what comes out of a car exhaust

[Elle]

What comes out of a car's exhaust is a combination of gases and particles, primarily the result of burning gasoline or diesel fuel in the engine. The main components are nitrogen (N2), carbon dioxide (CO2), and water vapor (H2O), which are generally harmless. However, the exhaust also contains harmful pollutants due to the imperfect combustion process. These include carbon monoxide (CO), a poisonous gas; unburned hydrocarbons (HC); nitrogen oxides (NOx), which contribute to smog and acid rain; and particulate matter (PM), which are tiny soot particles.

Modern vehicles are equipped with sophisticated emission control systems to drastically reduce these harmful emissions. The catalytic converter is the key component, using chemical reactions to convert CO, HC, and NOx into less harmful CO2, nitrogen, and water. A diesel particulate filter (DPF) in diesel engines traps soot particles. The specific composition of exhaust gas depends heavily on the engine's condition, fuel type, and driving style. A well-tuned engine with a functioning exhaust system will produce mostly clean byproducts, while a faulty engine can emit visible black smoke (excess soot/fuel) or blueish-white smoke (burning oil).

The following table illustrates the typical composition of exhaust gases from a modern gasoline engine with a functioning catalytic converter.

Component	Chemical Formula	Typical Concentration (by volume)	Primary Source & Impact
Nitrogen	N₂	~70%	From ingested air; inert and harmless.
Carbon Dioxide	CO₂	~10-12%	Product of complete fuel combustion; a greenhouse gas.
Water Vapor	H₂O	~10-12%	Product of combustion; harmless.
Oxygen	O₂	~0.5-2%	Excess oxygen from air; harmless.
Carbon Monoxide	CO	< 0.5%	Product of incomplete combustion; toxic.
Nitrogen Oxides	NOx	< 0.25%	Formed under high heat in the cylinder; contributes to smog.
Unburned Hydrocarbons	HC	< 0.05%	Unburned fuel; contributes to smog.
Particulate Matter	PM	Minimal	Tiny soot particles; regulated by emissions standards.

[ClaytonMarie]

Basically, it's what's left after your engine burns gas. You get a lot of harmless stuff like water vapor and nitrogen from the air. But you also get the bad leftovers: carbon monoxide, which is dangerous to breathe, and other chemicals that cause smog. That's why cars have catalytic converters—to clean up most of the nasty stuff before it leaves the tailpipe. If you see thick smoke, that's a sign something's wrong with the engine.

[AdrielMarie]

When I think about exhaust, I think about that smell from older cars. It's not just a smell; it's a mix of chemicals. My dad, a mechanic, always said a healthy car's exhaust should be almost invisible. If you see black smoke, it's burning too much fuel. Blue smoke means it's burning oil, which is a big problem. The white smoke on a cold morning is usually just water vapor, which is normal.