Catalytic Converter: Description and Operation
RA4900KE30015X 0015 - Function Description: Catalytic Converter KE-Injection
49-0015 Catalytic converter
The 3-way catalytic converters are located in the exhaust system upstream of the mufflers.
The following versions are fitted:
^ Pre-catalytic converter in front exhaust pipes (depending on engine)
^ Underfloor catalytic converter at frame floor
Pre-catalytic converter
Underfloor catalytic converter
The task of the three-way catalytic converter is to convert the three pollutants contained in the exhaust gas of gasoline engines of carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) into the harmless compounds of water (H2O), carbon dioxide (CO2) and nitrogen (N2).
The term catalyst comes from the Greek and designates the principal element for the catalysis which initiates chemical reactions without itself being consumed. In the 3-way catalytic converter these are the rare metals platinum (Pt) and rhodium (Rh).
The catalytic converters consist essentially of three principal parts:
^ Carrier body, also known as the monolith, either made of high-strength ceramic mounted flexibly on a wire mesh (underfloor catalytic converter) or of stainless steel resistant to high temperatures (pre-catalytic converter).
1 Underfloor catalytic converter
2 Wire mesh
3 Ceramic monolith
^ Intermediate layer (washcoat) for enlarging the surface. As a result of the pore structure the surface on the monolith is enlarged approximately 7000 times.
^ Catalytically effective layer of platinum (Pt) and rhodium (Rh).
Platinum accelerates the oxidation of hydrocarbons and carbon monoxide while rhodium accelerates the reduction of the oxides of nitrogen.
The content of rare metals contained in a catalytic converter is approximately 2 - 3 grammes.
Pre-catalytic converter with metal monolith
A Intermediate layer (washcoat) with platinum (Pt) and rhodium (Rh)
B Ceramic monolith
Function
The exhaust gases flow through the catalytic converter where they come into contact with the rare metals (Pt, Rh).
The following chemical reaction occurs:
As a result of oxidation, carbon monoxide (CO) is converted into carbon dioxide (CO2) and hydrocarbons (HC) into water (H2O), and by reduction, the oxides of nitrogen (NOx) are converted into nitrogen (N2). The decisive factors for a high conversion rate of the pollutants are the temperature and the exhaust residual content in the gas.
Oxygen is required for the oxidation of CO and HC. The reduction of the oxides of nitrogen takes place in the absence of oxygen.
Chemical reactions:
2 CO+O2 -> 2 CO2
2 C2H6+7 O2 -> 4 CO2+6 H2O
2 NO+2 CO -> N2+2 CO2
This fluctuation of the oxygen portion with exhaust gas is achieved with the oxygen sensor and the lambda closed-loop control (see appropriate sections). Only as a result of these oxygen fluctuations is it possible to chemically convert the three afore mentioned exhaust elements in the catalytic converter.
Voltage signals of oxygen sensor
A Rich fuel/air mixture
B Lean fuel/air mixture
The catalytic process, i. e. the chemical reaction, in the catalytic converter begins from approx. 250 °C. Excessively high temperatures cause a thermal overload.
Conversion rate of pollutants in catalytic converter
Only unleaded fuel should be used in vehicles fitted with a catalytic converter and oxygen sensor. Lead additives (Pb) in the fuel are deposited on the chemically reacting surface of the catalyst (Pt, Rh) and of the oxygen sensor, rendering the system ineffective.
Catalytic converter system version
