Spill factors of 3.2, 0.87, and 0.87 g/gal were used for Scenarios 1, 2, and 3, respectively. The spill factors reflect ARB's new rules for vehicle refueling spillage.
The API study does not indicate an increase in spillage from self-service fueling and topping off. Spilled fuel evaporates completely and is counted as fugitive emission.
Spillage rates for other liquid fuels were based on the premise that the average spill volume remains constant. Several fuel stations were surveyed and an average gasoline fill volume of 8.8 gallons was observed. This is consistent with the 9.2 gal observed by Morgester. An increase in fuel tank capacity for alternative-fueled vehicles is expected, but not enough to completely make up for the reduction in energy content. Table 4-27 shows the estimated average fill volume for the fuels in this study. LPG is also shown for comparison. Since the fuels have different densities, the spill mass was adjusted for each fuel.
Table 4-27: Vehicle Fuel Spillage Parameters
Average Tank Fill (gal)
Vapor space NMOG mass
Vapor emissions in this study are determined from modeled vapor concentrations. The fuel temperature used to determine vapor concentrations was selected to be consistent with ARB's inventory for fueling station emissions.
The vapor concentration in the tank vapor space is the basis for fuel transfer emission calculations in AP-42 and provides insight into the temperature conditions for vapor emissions. Vapor space concentrations are modeled to from equilibrium vapor concentration. The extent of vapor saturation is reflected by the saturation factor. For vapor recovery systems a saturation factor of 1.0 or completely saturated vapor is assumed in AP-42. ARB (Asregadoo 1992) bases the vapor space concentration on test data. The vapor space gas concentration represents the uncontrolled emissions from tank truck unloading (underground tank working losses), and vehicle tank working losses.
Vapor space concentrations from liquid fuels were estimated from the ideal gas law. Given a molar volume of 379.6 ft3/lb mole at 60F, the equilibrium vapor (Ve) in a tank head space can be calculated from the following equation: