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As a car drives, its engine cycles continuously, powered by the repeated explosion of gasoline inside its cylinders. It is with these explosions that nearly all auto emissions occur.

The explosion of gasoline inside a vehicle′s engine is caused by a chemical reaction between gasoline and oxygen. Automotive gasoline is a hydrocarbon in the form of octane which means that each molecule of gasoline consists of 8 carbon atoms flanked on each side by hydrogen atoms. Oxygen gas consists of 2 oxygen atoms.

Because oxygen and gasoline do not react without a catalyst, a spark from the engine′s sparkplugs provides the compounds with enough energy to create a reaction. When this happens, the carbon from gasoline donates its electrons to the oxygen gas producing carbon dioxide, water vapor, and heat.

While heat rapidly expands the air inside the engine to produce motion, carbon dioxide and water vapor are removed from the engine as exhaust. This exhaust circulates through the vehicle′s exhaust system and is eventually released into the atmosphere as emissions.