How can we control automobile exhaust emission?

Emission Control Techniques : All types of automobile exhaust emission is harmful for our environment.so for maintain a pollution free environment automobile emission control is must.Basically there are three types of automobile emission control techniques,which are-
                                                                                               
Pre-Engine System : Hot Air Intake System

In-Engine System : MPFI, EGR,λ- sensor                                         
Post-Engine System : Thermal reactor, Catalytic Converter, DPF



Manufacture needs to attain the tail pipe emission as per the standard, which
may be done involving one or more of the systems.


Thermal Reactors :
Thermal Reactor is a device which is very effective for controlling hydro carbon and carbon monoxide.
They provide more time for the exhaust at about 600°C, allowing more complete combustion of some exhaust components. Mainly involves oxidation of HC and CO.

Exhaust gas Recirculation (EGR):
Exhaust gas recirculation is another effective and efficient method for controlling automobile emission.The percentage of exhaust gas is to be recirculated can be obtain from this
equation-
EGR % = {mEGR/(mf+ma)} x 100

EGR ensures lower temperature and lower NOx formation, at the cost of –(a)lower volumetric efficiency, (b)thermal efficiency and (c)higher HC formation.
EGR is typically done at the rate of 10-15% of the intake, not exceeding 30%.EGR is done on top of the normal residual fraction of exhaust gas.EGR is not done at idling, EGR is not done at WOT.In CI engines EGR is less common as abrasive soot particles may re-enter the engine.


Catalytic Converters (CC):
Now a days catalytic converter is the most effective device for controlling automobile exhaust emission.Typically used for SI engine emission control.There are two types
of catalytic converters.they are-

2-WAY Catalytic Converter : Removes CO and HC
3-WAY Catalytic Converter : Removes NOx, CO and HC
so 3-way catalytic converter is more effective,besouse this is able to control all three types of emission.
Catalytic Converter Efficiency = {1 – (mexhaust out / mexhaust in)} x 100 %
A CC may have different removal efficiencies for different components.

Diesel Particulate Filter (DPF) :

This is mainly use to reduce PM emissions from diesel engines.DPF consists of -(a) Metallic casing (b) Substrate(Cordie rite/SiC) (c) Regeneration system (d) Monitoring system.
As the DPF is used the pressure drop increase with time.The rate of increase depend on flow rate and particulate concentration.Monitoring system is needed to control when to start regeneration or protect the engine.

Diesel Oxidation Catalyst(DOC):
They are similar to catalytic converter.Coating of Pd/Pt are provided on the ceramics substance/honeycomb
Catalyst oxidize pollutant like HC and CO to less harmful gases.It does does not need regeneration as long the catalyst are active.For proper operation of DOC the sulphur content in diesel should be very low, at least below 500 ppm or 0.05% by weight.

Selective Catalytic Reduction (SCR):
SCR is the process whereby a reluctant such as ammonia or urea is mixed with NOx emissions then passed through a special flow-through catalyst to create a reduction process. During the process, the ammonia-NOx mixture is converted into harmless nitrogen and water.