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Purification of Exhaust Gases. Removal of pollutants from exhaust gas after they leave the engine cylinder can be done either by using a thermal reactor or by using a catalytic converter

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Purification of Exhaust Gases

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Purification of exhaust gases

Purification of Exhaust Gases

Removal of pollutants from exhaust

gas after they leave the engine cylinder can be done either by using a thermal reactor or by using a catalytic converter

In order to oxidize HC in the gas phase without catalyst a residence time of the order of 50ms and temp of about 6000 C is required.

Whereas to oxidize CO temp in excess of 7000C is required.

Thermal reactors

Thermal Reactors

  • HC and CO can be reduced by using thermal reactors.

  • Reactors are designed to reduce heat loss of the gas in the exhaust manifold and increase residence time of the gas in the exhaust manifold.

  • Typically a thin steel liner acts as reactor core. Heat losses are minimized by insolating the reactor core.

  • The reactor core is kept in a CI casing.

Disadvantages of thermal reactors

Disadvantages of Thermal Reactors

  • To increase the residence time of the burned gases reactor vol has to be large.

  • To achieve the required temp of oxidation some time is required.

  • Air injection requirement makes the system complicated.

  • NOx reduction is not possible.

  • Reactor material has to withstand high very temp

Catalytic converter system

Catalytic Converter System

Four basic designs have been developed

  • Oxidation Catalytic Converter

  • Dual bed Catalytic Converter

  • Three way Catalytic Converter and

  • Denox (lean burn) Catalytic Converter

Oxidation catalytic converters

Oxidation Catalytic Converters

  • Oxidation catalytic converters are the simplest form of CCs.

  • It can oxidize CO and HC to form CO2 and H2O.

  • The air required for oxidation process can be supplied either by using lean mixture or supplying secondary air injection.

  • Normally used in diesel engines.

  • They cannot remove NOx. Also they are not capable of removing soluble PMs.

Dual bed catalytic converters

Dual Bed Catalytic Converters

  • A reduction catalyst is fitted to minimize NOx and then an oxidation catalyst is used to oxidize CO and HC.

  • Engine must be operated at rich mixture.

  • Therefore the system has certain drawback.

Denox catalytic converters

Denox Catalytic Converters

  • Denox catalytic converters allow not only CO and HC reduction but also NOx reduction at lean mixture. They are currently at development stage.

Three way catalytic converters

Three way catalytic converters

  • Most of the SI engines use 3 way CC since they allow HC, CO & NOx to be converted simultaneously.

  • The main components of a 3 way CC are

  • Substrate (monolith)

  • Washcoat

  • Catalyst (noble metal)

  • Support and housing

Three way catalytic converters1

Three way catalytic converters

Three way catalytic converters2

Three way catalytic converters

Three way catalytic converters contd substrate

Three way catalytic converters --Contd. (Substrate)

  • Substrate is a ceramic honeycomb structure held in a metal can or housing.

  • Noble metal (catalyst) is impregnated into a highly porous washcoat about 20 µm thickness that is applied to the passage way walls.

  • A typical monolith has a square section passage way of inside dimensions of about 1 mm separated by thin porous wall of thickness (0.15 to 0.3mm). Number of cells per sq. cm. varies between 30 to 60.

  • Ceramic mat must be highly thermal resistant.

  • It should have mechanical strength.

Metal substrate

Metal Substrate

  • Now a days metallic substrates are also available. Their advantages are:

  • Increased conversion efficiency

  • Longer life

  • Lower wall thickness (0.04 to 0.06 mm)

  • More No. of cells can be use per unit area.


    Washcoat is applied to substrate material. It has a surface area of 100-200 m2/g.



  • Catalysts used in a 3 way CC are Pt, Pd & Rh.

  • A very small amount of Pt, Pd & Rh is impregnated into highly porous alumina washcoat.

  • For oxidation of CO and HC a mixture of Pt and Pd is most commonly used.

  • For oxidation of CO, olefins and CH4 specific activity of Pd is higher than that of Pt.

  • For oxidation of aromatic compounds Pt and Pd have similar activity.

  • For oxidation of paraffinic HCs (<C3) Pt is more active than Pd.

Catalysts contd

Catalysts Contd.

  • Noble metals sinter rapidly at 5000 C to 9000 C.

  • Noble metals are dispersed as finely as possible in alumina which prevents particle to particle metal contact and supress sintering.

  • Particle size of noble metal is 50 nm. This can increased to about 100 nm when exposed to high temperature.

Catalysts contd1


  • Pt/Pd = 2 is typical in a three way catalytic converter.

  • Concentration of noble metal is about 1-2 g/dm3

Conversion efficiency of a cc

Conversion efficiency of a CC

Conversion efficiency =

(min – mout)/min = 1 – mout/min

Conversion efficiency of a new catalytic converter is 98 – 99 % for CO and 95% for HC

Effect of a f on conv of cc

Effect of A/F on ηconv of CC

Light off temperature

Light off temperature

Light off temperature of a catalytic converter is the temperature at which the catalytic converter become 50% effective.

Typical light off temperature is 250 -3000 C

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