How Catalytic Converters Work

To help reduce the emissions further, they have developed an interesting device called a catalytic converter, which treats the exhaust before it leaves the car and removes a lot of the pollution.

Pollutants Produced by a Car Engine

In order to reduce emissions, modern car engines carefully control the amount of fuel they burn. They try to keep the air-to-fuel ratio very close to the stoichiometric point, which is the calculated ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For gasoline, the stoichiometric ratio is about 14.7:1, meaning that for each pound of gasoline, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving.

The main emissions of a car engine are:

• Nitrogen gas (N2) - Air is 78-percent nitrogen gas, and most of this passes right through the car engine.

• Carbon dioxide (CO2) - This is one product of combustion. The carbon in the fuel bonds with the oxygen in the air.

• Water vapor (H2O) - This is another product of combustion. The hydrogen in the fuel bonds with the oxygen in the air.

The combustion process is never perfect, some smaller amounts of more harmful emissions are also produced in car engines:

• Carbon monoxide (CO) - a poisonous gas that is colorless and odorless

• Hydrocarbons or volatile organic compounds (VOCs) - produced mostly from unburned fuel that evaporates

Sunlight breaks these down to form oxidants, which react with oxides of nitrogen to cause ground level ozone (O3), a major component of smog.

• Nitrogen oxides (NO and NO2, together called NOx) - contributes to smog and acid rain, and also causes irritation to human mucus membranes

These are the three main regulated emissions, and also the ones that catalytic converters are designed to reduce.

How Catalytic Converters Reduce Pollution

Most modern cars are equipped with three-way catalytic converters. "Three-way" refers to the three regulated emissions it helps to reduce -- carbon monoxide, VOCs and NOx molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required (they are very expensive).

A three-way catalytic converter: Note the two separate catalysts.

There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads. Most cars today use a honeycomb structure.

Ceramic honeycomb catalyst structure