Car exhaust is the cloud of gases, usually barely visible, that puffs out of the tailpipes of our cars. It is the product of burning gasoline, and it has several components.
A car's exhaust starts it life in the engine, specifically in the pistons. During each piston's power stroke, a mixture of fuel and air is rapidly burned. This produces a combination of heat, which is dissipated by the car's radiator, mechanical energy, which is transferred to the wheels and some chemical waste products, which are flushed out as exhaust.
Pure gasoline is a combination of hydrocarbon chemicals, which are compounds made up entirely of the elements carbon and hydrogen. When pure gasoline is burned under ideal conditions, the carbon and hydrogen combine with oxygen from the air. This reaction releases heat and converts the hydrocarbons into carbon dioxide gas and ordinary water.
An engine does not really burn gasoline under ideal conditions. Each piston performs a power stroke hundreds of times per minute, so there is very little time in each ignition cycle for the fuel to burn. Atmospheric nitrogen is burned with the fuel-air mixture, and gasoline usually contains impurities such as sulfur. This results in a more complex chemistry for the car exhaust. Car exhaust usually contains carbon dioxide, water, carbon monoxide, and various nitrogen or sulfur oxides. The first two components are harmless. The last two can cause serious problems.
The pollutants in car exhaust, carbon monoxide and nitrogen and sulfur oxides, impact us in different ways. Carbon monoxide gas is what makes car exhaust so dangerous. Even if it's inhaled in moderate doses it can cause death. Nitrogen and sulfur oxides combine with other gasses in the atmosphere to form acid rain.
To ameliorate the damage that exhaust pollutants can cause, modern cars are equipped with a catalytic converter. This device "scrubs" the exhaust by forcing the gasses into a secondary series of chemical reactions before they are released into the atmosphere. The carbon monoxide is forced to complete its reaction with the air, to become carbon dioxide. Nitrogen and sulfur oxides are converted into less damaging compounds, and any unburned hyrdocarbons are converted into carbon dioxide and water. It's not a perfect system, but it works reasonably well.
