What Is the Function of a Gas Cap?

Even more important than the fuel bay door, a car's gas cap keeps gasoline in and the environment out.

When you open the fuel bay door on any vehicle, the gas cap is readily apparent. It is the physical seal that keeps gasoline inside your car's tank and the environment out. The gas cap performs three functions: improving safety, increasing fuel economy and reducing emissions.

Safety

Gasoline is highly volatile, meaning liquid gasoline vaporizes into a gas readily at room temperature. Since gas vaporizes easily, if your tank was not closed by a tight cap, highly flammable gasoline vapor would constantly leak from your tank.

Emissions

Gasoline normally burns cleanly within a vehicle's engine. If gasoline vapor was to escape without being burned, it would damage the atmosphere like many other volatile hydrocarbons. The gas cap prevents leakage of atmosphere-depleting vapors.

Fuel Economy

For every gallon of gasoline vapor that escapes from an open tank, a gallon of usable fuel is wasted. Gas caps make sure the gas you pay for at the pump actually reaches your engine.

References

• "Stant Manufacturing Products": Gas/Fuel Caps

Resources

• "Autoevolution": Emissions: Gasoline vs. Diesel vs. Bioethanol