Automotive refueling is gaining greater importance because fuel vapors released during refueling are believed to increase ozone levels in urban areas. As a step towards On-Board Refueling Vapor Recovery (ORVR) designs, vapor generation and transport during refueling needs to be understood to develop recovery techniques. The objective of the present study is to understand the fluid flow inside the automotive gas tank filler pipe using commercially available Computational Fluid Dynamics (CFD) software. This effort is expected to yield detailed flow field information, including air entrainment. The phenomena of well-back, the process of fuel flooding the filler pipe and flowing backwards at the filler pipe mouth, and the pressure transients inside the tank leading to premature nozzle shut-off were examined. The current work includes unsteady CFD simulation with gasoline and air as the working fluids. The fuel tank was modeled with a vent pipe, and due to its small diameter, pressure build-up inside the tank was observed. To simulate multiphase flow, Volume of Fluid (VOF) model was used. Three flow rates were considered: 15, 30,45 and 60 liters/min (lpm). Turbulence modeling was done using RNG k-ε model. The results show that air entrainment depends on gasoline flow rate. Design of the vent pipe and the tank pressure were found to be important variables that affected air entrained during the refueling process. The simulation successfully showed well-back when the tank was completely filled with gasoline. The simulation results show good agreement with previous experimental work, demonstrating that CFD can be used as a design tool to shorten the design, development and cost reduction cycle of nozzle, filler pipe and tank system. Copyright © 2001 Society of Automotive Engineers, Inc.