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EMISSION SYSTEM IN VEHICLES AND MUFFLERS

 

Contents of Exhaust gas:

  • The largest part of most combustion gas is
    • Nitrogen (N2 ), 
    • Water vapor (H2O) and 
    • Carbon-dioxide (CO2 ) these are not toxic

  • A relatively small part of combustion gas is toxic substances, such as 
    • Carbon Monoxide (CO) from incomplete combustion, 
    • Hydrocarbon (HC) from un-burnt fuel, 
    • Nitrogen Oxide (NOx ) from excessive combustion temperatures 
    • Particulate matter (PM). 

Treatment of Exhaust gas:

  • The Exhaust gas has more pollutant so that it should be reduced to prevent the pollution form the exhaust gas
  • There are some treatment carried out for Exhaust gases are below    

    • Exhaust Gas Recirculation(EGR)
    • Selective Catalytic Reduction(SCR)

Exhaust gas Recirculation(EGR) : 

  • The diesel engines to run more efficiently cause them to run at higher temperature this leads to the creation of Nitrogen Oxides (NOx).

  • The Combination of Nitrogen and Oxygen in the air to form Nitrogen Oxide.
  • In order to reduce NOx an engine should run at a lower temperature this can be achieved by Reducing the amount of Oxygen in the cylinder
  • Exhaust Gas Recirculation is an efficient method to reduce NOx emissions from the engine. 
  • It works by recirculating a quantity of exhaust gas back to the engine cylinders.

Working Principle

  • EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders.
  • This dilutes the Oxygen in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in cylinder temperatures.

  • NOx is produced in high temperature mixtures of atmospheric nitrogen and oxygen that occur in the combustion cylinder, and this usually occurs at cylinder peak pressure

Selective Catalytic Reduction(SCR) :

  • Selective Catalytic Reduction (SCR) is an emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine.
  • The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid (DEF).
  • The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2).
  • Natural components of the air we breathe, which is then expelled through the vehicle tailpipe.
  • SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere.
  • It is called "selective" because it reduces levels of NOx using ammonia as a reductant within a catalyst system.
  • The chemical reaction is known as "reduction" where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water and tiny amounts of CO2.
  • The DEF can be rapidly broken down to produce the oxidizing ammonia in the exhaust stream.

The Schematic diagram of Exhaust system.  

Catalytic Converter 

  • Catalytic converter makes use of two catalysts to convert harmful gases to harmless gases. • 
  • They are: 
    • Reduction Catalyst and 
    • Oxidation Catalyst

  • The reduction catalyst is made of platinum and rhodium
  • The oxidation catalyst is made of platinum and palladium
  • Both the catalysts have a ceramic honeycomb structure. 


  • The exhaust gases are first sent over the reduction catalyst.
  • It converts oxides of nitrogen (NOx ) to nitrogen (N2 ) and oxygen (O2 ).
  • Exhaust gases that are free of oxides of nitrogen (NOx ) are then sent over the oxidation catalyst
  • The oxidation catalyst coverts carbon-monoxide (CO) and hydrocarbons (HC) in the gases into carbon-di-oxide (CO2 ) and water (H2O).

Muffler

  • The purpose of the Muffler (silencer) is to reduce the noise of the exhaust gas before it is released to the atmosphere.
  • This leads not only to discharge noise at the exhaust outlet, but also to noise radiation from exhaust pipe and silencer surfaces.
  • The muffler is a set of tubes. These tubes are designed to create reflected waves that interfere with each other or cancel each other out.

  • The exhaust gases and the sound waves enter through the center tube.
  • They bounce off the back wall of the muffler and are reflected through a hole into the main body of the muffler.
  • They pass through a set of holes into another chamber, where they turn and go out the last pipe and leave the muffler.

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