How do automobiles produce nitrogen oxides?

Gasoline Contains Hydrocarbons

Most of the chemicals in gasoline are hydrocarbons. Hydrocarbons are compounds composed of mostly carbon and hydrogen atoms:

        `~C_xH_y`

In the formula for a hydrocarbon, "x" and "y" are replaced by numbers equal to the number of carbon and hydrogen atoms in the compound.

Combustion Reactions

Hydrocarbons react with oxygen to form water and carbon dioxide:

        `~C_xH_y` + `~O_2` -> `~H_2O` + `~CO_2`

This type of reaction is called a combustion reaction.

The oxygen for this reaction can be obtained from air. Air is composed of 21% oxygen, 78% nitrogen, and 1% other gases. 

Engine Combustion

In a perfect engine under ideal conditions, the hydrocarbons in gas would react with the oxygen in the air while the large amount of nitrogen in the air would pass through the engine and return to the atmosphere. 

Unfortunately, in real engines, some of the nitrogen from the air reacts with the oxygen to form nitrogen oxides (`~NO_x` ):

        `~N_2` + `~O_2` -> `~NO_x`

98% of the nitrogen oxides produced by engines is in the form of NO. High temperature and pressure make the production of nitrogen oxides more likely. Faulty engine systems can also increase nitrogen oxide emissions.

The combustion reaction in automobiles occurs as follows:

• 
  Intake Stroke: Air and gasoline move to a low pressure area created the downward motion of a piston.

• 
  Compression Stroke: As the piston moves upwards, the pressure increase heats the mixture of air and gasoline.

• 
  Power Stroke: The spark plug ignites the mixture of air and gasoline, triggering the combustion reaction.

• 
  Exhaust Stroke: The exhaust valve opens releasing the combustion products and nitrogen oxides.