This paper explores an infrequently encountered hazard associated with liquid fuel tanks on gasoline-powered equipment using non-vented fuel caps. Depending on the location of fuel reserve tanks, waste heat from the engine or other vehicle systems can warm the fuel during operation. In the event that the fuel cap is not vented and if the fuel is sufficiently heated, the liquid fuel may become superheated and pose a splash hazard if the fuel cap is suddenly removed. Accident reports often describe the ejection of liquid from the fuel tank opening as a geyser. This geyser is a transient, two-phase flow, vertical jet of flashing liquid. This could create a fire hazard as the geyser could result in splashing flammable liquid onto any bystanders.

Many fuel tank systems are vented to ambient through the fuel tank cap and, in addition, may incorporate other features that contribute to pressure relief. Venting of the pressurized vapor inhibits the vapor-liquid mixture in the fuel tank from achieving thermodynamic equilibrium, thus preventing the formation of a superheated liquid. It has been empirically determined that flashing two-phase flow can be prevented by keeping the fuel tank pressure below 1.5 psig. However, if the cap is not vented or vents at a lesser rate than the rate of liquid vaporization, pressure in the tank can rise and the flammable liquid can become superheated.

This phenomenon is explored here to facilitate a better understanding of how the hazard is created. The nature of the hazard is explained using thermodynamic concepts. The differences in behavior between a closed system and an open system are discussed and then illustrated through experimental results obtained from two sources: experiments with externally heated fuel containers and operation of a gasoline-powered riding lawn mower. The role of the vented fuel cap in preventing the geyser phenomenon is demonstrated.

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