IC Engine Emissions and Emission Control

Unit - III

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.

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.

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.

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.

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.

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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 sCaxHyaCOaOHC 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 21 )(

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Any carbon not oxidized in the cylinder ends up as soot in the exhaust!

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.

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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).

The active catalyst material is impregnated on the surface of catalyst substrate or support. The functionof 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 dropAbility to withstand mechanical shocks and vibrations at the operating temperatures under roadconditions for long life and durability

The following types of catalysts supports are used;PelletsMonolithic supportsCeramic monolithsMetal monoliths

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.

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. Followingare 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 catalystsPalladium 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

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.

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).

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 EGRReduced NOxPotential reduction of throttling losses on spark ignition engines at part loadImproved engine life through reduced cylinder temperatures (particularly exhaust valve life)

• 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.

• 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.

Particulate Trap