Statistical variation analysis of fuel spray characteristics under cross-flow conditions

Owing to cavitation inside the nozzle, spray breakup, and complex flow fields in the cylinder of a spark ignition engine, a certain degree of cycle-to-cycle variation (CCV) remains in different spray cycles under the same operating conditions, thereby affecting the spark ignition engine significantly. Therefore, the effect of cross-flow on the cyclic variation of the spray characteristics is investigated in this work. The CCV of fuel spray characteristics is evaluated via the statistical analysis of 30 repeated experiments under atmospheric pressure. First, the coefficient of variation (COV) is used to assess variations in the spray penetration, area, and optical thickness. Moreover, the effects of injection pressure and cross-flow velocity on the statistical characteristics of the spray are analyzed at the end of injection (EOI). Finally, a dimensionless "Ins number", which is defined as the parameter of any spray characteristic at a certain time divided by the average experimental value, is proposed to evaluate the spray variation characteristics under cross-flow conditions. Therefore, when the absolute value of the Ins number approaches 1, the CCV of the spray characteristic parameters is low. Results show that the COV of the spray penetration is the largest in the initial stage of injection and then decreases gradually. Meanwhile, the COV of the vertical penetration can be categorized into two stages. Furthermore, it is discovered that the variation in the horizontal penetration is enhanced by the cross-flow and injection pressure at the EOI. In addition, in terms of the COV of the optical thickness, the variation on the windward side is greater, and the cross-flow enhanced the variation in the spray tip region. Most of the Ins numbers of the spray characteristics range from 0.9 to 1.1 (+/- 10%) in this study, and they can provide data support for improving the accuracy of empirical prediction equations and models.