This document discusses emissions and emission control strategies in internal combustion engines. It covers the formation of various emissions like carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons, and particulates in both spark ignition (SI) and compression ignition (CI) engines. It also discusses emission control methods like catalytic converters and exhaust gas recirculation (EGR). The key points are: emissions form due to incomplete combustion and high temperatures; a three-way catalytic converter controls CO, HC, and NOx using platinum, palladium and rhodium; and EGR reduces NOx by lowering combustion temperatures but increases particulates.

1. IC Engine
Emissions and Emission Control
Unit - III

5. Formation of CO in IC Engines
• Formation of CO is well established.
• Locally, there may not be enough O2 available for complete
oxidation and some of the carbon in the fuel ends up as CO.
• The amount of CO, for a range of fuel composition and C/H ratios, is
a function of the relative air-fuel ratio.
• Even at sufficient oxygen level, high peak temperatures can cause
dissociation.
• Conversion of CO to CO2 is governed by reaction
HCOOHCO  2
• Dissociated CO may freeze during the expansion stroke.
The highest CO emission occurs during engine start up (warm up) when the engine is run
fuel rich to compensate for poor fuel evaporation.

6. Formation of CO in CI Engines
• The mean air-fuel mixture present in the combustion
chamber per cycle is far leaner in the diesel engine than in the
SI engine.
• Due to a lack of homogeneity of the mixture built up by
stratification, however, extremely “rich” local zones are exist.
• This produces high CO concentrations that are reduced to a
greater or lesser extent by post-oxidation.
• When the excess-air ratio increases, dropping temperatures
cause the post-oxidation rate to be reduced.
• The reactions “freeze up”.
• However, the final CO concentrations of diesel engines
therefore are far lower than in SI engines.
• The basic principles of CO formation, however, are the same
as in SI engine.

7. Hydrocarbon Emission Sources for CI Engines
Overmixing of fuel and air - During the ignition delay period
evaporated fuel mixes with the air, regions of fuel-air mixture are
produced that are too lean to burn.
Some of this fuel makes its way out the exhaust.
Longer ignition delay more fuel becomes overmixed.
Undermixing of fuel and air - Fuel leaving the injector nozzle at low
velocity, at the end of the injection process cannot completely mix
with air and burn.

10. NOx Formation in I.C. Engines
Three chemical reactions form the Zeldovich reaction are:
Forward rate constants:
 
 
 Tk
Tk
Tk
f
f
f
/450exp101.7
/4680exp108.1
/38370exp108.1
10
,3
7
,2
11
,1



Zelodvich reaction is the most significant mechanism of NO
formation in IC engines.

13. Particulates
• A high concentration of particulate matter (PM) is manifested as
visible smoke in the exhaust gases.
• Particulates are any substance other than water that can be
collected by filtering the exhaust, classified as:
• Solid carbon material or soot.
• Condensed hydrocarbons and their partial oxidation products.
• Diesel particulates consist of solid carbon (soot) at exhaust gas
temperatures below 500oC, HC compounds become absorbed
on the surface.
• In a properly adjusted SI engines soot is not usually a problem .
• Particulate can arise if leaded fuel or overly rich fuel-air mixture
are used.
• Burning crankcase oil will also produce smoke especially during
engine warm up where the HC condense in the exhaust gas.

14. 14
The soot formation process is very fast.
10 – 22 C atoms are converted into 106 C atoms in less than 1 ms.
Based on equilibrium the composition of the fuel-oxidizer mixture soot ,
formation occurs when x ≥ 2a (or x/2a ≥ 1) in the following reaction:
Mechanism of Formation of Particulates (soot)
)()2(
2
2 22 sCaxH
y
aCOaOHC yx 
Experimentally it is found that the critica C/O ratio for onset of soot
formation is between 0.5 and 0.8.
The CO, H2, and C(s) are subsequently oxidized in the diffusion flame
to CO2 and H2O via the following second stage.
OHOHCOOsCCOOCO 2222222
2
1
)(
2
1

Any carbon not oxidized in the cylinder ends up as soot in the exhaust!

23. Emissions Control
• Three basic methods used to control engine emissions:
• 1)Engineering of combustion process -advances in fuel
injectors, oxygen sensors, and on-board computers.
• 2) Optimizing the choice of operating parameters -two Nox
control measures that have been used in automobile engines
are spark retard and EGR.
• 3) After treatment devices in the exhaust system -catalytic
converter.
23

29. 29
Anatomy of Catalytic Converter
•All catalytic converters are built in a honeycomb or pellet geometry
to expose the exhaust gases to a large surface made of one or more
noble metals: platinum, palladium and rhodium.
•Rhodium used to remove NO and platinum used to remove HC and
CO.
Lead and sulfur in the exhaust gas severely inhibit the operation
of a catalytic converter (poison).

31. The active catalyst material is impregnated on the surface of catalyst substrate or support.
The function
of catalyst substrate is to provide maximum possible contact of catalyst with reactants.
Following arethe main requirements of catalyst substrate:
High surface area per unit volume to keep a small size of the converter Support should be
compatible with coating of a suitable material (washcoat) to provide high surface area and
right size of pores on its surface for good dispersion and high activity of thecatalyst.
Low thermal capacity and efficient heat transfer properties for quick heat-up to working
temperatures.
Ability to withstand high operating temperatures up to around to 1000º C.
High resistance to thermal shocks that could be caused by sudden heat release when HC
from engine misfire get oxidized in the converter.
Low pressure drop
Ability to withstand mechanical shocks and vibrations at the operating temperatures under
road
conditions for long life and durability
The following types of catalysts supports are used;
Pellets
Monolithic supports
Ceramic monoliths
Metal monoliths

33. Three way catalytic convertor
• A catalyst forces a reaction at a temperature lower than normally
occurs.
• As the exhaust gases flow through the catalyst, the NO reacts
with the CO, HC and H2 via a reduction reaction on the catalyst
surface.
• NO+CO→½N2+CO2 , NO+H2 → ½N2+H2O, and others
• The remaining CO and HC are removed through an oxidation
reaction forming CO2 and H2O products (air added to exhaust
after exhaust valve).
• A three-way catalysts will function correctly only if the exhaust gas
composition corresponds to nearly (±1%) stoichiometric
combustion.
• If the exhaust is too lean – NO is not destroyed
• If the exhaust is too rich – CO and HC are not destroyed
• A closed-loop control system with an oxygen sensor in the
exhaust is used to A/F ratio and used to adjust the fuel injector so
that the A/F ratio is near stoichiometric.

34. The oxides of base metals such as copper, chromium, nickel, cobalt etc.
have been studied. The base metal oxides are effective only at higher
temperatures. In addition, they sinter and deactivate when subjected to
high exhaust gas temperatures experienced at high engine loads. Their
conversion efficiency is severely reduced by sulphur dioxide produced by
sulphur in fuel.
The noble metals platinum (Pt), palladium (Pd) and rhodium (Rh) were
found to meet the above mentioned performance requirements. In
practice, only the noble metals are used although these are expensive.
Mixtures of noble metals are used to provide higher reactivity and selectivity of conversion.
Following
are typical formulations;
Pt : Pd in 2:1 ratio for oxidation catalysts
(Pt + Pd): Rh in ratio of 5 :1 to 10: 1 for simultaneous oxidation and reduction such as in 3-
way catalysts
Palladium has higher specific activity than Pt for oxidation of CO, olefins and methane.
For the oxidation of paraffin hydrocarbons Pt is more active than Pd. Platinum has a higher
thermal resistance to deactivation. Rhodium is used as a NOx reduction catalyst when
simultaneous conversion of CO, HC and NO is desired as in the 3-way catalytic converters.
The amount of noble metal used typically varies from about 0.8 to 1.8 g/l (25 to 50 g/ft3) of
catalytic

35. Properties of Catalyst
• The active catalyst material is required to posses
the following main characteristics
• High specific reaction activity for pollutants
• High resistance to thermal degradation
• Good cold start performance, and
• Low deactivation caused by fuel contaminants
and sulphur Other desirable requirements are
low
• cost.

36. Exhaust Gas Recirculation-EGR
• NOx Emissions
• In many countries around the world, the emissions of NOx from diesel
and gasoline vehicles are restricted. NOx is formed in the combustion
chamber of engines, when high temperatures cause oxygen and
nitrogen (both found in the air supplied for combustion) to combine.
• Exhaust Gas Recirculation
• A widely adopted route to reduce NOx emissions is Exhaust Gas
Recirculation (EGR). This involves recirculating a controllable proportion
of the engine's exhaust back into the intake air. A valve is usually used
to control the flow of gas, and the valve may be closed completely if
required.
• The substitution of burnt gas (which takes no further part in
combustion) for oxygen rich air reduces the proportion of the cylinder
contents available for combustion. This causes a correspondingly lower
heat release and peak cylinder temperature, and reduces the formation
of NOx. The presence of an inert gas in the cylinder further limits the
peak temperature (more than throttling alone in a spark ignition
engine).

37. The gas to be recirculated may also be passed through an EGR cooler, which is
usually of the air/water type.
This reduces the temperature of the gas, which reduces the cylinder charge
temperature when EGR is employed.
This has two benefits- the reduction of charge temperature results in lower peak
temperature, and the greater density of cooled EGR gas allows a higher proportion
of EGR to be used.
On a diesel engine the recirculated fraction may be as high as 50% under some
operating conditions.
Advantages of EGR
Reduced NOx
Potential reduction of throttling losses on spark ignition engines at part load
Improved engine life through reduced cylinder temperatures (particularly exhaust
valve life)

38. • Disadvantages and Difficulties of EGR
• Since EGR reduces the available oxygen in the cylinder, the
production of particulates (fuel which has only partially
combusted) is increased when EGR is applied. This has
traditionally been a problem with diesel engines, where the
trade-off between NOx and particulates is a familiar one to
calibrators.
• The deliberate reduction of the oxygen available in the
cylinder will reduce the peak power available from the
engine. For this reason the EGR is usually shut off when full
power is demanded, so the EGR approach to controlling
NOx fails in this situation.
• The EGR valve can not respond instantly to changes in
demand, and the exhaust gas takes time to flow around the
EGR circuit. This makes the calibration of transient EGR
behavior particularly complex- traditionally the EGR valve
has been closed during transients and then re-opened once
steady state is achieved. However, the spike in NOx /
particulate associated with poor EGR control makes
transient EGR behavior of interest.

39. • The recirculated gas is normally introduced into the intake system
before the intakes divide in a multi-cylinder engine. Despite this,
perfect mixing of the gas is impossible to achieve at all engine
speeds / loads and particularly during transient operation. For
example poor EGR distribution cylinder-to-cylinder may result in
one cylinder receiving too much EGR, causing high particulate
emissions, while another cylinder receives too little, resulting in
high NOx emissions from that cylinder.
• Although the term EGR usually refers to deliberate, external EGR,
there is also a level of internal EGR. This occurs because the residual
combustion gas remaining in the cylinder at the end of the exhaust
stroke is mixed with the incoming charge. There is therefore a
proportion of internal EGR which must be taken into account when
planning EGR strategies. The scavenging efficiency will vary with
engine load, and in an engine fitted with variable valve timing a
further parameter must be considered.

40. Particulate Trap