Numerical And Experimental Investigation Of Swirl-Tumble Combined Intake Flow On In-Cylinder Flow And Combustion Process Of Gasoline Engine

As the main power source of passenger cars,gasoline engines often operate under part loads in cities with low fuel economy.It is of great significance to improve the fuel economy of gasoline engines under part load.This paper proposes a technical route of swirl-tumble with dilution combustion to improve the fuel economy of gasoline engines under partial load conditions,and the underlying mechanism of its better stability of the swirl-tumble flow field is analyzed.Based on multi-cycle large eddy simulations,the cycle-to-cycle variation(CCV)of the flow field and the distribution of air/fuel mixture in the cylinder is analyzed,and special attention is paid to the region around the spark plug.The results show that in the tumble-only condition,the airflow in the cylinder rotates around an axis that is perpendicular to the axis of the cylinder,and the intake jet almost hits the piston crown perpendicularly,resulting in violent flow separation and high CCV.In contrast,in swirl-tumble flow condition,the rotation axis of the air flow is a straight line at an acute angle with the cylinder axis,and air flow has a certain inclination angle when it contacts the piston crown,so the flow separation is weak with lower CCV.At the spark timing,compared with the tumble-only condition,the swirl-tumble flow will produce a stable local over-concentrated area of the air/fuel mixture in the cylinder with lower CCV,and meanwhile,the swirl-tumble flow has lower CCV in the velocity field near the spark plug.In addition,compared with tumble-onlycondition,swirl-tumble flow can produce denser air/fuel mixture with lower CCV near the spark plug.Furthermore,based on the POD(Proper Orthogonal Decomposition),it is found that the swirl-tumble flow improves the stability of the overall flow field mainly by suppressing the CCV of the coherent structures.Swirl-tumble flow can also maintain the energy ratio of the mean flow field at the end of compression compared with the tumble-only condition,and can reduce the cyclic variation of the flow field.Based on the above results of the numerical simulations,the author further conducts a gasoline engine bench test to study the effects of low tumble flow,high tumble flow,and swirl-tumble flow on the combustion characteristics,the fuel economy,and the emission characteristics in the condition of exhaust-gas and air dilutions under a part load.The engine speed is 2000 RPM and the torque is 50 N·m.The results show that under air dilution,compared with the original engine with low tumble,of which theλ(excess air coefficient)limit is 1.37,the high tumble flow can extend theλlimit to 1.44,reduce the BSFC(brake specific fuel consumption)by 10%;the swirl-tumble can extend theλlimit to 1.51,reduce the BSFC by 12%.Under exhaust gas dilution,compared with the original engine with low tumble,of which the EGR limit is 23%,the high tumble flow can extend the EGR limit to 27%,reduce the BSFC by 5%;the swirl-tumble can extend the EGR limit to more than 28%,reduce the BSFC by 8%.Compared with the original engine with low tumble,the combination of high tumble and diluted combustion can reduce the NO_x and CO emissions by up to 69%and 89%,increase the HC emissions by up to 74%,respectively;the combination of swirl-tumble and diluted combustion can reduce NO_x and CO emissions by up to 63%and 87%,increase the HC emissions by up to 44%,respectively.Therefore,significant economic improvement and emission reduction can be achieved by introducing swirl-tumble and diluted combustion.