Aftertreatment Modelling Basics
GASOLINE VS DIESEL EMISSIONS:
1.POLLUTANTS:
* DESCRIPTION:
The Emissions from the engines is purely based upon the "AFR(Air fuel Ratio)", in which the Air-fuel mixture determines the combustion efficiency and also the emissions . In which the AFR doesnt remains constant and also it varies from gasoline engine to diesel engine.
GASOLINE ENGINE :
* So Gasoline engine always to tries to maintain the 'Equivelence ratio' to '1'
So the AFR some around the stiochiometric Airfuel ratio . And hence the gasoline can run in both lean condition and also to rich condition ,but it is always stick around the equivelence ratio of '1'.
* In Gasoline engine , Lean conditions always gives the better fuel effiiency ,
but lean run increases the temperature of the chamber and the high temperature give rise to Nox emissions .
* But Rich mixture in SI gives less Nox emissions but bad fuel efficiency.
* So as gasoline runs on both the rich and lean condition , the common emissions will be :
* CO,HC,NoX(No2,No)
* The reason why Rich condition is necessary for the gasoline engines , is the exhaust temperatures of the gasoline engine is high around 900 to 1000 deg celc
* Hence the high exhaust temperatures causes the exhaust manifold damaged
and so this can be avoided by reducing the cylinder temperature and by injecting more fuel causes the temperature to be reduced in the cylinder and also the combustion efficiency.
DIESEL ENGINE :
* Diesel engines always requires high Compression ratio , to get the good air and fuel mixture .
* And also the diesel has high cetane number(45-55) and so it has the good burning characteristics .
* And so the diesel engines always run in the lean condition to control the dynamics , thus here the diesel is injected in less amount and the air is more in quantity .
* And so the temperature is high always in diesel engine , as the temperature is high ,the "NOX emissions" are the major components of the emissions.
* And also there will be the contents of CO and HC in less amount .
2.AFTERTREATMENT DEVICES :
GASOLINE ENGINES :
* 3 WAY CATALYTIC CONVERTER
DIESEL ENGINES :
* DOC,DPF,SCR
THREE WAY CATALYTIC CONVERTER:
* As the three main components of emissions , CO,HC,Nox are treated in a single housing ,it is known as 3way catalytic converters , as it doesnt consists of much NOX emissions it can be easily treated with the 3way cat converters.
DOC(DIESEL OXIDATION CATALYSTS):
* DOC is mainly for the oxidation of CO and HC from the emissions of diesel engine.
DPF(DIESEL PARTICULATE FILTER):
* DPF is mainly for the particulate matters , which filters the particulate matters.
SCR(SELECTIVE CATALYTIC REDUCTION):
* This setup is mainly for the NOX emissions , in which the DEF(diesel exhaust fluid) is used for the reduction of Nox emissions and the most common DEF used is "UREA" .
these three setup are combinely used for the diesel aftertreatment purpose.
3.EXAMPLES:
SCR :
TWC :
DOC :
DPF :
4.STRATEGIES USED FOR REDUCING TAIL PIPE EMISSIONS :
TWO MAIN STRATEGIES USED IN EMISSION CONTROL :
* IN-CYLINDER STRATEGY
* AFTERTREATMENT STRATEGY
1.IN-CYLINDER STRATEGY:
* Controlling the fuel injection and also the design of piston bowl
* Using Exhaust gas recirculation (EGR)
* Downsizing technology
* Design Optimization
2.AFTER-TREATMENT STRATEGY :
* This aftertreatment strategy is necessary to achieve the regulations and these are the chemical plants to make the exhaust gas affordable to the environment.
1.SCR
2.DPF
3.DOC
4.3 WAY CATALYTIC CONVERTER
these are the devices used in the aftertreatment technology.
REACTIONS FOR TWC AND SCR FROM EXAMPLES :
REACTIONS IN TWC :
REACTIONS IN SCR :
DOC-SAMPARA AND BISSET :
Reactions modelled in the catalytic converter are :
SPECIES :
* The inlet species to DOC is also modelled and defined as follows ;
And there will be inlet and outlet cone before and after the catalysts :
Inlet cone dimensions are as follows ;
CONVERSION EFFICIENCY MODEL :
The emissions to the outlet (Environment) :
Conclusion :
Thus this aftertreatment models are created according to the temperature and the reaction rates of the reactants and their characteristics , so this is must necessary for the engines to safely emit their combusted products to the environment by obtaining the " zero emissions " .