the fact that multidimensional model of engine combustion in cylinder can be more accurate to reflect the processes of engine combustion.Using the FORTésoftware and the chemical reaction kinetics mechanism of multicomponent diesel fuel,the NG-diesel engine combustion process model is established in accordance with the experiment data of Z6170 type engine test bench.50%and 75%load of diesel engine and 75%load with 75%substitution rate are chosen to compare the simulations and experiments to validate the rationality of this model.Finally,the effects of initial conditions and the injection strategy on the natural-diesel engine combustion under different loads conditions are studied.The simulation results indicate that the initial temperature and pressure have great influence on the diesel injection state and fuel flow rate under the condition of75%load with 65%substitution rate.The ignition time in advance with the higher temperature and pressure in-cylinder,leading to the big change of NO_x,CO and unburned hydrocarbon emissions.The optimal initial temperature and pressure can be chosen under this load conditions on basis of the combustion characters and emissions,and they are 360K and 0.185 MPa respectively.Under different load conditions,the highest temperature and pressure in-cylinder rise and ignition time gets earlier,leading to the increasing of NO_x and decreasing of CO.With the increase of load,the temperature and pressure in cylinder rise and combustion duration time extend,highest pressure rise rate and total fuel consumption decrease.The optimal fuel initial injection time can be chosen under different load conditions.The injection time is in advance with the increase of load.However,it delays to-22~oCA at 100%load condition.As the substitution rate increases when the substitution is less than 65%at 50%and 75%load conditions,the highest temperature and pressure in-cylinder rise.The time of CA10 delays but CA50 gets earlier,leading to the fuel combustion duration time decreases.NO_x increases and CO first increases but then decreases.When the substitution rate is more than 65%,the state of combustion in cylinder gets worse.At100%load condition,the substitution rate should be lower than others to make the combustion and emissions better.