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Internal Combustion EnginesEmissions & Air Pollution: 3

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Internal Combustion Engines

Emissions & Air PollutionLecture 3


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Outline

• In this lecture we will discuss emission control strategies:– Fuel modifications– Engine technology– Exhaust gas aftertreatment

• We will become particularly familiar with catalytic converters and diesel particulate filters.


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Fuel Modifications• For modern low emission standards the

fuel quality has a considerable role to play.• However, the extent to which changes in

fuel properties can influence the exhaust emissions is a subject of much debate.

• Since fuel properties are generally interrelated, it is very difficult to draw general conclusions on how changes in a single fuel property can affect the exhaust emissions.


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Fuel Modifications• Moreover, the sensitivity to fuel property

changes can depend on the engine technology and the test drive cycle used.

• Regulations are already in place to control the density, viscosity, volatility, sulphur content and cetane/octane ratings of the fuel.

• Oxygenated fuels can significantly reduce HC and particulate emissions through the oxidation of soot precursors in fuel rich core regions of the injected fuel spray.


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Engine Technology• Engine-based modifications have been the main

strategy adopted by the automotive industry over the past two decades.

• These modifications have reduced automotive emissions by over 90% over the past two decades.

• Exhaust Gas Recirculation (EGR) has proven to be a very effective way to cut NOx emissions.

• The main principle is to dilute the intake charge with inert exhaust gases, and thereby lowering the flame speed and reducing the peak temperature of the combustion process.


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EGR


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EGR• However, increased EGR rates can lead to increased

particulate emissions due to air deficiency.• This problem can be overcome by cooling the EGR prior

to the mixing with inlet air, which increases the mass of the air charge and consequently, the oxygen concentrations.

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EGR rate (%)

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NOx cooled

PM uncooled

PM cooled


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Engine Modifications• Turbocharging also increases the density of the charge

air and thus can lead to lower CO and particulate emissions.

• Careful design of the combustion chamber can improve the in-cylinder air movement leading to better A/F mixtures.

• The use of injectors with more and smaller holes leads to finer atomisation of the fuel.

• Increasing the injection pressure also acts in the same direction.

• The introduction of advanced fuelling technologies capable of achieving precise control of injection timing and fuelling has led to impressive reductions of the particulate and NOx emissions.


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Exhaust Gas Aftertreatment• Although exhaust aftertreatment is relatively new

for diesel passenger cars, it has been introduced for gasoline vehicles since the 1970s in the form of catalytic converters.

• This is mainly due to the relatively higher CO, HC and NOx emissions from gasoline engines in comparison with diesel engines, in addition to the -then- wider spread of gasoline passenger cars.

• The catalytic converter proved to be the most effective method of controlling emissions from S.I. engines.


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Catalytic Converters• Catalytic converters contain no moving parts, no

electric power input and no mechanical interference with the operation of the engine or the combustion process.

• A catalyst is a substance that accelerates the velocity of a chemical reaction while it remains unaffected by the reaction itself.

• The main effect can be witnessed through the reduction of the threshold temperature at which the reaction takes place.


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Catalytic Converters• Catalysts such as platinum (Pt), palladium (Pd)

and rhodium (Rh) promote the oxidation / reduction of CO, HC and NOx and thus have been the primary catalysts used for automotive catalytic converter applications.

• To ensure best conditions of interaction between the exhaust gas and the catalyst material, the surface area on which the catalyst material is dispersed should be maximum.

• The optimum arrangement has been through a ceramic monolithic structures with very fine pores.


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Catalytic Converters


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Catalytic Converters• 3-way catalytic converters reduce CO, HC and NOx

simultaneously.• The reactions through which CO and HC are reduced

require a relatively lean environment (excess air).• On the other hand, reactions through which NOx

emissions are reduced require a relatively rich environment (less air).

• As a result of this, and since all three emissions need to be reduced simultaneously, catalytic converters need to operate over a narrow window around the stoichiometric A/F.

• Operation beyond this window can result in a large decrease in the efficiency of the reduction of either CO and HC or NOx.


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Catalytic Converters


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Catalytic Converters• Cold start:

– At the start of the operation of the engine, the temperatures of the exhaust gases are under the threshold required for the operation of the converter.

– This problem can be overcome by placing the converter closer to the exhaust manifold or by using electrical heating.

• Ageing and poisoning: – The efficiency of the converter drops with time due to

the reduction of the precious metal surface area due to thermal effects or the suppression of the precious metal by foreign metals present in the exhaust (poisoning).


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Diesel Exhaust Aftertreatment• Diesel aftertreatment is quite different from

gasoline aftertreatment due to the different exhaust composition.

• In the diesel exhaust, particulates are the major problem that need to be solved, while this problem is not present in gasoline exhaust.

• The other main difference is that the diesel exhaust is always very lean which means that reduction of NOx emissions using conventional catalytic conversion is ineffective.


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Diesel Exhaust Aftertreatment• On top of that, engine-based measures that

target low NOx emissions usually result in an increase in the particulates emissions and vice-versa.

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HC + NOx (g/km)

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Engine Modif ications

Euro 3

Euro 4

Euro 2


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Diesel Exhaust Aftertreatment• Lean NOx catalysts use HC as the reduction

agent instead of O2 and thus can achieve better conversion efficiencies.

• However, their application requires controlled increase of HC level and are very sensitive to the presence of sulphur.

• Selective Catalytic Reduction (SCR) systems use chemicals such as ammonia or urea as reducing agents instead of HC.

• However, their application still faces many practical limitations.


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Diesel Particulate Filters• Diesel Particulate Filters (DPFs) are generally

manufactured in cylindrical shapes and consist of square longitudinal channels as in catalytic converters.

• The difference is that these channels are alternately blocked, so that half of them are blocked from one end and the other half are blocked from the other end.

• As a result, the exhaust gas is forced to pass through the porous wall which acts as a trap or filter for the particulates.


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Diesel Particulate Filters


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Diesel Particulate Filters• DPFs proved to have excellent durability and

filtration characteristics. • A typical SiC DPF should have a lifetime of

around 80,000 km, with a filtration efficiency in excess of 90% on gravimetric basis.

• This efficiency can increase to as high as 98-99% as the DPF is put in use due to the accumulation of particulates.

• Nevertheless, there are two main obstacles facing the successful application of DPFs: the pressure drop and the regeneration process.


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Diesel Particulate Filters• A significant drop in the static pressure occurs

across the DPF because of the resistance the exhaust flow experiences.

• As the DPF is put in use the particulates start accumulating, forming a layer that acts as a second wall adding to the resistance meeting the exhaust flow.

• This leads to a steady increase in the pressure drop across the DPF as a function of time, with a rate that is dependent on the loading conditions


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Diesel Particulate Filters• Acceptable back pressure levels should be

maintained to avoid a negative impact on the fuel economy and drivability. Consequently, the filter needs to be, either continuously or periodically, cleaned by regenerating the collected particulates.

• During regeneration the particulates are oxidised in an exothermic reaction to produce CO2 and water vapour. The rate of this reaction is exponentially related to the temperature of the exhaust gases.

• Regeneration should be independent of the driver and the duty cycle, and should not affect drivability or other performance parameters.


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Diesel Particulate Filters• Unfortunately, under normal driving conditions

the rate of the reaction is negligible since diesel exhaust temperatures are relatively low.

• This means that the regeneration process should be assisted by either: – Decreasing the reaction temperature through using

catalysed DPFs or fuel additives.– Increasing the exhaust gas temperatures with the aid

of external means such as electrical heaters or controlled fuel post injection.

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