W3S5-Emissions _ Aftertreatment Systems2

8/18/2019 W3S5-Emissions _ Aftertreatment Systems2

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© IFPEN / IFP School 2014

Sustainable MobilityTechnical and environmental challenges for the automotive sector

Week 3 – Session 5 – Emissions and Aftertreatmentsystems: Pollutant reduction at source

Prakash Chandra Dewangan


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W3 – S5 – Emissions and aftertreatment systems2 p. 1


In order to reduce the pollutants, there are two options; first, reduce where the pollutants are

formed, that is to say, reduction at source or at the engine. To do this, we could work on engine

design, geometries, dimensions etc.; or, add specific technologies to the engine; or we could

include alternative combustion as we discussed in the previous video. We could also work on how

to use the engine at different conditions known as calibration; and also, on how we control the

engine in different circumstances.

The second option is to let the pollutants be produced in the engines and treated later with special

systems known as aftertreatment systems.

In general, the pollutants are reduced with the combination of these two options.

In the discussion of the first option, examples of some of the main technologies which are part of

engine design today will be discussed. Everything else, combustion, calibration and controls are out

of the scope of this introductory coursework.

A recap of pollutant formation: from the discussion of pollutant formation, we know that pollutants

are either the product of incomplete combustion or the by-product of combustion.

Also, we know that the gas composition and temperature together guide the combustion process

as seen on the right of this slide.

We have also seen that CO is produced in rich combustion and HC is produced due to incompletecombustion. Sometimes, especially in gasoline engines, fuel is trapped in engine corners where the


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flame does not reach it during combustion giving HC emissions. Soot is a product of a very rich mix

at a high temperature, and NOx is a product of a lean mix at a very high temperature.

To reduce pollutants at source, we must manage both gas composition and temperature during

combustion.

We will mainly discuss technologies for soot and NOx reduction at source. Because today emissions

are a bigger problem for diesel engines than for gasoline engines, and as we have already seen,

soot and NOx are the main emissions for diesel.

The first example is ‘Exhaust Gas Recirculation’ or ‘EGR’. This technology is mainly used to reduce

NOx at source.

As the name suggests, part of the exhaust gases are reintroduced at the entrance of the engine.

Due to CO2 produced in the combustion, there is an excess of CO 2 in the exhaust gases; therefore,

using EGR, the amount of CO2 in the mixture inside the engine increases and as a result the relative

amount of O2 reduces. As CO2 is not involved in the combustion, EGR increases the fraction of

unreactive gases in the mixture. These unreactive gases act as a heat sink during combustion and as

a result, combustion temperature goes down.


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As the temperature goes down, NOx decreases, as we can see in the graph. Although the desired

effect is shown by the arrow on the graph below:

the actual effect of EGR is as in the graph below. As a result, combustion enters the soot zone,resulting in an increase in soot and, in turn, increased smoke. This is the down side of EGR

EGR is a standard for modern diesel engines and is increasingly being used in modern gasoline

engines with direct injection.


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As a second example of pollutant reduction technologies, we will discuss the Common Rail Direct

Injection system; also known as CRDi.

It is mainly for soot reduction. The idea is to inject the fuel at a very high pressure into the engine.

The pressure can go as high as 2000 bar for diesel engines. Fuel injected with this high pressure

atomizes rapidly giving very small droplets which mix quickly with the air around the jet. It can be

seen in the blue picture (below) of the simulation of high pressure jet injection in a gas. In the

middle, you can see a typical Common Rail system with injectors in green and a “common rail”

attached to them at the back.

In the case of diesel engines, as the jet is at a very high pressure, it could reach the walls causing

droplet deposit at the wall. To avoid this, a swirl motion is introduced inside the engine.

The use of swirl and high pressure injection ensures better mixing, which as a consequence avoids

pockets of rich zones.


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Consequently, local richness decreases, as shown by the arrow in the graph below, and this reduces

soot.

However, in reality, instead of following the arrow in the graph above, we follow the arrow in the

graph below, towards the zone of NOx formation. Therefore, as one of the drawbacks of the CRDi

system, NOx rises.

The last technology we will see is the alternative fuels.

Fuels like ethanol and natural gas are less carbon intense in comparison to gasoline or diesel. They

have a higher number of hydrogen atoms per carbon atom. Therefore, during their combustion,

more H2O is produced than CO2.

Consequently, they tend to produce lower CO2 when compared with the combustion of gasoline or

diesel.

Also, these gases have smaller and simpler molecules giving cleaner combustion. This results in less

soot formation.


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Due to these advantages, these fuels mixed with more conventional fuels are often used in certain

engine technologies. These mixed fuels are called “dual fuels”; prominent examples being ethanol

with gasoline; natural gas with gasoline; and natural gas with diesel.

W3S5-Emissions _ Aftertreatment Systems2

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