The exhaust gases emitted into the atmosphere are a combination of burned and unburned fuel. To understand the exhaust emission and its composition, we must review some basic chemistry.
When the air/fuel mixture is introduced into the engine, we are mixing air, composed of nitrogen (78 percent), oxygen (21 percent) and other gases (1 percent) with the fuel, which is 100 percent hydrocarbons (HC), in a semi-controlled ratio. As the combustion process is accomplished, power is produced to move the vehicle while the heat of combustion is transferred to the cooling system. The exhaust gases are then composed of nitrogen, a diatomic gas (N$SUB2), the same as was introduced in the engine, carbon dioxide (CO$SUB2), the same gas that is used in beverage carbonation, and water vapor (H$SUB2O). The nitrogen (N$SUB2), for the most part, passes through the engine unchanged, while the oxygen (O$SUB2) reacts (burns) with the hydrocarbons (HC) and produces the carbon dioxide (CO$SUB2) and the water vapors (H$SUB2O). If this chemical process would be the only process to take place, the exhaust emissions would be harmless. However, during the combustion process, other compounds are formed which are considered dangerous. These pollutants are hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx) oxides of sulfur (SOx) and engine particulates.