A theoretical model based on a nonlinear ordinary differential equation was developed, which can estimate the atomization process of fuel droplets after the wall impingement. The phase-space trajectory of the equation for droplet deformation and oscillation varies from oval to parabola with increasing impact velocity. Four different regimes for droplet diameter distribution are derived from this complex feature of the equation. The amount of liquid film remaining on the wall and the number of droplets are estimated from the related mass and energy conservation laws. The model is called the Oval-Parabola Trajectories (OPT) model in the present report.Comparisons made with some fundamental experimetal data confirm that this mathematical model is effective in a velocity range from 2m/s to 40m/s and in a diameter range below 300 micrometers.A previously reported numerical code based on the multi-level formulation and the renormalization group theory is combined with the OPT model and the TAB model. The visualizations reemerged by computations indicate that secondary atomization behavior on valve surfaces plays a significant role in the fuel mixture formation in the cylinder of spark-ignition engine.