catalytic converter

CATALYTIC CONVERTER

What is catalytic converter?

The principle of the catalytic converter package is to control the emission levels of various pollutants by changing the chemical characteristics of the exhaust gases. In contrast to the thermal reactors, efficient catalytic oxidation catalysts can control HC and CO emissions almost completely at temperature equivalent to normal exhaust gas temperatures.

Thus the loss in fuel economy necessary to increase the exhaust temperature is avoided. Catalyst materials, e.g. platinum or platinum and palladium are applied to a ceramic support which has been treated with an aluminium oxide wash coat. This results in an extremely porous structure providing a large surface area to stimulate the combination of oxygen with HC and CO. The oxidation process converts most of these components to water vapour and carbon dioxide.

The catalysts used for these converters are closely guarded secrets. The catalyst for NOx is the first element in the gas flow path and does not cause any heat release. The HC/CO catalyst is the next. Its heat release is so great that there is a risk of overheating and burning of the element. This requires air injection and hence a secondary air pumps. Experiments with various types of converters have led to the conclusion that the axial flow from is superior to the radial flow type.

The converter should be able to provide the largest possible surface for gas flow and provide a sufficient reaction rate without unduly increasing the back pressure which affects the drivability of the engine. In order to increase the converter life to about 80000kms, a bypass valve, ahead of the converter is used. This is operated by an electric motor controlled by sensors for speed, throttle opening, engine coolant temperature and HC/CO catalytic converter temperature. This cuts off the converters at the present value and release the untreated exhaust into the atmosphere but only under less critical conditions.

The below converter is three way monolith converter. The front bed or inlet is treated with platinum and rhodium and is termed a reducing catalyst. The rear bed is coated with palladium and is referred to as oxidizing catalyst.

Exhaust gases first pass through the reducing catalyst. This causes the levels of NOx to be reduced. Pressurised air from the air injection system is forced into the space between the catalyst beds. The extra air supplies additional oxygen and causes greater oxidation of the gases.

As the treated exhaust gases from the first bed continue flowing, they eventually pass through the conventional oxidation catalyst made of palladium and platinum. Here hydrocarbons and carbon monoxide emissions are reduced.