A Numerical Study On Split Injection Strategies For Internal Combustion Engines Fueled With Gasoline/Hydrogenated Catalytic Biodiesel Blends

At present,with the intensification of energy crisis and the increasingly stringent emission regulations in the internal combustion engine industry,the development of advanced combustion technology with high efficiency and low emission is an important development direction to deal with these problems.For this reason,scholars at home and abroad are putting forward new combustion models,such as homogeneous charge compression ignition,low temperature combustion,premixed charge compression ignition,reactivity controlled compression ignition and gasoline compression ignition.Among them,gasoline compression ignition can obtain the similar thermal efficiency as diesel engine,which is a new combustion mode with great development potential.However,gasoline compression ignition will produce higher pressure rising rate due to the long ignition delay period of gasoline at high load.Therefore,a small proportion(30%)of hydrogenated catalytic biodiesel was added to gasoline in order to form blends for internal combustion engine.The cetane number of hydrogenated catalytic biodiesel is more than 100 which can greatly improve the ignition performance.The split injection of the blends can improve combustion roughness at high load.It is worth to further study how the pre-injection mass ratio,pre-injection timing,main injection timing,injection pressure and other parameters impact the combustion and emissions.In this paper,the effects of split injection strategies on the performance and emission of gasoline/hydrogenated catalytic biodiesel engine are investigated by numerical simulation,which can provide references for combustion system design.Firstly,a numerical model is established to describe the in-cylinder flow,spray and combustion processes of gasoline/hydrogenated catalytic biodiesel.Meanwhile,the credibility of the model is verified by experimental data.The impacts of pre-injection timing and mass ratio,as well as main injection timing of split injection on the combustion characteristics and emission are numerically evaluated when the injection pressure is 60 MPa.The results show that split injection can reduce the pressure rising rate and decrease the combustion noises when compared with single injection.In addition,the rate of premixed combustion reduces with the advances of pre-injection timing,which is beneficial to reduce the combustion noises.However,very early pre-injection timing results in the mixture located at the top of the piston crown,which will lead to the high hydrocarbon emission.A proper pre-injection timing range is-60~-20°CA ATDC.Thirdly,with the increase of pre-injection fuel mass ratio,heat release rate,peak pressure rising rate and NOx emission increase,while the fuel consumption rate decreases.A proper pre-injection fuel mass ratio range is 5~15%.Moreover,as the retarding of the main injection timing,the in-cylinder heat release rates of the premixed combustion,the peak pressure rising rate and NOx emission decrease,while the fuel consumption rate increases.It is found that the more appropriate main injection timing range is-10 ~-6 ° CA ATDC.In addition,the effects of increasing injection pressure on the combustion and emission characteristics of split injection gasoline/hydrogenated catalytic biodiesel engines are investigated.The results show that with the increase of injection pressure,the pre-injection fuel becomes more homogeneous mixture.It can also accelerate the diffusion combustion of the main injection fuel,which is beneficial to improve the thermal efficiency of the engine and reducing the soot emission.But it also brings some problems,such as the increase of maximum pressure,in-cylinder pressure rising rate and NOx emissions.When the injection pressure is increased from 60 MPa to 80 MPa,the indicated thermal efficiency increases significantly under various main injection timing.However,when the injection pressure is further increased to 100 MPa,although it has some effects on the improvement of soot emission,the thermal efficiency will not be significantly improved.Considering the effects of injection pressure and split injection strategies on the maximum in-cylinder pressure,pressure rising rate,indicated thermal efficiency,NOx and soot emissions,the optimum scheme is 80 MPa injection pressure,10% pre-injection fuel mass ratio and-6 °CA ATDC main injection timing.