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  Catalyst and Exhaust Systems  

Catalyst and Exhaust System


The Catalytic Converter and Exhaust Systems (Figure 135) work together to control the release of harmful engine exhaust emissions into the atmosphere. The engine exhaust gas consists mainly of Nitrogen (N) , Carbon Dioxide (CO2) and Water Vapor (H2O) . However, it also contains Carbon Monoxide (CO) , Oxides Of Nitrogen (NOx) , Hydrogen (H) , and various unburned Hydrocarbons (HCs) . CO, NO(x), and HCs are major air pollutants, and their emission into the atmosphere must be controlled.

The exhaust system generally consists of an exhaust manifold, front exhaust pipe, upstream Heated Oxygen Sensor (HO2S) , rear exhaust pipe, downstream HO2S, a muffler and an exhaust tailpipe. The catalytic converter is installed between the front and rear exhaust pipes. Catalytic converter efficiency is monitored by the On Board Diagnostic (OBD II) system.

Generic Catalyst and Exhaust System

Catalytic Converter
A catalyst is a material that remains unchanged when it initiates and increases the speed of a chemical reaction. A catalyst will also enable a chemical reaction to occur at a lower temperature. The concentration of exhaust gas products released to the atmosphere must be controlled. The catalytic converter assists in this task. It contains a catalyst in the form of a specially treated honeycomb structure saturated with catalytically active precious metals. As the exhaust gases come in contact with the catalyst, they are changed into mostly harmless products. The catalyst initiates and speeds up heat producing chemical reactions of the exhaust gas components so they are used up as much as possible.

Exhaust System
The purpose of the exhaust system is to convey engine emissions from the exhaust manifold to the atmosphere. Engine exhaust emissions are directed from the engine exhaust manifold to the catalytic converter through the front exhaust pipe. An HO2S is mounted on the front exhaust pipe before the catalyst. The catalytic converter reduces the concentration of Carbon Monoxide (CO) , unburned Hydrocarbons (HCs) and Oxides of Nitrogen (NO) in the exhaust emissions to an acceptable level. The reduced exhaust emissions are directed from the catalytic converter to a muffler through the rear exhaust pipe. Another HO2S is mounted on the rear exhaust pipe. Lastly, the exhaust emissions are directed to the atmosphere through an exhaust tailpipe.

The downstream HO2S may be located after the light off catalyst or underbody catalyst. The underbody catalyst may be in-line with the light off catalyst, or the underbody catalyst may be common to two light off catalysts, forming a "Y" pipe configuration. For an exact configuration of the catalyst and exhaust system, refer to Exhaust System.

Three Way Catalytic Converter
The Three Way Catalytic (TWC) converter contains either platinum (Pt) and Rhodium (Rh) or Palladium (Pd) and Rhodium (Rh) . The TWC converter catalyzes the oxidation reactions of unburned HCs and CO and the reduction reaction of NO. The three-way conversion can be best accomplished by always operating the engine air fuel/ratio at or close to stoichiometry.

Exhaust Manifold/Runners
The exhaust manifold runners collect exhaust gases from engine cylinders. The number of exhaust manifolds and exhaust manifold runners depends on the engine configuration and number of cylinders.

Exhaust Pipes
Exhaust pipes are usually treated during manufacturing with an anti-corrosive coating agent to increase the life of the product. The pipes serve as guides for the flow of exhaust gases from the engine exhaust manifold through the catalytic converter and the muffler.

Upstream Heated and Downstream Heated Oxygen Sensors
The HO2S provide the Powertrain Control Module (PCM) with voltage and frequency information related to the oxygen content of the exhaust gas.

In addition to providing the PCM with indications of how rich/lean the engine is operating, the upstream HO2S signal serves as an input to the HO2S monitor. The downstream HO2S signal is an input to the Catalyst Efficiency monitor.

Mufflers are usually treated during manufacturing with an anti-corrosive coating agent to increase the life of the product. The muffler reduces the level of noise produced by the engine, and it also reduces the noise produced by exhaust gases as they travel from the catalytic converter to the atmosphere.