See Figures 1 and 2
Engine exhaust consists mainly of Nitrogen (N 2 ), however, it also contains Carbon Monoxide (CO), Carbon Dioxide (CO 2 ), Water Vapor (H 2 O), Oxygen (O 2 ), Nitrogen Oxides (NOx) and Hydrogen (H), as well as various unburned Hydrocarbons (HC). Three of these exhaust components, CO, NOx and HC, are major air pollutants, so their emission to the atmosphere has to be controlled.
The catalytic converter, mounted in the engine exhaust stream, plays a major role in the emission control system. The converter works as a gas reactor whose catalytic function is to speed up the heat producing chemical reaction between the exhaust gas components in order to reduce the air pollutants in the engine exhaust. The catalyst material, contained inside the converter, is made of a ceramic substrate that is coated with a high surface area alumina and impregnated with catalytically active, precious metals.
All vehicles use a 3-way catalyst and some also use with a conventional oxidation catalyst. The conventional oxidation catalyst, containing Platinum (Pt) and Palladium (Pd), is effective for catalyzing the oxidation reactions of HC and CO. The 3-way catalyst, containing Platinum (Pt) and Rhodium (Rh) or Palladium (Pd) and Rhodium (Rh), is not only effective for catalyzing the oxidation reactions of HC and CO, but it also catalyzes the reduction of NOx.
The catalytic converter assembly consists of a structured shell containing a monolithic substrate; a ceramic, honeycomb construction. In order to maintain the converter's exhaust oxygen content at a high level to obtain the maximum oxidation for producing the heated chemical reaction, the oxidation catalyst usually requires the use of a secondary air source. This is provided by the pulse air or thermactor air injection systems.
The catalytic converter is protected by several devices that block out the air supply from the air injection system when the engine is laboring under one or more of the following conditions: