Research On The Influencing Factors Of The Flow Field In The Intake And Compression Stroke Of A Four-valve Diesel Engine

Combustion and emission performance of diesel engine are determined by air intake system that include intake manifolds,intake ports and valves,optimization on structural parameters of which contributes to the formation of in-cylinder eddy current distribution and mixture,thus affecting the overall performance of the engine.In this paper,with intake manifolds,intake ports and valves of an high pressure common rail diesel engine as research subject,theoretically based on CFD theory,in-cylinder flow characteristics during intake process in the diesel engine were investigated using steady flow test rig for inlet port and 3-D simulation in order to optimize parameters of relevant components of air intake system,finally improving inlet flow coefficient and swirl ratio.First,three simulation scheme were set up by modifying transitional radius of the helical port and sectional area of tangent intake duct based on the original model.Overall 1-D dynamic model was established,also,inlet,outlet and wall boundary conditions of the schemes were obtained.Then models are simulated,in order to get distribution of swirl and flow velocity along with flow coefficient and swirl ratio.Results show that when valve opening is at its maximum,it contributes to the formation of the in-cylinder swirl to modify sectional area of tangent intake duct,since high flow rate region expands,which makes air inflow becomes larger.However,when valve opening is relatively small,interference between airflow of helical port and tangent intake duct is rather serious.Also,there is complicated gas flow around outlets of air intakes,cylinder head and valves during whole intake process and swirl of the gas follow some certain rules as gas inside the cylinder move downwards.Then,effect of different intake valve chamfer on in-cylinder flow characteristics was studied using steady flow test rig and steady state numerical simulation.Three different simulation and experiment schemes are proposed.In the meanwhile,configuration,principle and operation steps of the steady flow test rig was presented.Results show that results of simulation agree well with those of steady flow test,which shows the feasibility of the simulation approach.Compared to the original valve chamfer,there are more swirls and air inflow,making gases mixed and combust thoroughly in engine with smaller valve chamfer.Besides,valve chamfer has a great effect on in-cylinder flow characteristics as valve stroke is relatively small;however,when valve stroke is relatively large,the opposite is the case.In addition,the effect of parameters of intake manifolds on difference between air inflow of each cylinder was investigated,especially length and diameter of inlet manifold,length of air intake branch,volume of resonator cavity and finally optimization scheme of intake manifold structure was obtained through orthogonal test method.Model of original intake manifold was simplified to establish new model of intake manifold-air inlet-cylinder,and relevant boundary and initial conditions are given.Then the models are simulated based on steady state method.Finally,distribution of gas flow inside intake manifolds and cylinders as well as difference between air inflow of each cylinder of different schemes are obtained.Results show that it could significantly reduce air-intake nonuniformity of engines to increase length of inlet manifold and air intake branch and to use inlet manifold with small diameter as well as resonator cavity with a small volume.