The Science of Tailpipe Emissions

An internal combustion engine burns a mixture of gasoline and oxygen to create power. In a perfect, impossible world, this combustion process would produce only three harmless things: Water (H2O), Carbon Dioxide (CO2), and Nitrogen gas. However, because engines are imperfect machines dealing with varying temperatures and fuel qualities, they inevitably produce toxic byproducts.

During a state emissions test, a metal sniffer probe is placed into the tailpipe while the engine is running on a dynamometer (treadmill) to measure the exact parts-per-million (ppm) of these three specific toxic gasses.

1. Hydrocarbons (HC) - Unburned Fuel

Gasoline is a hydrocarbon. If a droplet of gasoline enters the engine cylinder but fails to ignite, it gets pushed out of the exhaust valve as raw, unburned fuel. This is what you smell when you start an old classic car in a garage. High HC readings on an emissions test mean your engine has a Misfire. This is almost always an electrical problem, such as a dead spark plug, a failing ignition coil, or a severed spark plug wire preventing the fuel from detonating.

2. Carbon Monoxide (CO) - Choked Combustion

Carbon Monoxide is a deadly, odorless gas created when the engine burns gasoline, but doesn't have enough oxygen to complete the burn cleanly. In automotive terms, this means the engine is running "Rich" (too much fuel, not enough air). This is commonly caused by a severely clogged engine air filter, a stuck-open fuel injector dumping excess gas, or a failed Oxygen (O2) sensor feeding incorrect data to the engine computer.

3. Nitrous Oxides (NOx) - Extreme Heat

The air we breathe is mostly Nitrogen. Normally, Nitrogen just passes through the engine harmlessly. However, if the combustion chamber gets too hot (above 2,500°F), the extreme heat physically forces the Nitrogen to bond with Oxygen, creating Nitrous Oxides (NOx). NOx is the primary chemical responsible for thick brown city smog. High NOx readings mean the engine is running dangerously hot, usually due to a broken EGR (Exhaust Gas Recirculation) valve or a severely leaking head gasket.