Light My Fire

READER: I read with interest in one of your previous columns about the danger of cutting or welding on a fuel tank that has been used. The amount of fuel vapor still floating around in the tank must be small because I have seen people throw a match in a bucket half full of gas, and there was no explosion but only a harmless flame burning on top of the gas.

STEVE: You are correct. The flame burns without an explosion because the top of the bucket is open (will not contain expanded pressure), and only a small amount of vapor from the top of the liquid gas in the bucket can find oxygen to create a burn. The only thing that allows for such a massive explosion of a tank is that the tank contains the growing pressure after ignition until the tank explodes when the fuel vapor surrounded by oxygen is ignited with the torch. Oxygen is a big player with any burn. The gasoline engine in your truck requires around 10,000 gallons of air to properly burn 1 gallon of gas, so a little bit of gas vapor that can find plenty of oxygen with an igniter will blow. This is exactly what happens in your truck’s gas engine. When the spark plugs spark, the cylinder area contains the explosion, providing the force to blow the piston down, creating power to move the truck.

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— Email Steve Thompson care of askthemechaniccolumn@gmail.com, and be sure to include your contact information and phone number.

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