Why does gas escape when opening the car fuel tank cap?
2 Answers
When opening the car fuel tank cap, the escaping gas is gasoline vapor. Due to gasoline's high volatility and constant sloshing during driving, it evaporates faster, creating pressure inside the tank. This phenomenon mostly occurs in summer and is normal. During routine refueling, try not to fill the tank completely to avoid this situation. A faulty carbon canister can also cause gas to escape when opening the fuel tank cap, especially in summer when temperature spikes prevent the carbon canister from promptly balancing internal and external tank pressure.
When the car fuel tank cap is opened and gas comes out, it is mainly due to the pressure buildup inside the tank from fuel vaporization under high temperatures. The fuel tank is a sealed system. When the car is driven or parked under sunlight, the rising temperature causes some liquid fuel to turn into gas, increasing internal pressure. The moment the cap is opened, the pressure difference causes the gas to rush out suddenly, similar to opening a shaken can of soda. This vapor primarily consists of unburned gasoline components, hence the gasoline smell. While this is normal, it can be slightly dangerous, especially if the cap is opened immediately after driving. Modern vehicles are equipped with an Evaporative Emission Control (EVAP) system to recycle these vapors and prevent pollution, but the system may not perfectly handle all pressure fluctuations. To minimize gas release, it's best to park in a shaded area for a few minutes before opening the tank cap when refueling. Remember, this phenomenon is common and nothing to worry about, but always ensure the engine is off and avoid smoking or open flames near the fuel tank to prevent accidents.
