The Catalytic Converter Syistem

By the mid 1970s vehicle exhaust emission controls were unable to comply with EPA regulated limits, so the catalytic converter (CC) was introduced. Today the catalytic converter is the major component used to reduce vehicle exhaust emissions. In its simplest form the CC is used to chemically alter and convert HCs (hydrocarbons), CO (carbon monoxide), and NOx (ox-ides of nitrogen) into much friendlier gases by employing precious metals such as platinum, palladium, and rhodium within their internal construction. The CC can then convert:

• HCs and oxygen (O2) into CO2 (carbon dioxida) and water vapor (H2O).
• CO and oxygen into CO2.
• NO (nitric oxide) and hydrogen (H2) into N2 (nitrogen) and water vapor (H2O).

For these chemical reactions to occur within the CC, we require a certain exhaust gas temperature (light-off) flowing into and throungh the CC. Typically between 572oF and 662oF (300-350oC) CC light-off occurs where CC efficiency will now exceed 50%.

Figure 1. Location of the Catalytic Converter System

Catalytic Converter Types

The type of CC will vary depending upon the year of the vehicle. The three common types include:

Figure 2. Catalytic Converter

1. COC (conventional oxidation catalyst), used on earlier model vehicles. Technologically however these earlier models could only convert HCs and CO, therefore secondary air injection was used to supply excess oxygen to initiate the needed chemical conversion. Due to the fect that this type of CC could not effectively convert NOx, the air/fuel ratio was set to run slightly rich to reduce NOx emissions.
2. Combination TWC + COC (three-way catalyst and conventional oxidation catalyst), was the second generation of CC used when EPA exhaust emissions regulations became much more stringent. The adoption of the TWC permitted conversion of HC and CO conversions. Secondary air was still injected downstream into the middle of the CC through a pipe to supply the needed excess oxygen required by the COC. This permitted the TWC to operate using a stoichiometric (14.7 : 1) air/fuel ratio.
3. Today's technologically advanced vehicles designed to operate under closed-loop conditions of stoichiometry (14.7 : 1 air/fuel ratio) employ a TWC design that function to convert HCs and CO as well as a reduction function to convert NOx. This is achieved through the addition of "ceria" to store and release oxygen so that oxidation and reduction functions can occur within the TWC.

Taken From The Book : Automotive Electronics and Computer Systems (Robert N. Brady)