| Gasoline direct injection(GDI) engines have been gradually widely applied because of their low fuel consumption and high thermal efficiency. Compared with port-fuel-injected(PFI) engines, the injection pressure of GDI engines is usually between 6-10 MPa, which leads to higher possibility of fuel impinging on the cylinder wall, resulting in worse emissions. Furthermore, it faces the problems of the shorter time of fuel atomization and larger cycle-to-cycle variation(CCV) fuel/air mixture which induces larger CCV of combustion with declined engine performance. Therefore, the formation of homogeneous fuel/air mixture makes a great co ntribution to generate normal combustion in GDI engines and the intake airflow is an important factor for the formation of homogeneous mixture.In this paper, particle image velocimetry(PIV) technique was used to study the in-cylinder tumble motion under three different intake airflow conditions in a optical engine with variable tumble which was transformed from a GDI engine. The proper orthogonal decomposition(POD) was applied to study the in-cylinder tumble motion and its effect on the CCV in cylinder flow without spray. In order to investigate the influence of intake airflow on the spray process, the cases with spray under high and low tumble intensity conditions were further investigated. Furthermore, the homogeneity degree of fuel/air mixture was quantified according to entropy analysis of the spray droplets based on different light intensity through Mie scattered technique.The tumble ratio was controlled by a flap in the manifold and a baffle in the intake port. When the flap is closed, a single large-scale tumble flow is formed at 90°CA ATDC. The tumble ratio increases to 2 from 0.5 when the intake valves is closed. According to POD analysis, the coherent part is the main factor for the cyclic variation of the airflow which can be reduced by the large-scale stronger tumble motion. Among the reconstructed flow fields, the mean part occupies about 50% of total energy which dominate the fundamental characteristics of the flow field. The coherent part contains the most fluctuation energy of the flow field. As the tumble ratio is risen, the maximum energy ratio of the coherent part decreases from 30% to 14%.The CCV of the coherent part is weakened dramatically, inhibiting the CCV of whole in-cylinder airflow.In low tumble ratio conditions, the sprays accelerate the formation of large-scale tumble flow, while they have little impact in high tumble ratio conditions. Before 1.0ms after start of injection(ASOI), the vertical velocity is increased that lengthens the spray penetration quickly. Meanwhile, the decreased transverse velocity widens spray cone angle gradually. After the finish of spray, the increase level of penetration slows down and spray cone angle is wider because of the tumble flow. When the flap is closed, the single large-scale tumble increases the maximum shear strain rate up to 2400 s-1 and the maximum vorticity increases to about 6000 s-1 by rolling up of the airflow, which contributes to wider spray cone angle and shorter penetration distance. The statistics of the measurement data illustrate that a single large-scale tumble can promote the formation of homogenous mixture and reduce the cyclic variations of fuel/air mixture. |
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