Catalytic Converter - RA4900HFM0005X(49-0005)

The 3-way catalytic converters are designed as underfloor catalytic converters and are located in the exhaust system upstream of the center and rear mufflers.

Fig 1: Identifying 3-Way Catalytic Converters

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The purpose of the catalytic converter is to convert the three pollutants of carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NO_x ) contained in the exhaust of gasoline engines, into the harmless compounds of water (H₂ O), carbon dioxide (CO₂ ) and nitrogen (N₂ ).

The catalytic converter contains a catalyst, a word coming from the Greek and which designates the element essential for catalyst which triggers chemical reactions without itself being consumed. These catalysts in the 3-way catalytic converter are the rare metals platinum (Pt) and rhodium (Rh).

Fig 2: Identifying Catalytic Converter Is Convert Pollutants

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The catalytic converter consists essentially of three main elements.

• Substrate (monolith) made of high-strength ceramic flexibly mounted on a wire mesh
  Fig 3: Identifying Ceramic Monolith And Wire Mesh

  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
• Intermediate layer (washcoat) for enlarging the surface. As a result of the pore structure, the surface on the monolith is enlarged approximately 7000 times over.
• Catalytically active coating of platinum (Pt) and rhodium (Rh).

  Platinum promotes the oxidation of hydrocarbons and carbon monoxide, rhodium the reduction of the oxides of nitrogen.

  The share rare metals contained in a catalytic converter is about 2 to 3 grams.

  Fig 4: Identifying Intermediate Layer For Washcoat

  Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Function 

Fig 5: Identifying Exhaust Gases Flow Function And Chemical Reaction

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The exhaust gases flow through the catalytic converter and, in so doing, come into contact with the rare metals (Pt and Rh). The following chemical reactions are produced:

• Oxidation, which converts carbon monoxide (CO) and hydrocarbons (HC) into water (H₂ O).
• Reduction, which converts oxides of nitrogen (NO_x ) into nitrogen (N₂ ).

The presence of oxygen is required for the oxidation for CO and HC while an oxygen deficiency is necessary for the reduction of the oxides of nitrogen.

This fluctuation between an oxygen-poor and oxygen-rich exhaust gas is achieved by altering the fuel-air mixture within narrow limits around λ = 1.

Chemical reactions:

2 CO +O₂ → 2 CO₂

2 C₂ H₆ +7O₂ → 4 CO₂ +6H₂ O

2 NO +2 CO → N₂ +2 CO₂

The decisive elements for a high conversion rate of the pollutants are the temperature and the residual oxygen content in the exhaust gas.

The ratio of the fuel-air mixture is designated with lambda (λ), in which the following mean:

λ < 1 rich mixture

λ > 1 lean mixture

λ = 1 full combustion

(stochiometric air-fuel ratio, 14.7 mass parts of air to 1 mass part of fuel)

This fluctuation of the oxygen portion in the exhaust gas is achieved by the oxygen sensor and the lambda closed-loop control.

The three exhaust pollutants stated can be chemically converted in the catalytic converter only by fluctuating the residual oxygen content in this manner.

Voltage signals of oxygen sensor

Fig 6: Voltage Signals Of Oxygen Sensor

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The catalytic process, ie. the chemical reaction, in the catalytic converter begins from a temperature of 250°C. Temperatures which are too high result in excessive thermal stresses.

Conversion rate of pollutants in the catalytic converter

Fig 7: Control Range For Catalyst Window

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Vehicles fitted with a catalytic converter and oxygen sensor should be operated only with unleaded fuel. Lead additives (Pb) in the fuel form deposits on the chemically active surface of the catalyst and of the oxygen sensor, rendering the system ineffective.

Fig 8: Identifying Catalyst And Oxygen Sensor Chemically Reactive Surface

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.