pollutant formation

7/28/2019 Pollutant Formation

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Pollutant Formation and Control

2103471 Internal Combustion Engine


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Pollutant Formation and Control

All IC engines produce undesirable emissions as a result of combustion.

The emissions of concern are unburned hydrocarbons (UHC), carbon

monoxide (CO), oxides of nitrogen such as nitric oxide and nitrogen dioxide

(NOx), sulfur dioxide, and solid carbon particulates.

These emissions pollute the environment and contribute to acid rain, smog

odors, and respiratory and other health problems.

HC emissions from gasoline-powered vehicles include a number of toxic

substances such as benzene, polycyclic aromatic hydrocarbons (PAHs),

1,3-butadiene and three aldehydes (formaldehyde, acetaldehyde, acrolein).

Carbon dioxide is an emission that is not regulated but is the primary

greenhouse gas responsible for global warming.


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Emissions Control

The current emission limits for HC, CO and NOx have been reduced to 4%,

4% and 10% of the uncontrolled pre-1968 values, respectively.

Three basic methods used to control engine emissions:

1) Engineering of combustion process - advances in fuel injectors, oxygensensors, and on-board computers.

2) Optimizing the choice of operating parameters - two NOx control measures

that have been used in automobile engines since 1970s are spark retard and

EGR.

3) After treatment devices in the exhaust system - catalytic converter


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During the 1940s air pollution as a problem was first recognized in the LosAngeles basin.

Two causes of this were the large population density and the natural weather

conditions. Smoke and other pollutants combined with fog to form smog.

In 1966 HC and CO emission limits were introduced in California.

All of North America usually follows Californias lead (all US in 1968).

By making more fuel efficient engines and with the use of exhaust after

treatment, emissions per vehicle of HC, CO, and NOx were reduced by

about 95% during the 1970s and 1980s.

Automobiles are more fuel efficient now (2x compared to 1970) but there are

more of them and the trend is to larger SUVs, as a result fuel usage is

unchanged over this period.

Historical Perspective


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Ontario Drive Clean Program

In Ontario every vehicle must undergo a tail pipe emission test every other

year to check compliance with regulation:

Nitrogen Oxide 984 ppm @ 3000 rpm

Carbon Monoxide 0.48% @ 3000 rpm and 1.0% @ 800 rpm

Unburned hydrocarbons 86 ppm @ 3000 rpm and 200 ppm @ 800 rpm

Particulates (diesels only at present) 30% opacity

Evaporative Emissions (SI only at present)


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Test results between 1999 and March 2004

Light-Duty Program*: 14.6% failed test

Heavy-Duty Diesel**: 4% failed test

Heavy-Duty Non-Diesel**: 27.3% failed test

* 6 million vehicles (automobiles, vans, SUVs, pick-ups) in program

** 200,000 vehicles in program

Ontario Drive Clean Program Stats


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Emission Formation In S.I. EngineThis section provides an introduction to

vehicle emissions components.


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Principle S.I. Engine Exhaust Constituents

The chemical reaction that takes place in the enginecylinder when air is mixed with fuel and ignited is calledcombustion. Although combustion has been studied formany years, scientists and engineers are still unable to

fully predict the behavior of this complicated process. Several variables must be considered in determining the

correct levels of fuel, air, and ignition required forefficient combustion.

Port or direct fuel injection has improved the distributionof the fuel in the cylinders; mass air flow metersaccurately determine the amount of air flowing into anengine; and distributor-less electronic ignition systems

have improved the reliability and accuracy of ignitionsystems.

The entire power-producing process has considerablyreduced emissions and improved driveability over the

last 20 years.


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Consider the combustion of octane, C8H18. Under idealconditions, all of the hydrocarbon fuel consumed by the

engine would be converted to CO2, H2O, and N2. Sinceambient air contains approximately 21% oxygen and78% nitrogen by volume, each mole of oxygenconsumed involves (78/21) = 3.7 moles of N2.

With this information, the ideal combustion equation for afuel such as octane can be written by balancing thenumber of moles of each constituent on either side of the

combustion equation: C8H18 + (12.5)O2 + (12.5)(3.7)N2 --> (8.0)CO2 +

(9.0)H2O + (47.0)N2



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Hence, ideal combustion of octane producesapproximately 13% carbon dioxide, 14 % water vapor,

and 73 % nitrogen. However, actual vehicle exhaust gasalso contains unburned hydrocarbons (HC), carbonmonoxide (CO), oxides of nitrogen (NOx), aldehydes,and various other constituents. The emissions in exhaustas it leaves the engine are called engine-out or feedgasemissions; they are carried through a catalytic converter,and the net output to the air is called tailpipe emissions.

The ratio of air mass to fuel mass is called air fuel ratio

(A/F). The stoichiometric or theoretical A/F is defined asthe minimum amount of air that supplies sufficientoxygen for the complete combustion of all the carbon,hydrogen and any other elements in the fuel that may

oxidize. For example, the stoichiometric A/F for the idealcombustion of octane can be calculated as follows:



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On a molar basis:

(A/F)mole = (12.5 moles O2

+ 47.0 moles N2)

(1 mole of fuel)

(A/F)mole = 59.5 mole air/mole fuel

On a mass basis:

(A/F)mass = (59.5 mole air/mole fuel)*(28.97 kg/mole air)

(114.2 kg/mole fuel)

(A/F)mass = 15.0 kg air/kg fuel



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In current combustion control systems, air flow iscontrolled by throttle position. Closing the throttle platerestricts airflow, thereby reducing power output.

Fuel flow is regulated by electronic control of the amountof time the solenoid fuel injectors are open.

The electronic engine control system measures airflow

and adjusts the injector control to maintain A/F withinfairly narrow limits around the stoichiometric A/F.

A/F ratio data is often presented as fuel to air ratio, F/A,

or as the equivalence ratio, commonly defined in the twoforms shown below:

= (A/F)/(A/Fstoich)

= (F/A)/(F/Astoich

)



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It is important to define what measure is being used indescribing the relationship between A/F and otherparameters.

Keys for remembering the trends in each measure of A/Fis:

Measure Leaner

Combustion

Richer

Combustion

A/F Bigger Smaller

F/A Smaller Bigger Bigger Smaller

Smaller Bigger



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Under normal operating conditions, stoichiometricoperation is maintained by control electronics.

When the control system is operating in closed-loopmode, it utilizes feedback signals from the heatedoxygen sensors. These signals provide a means for

determining how close the vehicle is to stoichiometricoperation.

When the control system is operating in open loop

mode, this feedback signal is not used. In open loopmode, the control electronics uses default operationparameter values for determining correct operation.



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Emission Formation Mechanisms

This section discusses the formation of HC, CO, NOx,

CO2, and aldehydes and explains the effects of designparameters.


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(i) Hydrocarbon Emissions

Hydrocarbon emissions result from the presence ofunburned fuel in the engine exhaust.

HC emissions are various compounds of hydrogen,carbon, and sometimes oxygen. They are burned orpartially burned fuel and/or oil. HC emissions contributeto photochemical smog, ozone, and eye irritation.

However, some of the exhaust hydrocarbons are notfound in the fuel, but are hydrocarbons derived from thefuel whose structure was altered do to chemical reactionthat did not go to completion. For example: cetaldehyde,

formaldehyde, 1,3 butadiene, and benzene all classifiedas toxic emissions.


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(i) Hydrocarbon Emissions There are several formation mechanisms for HC, and it

is convenient to think about ways HC can avoidcombustion and ways HC can be removed; we will

discuss each below. Most of the HC input is fuel, and most of it is burned

during normal combustion.

However, some HC avoids oxidation during this process. About 9% of the fuel supplied to the engine is not burnedduring the normal combustion phase of the expansionstroke.

Only 2% ends up in the exhaust the rest is consumedduring the other three strokes.

As a consequence hydrocarbon emissions cause adecrease in the thermal efficiency, as well as being an

air pollutant.


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The processes by which fuel compounds escapeburning during normal S.I. combustion are:

1. Fuel vapor-air mixture is compressed into thecombustion chamber crevice volumes.

2. Fuel compounds are absorbed into oil layers on the

cylinder liner.3. Fuel is absorbed by and/or contained within

deposits on the piston head and piston crown.

4. Quench layers on the combustion chamber wall areleft as the flame extinguishes close to the walls.

5. Fuel vapor-air mixture can be left unburned if theflame extinguishes before reaching the walls.


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6. Liquid fuel within the cylinder may not evaporate andmix with sufficient air to burn prior to the end ofcombustion.

7. The mixture may leak through the exhaust valveseat.

Each of these seven processes allows some fraction of the

fuel to escape normal combustion. However, there are anumber of hurdles which the HC must pass before it canmake it into the exhaust system

E i i F ti M h i


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The processes by which HC can oxidize or exit the cylinderare:

1. Outflow of unburned mixture from the crevices, mixingwith the hot burned gases; some of this will oxidize.

2. Diffusion of HC vapor from oil layers and deposits intothe burned gases; some will oxidize.

3. Mixing of wall quench and burned gases; some willoxidize.

4. Exhaust blowdown process carries some HC into theexhaust.



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5. Displacement of gases by the piston during theexhaust stroke will transport an additional fraction into

the exhaust.6. Unburned HC leaving the cylinder mixes with hot

exhaust gases; some of this will oxidize in the port andmanifold.

The formation and exhausting of HC is very complex. Norigorous models of HC emissions exist today, although

many labs are working on the problem.



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Hydrocarbon Emission Sources for SI Engines

The six principal mechanisms are believed to be responsible for

hydrocarbon emissions:

% fuel escaping

Source normal combustion % HC emissions

Crevices 5.2 38

Oil layers 1.0 16

Deposits 1.0 16

Liquid fuel 1.2 20Flame quench 0.5 5

Exhaust valve leakage 0.1 5

Total 9.0 100



E i i F ti M h i


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Crevices these are narrow regions in the combustionchamber into which the flame cannot propagate becauseit is smaller than the quenching distance.

Generally, any volume with a dimension less than onemillimeter across will form a crevice.

Crevices are located around the piston, head gasket,spark plug and valve seats and represent about 1 to 2%of the clearance volume.

With walls so close together, the flame cannot burn intothe crevice, so mixture in the crevices escapes primarycombustion.

E i i F ti M h i


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Figure 1 illustrates the various crevice volumes which

exist in an engine.

Figure 1 Crevice Volume Sites.

E i i F ti M h i


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The crevice around the piston is by far the largest.

Crevices typically amount to about 2% of the cylinderclearance volume. Fuel-air mixture is compressed intothe crevices during the compression stroke and alsoduring combustion (density higher than cylinder gassince gas is cooler near walls) and released during

expansion.

Crevice

Piston ring



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At peak pressure, calculations suggest as much as 6-8%of the mixture may be trapped in the crevices. This is

much higher than the crevice volume fraction becausethe gases in the crevices cool rapidly to something closeto wall temperature, which is much lower than cylindertemperature at peak pressure. As a result of the lower

temperature, their density is 3-4 times higher than theburned gases.

As the piston travels downward, pressure in the cylinder

decreases and mixture comes back out of the crevices.

Crevices are estimated to cause roughly 40% of HCemissions.


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The second HC mechanism is related to the thin layerof oil which is always present on the cylinder walls.

Since the piston ring is not 100% effective in preventingoil migration into the cylinder above the piston, oil layersexist within the combustion chamber. This oil layer traps

fuel and releases it later during expansion. Although the rings scrape off most of the oil, there must

be a tiny layer to provide lubrication for the rings on theup strokes.

This thin layer (on the order of tens of microns inthickness) absorbs HC. Oil layers are thought to causeroughly 10-30% of HC emissions.



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Figure 2 Unburned Hydrocarbon Solubility in Oil.


Figure 2 shows the effect on HC of fuel solubility in oil.Oils with higher fuel solubility increase HC emissions.Unfortunately, practical oils - i.e., those which prevent

mechanical wear acceptably, have a very limited rangeof solubilities so that we cannot do too much about thismechanism.



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(i) Hydrocarbon Emissions Deposits - With continued use carbon deposits build up

on the valves, cylinder and piston head. These deposits

are porous with pore sizes smaller than the quenchingdistance so trapped fuel cannot burn. Deposits can

absorb fuel in a manner similar to oil. However, deposits

are not present on new engines but accumulate withmileage. The fuel is released later during expansion.

Operating conditions also affect the deposition and

removal of deposits. As a result, the effect of deposits onHC emissions is rather difficult to identify.



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Hydrocarbon Exhaust Process

(i) Hydrocarbon Emissions When the exhaust valve opens the large rush of gas

escaping the cylinder drags with it some of the hydrocarbons

released from the crevices, oil layer and deposits.

During the exhaust stroke the piston rolls the hydrocarbons

distributed along the walls into a large vortex that ultimately

becomes large enough that a portion of it is exhausted.

Blowdown ExhaustStroke




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Hydrocarbon Exhaust Process

(i) Hydrocarbon Emissions The first peak is due to blowdown and the second peak is

due to vortex roll up and exhaust (vortex reaches exhaust

valve at roughly 290o)


Exhaust

valve

opens

Exhaust

valve

closes

TCBC



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(i) Hydrocarbon Emissions Liquid fuel For some fuel injection systems there is a

possibility that liquid fuel is introduced into the cylinder

past an open intake valve. The less volatile fuelconstituents may not vaporize (especially during enginewarm-up) and be absorbed by the crevices or carbondeposits.



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(i) Hydrocarbon Emissions Flame quenching It has been shown that the flame

does not burn completely to the internal surfaces, theflame extinguishes at a small but finite distance from the

wall. Most of this gas eventually diffuses into the burnedgas during expansion stroke.

Flame quenching is another HC producing mechanism

but one not very well understood. Under extremeconditions of lean A/F, high exhaust gas recirculation(EGR), or high residual gas fraction, the flame may notpropagate fully across the chamber. Very severe

decelerations may produce such conditions. Figure 3 summarizes the path of HC through the engine.

The percentages are rough estimates of the percent ofthe input fuel which goes through each path for a typicalpartial load condition.

E i i F ti M h i


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Figure 3 Hydrocarbon Path Through Engine.



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(i) Hydrocarbon Emissions HC emissions are also dependent on several other

parameters such as spark advance, A/F ratio, and

EGR. High advance generally increases HC emissions,

primarily because the temperature of the charge is lowerduring the expansion stroke. This reduces oxidationduring expansion as the HC comes off the walls and outof the crevices.

Lower A/F ratios (richer combustion) cause more HC

emissions because there is less O2 to oxidize fuel. Verylean A/F ratios cause HC emissions to increase due tomisfires, partial burns, and reduced temperature duringexpansion.



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Figure 4 EGR Effect on

Hydrocarbon Formation.

Figure 4 depicts HC formation versus EGR rate.

High EGR causes an increase in HC emissions. Theincrease is gradual at first, but the curve becomes steep

as the combustion degrades.

In fast burn engines, the combustion degradation sets inat higher EGR rates than in slow burn engines.



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(ii) Carbon Monoxide

Formation of CO is well established. Under someconditions, there is not enough O2 available for complete

oxidation and some of the carbon in the fuel ends up asCO.

The amount of CO, for a range of fuel composition andC/H ratios, is a function of the relative air-fuel ratio. Evenwhen enough oxygen is present, high peak temperaturescan cause dissociation - chemical combustion reactionsin which carbon dioxide and water vapor separate intoCO, H2, and O2. Conversion of CO to CO2 is governed

by reactionCO + OH CO2 + H

Dissociated CO may freeze during the expansion stroke.

Figure 5 shows CO production as a function of A/F ratio.



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Figure 5 Air/Fuel Ratio Effects on Carbon Monoxide Concentration.



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(ii i) Oxides of Nitrogen

NOx is a generic term for the compounds nitric oxide(NO) and nitrogen dioxide (NO2). Both are present to

some degree in the exhaust, and NO oxidizes to NO2 inthe atmosphere.

NOx contributes to acid rain and photochemical smog; itis also thought to cause respiratory health problems at

atmospheric concentrations found in some parts of theworld.

To understand NOx formation, we must recognizeseveral factors that affect NOx equilibrium. Rememberthat all chemical reactions proceed toward equilibrium atsome reaction rate.



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In SI engines the dominant component of NOx is NOforms as a result of dissociation of molecular nitrogen

and oxygen. Since the activation energy of the critical elementary

reaction O+N2NO+N is high the reaction rate is verytemperature dependent, w''~ exp (-E/RT)

Therefore NO is only formed at high temperatures andthe reaction rate is relatively slow.

At temperatures below 2000K the reaction rate is

extremely slow, so NO formation not important.

Equilibrium NO (which comprises most of the NOxformation) is formed at a rate that varies strongly with

temperature and equivalence ratio, as shown in Figure 6.



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Figure 6 Temperature and Equivalence Ratio Effects on NO Formation Rate.



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Since the cylinder temperature changes throughout the

cycle the NO reaction rate also changes.

In figure 6, much of the mixture in the cylinder is burnedessentially adiabatically (without loss or gain of heat). Aswe can see, more NO will be formed rapidly during theengine cycle when the peak temperature is highest and/or

when the equivalence ratio is slightly lean of stoichiometry. During the combustion process, O2 and N2 are heated to

high temperatures, and NO is formed very rapidly at the

peak temperatures. Then, during expansion, the temperature drops so rapidly

that the reaction rate slows before the concentration cango all the way down; the NO concentration freezes at a

level typical of equilibrium at an elevated temperature.

SI Engine In-cylinderNO Formation


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Each fluid element burns to its AFT based on its initialtemperature, elements that burn first near the spark plug

achieve a higher temperature.

Since the chemistry is not fast enough the actual NOconcentration tends toward but never achieves the

equilibrium value.IfNO concentration is lower than equilibrium value NO forms

IfNO concentration is higher than equilibrium value NO decomposes

Once the element temperature reaches 2000K the reaction

rate becomes so slow that the NO concentration effectively

freezes at a value greater than the equilibrium value.

The total amount ofNO that appears in the exhaust is

calculated by summing the frozen mass fractions for all the

fluid elements: =1

0dxxx NONO


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x = 0

-15o (x = 0)x = 1

25o (x = 1)

x = 0

x = 1

Equilibrium concentration:

based on the local temperature, pressure,

equivalence ratio, residual fraction

Actual NO concentration:

based on kinetics

(assuming no mixing of fluid elements)



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Figure 7 Air/Fuel Effect on NO

Formation.

One would expect the peak NO concentrations to

coincide with highest AFT.

Figure 7 shows feed-gas emission of NOx versus A/F

ratio. A peak occurs at about 16:1 A/F.

Under rich conditions, the amount of available O2 is

less than optimal for NOx formation. Under leaner conditions, the peak temperatures drop.



(ii i) O id f Nit


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Typically peak NO concentrations occur for slightly leanmixtures that corresponds to lower AFT but higher oxygen

concentration.

Effect of Equivalence Ratio on NO Concentration



(ii i) O id f Nit


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Exhaust NO Concentration Reduction

Since the formation of NO is highly

dependent on cylinder gastemperature any measures taken to

reduce the AFT are effective:

increased residual gas exhaust gas recirculation (EGR)

moisture in the inlet air

In CI engines the cylinder gastemperature is governed by the load

and injection timingIDI/NA indirect injection

naturally aspirated

DI/NA direct injectionnaturally aspirated




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Exhaust Gas Recirculation is used to reduce NOx. Byrecirculating a fraction of the exhaust into the intakecharge, the heat capacity (Cp) of the mixture in thecylinder is increased.

This means that the temperature of the charge increases

less during compression (including the compression ofunburned charge during combustion). As we have seen,even a small drop in peak temperature can have adramatic effect on NO formation.

Thus, EGR has a strong effect on NO, as shown inFigure 8. A 2:1 reduction is obtained with as little as 10%EGR.



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Figure 8 EGR Effect on NO Formation.




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Unfortunately, EGR also reduces flame speed.

Excessive EGR rates can slow burn rate so drastically

as to cause surge, misfire, and other combustionproblems.

However, EGR can increase fuel economy.

When used optimally, EGR can reduce NO and improvefuel economy while avoiding rough engine operation.

This optimum varies with speed, load, and enginedesign.

NOx also varies with spark timing. More spark advancemeans that parts of the mixture spend more time at hightemperatures, so more NO is formed.



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Effect of Various Parameters on NO Concentration

Increased spark advance and intake manifold pressure

both result in higher cylinder temperatures and thus

higher NO concentrations in the exhaust gas

= 0.97

= 1.31

= 1.27

= 0.96

Pi= 354 mm HgPi= 658 mm Hg



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(iv) Carbon Dioxide While not normally considered a pollutant, CO2 may

contribute to the greenhouse effect. Proposals to reduceCO2 emissions have been made.

CO2 controls strongly influence fuel economyrequirements.

(v) Aldehydes

Aldehydes are the result of partial oxidation of alcohols.They are not usually present in significant quantities in

gasoline-fueled engines, but they are an issue whenalcohol fuels are used.

Aldehydes are thought to cause lung problems. So far,little information of engine calibration effects on aldehyde

formation is available.

Fuel and Combustion Control

For S I Engine


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For S.I. Engine

Fuel Injection Technology

Fuel injectors are solenoid valves which deliver andatomize precisely controlled amounts of fuel into an airintake system.

The operation of fuel injectors is controlled by thepowertrain control module (PCM) or engine control unit

(ECU). Use of fuel injection provides for better cold-driveability,

lower emissions, and eliminates dieseling when the key

ignition is turned off.



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The three principle fuel injection systems are as follows: Central Fuel Injection (CFI)--Injector delivers fuel

above the throttle plate.

Multiport Fuel Injection (MFI)--Injector placed neareach intake valve; injectors arranged on commonfuel rails open and close simultaneously.

Sequential Electronic Fuel Injection (SEFI)--

Injector placed near each intake valve; injectoroperation matched to operation of correspondingIntake valve. SEFI is also called SFI (SequentialFuel Injection).

SEFI provides better performance, driveability, and loweremissions than CFI and MFI.



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Figure 9 shows the various components of a fuel injector. Pressurized fuel enters the fuel injector at the top, then

passes through a filter and into the injector body, where a

needle blocks it from passing out the injector orifice.When the solenoid activates, it pulls back the needle,thus allowing fuel to pass out of the injector through themetering plate. When the coil is de-energized a spring

pulls the needle back to its seated position, therebyclosing off the exit orifice to further fuel flow. The amountof fuel delivered depends on how long the injector isopen.

Typical fuel injection "pulse width"--the amount of time thefuel injector is delivering fuel--is between one and fifteenmilliseconds, depending on what the PCM commands.

Heated Oxygen Sensors


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The Heated Oxygen Sensor (HO2S, formerly known asHEGO), shown in Figure 10, is located in the exhaustmanifold and monitors the oxygen content in theexhaust. This information is used by the PCM to controlthe fuel metering to keep the desired air-fuel mixtureentering the cylinder chamber.

An HO2S has a ceramic surface coated in platinum. This

ceramic surface can conduct oxygen ions when exposedto a gas of at least 300 oC, and when the oxygen sensoris internally heated it is functional for exhaust gastemperatures as low as 200 oC. When the surface is

exposed to both the exhaust gas and ambient air, avoltage signal is generated indicative of how muchoxygen is in the exhaust.



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The Heated Oxygen Sensor (HO2S, formerly known asHEGO), shown in Figure 10, is located in the exhaustmanifold and monitors the oxygen content in theexhaust.

This information is used by the PCM to control the fuelmetering to keep the desired air-fuel mixture entering thecylinder chamber.

An HO2S has a ceramic surface coated in platinum. Thisceramic surface can conduct oxygen ions when exposedto a gas of at least 300 oC, and when the oxygen sensor

is internally heated it is functional for exhaust gastemperatures as low as 200 oC. When the surface isexposed to both the exhaust gas and ambient air, avoltage signal is generated indicative of how much

oxygen is in the exhaust.



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Figure 9 Fuel Injector.

Figure 10 Heated Oxygen Sensor.



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HO2S are located in the exhaust pipe both before andafter the catalytic converter.

The HO2S before the catalytic converter gives thefeedback necessary to keep the desired air-fuel mixture.

The output of the HO2S after the catalyst is compared tothe output of the HO2S before the catalyst as anindication of catalytic converter efficiency with respect to

oxygen. This is used in the On-Board Diagnostics to determine

catalyst burnout.

Powertrain Control Module


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The powertrain control module (PCM) controls avehicle's powertrain (P/T) by monitoring electronic

signals generated by various sensors in order toascertain current powertrain conditions.

Based on these P/T conditions, the PCM'smicroprocessing unit will signal various actuators to

perform certain control functions according to acomputer program referred to as the vehicle's enginecontrol calibration.

This calibration is composed of smaller programs calledstrategies.



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Different strategies are used to control engine operationfor the following: Engine Cranking;

Cold Start/Warm Up;

Cold Driveaway; Warm Driveaway;

Warm Cruise;

Part-Throttle Acceleration; Full-Throttle Acceleration;

Deceleration; and

Warm-Idle.

The strategies, stored in the PCM's Programmable ReadOnly Memory (PROM) chip, contain the logic to controlthe powertrain. Strategy development is an integral partof the development of a powertrain.



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Within each strategy there exists a vast quantity ofvariables used by the logic.

Certain variables, referred to as Random AccessMemory (RAM) variables, are set by calibratorsduring vehicle operation to reflect current signalstatus.

Other variables, referred to as Read Only Memory

(ROM) variables, are set by calibrators duringdevelopment for use as references in the logic.

We can significantly enhance a powertrain'sperformance by setting the ROM variables at optimum

values during strategy development.

We refer to this process as engine calibration.

Exhaust Gas Recirculation (EGR)

Th EGR t h i Fi 11 i i l d t


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The EGR system, shown in Figure 11, is mainly used to

reduce nitrogen oxides (NOx) which are formed in the

combustion chamber at high temperatures.

Figure 11 EGR System.


The EGR system recirculates already combusted exhaust


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The EGR system recirculates already combusted exhaust

from the exhaust manifold back to the air-fuel mixture andis drawn into the cylinder chamber. This reduces the peakcombustion temperature in the cylinder chamber. Thistemperature reduction in turn reduces NOx formation.

The EGR flow is directed by a solenoid valve that iscontrolled by the Powertrain Control Module (PCM). ThePCM uses inputs such as engine speed, intake-manifold

pressure, and engine temperature to determine how muchexhaust flow to meter back to the air-fuel mixture.



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Disadvantages of EGR are increases in hydrocarbonemissions and fuel consumption.

Consequently, EGR is not used during conditions of lowNOx formation such as idle.

EGR is also not used during wide open throttle (WOT)when full power is needed for acceleration.

Emission Formation In C.I. Engine


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For many years, diesel engines have had a reputation ofgiving poor performance and producing black smoke, anunpleasant odor, and considerable noise.

However, it would find it difficult to distinguish todaysmodern diesel car from its gasoline counterpart. Fordiesel engines the emphasis is to reduce emissions ofNOx and particulates, where these emissions are

typically higher than those from equivalent port injectedgasoline engines equipped with three-way catalysts.

Catalyst of diesel exhaust remains a problem insofar as

research has not yet been able to come up with aneffective converter that eliminates both particulate matter(PM) and oxide of nitrogen (NOx).

Principle C.I. Engine Exhaust Constituents

Acquired load of diesel engines is accomplished by


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Acquired load of diesel engines is accomplished bymodifying the fuel quantity supplied, whereas the airquantity per work cycle is drawn in without any throttling.The air to fuel mixture ratio is therefore adjusted toachieve load required.

This means that the mixture in its working range is madeleaner or richer, respectively. Depending on friction

power and the combustion process of the engine, themixture setting, as shown in Figure 12,may made leanerup to levels of = 10.

This diesel-engine processes is referred to as qualitymanagement.



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Figure 12 Excess air ratio map of a diesel engine.


I th ith SI i th i /f l ti


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In the same manner as with SI engines, the air/fuel ratio

of the diesel engine has a significant impact on the level

of pollutant concentrations but this parameter is not

freely available for minimizing pollution.

Figure 13 shows an example of the dependence of major

pollutant concentrations on the air /fuel ratio in the case

of a direct-injection diesel engine.



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Figure 13 Exhaust concentrations as a function of air/fuel ratio.



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Carbon monoxide: 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 themixture built up by stratification, however, extremelyrich local zones are exist. This produces high COconcentrations that are reduced to a greater or lesser

extent by post-oxidation. When the coefficient increases, i.e. when the excess-air

ratio increases, dropping temperatures cause the post-

oxidation rate to be reduced. The reactions freeze up.The CO concentrations of diesel engines therefore arefar lower than in SI engines.

The basic principles of CO formation, however, are the

same as in SI engine.

Carbon Monoxide



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Carbon monoxide appears in the exhaust of fuel rich running engines.

For fuel rich mixtures there is insufficient oxygen to convert all the

carbon in the fuel to carbon dioxide.

C8H18-air

Carbon Monoxide



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Carbon Monoxide The C-O-H system is more or less at equilibrium during combustion

and expansion.

Late in the expansion stroke when the cylinder temperature gets

down to around 1700K the chemistry in the C-O-H system becomesrate limited and starts to deviate from equilibrium.

In practice it is often assumed that the C-O-H system is in

equilibrium until the exhaust valve opens at which time it freezes

instantaneously.

The highest CO emission occurs during engine start up (warm up)

when the engine is run fuel rich to compensate for poor fuel

evaporation.

Since CI engines run lean overall, emission of CO is generally low

and not considered a problem.



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Unburned hydrocarbons:

Since the air-fuel mixture is not homogeneous

throughout, extremely too lean (high excess air ratios)

are present in certain zones during the dieselcombustion process.

The leaner the air-fuel mixture, the lower is the local

temperature. This means that chemical reactions proceed fairly slowly

or may even freeze up, thus leading to increased HC

emissions. On the whole, HC concentrations of diesel engines are

lower than those of SI engines.

Emission Formation MechanismUnburned hydrocarbons:


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Crevices - Fuel trapped along the wall by crevices, deposits, or oil due toimpingement by the fuel spray (not as important as in SI engines).

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.

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.

ExhaustHC,

ppm

C

Emission Formation Mechanism

Unburned hydrocarbons:


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Note for the direct injection diesel the hydrocarbon emission are the worst at

light load (long ignition delay)

y



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Nitrogen oxides: NOx concentrations are lower than in SI engines; the

share of NO2 in the NOx emissions is slightly higher.

When comparing an SI engine with three-way closed-loop catalytic converter with a diesel engine with

oxidation catalyst, it is found that NOx concentrations of

the SI engine are lower.

The type of combustion process has a significant effect

on nitrogen oxide formation.

Emission Formation MechanismsNitrogen oxides:


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Diesel engine with IDI combustion chamber: Inengines with an IDI combustion chamber, combustioninitially occurs in the pre-chamber or swirl chamberunder conditions of extreme oxygen deficiency. Thisgenerates high temperatures, but NOx levels are lowdue to a lack of oxygen. This process is reversed in themain combustion chamber. Extreme excess air ratiosand, hence, low temperatures also result in low NOx

formation.


Nitrogen oxides:


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Diesel engine with DI combustion chamber: The

direct-injection diesel engine does not have the above

features that keep NOx emissions low. As a result, NOx

formation is approximately twice as high as with an IDI

combustion chamber engine. When exhaust recirculation

(EGR) systems are used, both systems show virtually

identical NOx levels as the direct-injection diesel engineis far more compatible with large re-circulated exhaust

quantities (more than 50% of the fresh charge).


Sulphur compounds:


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Sulphur compounds are caused by the sulphur contentof the diesel fuel.

According to modern world-wide fuel regulations, this

content may have to be reduced to below 0.05 weightpercent.

When combined with the water produced during the

combustion process, SO2 produces sulphuric acid. Sulphur compounds cause problems with regard to acid

rain and particulate formation via sulphates.

Emission Formation MechanismsParticulates:


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A high concentration ofparticulate matter(PM) ismanifested as visible smoke in the exhaust gases.

Particulates are any substance other than water that can be

collected by filtering the exhaust, classified as:1) solid carbon material or soot

2) condensed hydrocarbons and their partial oxidation products

Diesel particulates consist of solid carbon (soot) at exhaustgas temperatures below 500oC. HC compounds becomeabsorbed on the surface.

Particulate can arise if leaded fuel or overly rich fuel-airmixture are used. Burning crankcase oil will also producesmoke especially during engine warm up where the HCcondense in the exhaust gas.


Particulates:


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According to the California Air Resources Board (CARB),particulate matter is defined as all exhaust componentsthat are deposited on a defined filter after having been

diluted with air to a temperature below 51.7o

C. Basically, soot emissions also are part of particulate

emissions.

Soot formation occurs at extreme air deficiency. This airor oxygen deficiency is present locally inside dieselengines. It increases as the air/fuel ratio approaches avalue = 0.



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Most particulate material results from incompletecombustion of fuel HC which occurs in fuel rich mixtures.

Based on equilibrium the composition of the fuel-oxidizer

mixture at the onset of soot formation occurs when x 2a(orx/2a 1) in the following reaction:

i.e. when the (C/O) ratio exceeds 1. Experimentally it is

found that the critical 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

)()2(2

2 22 sCaxHy

aCOaOHC yx +++

OHOHCOOsCCOOCO 2222222 2

1)(2

1

+++


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


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the exhaust! Soot is produced by thermal cracking of long-chain

molecules at oxygen deficiency. A separation of hydrogenleads to C-structures showing an increasing lack of

hydrogen. Acetylene and other polymerization processes lead to

formation of molecules rich in carbon that form sootparticulates.

Once soot has formed, it can be oxidized only to a limitedextent.

The process described here is shown schematically in

Figure 14. The logarithm of the molecular mass is shown asa function of the hydrogen contents of the hydrocarboncomponents.

Soot formation produces molecules with an increasingly low

hydrogen content and higher weight that will finallyagglomerate to form soot particulates.


Particulates:


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Figure.14 Potential soot formation process.


Particulates:


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Particulates consist of solid (organically insoluble) andliquid (organically soluble) phases.

The solid phase consists of

Soot in the form of amorphous carbon, ash, oiladditives, corrosion products and abrasion products.

Sulphates and its molecule-bound water.

The liquid phase consists of

fuel and lubricant contents that are, usually, combined

with soot. The hydrocarbons contained in the hot

exhaust still are largely gaseous and are convertedinto a liquid, organically soluble phase (particulates)

only after having been cooled by turbulent intermixing

with air.


Particulates:

Th i f h ti l t i 0 01 t 1 d


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The size of such particulates is approx. 0.01 to 1m andabove. Most particulates have a size below 0.3m and

some of them can therefore penetrate into the lungs.

Particulate composition is largely dependent on theoperating point and the combustion process.

Figure 15 shows a typical particulate composition of

diesel engine exhaust gas.


Particulates:


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Figure 15. Particulate composition of diesel engine exhaust.

Particulates are a major emissions problem for CI engines.

E h t k li it th f ll l d ll i l ti t b t 0 7

Particulates and CI Engines


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Exhaust smoke limits the full load overall equivalence ratio to about 0.7

An outstanding problem for diesel engine designers is that in order to reduce

NOx one wants to reduce the AFT but this has the adverse effect of decreasing

the amount of soot oxidized, or increases the amount of soot in the exhaust.

= 0.7

= 0.5

= 0.3

One technique for measuring particulate

involves diluting the exhaust gas with

cool air to freeze the chemistry before

measurements

An example of this dilemma is changing the start of injection, e.g., increasing

the advance increases the AFT

Particulates and CI Engines


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the advance increases the AFT

Crank angle bTC for

start of injection

Fuel and Combustion Control For C.I. Engine

I i l t i t i i l i l ti th t


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Increasingly stringent emission legislation means that

the high speed CI engine will have to employ advanced

fueling technologies to meet these requirements

principally by employing systems which use very highfuel injection pressures with electronic control of

injection timing and fueling.

These large pressure differences across the injectornozzle are required so that the injected liquid fuel enters

the chamber at sufficiently high velocity to:

Atomize into small-sized droplets to enable rapidevaporation.

Traverse the combustion chamber in the time available

and fully utilize the air charge.

Electronic Fuel Injection Technology

DI diesel engines will use electronic fuel injection


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DI diesel engines will use electronic fuel injection

systems. Replacing traditional mechanical systems,

which are now approaching the end of their effective

life. In what are termed full authority fuel injectionsystems, the benefits include emission reduction,

improved driveability. Smoother idling, integration with

other vehicle systems, self-diagnostics, andperformance checks.

The benefits of higher injection pressure on particulates

and smoke are shown in Figure 16.

Electronic Fuel Injection Technology


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Figure 16 Benefits of higher mean injection pressure

on particulate and smoke.

Electronically Controlled Distributor Pumps

The fuel injection pumps are able to vary the fueling and

injection timing using electro hydraulic devices


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injection timing using electro-hydraulic devices. The cam ring is rotated to vary injection timing using a

hydraulic actuator.

The working fluid is diesel fuel, the pressure beingregulated by a solenoid valve acting on a pulse widthmodulation (PWM) signal from the electronic control unit(ECU).

Fueling is varied by moving the rotor mechanism axiallyusing a second actuator. Or a solenoid-operated spillvalve Injection timing is still set by rotating the cam ringthe use of the solenoid-operated spill valve may allowrate shaping and pre-injection.

Electronically Controlled Distributor Pumps

If the cam profile has a varying rate of rise, the injection

rate can be varied by using the appropriate section of the


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rate can be varied by using the appropriate section of thecam. Errors in the timing of the valve operation willinfluences start of injection timing, injection rate, andinjected quantity.

In addition to controlling the fuel pump, the ECUsupervises a EGR control, inlet throttle control (if fitted),and turbocharger wastegate or variable geometryturbocharger (if fitted).

The ECU will generally have other abilities including on-board diagnostics (OBD), cruise control, and networkingwith other controllers. The advent of electronic controlhas opened up a variety of alternative approaches.

Electronic Unit Injector (EUI) System

The EUI is a fueling system. Both pump and injector are

combined in a single unit for each cylinder (Figure 17)


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combined in a single unit for each cylinder (Figure 17). Each pumping plunger is driven directly by the engines

camshaft, and this enables very high injection pressures

(in the region of 1500-2000 bar) to be obtained. The main benefits of these very high injection pressure

are smaller droplet sizes, better fuel-air mixing, andreduced ignition delay.

Electronic Unit Injector (EUI) System


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Figure 17 Schematic of EUI system.

Electronic Unit Injector (EUI) System The timing and quantity of fuel can be adjusted to give

optimum conditions over the complete engine operatingprofile


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optimum conditions over the complete engine operatingprofile.

The system also has the capability to act as a pilotinjection device to reduce NOx and engine noise.

Sensors provide information to the electronic control unit(ECU) on the relevant functions of engine operation,which are: Acceleration position

Engine speed Camshaft position

Inlet manifold temperature and pressure

Coolant temperature

This information is continuously compared with theoptimum values stored in the ECU memory. The result ofthat comparison is translated into signals instructing theunit injector solenoid-actuated spill valve system to

deliver the fuel at the timing required by the engine.

Caterpillar HEUI System

This hydraulic electronic unit injector (HEUI) requires no

mechanical actuation or mechanical control devices


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mechanical actuation or mechanical control devices Inherent features of the HEUI fuel system include

injection pressure control independent of engine speed

or load, totally flexible injection timing, and full control ofinjection parameters.

High-pressure oil from the pump flows to a manifold or

rail, which is connected to the injectors. A solenoidoperates a control valve to start and end the injection

process. Initial injection rate shaping is achieved with a

Ported spill control device in the plunger and barrel; thisis called PRIME (preinjection metering).

The HEUI system is time based, so injection

characteristics are independent of engine speed.

Caterpillar HEUI System


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Figure 18 caterpillar HEUI system.

The Common Rail System

The common rail system in which high-pressure fuel is

fed into a manifold (or rail), which then serves individuali j t (Fi 19)


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fed into a manifold (or rail), which then serves individualinjector (Figure 19).

Figure 19 Schematic of a common rail system


The injector is simply a solenoid-actuated nozzle, which

is opened under the control of the ECU when injection isrequired


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p jrequired.

In principle the system is very simple, a low-drive torquemechanical pump pressurizes the distribution rail withdiesel fuel at the desired injection pressure Short pipesconnect the rail to each injector. There is no pressureintensification within the injector.

The injection process is controlled by a solenoid valve ineach injector.

The final fuel injected quantity being dependent on the

opening period and system pressure. The solenoid is controlled by the ECU, as is the railpressure via a pressure sensor.


The key advantage of the common rail fuel system

compared to a traditional pump-line-nozzle system, isthat injection pressure is independent of engine speed


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p p p y ,that injection pressure is independent of engine speedand load.

Injection timing, rate, and duration can be variedprecisely over the operating range.

At part load the higher injection pressure available givesa reduction of particulate emissions. The lower

particulate emissions allow higher EGR rates, so theNOx emissions can be reduced, leading to a superiorNOx particulates trade off.

Pilot injection for combustion noise control is feasiblewith common rail. Small fuel quantities of 1 to 2 m3 /stroke can be injected before the main injection.


The high-pressure pump acts continuously and so has a

more uniform drive torque than the highly cyclic loadsimposed by distributor and unit injector pump


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q g y yimposed by distributor and unit injector pump.

A comparison between the capabilities of pump-line-nozzles systems, EUIs, common rail systems is shown inFigure 20.

Figure 20 Comparison of different electronic fuelling system capabilities.

Exhaust Gas Recirculation Exhaust gas recirculation (EGR) is a well-proven

technique for the effective suppression of NOxemissions. The reductions in emissions due to the


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influence of EGR are shown in Figure 20.

Figure 20 General trend on emissions due to the influence

of EGR in DI diesel engine.

Control of EGR EGR systems consist of an electronically controlled,

vacuum-operated poppet valve mounted mounted ont ehexhaust or the intake manifold.


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A current-to-vacuum transducer (CVT) controls theamount of vacuum applied to the EGR valve diaphragm,

which in turn lifts the poppet off its seat. The engine ECU contains schedules (look-up tables),

which relate engine speed, fueling rate, and mass airflow (MAF).

The MAF sensor is located in the air intake system andprovides the information to determine EGR valveposition and, hence, EGR mass flow is calculated by

implication.

Control of EGR There are a number of disadvantages with this method,

namely:

Volumetric characteristics of the engine can drift with


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gtime due to wear and tear and the build-up of deposits

The flow capacity of the EGR system can be affected

by partial blocking through deposit build-up (i.e., soot,particulates, and so on and wear of the EGR valve)

Blockages in the inlet air filter can seriously affect

EGR mass flow Blockages in the exhaust can cause increased

exhaust pressure


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After Treatment Devices

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.


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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).

Three-way Catalytic Converter

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


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HC and H2 via a reduction reaction on the catalyst surface.e.g., NO+CON2+CO2 , NO+H2 N2+H2O, and others

The remaining CO and HC are removed through an oxidation reaction formingCO2 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 are 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

determine the actual A/F ratio and used to adjust the fuel injector so that the

A/F ratio is near stoichiometric.

Effect of Mixture Composition


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Since thermal efficiency is highest for slightly lean conditions it may seem thatthe use of a catalytic converter is a rather severe constraint.

The same high efficiency can be achieved using a near stoichiometric mixture

and diluting by EGR

Effect of Temperature

The temperature at which the converter becomes 50% efficient is referred to

as the light-off temperature.

The converter is not very effective during the warm up period of the engine


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Catalytic Converter for Diesels

For Diesel engines catalytic converters are used to control HC and CO, but

reduction of NO emissions is poor because the engine runs lean in order to

avoid excess smoke.


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The NO is controlled by retarding the fuel injection from 20o to 5o before TC in

order to reduce the peak combustion temperature.

This has a slight negative impact, increases the fuel consumption by about 15%

pollutant formation

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