Modeling of Fuel Permeation in Multilayer Automotive Plastic Fuel Tanks

A numerical simulation model for predicting the fuel hydrocarbon permeation as well as the barrier layer thickness optimization for multilayer plastic fuel tanks is presented. The diffusion model is based on Fick's laws of diffusion for a steady/unsteady state permeation regime through a multilayer polymeric wall under isothermal condition. A continuum approach based on an homogenization technique is used to model solvent diffusion through an n layer film. The hydrocarbon flux determination through the multilayer film is solved using homogenization techniques that ensure continuity of partial pressure at the polymer-polymer inter-diffusion interface. Since the pinch-off zone is known to be the major source of emission per unit area, a method has been developed to automatically detect it at the end of the extrusion blow molding process and the diffusion model is adapted to adequately evaluate the hydrocarbon permeation through this specific area. Finally a gradient-based algorithm has been implemented to get the optimal barrier layer thickness which satisfies the total hydrocarbon fuel emission constraint. The numerical validation in terms of hydrocarbon flux calculation under steady and non steady state is performed on academic case studies by comparing numerical predictions to the analytical ones. The illustration of the methodology and the gain in terms of weight and fuel permeation, during optimization iterations, will be presented for a plastic fuel tank (PFT) case study.