A Quasi-Steady Diffusion-Based Model for Design and Analysis of Fuel Tank Evaporative Emissions

In this paper, a fuel tank evaporation/condensation model was developed, which was suitable for calculation of evaporative emissions in a fuel tank. The model uses a diffusion-controlled mass transfer approach in the form of Fick's second law in order to calculate the average concentration of fuel vapor above the liquid level and its corresponding evaporation rate. The partial differential equation of transient species diffusion was solved using a separation of variables technique with the appropriate boundary conditions for a fuel tank. In order to simplify the solution, a quasi-steady assumption was utilized and justified. The fuel vapor pressure was modeled based on an American Petroleum Institute (API) procedure using either a distillation curve or a Reid Vapor Pressure (RVP) as an experimental input for the specific fuel used in the system. The advantage of this model compared to other published models is the fact that it is a non-equilibrium model that considers the effects of mass transfer between phases. The model was validated with transient data for both short (drive cycle) and long (diurnal cycle) durations. In addition, evaporation and condensation cycling simulations were performed.