Catalytic Converter: Description and Operation

CATALYST AND EXHAUST SYSTEM

Overview

HO2S Location:

The catalytic converter and exhaust systems 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, front heated oxygen sensor (HO2S), rear exhaust pipe, rear 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) system strategy in the powertrain control module (PCM). For specific OBD catalyst monitor information, refer to the Catalyst Efficiency Monitor. Catalyst Efficiency Monitor

The number of HO2S used in the exhaust stream and the location of these sensors depend on the vehicle emission certification level (LEV, ULEV, PZEV). The hybrid vehicle is a partial zero emission vehicle (PZEV) equipped with 2 separate HO2Ss that provide input to the PCM. The first sensor in the exhaust stream before the catalyst (HO2S11) is used for primary fuel control and the last sensor after the catalyst (HO2S12) is used to monitor the light-off catalyst.

Catalytic Converter
The catalytic converter contains a catalyst in the form of a specially treated ceramic honeycomb structure saturated with catalytically active precious metals. The purpose of the catalytic converter is to assist in controlling the concentration of exhaust gas products released into the atmosphere. The catalyst material remains unchanged when it initiates and speeds up the heat producing chemical reactions of the exhaust gases. As exhaust gases come in contact with the catalyst they are changed into mostly harmless products. Also, the catalyst enables the chemical reaction of the exhaust gases to occur at a lower temperature.

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 246°C to 301°C (475°F to 575°F). The light-off catalyst is located close to the exhaust manifold and lights off faster and reduces emissions quicker than the catalyst located under the body. Once the catalyst lights off, it quickly reaches the maximum conversion efficiency for that catalyst.

Exhaust System

HO2S Configuration:

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 across another H02S mounted in the rear exhaust pipe and then into the muffler. Finally, the exhaust emissions are directed to the atmosphere through an exhaust tailpipe.

The hybrid vehicle is a PZEV and has 2 separate HO2S in the exhaust stream. The first sensor is near the exhaust manifold (stream 1), and the second sensor (stream 2) is mounted after the light-off catalyst.

Underbody Catalyst
The underbody catalyst is located after the light-off catalyst. The underbody catalyst is in-line with the light-off catalyst. For an exact configuration of the catalyst and exhaust system, refer to Exhaust System.

Exhaust Manifold/Runners
The exhaust manifold runners collect exhaust gases from the engine cylinders.

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

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

Muffler
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. Mufflers are usually treated with an anti-corrosive coating agent during manufacturing to increase the life of the product.