Catalyst and Exhaust Systems
OverviewGeneric Catalyst And Exhaust System:
The Catalytic Converter and Exhaust Systems (Figure 133) 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, NOx, 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. (Refer to OBD II Monitors to Catalyst Efficiency Monitor-Federal Test Procedure for specific information.)
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 ceramic 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.
Light Off Catalyst
As the catalyst heats up, converter efficiency rises rapidly. The point at which conversion efficiency exceeds 50% is called catalyst light off. For most catalysts this point occurs at 475 to 575°F (246 to 301°C). A fast light catalyst is a Three Way Catalyst (TWC) that is located as close to the exhaust manifold as possible. Because the light off catalyst is located close too the exhaust manifold it will light off faster and reduce emissions quicker than the catalyst located under the body. Once the catalyst lights off, the catalyst will quickly reach the maximum conversion efficiency for that catalyst.
TWC Conversion Efficiency
TWC Conversion Efficiency Chart:
A TWC requires a stoichiometric fuel ratio, 14.7 pounds of air to 1 pound of fuel (14.7:1), for high conversion efficiency. In order to achieve these high efficiencies, the air/fuel ratio must be tightly controlled with a narrow window of stoichiometry. Devations outside of this window will greatly decrease the conversion efficiency (Figure 134). For example a rich mixture will decrease the HC and CO conversion efficiency while a lean mixture will decreases the NOx conversion efficiency.
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 (NOx) in the exhaust emissions to an acceptable level. The reduced exhaust emissions are directed from the catalytic converter through another HO2S mounted in the rear exhaust pipe and than on in the muffler. Lastly, the exhaust emissions are directed to the atmosphere through an exhaust tailpipe.
Hardware
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 NOx. 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. (Refer to the PCM Inputs for a description of how the HO2S operates.)
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. (Refer to the OBD II Monitors for specific information on these monitors.)
Muffler
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.