To prepare for the new and changing regulations to drive cycle and fuel properties, optimize current systems, and evaluate future configurations and calibrations, engineers at Ford Motor Co. and Visteon have developed a fuel vapor system model (FVSMOD). The intent was to investigate the broad interactions between heat transfer, vapor generation, and carbon canister vapor storage and purge. The model includes the gas tank, fuel pump, supply and return lines, fuel rail, vapor valve, vent lines, purge valve and line, and carbon canister (Figure 1).

Earlier models of fuel evaporation losses (developed by Wade and Koehl) focused on the experimental conditions of the evaporative emissions tests for the vehicles of their timeÑi.e. carbureted vehicles with nonrecirculating fuel systems. For these types of vehicles, the fuel in the tank rarely reached the boiling point, while the fuel in the carburetor float bowl almost always reached boiling during the soak period. Models of evaporative losses from the tank assumed equilibrium vapor and temperature in the vapor space as determined by the instantaneous temperature of the fuel and a suitable vapor pressure equation. Heating was used to increase the concentration of vapor in the tank and force it out by expansion. Carburetor float bowl losses were calculated based on empirical single-plate-distillation data with temperatures at the measured maximum. The results from the experiment were in agreement although the computation was somewhat complex due, in part, to the method used for estimating the fuel's vapor pressure.

Since the 1995 and 1996 model years, new enhanced evaporative emissions standards have been in place that address the generally higher fuel temperatures experienced by current fuel-injected vehicles. These standards address higher temperatures by extending the range of operating conditions to include running-loss testing at 35&#deg;C (95&#deg;F), and extended realtime diurnal testing over a wider temperature range than previously considered (22&#deg;-36&#deg;C or 72&#deg;-97&#deg;F). During the high-temperature running-loss portion of the test, vehicle tank temperatures may reach the boiling regime.

Figure 1. Schematic of the fuel vapor system model developed by Ford Motor Company and Visteon. Dashed lines indicate control volumes for the tank/vapor and supply/rail subsystems.

The FVSMOD is a simple, thermodynamically consistent representation of the complete fuel/vapor system, and it allows a thorough exploration of the key design and environmental parameters governing the performance of modern evaporative emission control systems. The model's approach is an extension of Wade's work, which assumes the fuel/vapor system to be in vapor-liquid equilibrium and exist at some average temperature and pressure. The driving force for vapor generation is heat transfer to the tank from the underbody and hot return fuel, in which the rail system is modeled as quasi-steady flow with specified heat transfer parameters and ambient temperatures along the fuel path. As a result of the heat transfer to the tank, fuel and vapor temperature increase. This causes expansion of the vapor and air in the vapor space along with an increase in partial pressure of the fuel vapor. The increased total pressure then results in flow from the tank.

Weathering of the fuel (degree of evaporation) and the resulting loss of vapor pressure can be calculated by performing a mass balance on the fuel vapors and by using the vapor pressure equation. This allows the model to simulate successfully the transition from evaporative vapor generation, in which the vaporization rate is dependent only on vapor pressure changes and expansion, to the boiling regime. In this regime, vapor pressure is governed by the vent system characteristics and evaporation depends primarily on heat input and fuel distillation characteristics.

The vent system was modeled as a passive network. Flows are assumed to be incompressible, quasi steady, and are empirically known functions of pressure drop, local density, and viscosity, which are, in turn, known functions of local composition, temperature, and pressure. The carbon canister acts as a sink source of vapor at the junction of the purge and vent lines. It is modeled as a one-dimensional packed bed with equilibrium absorption determined throughout the bed by convective transport together with a one-dimensional treatment of thermal transport with axial conduction and convection combined with lateral heat losses. A mass balance of vapor and air throughout the vent system and canister allows a final calculation of vapor escaping to the atmosphere from the canister vent side.

• Model validation
• Applications