Catalytic converter (page 2 of 2)

The catalytic converter’s housings are usually fabricated from stainless steel and are fully welded at all joints. Each catalytic converter contains at least one (sometimes more) element comprising of an extruded ceramic substrate which is formed into a honeycomb of small cells (typical density of 62 cells / cm²). The ceramic element is coated with a special surface treatment called ’washcoat’ which increases the surface area of the catalyst element by approximately 7000 times. A coating is applied to the washcoat which contains the precious elements Platinum, Palladium and Rhodium in the following relative concentrations: 1 Pt : 21.6 PD : 1 Rh

Catalytic converters used on low emission vehicles (LEVs) have active constituents of palladium and rhodium only, in the ratio 14PD: 1Rh.

The metallic coating of platinum and palladium oxidize the carbon monoxide and hydrocarbons and convert them into water (H₂O) and carbon dioxide (CO₂). The coating of rhodium removes the oxygen from nitrogen oxide (NO_x) and converts it into nitrogen (N₂).

CAUTION: Catalytic converters contain ceramic material, which is very fragile. Avoid heavy impacts on the converter casing.

WARNING: To prevent personal injury from a hot exhaust system, do not attempt to disconnect any components until the exhaust system has cooled down.

CAUTION: Serious damage to the catalytic converter may occur if a lower octane number fuel is used , and definately will occur if leaded fuel is used. Fuel tank filler necks are designed to accommodate only unleaded fuel pump nozzles.

CAUTION: Ensure the exhaust system is free from leaks. Exhaust gas leaks upstream of the catalytic converter could cause internal damage to the catalytic converter.

The function and efficiency of a catalytic converter is assessed by an OBD system by measuring the oxygen content of the exhaust gases before and after the catalytic converter. The relevant oxygen content is calculated from the signals of the pre-catalytic converter HO₂ sensor and the post-catalytic converter HO₂ sensor. A properly functioning catalytic converter consumes the oxygen contained in the exhaust gases that are introduced in order to convert the pollutants. The gases flowing into the catalytic converter are converted from CO, HC and NO_x into CO₂, H₂O and N₂. The catalytic converter is monitored once per trip.

In order to determine whether the catalytic converter is functioning correctly, the signal from the post-catalytic converter HO₂ sensor is observed over a period in which the pre-catalytic converter HO₂ sensor is operating (i.e. the pre-catalytic converter HO₂ sensor is oscillating). During this measurement period, the post-catalytic converter HO₂ sensor must remain relatively constant as the catalytic converter keeps the oxygen content of the emerging gases constant.

In the normal control range, the fluctuating air:fuel ratio in the exhaust gas before the catalyst results in an oscillating pre-catalytic converter HO₂ sensor signal. Exhaust-gas conversion / oxygen storage in the functioning catalytic converter results in a relatively constant signal in the post-catalytic converter HO₂ sensor. Depending on how the vehicle is being operated at the time of measurement and which catalytic-converter type or which coating is used, the signal is to be found in the “lean” or “rich” voltage range.

Note: New petrol-engined cars fitted with catalytic converters produce less than 10% of the toxic gases of pre-1993 cars.