We have known about the two different parts like a muffler and a resonator in the exhaust system till now.Now it's time to look at the device which is generally placed in between the exhaust manifold and the muffler or resonator.It is mandatory for every vehicle.There is more about to know the CAT.We will discuss it clearly.
WHAT IS IT FOR?
It is to reduce the number of harmful gases coming out of the engine.The toxic gases involved in the exhaust are Hydrocarbons (HC), CO(Carbon Monoxide), NOx gases(NO, NO2).It helps in reducing the emissions.
CONSTRUCTION:
Its construction involves 6 things.
1) Catalyst Support or Substrate or Core or monolith
2) Wash coat
3) Oxygen storage promoters
4) Nobel metals or Catalysts
5) Intumescent mat
6) Outer covering (shield)
1)CATALYST SUPPORT OR SUBSTRATE OR CORE OR MONOLITH:
It is a support for a catalyst to sit.It has honeycomb structure and made up of either ceramic or metal depends on the exhaust temperatures.For high temperatures, a metal core is used.Almost in all vehicles, a ceramic substrate is used.In 2 way CAT, there is only one Substrate whereas in 3 way CAT there are two cores.
WHAT IS IT FOR?
It is to reduce the number of harmful gases coming out of the engine.The toxic gases involved in the exhaust are Hydrocarbons (HC), CO(Carbon Monoxide), NOx gases(NO, NO2).It helps in reducing the emissions.
CONSTRUCTION:
Its construction involves 6 things.
1) Catalyst Support or Substrate or Core or monolith
2) Wash coat
3) Oxygen storage promoters
4) Nobel metals or Catalysts
5) Intumescent mat
6) Outer covering (shield)
1)CATALYST SUPPORT OR SUBSTRATE OR CORE OR MONOLITH:
It is a support for a catalyst to sit.It has honeycomb structure and made up of either ceramic or metal depends on the exhaust temperatures.For high temperatures, a metal core is used.Almost in all vehicles, a ceramic substrate is used.In 2 way CAT, there is only one Substrate whereas in 3 way CAT there are two cores.
Generally, cordierite ceramic substrate is used in most cats.
2)THE WASHCOAT (with the catalytic active material):
It is a carrier for catalytic materials and used to disperse the Nobel materials over the large surface area.Generally, the wash coat compounds are Aluminum Oxide(Al2O3), Titanium dioxide(Tio2), Silicon Dioxide(SiO2) or a mixture of both Silica(SiO2) and Alumina(Al2O3).
The catalytic materials are suspended in the washcoat before applying to the core(catalyst support or substrate or monolith).
The washcoat materials are mainly for forming the rough and irregular surfaces on the substrate which greatly increases the surface area of contact.This is better than the smooth surface of the bare substrate.This, in turn, maximizes the catalytically active surface available to react with exhaust.
This coating helps to retain the catalytically active surface available area and also prevents the sintering of the catalysts or Nobel materials even at high temperatures (1000°C).(source: Wikipedia)
3)OXYGEN STORAGE PROMOTERS:
These are mainly for storing and releasing the oxygen according to the need.Since oxygen is the significant molecule in the catalytic converter for happening of proper oxidation reactions.For this, these promoters help to store and release the oxygen.The oxides are mainly Ceria(CeO2) or Ceria-Zirconia(CeO2-ZrO2) solid solution.
4)NOBEL METALS OR CATALYSTS:
In cats, generally, Platinum, Rhodium(reduction catalyst), Palladium(oxidation catalyst) are used as catalysts.Whereas in the place of Platinum; Nickel, Cerium, Iron, Copper can also be used.But Nickel is not legal to use in European Union because it produces toxic Nickel Tetracarbonyl(Ni(CO)4).These all are called as Earth Stabilizers.
5) INTUMESCENT MAT:
The blocks of the substrate are generally covered with a mat which provides a proper insulation so that heat transfer to the outside is less.The name intumescent tells that it swells on exposure to heat, thus increasing its volume and decreasing its density.It insulates, seals and provides an unbreakable enclosure for the monolith.
6)OUTER SHIELD:
Generally, it is a stainless steel covering which provides an airtight seal for the whole body.Some other lighter materials can also be used as like Aluminium etc.The outer shield can also be coated or wrapped for providing better insulation.
TYPES:
1)2-way cat
2)3-way cat
WORKING :
1)2-way catalytic converter / oxi or oxidation cat:
This is used to eliminate the Hydrocarbons(CxH2x+2), Carbon monoxide (CO) and Carbon dioxide (CO2) from the exhaust.Since it eliminates 2 pollutants mainly Carbon Monoxide and Hydro Carbons, it is called as a 2 -way catalytic converter.
It has only one block of substrate coated with Platinum and Palladium catalysts which promote only oxidation.
The basic reactions occur in the 2-way catalyst are:
Many people want to delete it because of the belief that cats rob performance.Removing the catalytic converter is illegal and also remember that cats don't rob performance#
2)THE WASHCOAT (with the catalytic active material):
It is a carrier for catalytic materials and used to disperse the Nobel materials over the large surface area.Generally, the wash coat compounds are Aluminum Oxide(Al2O3), Titanium dioxide(Tio2), Silicon Dioxide(SiO2) or a mixture of both Silica(SiO2) and Alumina(Al2O3).
The catalytic materials are suspended in the washcoat before applying to the core(catalyst support or substrate or monolith).
The washcoat materials are mainly for forming the rough and irregular surfaces on the substrate which greatly increases the surface area of contact.This is better than the smooth surface of the bare substrate.This, in turn, maximizes the catalytically active surface available to react with exhaust.
This coating helps to retain the catalytically active surface available area and also prevents the sintering of the catalysts or Nobel materials even at high temperatures (1000°C).(source: Wikipedia)
3)OXYGEN STORAGE PROMOTERS:
These are mainly for storing and releasing the oxygen according to the need.Since oxygen is the significant molecule in the catalytic converter for happening of proper oxidation reactions.For this, these promoters help to store and release the oxygen.The oxides are mainly Ceria(CeO2) or Ceria-Zirconia(CeO2-ZrO2) solid solution.
4)NOBEL METALS OR CATALYSTS:
In cats, generally, Platinum, Rhodium(reduction catalyst), Palladium(oxidation catalyst) are used as catalysts.Whereas in the place of Platinum; Nickel, Cerium, Iron, Copper can also be used.But Nickel is not legal to use in European Union because it produces toxic Nickel Tetracarbonyl(Ni(CO)4).These all are called as Earth Stabilizers.
5) INTUMESCENT MAT:
The blocks of the substrate are generally covered with a mat which provides a proper insulation so that heat transfer to the outside is less.The name intumescent tells that it swells on exposure to heat, thus increasing its volume and decreasing its density.It insulates, seals and provides an unbreakable enclosure for the monolith.
6)OUTER SHIELD:
Generally, it is a stainless steel covering which provides an airtight seal for the whole body.Some other lighter materials can also be used as like Aluminium etc.The outer shield can also be coated or wrapped for providing better insulation.
TYPES:
1)2-way cat
2)3-way cat
WORKING :
1)2-way catalytic converter / oxi or oxidation cat:
This is used to eliminate the Hydrocarbons(CxH2x+2), Carbon monoxide (CO) and Carbon dioxide (CO2) from the exhaust.Since it eliminates 2 pollutants mainly Carbon Monoxide and Hydro Carbons, it is called as a 2 -way catalytic converter.
It has only one block of substrate coated with Platinum and Palladium catalysts which promote only oxidation.
The basic reactions occur in the 2-way catalyst are:
Oxidation Reactions:
#2CO + O2 =2CO2
#CxH(2x+2)+{(3x+1)÷2}O2=XCO2+(X+1)H2O
In 2- way cat a perforated pipe is placed for sending oxygen from the atmosphere for happening of oxidation in the substrate.
Since it cannot converts the NOx emissions it is not mostly used cat.
2)3-way catalytic converter:
To overcome the problem of 2-way cat regarding about NOx emissions, the invention of 3-way has come.It can also convert NOx gases along with the function of 2-way.Since it can convert 3 pollutants, it is called as a 3 -way catalytic converter.
It has two blocks of substrate namely block 1 and block 2.Here the block 2 is followed by block 1.The block 1(first substrate) is coated with Platinum and Rhodium which promotes reduction.So, it is called reduction substrate.By following the reduction substrate there is a second block which is coated with Platinum and Palladium which promotes oxidation.So it is called as oxidation substrate.
The basic reactions occur in 3-way catalyst are:
1.Reduction Reaction:
# 2NOx = N2 + XO2
2.Oxidation Reactions:
# 2CO + O2 =2CO2
#CxH(2x+2)+{(3x+1)÷2}O2=XCO2+(X+1)H2O
3.Combined Reaction:
# CO + NOx = CO2 + N2
In between the two blocks, a perforated pipe is placed for sending excess oxygen from the atmosphere for further oxidation in block 2 in the case of less oxygen content detected by the oxygen sensor present after the cat.Here the less oxygen content means there is more unburnt fuel(CxH2x+2) and carbon monoxide (CO) which remain as an unreacted compound due to lack of oxygen.An electrically driven or engine driven pump is used to send the oxygen from the atmosphere.
#The placement of cat is very important in automobiles.It requires a temperature of 426°C (800°F) for efficiently convert the gases.So these are mostly placed close to the engine to ensure fast heating.However such placing causes several problems like vapor lock (evaporation of fuel and stuck in the fuel hoses due to more heat.Mostly"petrol").
All the emitted gases like CO leads to increase in global temperatures and causes serious health problems whereas NOx (precursors to acid rain and smog) and Hydrocarbons (unburnt fuel) leads to environmental pollution.So it is our responsibility to use the catalytic converters for saving the environment and for good health.Many people want to delete it because of the belief that cats rob performance.Removing the catalytic converter is illegal and also remember that cats don't rob performance#
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