Research On The Influence And Optimization Of Miller Cycle On GDI Engine Mixture Formation At Low Temperature

The overall emission of gasoline vehicles has been greatly reduced,along with constantly updated and implemented emission regulations.Under cold start stage and other working conditions,the in-cylinder temperature is low,which easily leads to poor fuel evaporation,uneven air-fuel mixture,wall wetting phenomenon,serious emission and so on.Therefore,it is of great significance to further improve the performance of GDI engines to study the evaporation and mixture formation characteristics in the cold start stage.As one of the main measures for efficient and clean GDI engine in recent years,Miller cycle is mainly used at medium and low load conditions.Due to the late closing of the intake valve,the in-cylinder temperature is decreased,which will further deteriorate the fuel evaporation.Therefore,there is little research on the performance of Miller cycle engine under cold start condition.However,the early closing of intake valve has great potential to improve the quality of fuel atomization.In this paper,a GDI engine is selected as the original engine for bench test.A one-dimensional GT-Power model is established by the parameters of the original engine and the test results,which provides reliable boundary and initial conditions for the 3D simulation.Steady-state intake test,spray test in constant volume bomb and numerical simulation were carried out to verify the accuracy of inlet and exhaust ports geometric model and spray model,respectively.Five groups of inlet valve early closing Miller cycle valve lift lines were designed by interpolation method.Miller cycle engine model was established in CONGVERGE to explore the influence of Miller cycle on the development of wall films,in-cylinder flow and the formation quality of spray field in low temperature environment.The in-cylinder working process of the Miller cycle with different miller degrees and the original engine were studied under the condition of the total air intake mass unchanged.The results show that with the increase of Miller degree,the thick film area on the surface of the pit in piston top was effectively reduced under the action of slow turbulent dissipation in the cylinder,which improved the evaporation of wall films.Compared with the original engine,the wall films mass of EIVC60 and EIVC75 miller cycles decreased by 28% and 48% at 340℃A,which effectively avoid the ’pool fires’ phenomenon.The larger intake pressure significantly increases the in-cylinder tumble of GDI engine,and the most significant is the positive(clockwise)tumble ratio of EIVC75,with a peak value of 23.6 % higher than that of the original engine.Different in-cylinder flow directly affects the concentration field distribution.When the ignition time is reached,the concentration fields of the original engine is greatly different from that of EIVC60 and EIVC75.The rich air-fuel mixture of the original engine is mainly located in the intake side perpendicular to the X-axis section,and that of Miller cycle engines is located in the cross section perpendicular to the Y-axis.The evaporation of wall films and the quality of air-fuel mixture were optimized.Doubleinjection was applied to the GDI engine with EIVC75 miller cycle.The best injection timing and injection ratio with double-injection and Miller cycle were explored by controlling the single factor method.The results show that with the delay of the second injection timing,the film evaporation in the cylinder increases.When the second injection timing is later than 150℃A,the wall wetting begins to deteriorate.However,compared with single injection,the wall wetting of different double-injection schemes was improved.The second injection timing was set to 150℃A for studying the influence of different injection ratios.The ratio of 2:1 between the first injection mass and the second injection mass shows the highest film evaporation efficiency,and the residual wall films mass were only 0.054 mg at the ignition time.The double-injection scheme with the first injection mass greater than the second injection mass significantly improves the quality of the concentration field in the cylinder.At the ignition time,the selected Case5-2,Case5-4 and Case5-6 schemes were uniform mixture in most areas of the two cross sections passing through the central line of the cylinder.The relatively rich air-fuel mixture with equivalence ratio of 1.2-1.3 is conducive to stable ignition near the spark plug.